The equine encephalitis complex, including western, eastern, and Venezuelan equine encephalitis viruses (WEEV, EEEV, and VEEV), are New World alphaviruses in the family Togaviridae (Aguilar et al., 2011; Calisher, 1994). These viruses are collectively referred to as equine encephalitic viruses (EEVs) throughout this manuscript. These viruses are maintained in an enzootic cycle between mosquitoes and an animal host (e.g., rodents and/or birds) but can spill over into both humans and horses, causing illness (Ronca et al., 2016). In horses, mules, and donkey's, equine encephalitis can cause appetite loss, flu-like symptoms, and progress to muscle and nervous system degeneration (Guzmán-Terán et al., 2020). In humans, illness can include flu-like symptoms and can also progress to neurological deficits and seizures. These effects occur most commonly in children for whom EEVs are more likely to cause life-long behavioral deficits, seizures, and delirium (Falchek, 2012). Cases that progress to neurological deficits, including encephalitis require significant long-term care, which was estimated to cost $320,000 per individual in 1971, and $400,000 per year per individual in 1995 (Villari et al., 1995; Earnest et al., 1971). Based on the 2.54% inflation of the economy between 1995 and 2023 (Webster, 2024), this equates to ~$790,000 per year in 2023. Despite over 100 years of research on these viruses, there are no FDA-approved antiviral treatments or vaccines against EEV infection in humans. Ultimately, current treatment options are limited to supportive care. The lack of FDA approved therapeutics for EEVs is due to a variety of challenges, including the limited number of laboratories researching these viruses because of biosafety level limitations and the lack of adequate and standardized animal models.

Traumatic brain injury (TBI) is a leading cause of death and disability across the world, with an estimated 69 million people sustaining an injury each year (Dewan et al., 2018; Peterson et al., 2022). The CDC defines a TBI as “an injury that affects how the brain works” and may be caused by a “bump, blow, or jolt to the head, or a penetrating injury” (CDC, 2023a). Following the primary insult, “secondary injury” will occur in the minutes, hours, days, weeks, and years' post-injury. Secondary injury leads to chronic neurological deficits due to cell death, inflammation, and other consequences of primary injury. Additionally, TBI is a risk factor for several neurodegenerative disorders and neurological disorders and impairments (Dams-O'Connor et al., 2016; Delic et al., 2020; Rapoport, 2012; Howlett et al., 2022). Both primary and secondary injury lead to acute seizures in 1/5 individuals who receive a TBI and chronic post-traumatic epilepsy (PTE) occurs in ~1/50 cases (Fordington and Manford, 2020). Due to the heterogeneous nature of the TBI mechanism, pathophysiology, severity, and outcome, developing effective therapeutic strategies and treatments has yielded limited success, with no correct or universal FDA-approved treatments available (Kochanek et al., 2020; Nishimura et al., 2022). Several recent review articles and reports cover new technological advances in TBI research (Bowman et al., 2022; Bonanno et al., 2022; Lu et al., 2015; Ahmed, 2022). These focus on developments in imaging, biomarkers, and therapeutic approaches that offer insights into the pathophysiology and treatment of TBI. For instance, innovations in high-density neurophysiology monitoring systems, alongside advanced neuroimaging and bioinformatics, have enabled more precise tracking of brain injury progression and recovery. Technologies like high-resolution MRI, PET imaging, and blood-based biomarkers are helping researchers characterize injury severity and predict outcomes more accurately. Therefore, novel approaches to studying TBI and its related pathophysiology may enable therapeutic development.

Organophosphate compounds (OPs) are primarily divided into nerve agent OPs and pesticide OPs. Exposure to OPs causes the death of 300,000 people per year worldwide (Ahmad et al., 2024; Adeyinka et al., 2024), primarily from occupational exposure to pesticide OPs and exposures in underdeveloped countries. Challenges in identification of agent in a warfare setting make estimation of deadly nerve agent OP exposures difficult and likely underestimated (Costanzi et al., 2018; Gunnell et al., 2007) though there are recent high-profile examples of nerve agent OP use on civilians (Chai et al., 2017; Haslam et al., 2022; John et al., 2018; Morita et al., 1995). For this review we will focus on nerve agents (NAs) as these have significant implications as warfare agents (Mukherjee and Gupta, 2020). NAs irreversibly inhibit acetylcholinesterase activity, causing a buildup of acetylcholine in both the central and peripheral nervous systems that produces significant neurological changes (Costanzi et al., 2018). NAs are typically broken into classes, including the “German” G-series agents sarin (GB), soman (GD), tabun (GA), and the Russian “Venomous, Victory, or Viscous” V-series agents, which consists of VE, VG, VM, VR, and VX. Nerve agent mechanisms of action are consistent across the series, but the volatility and toxicity vary. V-series nerve agents, most notably VX, are less volatile at ambient temperature and generally regarded as more toxic than G-series agents, but this toxicity is typically associated with skin contact and persistent environmental hazard (Jang et al., 2015; Wiener and Hoffman, 2004; Rosenblatt et al., 1996). G-series agents are volatile at room temperature leading to more significant toxicity through inhalation or skin contact. An additional class, the Novichoks and A-series agents, will not be discussed in this review as their classification, chemical structures, and mechanisms are not well characterized by the literature. The little information that is present concerning this series is incomplete and from scrutinized sources, whereas G-series and V-series agents are relatively well characterized in literature. A-series and Novichok agents in particular display an extreme level of toxicity not observed in other nerve agents (Opravil et al., 2023; Noga and Jurowski, 2023).

Exposure to NAs can occur through inhalation, ingestion, or skin absorption (Wiercinski and Jackson, 2024). Small amounts of these chemicals can cause a variety of mild to moderate flu-like symptoms such as nausea, vomiting, confusion, headache, and weakness or other cholinergic symptoms such as watering eyes, drooling, blurred vision, increased heart rate, muscle spasms, and sweating (CDC, 2023b). The onset of symptoms is typically rapid at high doses, but long-term low dosage exposure can induce memory impairment, motor dysfunction, depression, and anxiety, with multiple lines of evidence from Tokyo Subway Attack victims, veterans, and exposed Iraqi civilians (Figueiredo et al., 2018). The mechanism by which long-term low-dose exposure of NA induces depression and behavioral changes is not well understood: however, neuronal damage and degeneration is commonly correlated with cognitive and motor dysfunction observed post-mortem (Figueiredo et al., 2018). Lower doses of NAs also can cause convulsion mediated at peripheral neuromuscular junctions, high levels of NA exposure almost invariably result in prolonged or repetitive central seizures (status epilepticus, SE) or death without immediate medical intervention (Hrvat and Kovarik, 2020). Unique to NAs, most long-term neurological changes are due to SE-induced neuronal damage, inflammation, and changes to neuronal signaling (Figueiredo et al., 2018) as NAs do not have directly toxic effects on neural cells (Aroniadou-Anderjaska et al., 2023).

Human case studies of VEEV, EEEV, and WEEV are extensively under-documented due to the initial symptoms often presenting as febrile illness as well as non-specific documentation due to these viral illnesses being classified under a larger umbrella of neuroinvasive viruses, including flaviviruses, West Nile virus (WNV), and Japanese encephalitis virus (JEV) (Calisher, 1994; Ronca et al., 2016). The number of reported VEEV, WEEV, and EEEV cases trace back to the 1930s, the same time frame in which these viruses were isolated and mosquito vectors were determined to be the route of transmission (Weaver et al., 2004a; Zacks and Paessler, 2010). The combined impact of WEEV, VEEV, and EEEV includes >300,000 cases, >300 recorded deaths, and 3,000 survivors with long-lasting neurological disorders, including paralysis, seizures, migraines, and depression (Crosby and Crespo, 2024) (Table 1). As previously mentioned, this is likely an underestimate due to many viral infections causing vague symptoms in humans, and therefore these infections may go undiagnosed and untested. A recent study identified elevated glial fibrillary acidic protein (GFAP) in patient serum from acute VEEV and Madariaga virus (MADV) infections; however this biomarker was not specific to alphavirus infection as bacterial cases with encephalitis also displayed elevated GFAP (Bartlett et al., 2024). EEV pathology in humans has been divided into three distinct phases consisting of a lymphotropic phase (early/short term), a neuroinvasive phase, and a neurodegenerative phase (late/long-term) [reviewed in Steele and Twenhafel (2010); Kehn-Hall and Bradfute (2022)]. Neurological complications can be attributed to both the neuroinvasion and replication of EEVs in the brain, as well as from the inflammatory response. Despite the large range of neurological deficits induced by these viruses, there is little known about the mechanism of neuroinvasion in humans. Similarly, it is debated whether the inflammatory process hallmarked by the increase of neutrophils in the central nervous system (CNS) is harmful or helpful (Peiseler and Kubes, 2019; Drescher and Bai, 2013). Therefore, we rely upon animal models of infection to study pathologic, behavioral, and molecular processes altered by infection and ultimately identifying therapeutics. Animal models of EEVs have previously been reviewed (Ronca et al., 2016; Steele and Twenhafel, 2010; Kehn-Hall and Bradfute, 2022); however, here, we seek to highlight the consequences of neuroinvasion and long-term neurological symptoms of EEVs in order to correlate them across different neurological diseases.

Animal models used for EEV research include non-human primates, rats, gerbils, mice, guinea pigs, rabbits, and hamsters, which have been extensively reviewed by others (Steele and Twenhafel, 2010; Kehn-Hall and Bradfute, 2022; Dremov and Solianik, 1977). Of these models, Guinea pigs, hamsters, and rabbits have high fatality rates, including severe lymphoid necrosis prior to invasion into the CNS; therefore, they are less ideal models for studying neurological sequelae. In this review, we focus on rodents (i.e. mice, rats, and some hamster models) and non-human primates (NHPs) as animal models of EEV infection, where neurological infiltration has been well established. It's also important to note that studies in animal models have primarily been reliant upon the utilization of a variety of different strains of VEEV, EEEV, and WEEV, including naturally evolving strains (VEEV Subtypes I-VI, VEEV Trinidad Donkey (TrD), EEEV Georgia Fatal, EEEV FL93-939, EEEV North American, WEEV MacMillan (MCM), WEEV IMP 181) as well as some attenuated viral isolates previously reviewed (e.g. VEEV TC-83) (Steele and Twenhafel, 2010; Sharma and Knollmann-Ritschel, 2019). Route of infection is also important to consider as there are multiple infection methods used including intranasal, aerosolization, and subcutaneous infection; however, there are different degrees of morbidity and mortality across the different infection routes. While EEVs are naturally transmitted via mosquito bite, there is a large interest in understanding pathogenesis that results from inhalational exposure. EEVs are readily aerosolized, VEEV was developed as a biological weapon, and a large number of laboratory acquired infections have occurred via VEEV aerosolization (Rusnak et al., 2018; Weaver et al., 2004b).

The National Institute of Neurological Disorders and Stroke classifies TBI as an external mechanical force that causes damage to the brain, potentially leading to temporary or permanent disability (NINDS, 2024). The most recent data suggests that over 200,000 TBI related hospitalizations occurred in 2020 and almost 70,000 TBI related deaths occurred in 2021 (CDC, 2024). TBI injury severity varies greatly both across human patients requiring varied animal models to replicate the disease processes and outcomes seen in people. TBI severity can be stratified by the Glasgow Coma Scale (GCS) into mild, moderate, and severe injuries (Vella et al., 2017; Jain and Iverson, 2023). Much of the human research conducted focuses on sports related injuries or combat related injuries (Fehily and Fitzgerald, 2017; Kim et al., 2023; Elder and Cristian, 2009). Even so, the majority of TBIs are caused by falls and frequently impact elderly individuals and young children (Faul et al., 2010). Further, males are over three times more likely to sustain a TBI than females, likely due to more prevalent high-risk behaviors and higher-risk jobs as this sex difference is only present in adults, not children (Alexis et al., 2022). The TBI injury cascade can be generally split into two categories: primary and secondary injury (Davis, 2000). The primary injury reflects the initial mechanical insult, such as axonal shearing, contusion, laceration, and skull fracture. The secondary injury includes the physiological aftermath that causes continued cell damage and death in the hours, days, and years following the initial impact. This includes BBB disruption, hypotension, hyperglycemia, hypoglycemia, changes in intracranial pressure, cerebral edema, peripheral immune cell infiltration, gliosis, and release of excitatory neurotransmitters. These mechanisms lead to the symptoms and outcomes seen in patients and replicated in animal models (Table 3). To replicate human TBI cases, animal models of TBI are adapted to study various injury types and severities (Xiong et al., 2013).

Animal models utilized for TBI research include rats, mice, pigs, rabbits, dogs, swine, sheep, ferrets, monkeys, and cats, which have been evaluated and critiqued by others (Xiong et al., 2013; Cernak, 2005; Ma et al., 2019). Non-human primate models of TBI are essential for accurate modeling of TBI impact on neural damage as these models most closely resemble the human brain; however, these models are limited (Barbay et al., 2021). The most common model of TBI is the rodent (mouse and rat), which we will focus on in this review. Additionally, it is essential to consider that TBI is a heterogeneous injury and that there are many injury models that replicate various pathologies, neurological manifestations, and severity levels. While the classification of TBI is under current refinement including the use of endophenotypes, we aim to review several known severity categories in humans (mild, moderate/severe) and compare human disease pathology to rodent models.

Early descriptions of human exposure to NAs occurred through laboratory exposure through inhalation of tabun (ethyl N,N-dimethylphosphoramidocyanidate) (Lopez-Munoz et al., 2008). Exposure to sarin (isopropyl methylphosphonofluoridate), soman (pinacolyl methylphosphonofluoridate), and VX are the most well documented in both patients and animal models (Moshiri et al., 2012). Unlike the aforementioned viruses or TBI models, the mechanism of action and the manifestation of disease is nearly identical across different NAs. The primary mechanism of action of all NAs is the inhibition of acetylcholinesterase activity, leading to accumulation of acetylcholine at neuronal synapses. The excess acetylcholine, termed cholinergic crisis, causes prolonged activation of nicotinic and muscarinic receptor activity, which induces neurotoxic symptoms including muscle cramping, paralysis, headaches, and more (Lopez-Munoz et al., 2008). The accumulation of acetylcholine at the synapses in both the peripheral nervous system (PNS) and CNS leads to widespread CNS damage, with damage in the limbic system, particularly the amygdala and hippocampus, being most noted in the literature (Prager et al., 2014; Shih et al., 2003; Miller et al., 2015). The dentate gyrus of the hippocampus is built of tightly packed cholinergic neurons, therefore, they are highly sensitive to acetylcholine accumulation. Communication between the amygdala and the hippocampus is essential for memory and anxiety behaviors, and these cholinergic neurons play a large role in signal transmission between these portions of the brain (Song, 2023); therefore, it is unsurprising that these regions are significantly impacted by nerve agents (Aroniadou-Anderjaska et al., 2009). While the action of all NAs is similar, the primary differences in disease manifestation are related to the concentration and duration of exposure (CDC, 2017). Unfortunately, high or prolonged low-dosage exposure is highly fatal without immediate medical intervention to mediate seizure damage. The G-series agents (soman and sarin mostly discussed here) are water-like in consistency and easily form vapors, while the V-series agents (VX primarily discussed here) which are a thicker consistency and typically do not vaporize, leading to prolonged presence in the environment (Fan et al., 2024). While not discussed here, it's worth noting that several research articles have used surrogates, which are nonvolatile chemical compounds that inhibit acetylcholinesterase activity but are not as toxic or as strictly controlled as NAs (Finnegan et al., 2021). These compounds are valuable for enabling more laboratory research for therapeutics and biomarker analysis, but are not as toxic as G and V series nerve agents, so they are not the focus of this review.

In the following sections, we will review the animal models of NA exposure, which consist of NHP, rat, and mouse models. Animal models have been reviewed previously (Figueiredo et al., 2018; Pereira et al., 2014). As briefly discussed in future sections, an NHP model is ideal for nerve agent research because humans and primates have conserved nicotinic acetylcholine receptors (Kendrick et al., 2021). Cynomolgus, African green, and rhesus monkeys, as well as common marmosets have been investigated (Despain et al., 2007). Some models that have not been as well evaluated, such as the baboon model, have shown significant airway decline and neuromuscular junction activity, but the effects were inconsistent due to anesthetization with phenobarbital which inhibits some of the clinical presenting signs of NA toxicity, including muscle fasciculation and seizures (Anzueto et al., 1990). Collectively, it has been suggested that NHPs are the best model for NA intoxication due to similar levels of organophosphorus metabolizing carboxylesterases to humans though new humanized mouse models are being developed to better mimic human responses and reduce some of the ethical challenges of NHP experimentation (Marrero-Rosado et al., 2021; Tressler et al., 2024).

Disease manifestations of EEVs, TBI, and NA include overlapping symptoms and neuropathologies, which highlight the potential for both research gaps in neurological disease mechanisms as well as therapeutic potentials. These illnesses induce mild symptoms such as fever, confusion, headache, shock, neck pain, vomiting, malaise, and chills and can progress to more severe symptoms such as stupor, left-sided weakness, thalamic enhancement, dysarthria, convulsions, seizures, paralysis, intellectual disability, as well as cognitive, motor, and behavioral changes (Figure 1). EEV infection and TBI have a few shared neurological sequelae, including intellectual disability, memory loss, and depression, while convulsions are shared amongst EEV infection and NA exposure. Neurological sequelae shared by TBI and NA exposure include loss of consciousness, slurred speech, partial or complete vision loss, fatigue, insomnia, and drowsiness. All three pathologies can result in paralysis, coma, muscle twitch, photophobia, sleep disorders, and seizures (Figure 1). Although these conditions impact a varying number of individuals worldwide, there are significant case fatalities and progression to neurological sequelae warrantying further evaluation (Table 1).

Immune system activation induced by NA, TBI, and EEVs follow relatively similar patterns; however, the timescale of activation based on the progression of the disease state varies drastically (Figure 2). In brief, primary acute damage driven by NAs occurs within the first 24 h and is primarily driven by immediate neurotransmitter dysregulation and seizures. Continued seizures after the initial injury can occur for days, months, or indefinitely if not mediated by antiseizure therapies. For TBI, the initial damage response recognition initiates a cytokine and chemokine response and glial cell activation in the first 4–5 days, but secondary damage can occur days and months after injury due to prolonged microglia activation and inflammation. EEVs initially evade the immune system and establish primary infection in lymphocytes in the first 4 days of infection, but as the virus spreads to the brain, there is an increase in immune mediators in both the PNS and CNS. While some is known about the immune response to NA and EEVs, much more is known about the immune response to TBI in both human and animal models.

While the immune response to EEVs has not been entirely elucidated, the replication and acute response to infection with VEEV have been explored (Skidmore and Bradfute, 2023). VEEV enters the cell via cell surface receptors, passes through the plasma membrane through endocytosis, and replicates within the host cytoplasm before genetic material is encapsulated and buds from the cell (Lundberg et al., 2017). All EEVs have a capsid protein which plays a significant role in dampening the host immune and antiviral responses to promote viral replication (Lundberg et al., 2017). Antivirals aimed at disrupting this activity of VEEV capsid have been tested in vitro (Thomas et al., 2018; Lundberg et al., 2017, 2018; Shechter et al., 2017; DeBono et al., 2019), but no data in animal models is available to date. Efficient replication strategies promote rapid spread into both PNS and established CNS infection, including in the brain within 24–72 h post-infection, depending on the route of infection (Phillips et al., 2016; Salimi et al., 2020) (Figure 2A). Both Type I and II interferon systems are likely initially upregulated by infection, but VEEV has been shown to disrupt nuclear localization of STAT1 and, therefore, reduce Type I and II Interferon systems. Treatment with IFNβ in both rats and NHPs significantly reduced VEEV levels in the CNS post-infection (Cwiklinska et al., 2020; Thorne et al., 2008).

VEEV infection in CH3 mice dampens cytokine levels at 1 DPI, whereas BALB/c mice have a slight upregulation at 1DPI and both models have significant upregulation of chemokines, cytokines, and upregulation of genes associated with interferon responsive genes by 6 DPI (Phelps et al., 2023). Five to six DPI, VEEV-infected mice also display decreased leukocytes and increased T-cells, monocytes, and neutrophils (Phelps et al., 2023). In addition to these gene expression studies, there have been some studies that have evaluated immune responses in mice in different rodent models and with a variety of different VEEV strains. It's proposed that attenuated strains of VEEV induce delayed cytokine expression profiles, but both attenuated and virulent strains of VEEV induce significant upregulation of INF-g, IL-6, IL-12, IL-10, and TNF-a (Grieder et al., 1997). IFNAR-1^−/− and IRF-2^−/− mice show accelerated viral replication, disease onset, and reactive oxygen species, highlighting that these genes play a role in viral dissemination and antiviral response (Schoneboom et al., 2000). Natural killer cells are early innate response immune effectors that induce IFN-y and cell death mechanisms (Taylor et al., 2012). Some reports suggest that natural killer cells also have neuroprotective roles; however, in VEEV, it appears that NK cells increase from 1 to 6 DPI and appear to worsen pathological outcomes of VEEV infection (Segal, 2007). Principal component analysis of a panel of chemokines, cytokines, and other immune and inflammatory markers distinctly separated VEEV-infected animals with pathology, highlighting that VEEV-induced pathology is highly correlated with inflammatory biomarkers (Phelps et al., 2023). Perivascular mononuclear cell infiltration, encephalitis, microglia activation, and neutrophil infiltration in the brain are well established at 7 DPI (Paessler et al., 2006). Collectively, it appears that CNS infiltration appears to occur at relatively the same time as T-cell, B-cell, Natural killer cells, neutrophil, and monocyte upregulation, only slightly later than cytokine and chemokine upregulation, which may promote severe CNS disease.

The time course of the immune response to TBI is biphasic (Figure 2B). Following the initial insult, tissue damage leads to the release of DAMPs (damage-associated molecular patterns). DAMPs stimulate resident cells to release chemokines and cytokines, which in turn recruit neutrophils to the injury site to contain the injury and remove debris. As the neutrophil population dwindles, monocytes begin to infiltrate, and glia become activated around the injury site to initiate reparative processes. T and B cells may also be recruited at later timepoints (Alam et al., 2020; Blennow et al., 2016; McKee and Lukens, 2016). Acute fever has been observed in one rodent models of TBI from day 1–4 post injury (Verduzco-Mendoza et al., 2023), which correlates with clinical case reports of fever after TBI, but this is inconsistently observed and not well characterized (Thompson et al., 2003). After injury, cytokines and chemokines are released, guiding circulating peripheral immune cells to the injury site. Microglia and astrocytes also become activated in response to injury and peripheral immune cell infiltration and can influence long-term outcomes (Loane and Kumar, 2016; Burda et al., 2016). This secondary injury can include but is not limited to, elevated intracranial pressure, ischemia, excitotoxicity, cell death, swelling, axonal injury, BBB disruption, and inflammation (Patel et al., 2023; Kochanek et al., 2000; Yi and Hazell, 2006).

Immune response after NA exposure is driven by convulsion and seizure damage (Figure 2C). Immediate consequences of nerve agent exposure are the inhibition of acetylcholinesterase, accumulation of acetylcholine at the synapse, and subsequent dysregulation of neurotransmitters GABA and glutamate function that leads to neurotoxicity (Figueiredo et al., 2018). Peripheral cytokine and leukocyte activation occur within 30 min post-seizure onset (de Araujo Furtado et al., 2012; Johnson et al., 2011). Proinflammatory cytokines (IL-1B, IL-6) and necrosis factors (TNF-α) increase as early as 2 h post-exposure to NA and peak between 6 and 24 h, depending on the agent and exposure concentration (Johnson and Kan, 2010; Chapman et al., 2006). Inflammation is accompanied by microglial and astrocyte activation, CNS inflammation, and peak injury within the first 48 h (Zimmer et al., 1997). Long-term functional and structural damage primarily dependent primarily depends on the duration and frequency of seizure post-exposure (Hrvat and Kovarik, 2020).

Documentation of chronic pathological changes is limited in humans and NHPs; therefore, to eliminate these gaps, we primarily focus on rodent models. While assessment of acute neuropathologies is crucial for animal model development and evaluation of biomarkers; chronic manifestations of disease are less well-understood.

Throughout this review, it has been highlighted that EEV infection induces prolonged neurological symptoms. Acute infection of VEEV yields neuronal necrosis, lesions in the thalamus and olfactory cortex, and perivascular cuffing and gliosis with severe inflammation in the hippocampus and cortex (Reed et al., 2004; Gleiser et al., 1961; Danes et al., 1973; Victor et al., 1956) (Figure 3 and Table 5). While viral replication is not dependent on BBB breakdown, hemorrhage, lesions, and BBB breakdown are observed in acute infection, which could promote worsened chronic outcomes (Cain et al., 2017; Fongsaran et al., 2024). The overarching similarity between VEEV and EEEV infections in mice is the neuronal damage; however, following infection with VEEV, neurons in the hippocampus and cerebellum show signs of morphological changes correlated with apoptosis, whereas damage following EEEV infection seems to induce widespread necrosis, which is uniformly fatal (Steele and Twenhafel, 2010; Honnold et al., 2015a,b). Pathological evidence of vasculitis, perivascular cuffing, edema, hemorrhage, and widespread neuronal necrosis across the brain has also been observed in a variety of different models (Steele and Twenhafel, 2010). The chronic manifestations of EEVs have only recently been explored, but it is evident that VEEV induces neuronal loss, astrocyte activation, neuromuscular deficits, and fear responses several months post-exposure (Ronca et al., 2016, 2017; Fongsaran et al., 2024).

Despite varying mortality rates and animal models associated with TBI injury (including ferret, mouse, rat, dog, sheep, swine, monkeys, and more) there are a variety of pathological changes which are consistent across the models, including cortical, hippocampal, thalamic degeneration, neuronal death, glial activation, neuroinflammation, excitotoxicity, and BBB disruption (Blennow et al., 2016; Loane and Faden, 2010; Liu et al., 2010) (Figure 3 and Table 5). Following the primary insult, ‘secondary injury' will begin to occur in the minutes, hours, days, weeks, and years post-injury. This secondary injury can include but is not limited to, elevated intracranial pressure, ischemia, excitotoxicity, cell death, swelling, axonal injury, BBB disruption, and inflammation (Burda et al., 2016; Loane and Faden, 2010; Dewan et al., 2018).

For NAs, the severity of the disease is almost exclusively dependent upon the duration of SE post-exposure (Aroniadou-Anderjaska et al., 2008). NA-induced pathology includes severe damage to the amygdala, the thalamus, and the hippocampus (Aroniadou-Anderjaska et al., 2008) (Figure 3 and Table 5). Neuronal loss, neuronal necrosis, and neuronal lesion abnormalities have also been observed (Redell et al., 2020; Apland et al., 2017). In models where prolonged seizures continue for longer durations without medical intervention, the degree of neurodegeneration-induced mortality is very high; therefore, the majority of studies provide a medical countermeasure against seizures to study sequelae.

Collectively, there are signs of neuroinflammation and neuronal loss across all of these models (Figure 3). Additional shared neuropathologies include gliosis, disruption of the BBB, and hippocampal damage. Perhaps the most striking and consistent deficits between the three conditions are associated with regions of the brain associated with memory, namely the hippocampus. This suggests that there are shared neurological pathways altered by infection or exposure, which medical countermeasures can target to alleviate long-term sequelae.

In human cases, symptoms of EEVs, TBI, and NAs often result in behavioral and emotional changes, but the mechanisms behind these personality alterations induced by injury are relatively unknown. Animal models are crucial for developing a better understanding behind the underlying cause and mechanisms which drive these responses (van der Staay et al., 2009). In animal models, neuron damage as a result of NAs, TBI, and EEVs is well documented across the brain with the most striking impacts in the hippocampus, amygdala and other regions of the limbic system (Williams et al., 2023; Shih et al., 2003; Aroniadou-Anderjaska et al., 2008; Palmer et al., 2016; Atkins, 2011). In recent years, there has been an increased interest in memory and anxiety behaviors in rodents and how this information can better inform therapeutic development, including related to how potential therapeutics impact areas of the brain that drive these interactions. In both rodent and human brains, memory formation occurs through three main steps: encoding, storage, and recall, which primarily occurs between sensory signal input from the cortex, signal transduction through the limbic system, and storage within the hippocampus (Joshi et al., 2019).

Behavioral tests to evaluate memory in rodents include both spatial and recognition memory tests, such as Novel Object Recognition, Morris water maze, y-maze and novel arm y-maze. In sarin-exposed (low dosage) and recovered rats with no visible weight difference or persisting symptoms, there is significant impairment of spatial learning and memory via the Morris water maze test (Shi et al., 2023). Further analysis of these animals showed significantly decreased acetylcholine activity in the mouse hippocampus and decreased dendritic spine density in the hippocampal neurons 21 days post-injury, supporting these behavioral deficits. Gene ontology analysis of differentially expressed genes highlighted significant upregulation of genes associated with neurodegenerative diseases, including Huntington's disease and Alzheimer's disease, and downregulation. On a more chronic timescale of 6 weeks post-injury, adult rats exposed to sarin showed poor spatial memory via Y-Maze and the Morris water maze in both animals presenting clinical signs and asymptomatic animals (Kassa and Vachek, 2002; Kassa et al., 2001; Filliat et al., 1999; Raveh et al., 2002). TBI memory deficits have been well established following mild and moderate TBI (Malkesman et al., 2013). The memory deficits appear to be primarily associated with challenges with working memory [reviewed in Paterno et al. (2017)] and more information could be gleaned through new technology such as opto- and chemo-genetics (Paterno et al., 2017). The primary behavior evaluated in most EEV models is neuromuscular function, not memory, as the biosafety challenges with setting up large maze systems in biocontainment adds several challenges and limitations. Several studies have highlighted there is significant viral invasion and damage to the hippocampus as early as 4 days post-infection (Ronca et al., 2017; Williams et al., 2023). Active avoidance, which evaluates fear memory, has been evaluated for VEEV-infected mice with mixed effects as they discovered a reduced latency between shock and escape compared to uninfected controls, but no differences between shock avoidance (Fongsaran et al., 2024). Other behavioral mazes, or the behavior response following EEV infection has not been explored.

Emotional instability and behavioral changes, such as anxiety, are a common complaint in inpatient cases of TBI, NA, and EEVs. In the brain, anxiety is most often associated with the limbic system, of particular importance is the amygdala, which receives information from the cortices and regulates fear, emotion, and motivation (AbuHasan et al., 2024). These changes have been most thoroughly explored in TBI rodent models. Anxiety behaviors of rodents following TBI have been previously reviewed (Malkesman et al., 2013; Tucker and McCabe, 2021). The majority of tests performed in TBI-inflicted animals report increased anxiety in both rats and mice, in both closed head, CCI, and FPI injuries, but there are some outliers (Tucker and McCabe, 2021; Tucker et al., 2017; Lapinlampi et al., 2020; Das et al., 2019). WEEV infection has often been likened to Parkinson's-like disease, and mice exposed to WEEV display abnormal movement gait and increased run duration, which could be indicative of both motor dysfunction and increased anxiety (Bantle et al., 2019). Unfortunately, there has been no specific investigation of anxiety in rodent models of EEVs, but fear memory tests indicate there is a potential alteration of fear anxiety pathways (Fongsaran et al., 2024). At both acute and chronic timepoints following sarin and soman exposure, surviving rats show increased anxiety-like behavior, assessed via open field and elevated T-maze, at both acute and chronic sarin and soman exposure (Sirkka et al., 1990; Baille et al., 2005; Mamczarz et al., 2010). Recordings of other rodent anxiety-like behaviors such as fleeing, hiding, ridged movement, or other prey responses to predators could provide further insight into anxiety in models where behavioral tests have not been performed (Zhao et al., 2023).

One important consideration for rodent research is the difference in behavioral outcomes in regard to the strain of animals being utilized. For example, sham-injured C57BL/6 mice scored significantly better than FVB/N and 129/SvEMS sham mice in motor function assessments (Fox et al., 1999). FVB/N and 129/SvEMS sham mice were not capable of learning in the Morris water maze or Barnes circular maze tasks. Further, C57BL/6 and BALB/c mice show distinct anxiety-like and depressive-like behaviors, pain perception, motor performance, and learning and memory (Mogil et al., 1999; Lucki et al., 2001; An et al., 2011; Garcia and Esquivel, 2018; Stelfa et al., 2022). These strain differences suggest one should carefully select rodent strains based on the research conducted. Collectively, there is indication based on neuropathology and reports of human clinical cases that there are changes in memory and anxiety which could be evaluated using rodent behavioral models.

Currently, no publicly available therapeutics or antiviral treatments are licensed for use in humans infected with VEEV, EEEV, or WEEV. The WHO report indicates no ongoing clinical trials for vaccines or therapeutics against EEVs; however, a recent review of preclinical therapeutics for VEEV is available (Ogorek and Golden, 2023). Several groups have investigated direct-acting antivirals, vaccines, and supportive treatments for injury or inflammation resulting from EEVs (Thomas et al., 2018; Lundberg et al., 2017, 2018; Shechter et al., 2017; DeBono et al., 2019; Ogorek and Golden, 2023; Jonsson et al., 2019; Panny et al., 2023; Lehman et al., 2022; Saikh et al., 2020; Risner et al., 2019; Gall et al., 2018; Carey et al., 2018, 2020; Barrera et al., 2021; Bakovic et al., 2021; Ahmed et al., 2020). Given this, the current treatments are limited to supportive care, a costly treatment with up to $4.6 million-dollar life-time care reported by an EEV infected patient (Villari et al., 1995). In the United States, a live attenuated version of VEEV (VEEV TC83) has been previously used to vaccinate military personnel and at-risk laboratory workers; however, there has been an estimated < 30% of those vaccinated developing flu-like illness and adverse reactions (Tigertt et al., 1962; Sewell, 1995). Immunocompetent species of mice and NHPs display similar tropism and CNS infection with one of the hallmarks of infection and major contributing factors to mortality being neuronal damage. Monoclonal antibodies have been used to protect NHPs from VEEV-induced death if administered 48 h post-infection and show some promise as a potential treatment; however, there were mutations in the virus detected in NHPs treated 24 h post-infection (Burke et al., 2019). Thus far, most medical countermeasures of EEVs have focused on the inhibition of viral replication. Prominent antivirals include quinazolinone compound, CID15997213, which inhibits viral replication of VEEV and WEEV; and a ML336 derivate BGDR-4, which has demonstrated over 90% protection against VEEV and EEEV if administrated within 48 h post-infection; although there are potential concerns with viral resistance which is concentration dependent (Jonsson et al., 2019; Chung et al., 2014). CID15997213 targets the viral nonstructural protein 2 (nsP2) (Chung et al., 2014), which contains protease and helicase activities, and BDGR compounds target nsP4, the viral RNA dependent RNA polymerase (Skidmore et al., 2020). The most recent BDGR derivative developed by this group, BDGR-49, shows 70% and 100% protection from 10x LD50 of EEEV (FL93-939) and VEEV (TrD), respectively (Cao et al., 2023). While there are no direct neuroprotective drug treatments for VEEV, there are treatments that have been investigated to reduce neurological damage in viral infection and neurodegenerative diseases. These include pretreatment or co-treatment at infection with melatonin, which significantly reduces mortality, apoptosis, and oxidative stress (Boga et al., 2012; Montiel et al., 2015). Current treatments are challenged by the short window required for adequate inhibition of the virus as well as the potential for viral mutations to overcome antiviral mechanisms. Other challenges are due to the unique feature of illness where many of the human cases that document long-term neurological deficits have mild flu-like symptoms or no symptoms in the early viral infection period, making it challenging to diagnose and prevent. Given this, there is an urgent need for both characterization of CNS infection and neuroprotective treatment options for acute and chronic illness manifestations.

As of this publication, there are no FDA-approved medications for the treatment of TBI (Food and Drug Administration, 2021). However, numerous interventions are used in the clinic to alleviate pain and symptomology. Like many other injuries, the elevation of the head has immediate effects as intracranial pressure is reduced as the cerebral spinal fluid is displaced, and venous outflow is increased (Sattur et al., 2023). Intracranial pressure monitoring using an instrument inside the patient's body is an additional option for those who show substantial neurological compromise but do not require immediate surgical intervention (Shim et al., 2023). Hyperventilation is a strategy used to reduce cerebral blood flow via vasoconstriction (Gouvea Bogossian et al., 2020). This is typically only used in severe cases when acute neurological decline is expected to occur. Antiepileptics are often used during the acute phase. However, there is no evidence that these drugs help to prevent PTE in the long term (Chang et al., 2003). The last resort in managing the injury is putting the patient into a medically induced coma, which reduces the metabolic demand in the brain (Galgano et al., 2017). In the most severe cases, often due to cerebral edema and/or severe bleeding, surgical intervention may be warranted. Surgical intervention typically involves a craniotomy over the brain region of interest, followed by the removal of the hematoma and vessel cauterization (Galgano et al., 2017). Altogether, the lack of FDA-approved medication or therapeutics suggests a need to better understand the underlying pathology.

Positive outcomes of NA illness induced by acute exposure are heavily reliant upon quick identification of the exposure and immediate treatment. In cases of high-dosage exposure to NAs, or a delay in the time between exposure and treatment, current treatment methods are inadequate at reducing or preventing long-term neurological deficits. Emerging NA biomarkers are crucial for quick identification of NA exposure which could aid in faster and more target specific treatments (Wang et al., 2022). Current treatments for NA-exposed patients typically consist of anti-seizure agents (i.e. benzodiazepine anticonvulsants such as diazepam and midazolam), atropine (a muscarinic acetylcholine receptor antagonist), and pralidoxime (a.k.a 2-PAM, an acetylcholinesterase reactivator) (Figueiredo et al., 2018; Lallement et al., 2001). Benzodiazepines are well characterized in the treatment of NA-induced seizure and are therefore the anticonvulsant drug of choice, while neurosteroids are only recently being explored (Reddy, 2024). Neurosteroids may be an effective treatment for NA-induced seizure, especially seizure that has become refractory to benzodiazepine treatment. However, more research is needed. Combination treatments such as combining the NMDA antagonist ketamine with anticholinergic medications such as atropine have indicated improved outcomes compared to just atropine alone (Marrero-Rosado et al., 2021). Prophylactic treatments, such as carbamate pyridostigmine bromide, have been explored as protective agents against inhibited cholinesterases (Kassa et al., 2001; Bajgar, 2004; Kassa et al., 2008; Gordon et al., 1978). Emerging anticonvulsant options are also being explored, including the neurosteroid, Ganaxolone, which aids in reactivation of GABA-A receptors (Reddy, 2024). Neurosteroids are a promising alternative to benzodiazepines to mitigate acute and refractory seizures. Collectively, several potential therapeutics focused on acetylcholinesterase inhibition could be utilized to pretreat against nerve agent toxicity (van Helden et al., 2011); however, the approval of these therapies are still in the early stages.

Neuroprotective therapies have been broadly defined as therapies that either reduce, prevent, or reverse permanent damage to neuron structure and/or function (Levi and Brimble, 2004; Mallah et al., 2020). Most commonly, neuroprotective agents mediate conditions such as Alzheimer's, Parkinson's, or Ischemia (Rehman et al., 2019). As discussed earlier, there are many therapies in the preclinical evaluation stage and some emerging therapies in later stages of the clinical approval pipeline which may be able to mediate some of the neurological deficits caused by EEV, TBI, and NA damage. Currently, the DrugBank online resource cites 43 drugs which are classified as neuroprotective agents (Knox et al., 2024; Wishart et al., 2018; Law et al., 2014; Knox et al., 2011; Wishart et al., 2006).

One area of potential therapeutic development would be guided toward novel treatments selective for modulating neurotransmitter activity to improve neuron signaling. Acetylcholinesterase therapies have proven beneficial in antipsychotic therapy of schizophrenia, as well as visual hallucinations and dementia in Parkinson's and Alzheimer's disease (Singh et al., 2012; Bittner et al., 2023). Gacyclidine, an NMDA receptor agonist, is a psychoactive drug that has been evaluated for a variety of neuroprotective mechanisms, including against OPs. One study evaluated GK-11 (Gacyclidine) treatment in NHPs exposed to OP levels at 8x the LD50 and found it protected animals from mortality (Golime et al., 2018). Similarly, there are a variety of neurotransmitter mediating therapies that aid with both negative regulation of signal transduction, such as Ziconotide which mediates chronic pain via calcium channel blockage (McGivern, 2007), and Tenocyclidin which mediates NMDA binding and provides anesthetic and dissociative effects in a variety of different situations including mediation of severe injuries such as spinal cord or brain injury (Radic et al., 2006). Mediation of the toxic effects of neurotransmitter dysregulation could help preserve neuron function and general immune responses to promote clearance of damaged regions of the brain following injury.

Oxidative stress is a driving mechanism behind many progressive neurodegenerative diseases as well as a potential therapeutic avenue against secondary injuries. Mitochondrial disruption is one of the drivers of this damage and has been identified in EEV (Keck et al., 2018), TBI (Hiebert et al., 2015), and NAs (Pearson and Patel, 2016). Antioxidants can help with scavenging and clearing radical oxygen species in the brain, as well as promoting the stability of naturally occurring antioxidants during phases of mitochondrial dysregulation (Ashok et al., 2022). Several antioxidants have been evaluated in clinical trials against neurological injuries and appear to be a safe and well-tolerated therapeutic option, these have been previously reviewed (Lee et al., 2020; Kelsey et al., 2010; Lalkovicova and Danielisova, 2016; Teleanu et al., 2019).

Combating the death of healthy cells while clearing damaged or misfunctioning cells is a major challenge for every organism. Neurodegenerative diseases almost always have alterations in programmed cell death (apoptosis), mitochondrial recycling (mitophagy), or cellular recycling (autophagy) mechanisms (Knox et al., 2024). For VEEV it has been shown there is an upregulation in apoptosis and mitophagy which ultimately worsens pathology in response to infection (Baer et al., 2016; Keck et al., 2017). Several cell death pathways are induced by TBI and OPNA and are likely a leading cause of secondary injury (Wu and Lipinski, 2019). While these mechanisms require energetic stress and cause the death of cells, autophagic mechanisms are often beneficial as they promote restoration of cellular function (Liao et al., 2022). Therefore, modulating cell death or recycling mechanisms is a challenge, and short-term administration of potential therapeutics could be an avenue of healthy cell death while also promoting the uncontrolled growth of dysfunctional cells. In both spinal cord injury and TBI, rapamycin treatment reduces neuron death through activation of autophagy and microglia activation (Husain and Byrareddy, 2020; Song et al., 2015; Li et al., 2019). In combination with lithium, rapamycin can provide synergistic effects inducing autophagy to enhance the clearing of protein aggregates associated with aging neurons (Sarkar et al., 2008; Cherra, 2008). Apoptosis, driven by caspase-3 activation, is associated with apoptosis in the CNS as well as several diseases, including TBI, spinal cord injury, and Alzheimer's disease (Khan et al., 2015). Activation of the neuronal-specific inhibitor of the apoptosis (IAP) family has been shown to reduce the loss of hippocampal neurons, which could potentially reduce pathology and memory deficits (Robertson et al., 2000). Further evaluation of apoptosis and autophagy mechanisms is necessary to determine specific time points post-injury where these therapies could be beneficial or harmful to patient outcomes.

While this is not an exhaustive list, these neuroprotective therapies highlight crucial areas for advancement in neuroprotective health. Further studies into the mechanisms behind neurological damage, especially those that target deficits common across multiple neurodegenerative diseases, such as memory loss or anxiety, could provide promising avenues for therapeutics.

Alzheimer's disease is the most common neurodegenerative disease driven by the accumulation of misfolded proteins. Unlike many other neurodegenerative diseases, the onset of Alzheimer's disease is not typically associated with genetic mutations but rather a combination of environmental and lifestyle (i.e., age and fitness) risk factors (Korczyn and Grinberg, 2024). Typically associated with increasing age, the initial symptoms of Alzheimer's disease are mild memory loss, which typically progresses into more severe memory loss over time, but each individual case has a different rate of disease progression. Therapeutic options for Alzheimer's disease are limited, but therapies currently include antipsychotics and mediators of abnormal neurotransmitter function which contribute to memory loss-induced psychosis (Eassa et al., 2023; Drevets and Rubin, 1989; Wang et al., 2023; Ricci et al., 2021). In recent years, there has been an increasing interest in the relationship between infectious agents and neurodegenerative disease due to a variety of shared factors, including neuron loss, increased cellular damage, and increased inflammation (Piekut et al., 2022). Memory deficits have been noted in clinical cases of NA, TBI, and EEV survivors. A recent study highlighted the similarities between VEEV and Alzheimer's utilizing early onset Alzheimer's animal model (Tg2576) revealed that alphavirus infection can speed up neurodegenerative phenotypes associated with Alzheimer's, including pro-damage cytokines TNF-alpha and IL-1B, fear memory formation, and amyloid beta plaque concentrations (Fongsaran et al., 2024). Similar links have been made between Alzheimer's disease and TBI, as there are signs of amyloid beta accumulation and an increase in the same cytokine profiles; however, it is still unclear what leads to the accumulation of these factors beyond the hypothesis that its driven by vascular dysfunction or ischemic damage from BBB disruption (Ramos-Cejudo et al., 2018). Several reversible acetylcholine esterase inhibitors have been utilized to treat Alzheimer's disease and have been proposed as potential prophylactic therapeutic options for acetylcholinesterase toxicity caused by NA exposure given that they can occupy acetylcholinesterase binding sites, therefore minimizing the effects of NA. One of these therapies, Galantamine, was used to rescue animals from death and provided notable prevention from sarin and soman-induced neurodegeneration (Golime et al., 2018; Albuquerque et al., 2006). Acetylcholinesterase-inhibiting therapies have greatly increased as a result of NA release, which can be used for the management of oxidative stress and neurotransmitter dysregulation caused by Alzheimer's disease through increasing acetylcholinesterase levels in the brain, though to a lesser extent than NAs (Singh et al., 2012). Collectively, the overlapping neuropathologies between Alzheimer's disease and the biological, physical, and chemical injuries discussed here further highlight the need for broad neuroprotective and neuroreparative therapies.

Parkinson's disease is the second most common neurodegenerative disease and is typically characterized by the reduction of dopaminergic neurons partially attributed to mitochondrial alteration, inflammation, and alpha-synuclein aggregation (Pardo-Moreno et al., 2023). Since its earliest discovery, WEEV infections in humans have been often likened to Parkinson's disease and there is significant dopaminergic neuron loss and a-synuclein protein aggregation (Bantle et al., 2019, 2021). Increased rigidity, tremors, and slowed movement have been observed in patients, and further pathological evaluation identified chronic microglia and astrocyte activation as well as aggregation of alpha-synuclein and neuron death (Bantle et al., 2019, 2021). In WEEV-infected patients, a Parkinson's therapy which inhibits decarboxylases (levodopa and trihexyphenidyl) is effective at reducing clinical disease indicating these drugs may be able to aid with long-term deficits (Oertel and Schulz, 2016). Other Parkinson's related therapies, including Caramiphen, Bupropion, and Scopolamine, have been explored for protection against soman in rats because of their ability to antagonize cholinergic and glutaminergic fluxes (Myhrer et al., 2008, 2013). Ultimately, these therapies reduced cognitive impairment induced by soman seizures, but co-administration of these drugs is typically required prophylactically to see robust results.

Recurrent seizures and epilepsy impact over 65 million people per year and have a variety of causative conditions (Fordington and Manford, 2020; Kanner and Bicchi, 2022). Benzodiazepines for antiseizure effects are most effective against NA-induced neurological deficits because, as discussed previously, seizures are the main cause of neurological damage. Emerging reversable cholinesterase inhibitor therapies which have been used for Parkinson's therapeutic such as physostigmine, are being evaluated for prophylactic effects against NA-induced damage with some success (Myhrer et al., 2008, 2013). Post-exposure therapies for NA revolve around the immediate administration of antiseizure medications as the standard of care, but these same antiseizure measures have only been minorly explored in other neuropathologies, including TBI and EEV infection. In the clinic, there has been the utilization of anticonvulsant therapy to treat reoccurring seizures post symptom onset from humans infected with EEEV, which promoted survival in three patients, but neurological deficits, including hemiparesis and psychomotor deficits, persisted (Carrera I. et al., 2013). In TBI, prophylactic administration of antiepileptic drugs is effective in controlling seizures post-moderate TBI if provided within 7 days post-injury; however, these therapies are not effective if administered after 7 days post-injury (Chang et al., 2003). The effect of antiepileptic therapies beyond the early acute phase of injury or infection appears to be poorly understood. Collectively, antiseizure therapies appear to be effective if administered prior to, or shortly after TBI, NA, or EEV, but are not useful against chronic symptoms. Although antiepileptic therapies have significantly improved over the last 150 years, these therapies still have reduced effectiveness overtime, which appears to also overlap with treating other recurrent seizure conditions; there are still major challenges with the development of tolerance over time, requiring increased dosage and increased adverse effects (Schmidt, 2009). Given these challenges, the development of robust and target selective anticonvulsive therapies are crucial for a large number of seizure-causing diseases.

Mood disorders are one of the most common disabilities worldwide, of which depression and sleep disorders are 18% (Dulawa and Janowsky, 2019) and 20% of the population, respectively (Chattu et al., 2018). Insomnia, nervousness, depression, anxiety, and irritability are common among many neurological diseases including many of the ones listed previously, and EEV, TBI, and NA. Depressive disorders has become known as the most common comorbidity, accompanying neurological disorders as well as cardiovascular, inflammatory, and metabolic disorders (Kochanek et al., 2020). Few mechanisms are known for the major drivers of these behaviors, but there are several categories of depression with different causes, whether that be prior injury, genetic susceptibility, or environmental changes (Remes et al., 2021). Given this, it is not surprising that there are many anti-depressant therapies available, with 5 major types based on mechanisms of action. The most common typically mediate neurotransmitters are dopamine, norepinephrine, and serotonin. While there is much variability in the effectiveness of antidepressants and insomnia treatment, there are some areas of promise for elevating long-term symptoms caused by neurological disease. Melatonin has often been evaluated as a potential neuroprotective therapy to mediate oxidative stress and enhance sleep (Chitimus et al., 2020). Treatment with melatonin significantly reduces symptomatic disease, replicating virus in the brain, and mortality in mice infected with VEEV (Boga et al., 2012; Montiel et al., 2015; Ahmad et al., 2023). In TBI studies, melatonin has both anti-inflammatory and antioxidant properties which reduce secondary injury and chronic neurological deficits (Blum et al., 2021). Albeit a different mechanism of action, the steroid hormone 17β-estradiol, E2, has demonstrated neuroprotective qualities, including reducing TBI-altered miRNAs and reducing inflammation and depressive-like behavior in mice (Sell et al., 2019). Although not tested in NAs, melatonin and other steroid treatments can also be effective at reducing apoptosis, reducing inflammation, and controlling reactive oxygen which are protective in peripheral nerve injuries (Uyanikgil et al., 2017). Uses of melatonin as a treatment option for VEEV, TBI, and NA are currently limited to animal models, but the usage of this naturally occurring and affordable therapy in humans could be a valuable asset to mediating chronic damage induced by these conditions.