Giant otters (Pteronura brasiliensis, Zimmermann 1780) are semiaquatic mammals and the largest among the fourteen mustelid species in the subfamily Lutrinae. They primarily feed on fish and inhabit freshwater ecosystems near tropical forests and floodplains (Duplaix, 1980). Endemic to South America, the species originally had a wide range of distribution of occurrence. However, intensive hunting for the fur trade between the 1940s and 1970s led to a range reduction of nearly 40% by the 1980s (Colodetti, 2014). Currently, populations persist in several countries, including Brazil, where it inhabits the Amazon, Cerrado, and Pantanal ecoregions (Rodrigues et al., 2018). Nevertheless, there is still no reliable estimate of the total population size, as available estimates are restricted to specific regions, and overall population trends remain uncertain (Wallace et al., 2025).

Many large carnivores, including giant otters, face significant threats leading to population declines, reductions in their geographic ranges, and habitat fragmentation (Ripple et al., 2014). In Brazil and across most of their range, the main threats to this top predator of aquatic ecosystems include water pollution, competition with human fisheries, diseases transmitted by domestic animals, and habitat degradation (Leuchtenberger et al., 2018). Climate change alters rainfall patterns and river flow, reducing fish availability and degrading denning sites (Groenendijk et al., 2021; Wallace et al., 2025). As a result of these threats, the species is classified as Endangered by the IUCN since 2000 (Groenendijk et al., 2021) and as Vulnerable on Brazil’s national Red List (Rodrigues et al., 2018).

Information on ophthalmic abnormalities in mustelids, including otters, is mostly limited to isolated cases (Williams, 1989; Williams et al., 2000; 2004; Dubey and Thomas, 2011). Some conditions linked to ocular diseases in wild animals include trauma (e.g. Kumar et al., 2015), nutritional factors (e.g. Williams et al., 2004; Lange et al., 2017), aging (e.g. Roth et al., 2004; Kumar et al., 2015), poor environmental or water quality (e.g. contamination through dieldrin pesticide as reported by Williams et al., 2004), parasites and infectious agents (Pinto et al., 2005; Dubey and Thomas, 2011).

Research on ophthalmic disorders (as well as epidemiological studies) in free-ranging giant otters remains limited, largely due to the challenges involved in capturing this species. The only published method employs a funnel-shaped net trap installed at den entrances (Silveira et al., 2011). While effective, this technique does not allow researchers to choose which individual from the group will be captured and they need to avoid groups with pregnant or lactating females. Additionally, its success also relies on specific environmental conditions, such as the dry season (when dens are more accessible), and certain den characteristics, like having few entrances, no submerged entry points, and entrances that are neither too close to the water nor too wide for effective trapping.

Conducting a detailed ophthalmic examination typically requires specialized optical devices or techniques to properly describe eye lesions. However, certain anatomical features of the eye, such as changes in the color of ocular tissues (e.g. opacity), can be easily detected through standard photography. Ophthalmic imaging has long been a crucial tool for documenting ophthalmic diseases (Bennett and Barry, 2009). External eye photography is a relevant approach for examining the ocular surface, eyes adnexa, and surrounding facial structures. It is often used in veterinary ophthalmology studies (Bennett and Barry, 2009; Colitz et al., 2010; Simeone et al., 2017) and is particularly useful when physical restraint of the animal is not achievable.

This study aims to document evidence of ophthalmic and associated systemic disorders in free-ranging giant otters using images and discuss potential causes.

Between December 2009 and October 2019, we recorded a total of 12 adult giant otters with presumed ophthalmic disorders (affecting the eyes, eyelids, and surrounding skin) in the Pantanal wetland, Brazil. Sex was determined for nine individuals (75%), of which five were female and four were male. Most cases (83.3%, n=10) were observed in the southern region of the Pantanal. Of the 12 otters, seven animals presented bilateral ocular disorders, two had disorders affecting only the left eye (OS, oculus sinister), and three had disorders in the right eye (OD, oculus dexter). Detailed information available on each individual, as well as their ophthalmic and systemic conditions, is provided in Table 2. The images of the eyes exhibiting alterations are presented in Figure 2.

Ocular anatomy and normal ophthalmic parameters in mustelids are relatively well-documented in the literature (e.g. Montiani-Ferreira and Freeman, 2022). However, information on ocular disorders in free-ranging Lutrinae remains limited. Notable exceptions include cases of ocular chemical burns caused by oil spills in sea otters (Enhydra lutris) (Lipscomb et al., 1993), and retinal dysplasia observed in Eurasian otters (Lutra lutra) (Williams et al., 2004). To our knowledge, there is no published data on ocular diseases in giant otters, so far. This lack of information may be attributed to several factors, including the Endangered status of the species (Groenendijk et al., 2021), their low population densities (Duplaix et al., 2015), and the challenges of developing an effective method of capturing affected individuals from the social group.

Giant otters are territorial social mammals that interact playfully and aggressively, particularly during agonistic encounters between groups, which can be extremely aggressive and often result in wounds and scars (Rosas and Mattos, 2003; Ribas and Mourão, 2004). This could explain the lesions observed in a giant otter in this study (I). Cases of myiasis in free-ranging giant otters likely resulted from intraspecific fights and ultimately led to the death of the affected individuals (Foerster et al., 2022; Gonchoroski et al., 2025). In aquatic mammals, such as cetaceans, playful behavior can lead to blunt or sharp trauma to the eyes and surrounding structures (Colitz et al., 2016), suggesting that similar mechanisms of injuries may also occur in otters, although no specific records currently exist.

Epidemiological studies on giant otters are limited, although, like other otter species, they are vulnerable to various pathogens commonly spread by domestic animals (Foster-Turley et al., 1990), such as canine distemper virus (White et al., 2013), leptospirosis (White et al., 2018), and canine parvovirus (Echenique et al., 2018). Systemic infections caused by Toxoplasma gondii can lead to significant morbidity and mortality in mustelids, as observed in southern sea otters (Enhydra lutris nereis) and black-footed ferrets (Mustela nigripes) (Burns et al., 2003; Conrad et al., 2005). In southern sea otters, T.gondii was detected in 52% of necropsied and 38% of live-captured animals (Conrad et al., 2005). Recently, a specific strain of T. gondii was identified as the cause of death in several sea otters in California (Shapiro et al., 2019). Clinical signs of toxoplasmosis in otters can include a range of neurological (ataxia, head tilt, tremors), ocular (corneal edema, uveitis, blindness), respiratory (nasal discharge, dyspnea), and general systemic signs (weight loss, lethargy, weakness) (Conrad et al., 2005; Shapiro et al., 2019). Based on observed clinical presentation, toxoplasmosis may be a potential diagnosis for individuals from giant otter groups inhabiting the same area of the Miranda River (D-F and K-M). However, these animals were not captured or sampled, preventing diagnostic confirmation. These groups inhabited areas near a large bridge adjacent to a riverside community, where increased proximity to humans and domestic animals might increase the risk of exposure to contaminated water or prey containing oocysts from feline feces.

Another infectious disease of concern is caused by canine adenovirus type 1, which can lead to incoordination, generalized ataxia, and bilateral ocular opacity, a condition known as the “blue eye” phenomenon. This pathology was recently described in a hoary fox (Lycalopex vetulus) from Brazil, co-infected with canine distemper virus (Silva et al., 2023). In otters, canine adenovirus type 1 infection can be fatal, as observed in a Eurasian otter showing anorexia, lethargy, and weight loss (Park et al., 2007). Sea otters can also be infected by this viral agent (Foster-Turley et al., 1990). Additionally, an outbreak of adenovirus infection causing acute hepatitis was reported in otariids (Inoshima et al., 2013). These studies suggest that semiaquatic mammals are susceptible to this pathogen, which is commonly found in unvaccinated domestic dogs. One giant otter in our study (identified as J1 and J2) presented systemic signs resembling that observed in the hoary fox, suggesting canine adenovirus as another potential etiological agent. In Brazil, canine adenovirus infection has become less common in urban areas due to widespread vaccination programs. However, in the Pantanal, domestic dogs are generally not vaccinated, and when vaccination occurs, it is usually limited to rabies.

The avian eye trematode Philophthalmus lachrymosus, which inhabits the palpebral conjunctiva, has been associated with severe conjunctivitis and blindness in capybaras (Hydrochoerus hydrochaeris), a species that frequently comes into close contact with avian definitive hosts (Pinto et al., 2005). Although we identified four apparently blind giant otters (D, J1/2, K, and L), there are no confirmed records of P. lachrymosus occurrence in the Pantanal or in other mammalian species. However, its presence in shared aquatic habitats may pose a risk to other susceptible mammals.

Cases of blindness in Eurasian otters in Britain have been linked to population declines potentially caused by exposure to organochlorine insecticides (Williams, 1989; Jefferies and Hanson, 2000). These compounds are known to induce hypothyroidism, which affects vitamin A metabolism leading to hypovitaminosis A (Jefferies, 1975). In mammals, vitamin A deficiency is associated with ocular discharge, conjunctivitis, keratitis, and corneal hyperkeratosis, conditions that could explain the blindness observed in British otters (Jefferies and Hanson, 2000).

In the Brazilian Pantanal, agricultural activities are increasing over recent years. According to MapBiomas, a platform for mapping land use and cover in Brazil, agricultural land for crops increased by approximately 100,000 hectares between 2021 and 2023. These data underscore growing pressure on the Pantanal and the urgent need to implement sustainable land use practices to protect this ecoregion. Although agrochemical residue studies in giant otters are lacking, elevated mercury concentrations have been reported in fur samples from giant otters living in the same area where blind otters were observed in the southern Pantanal (Miranda River), suggesting potential toxicological risks (Soresini et al., 2021). While high concentrations of mercury are known to contribute to neurological effects, including central blindness, there is no evidence that mercury causes the specific ophthalmic abnormalities observed in Pantanal otters. Nonetheless, other environmental contaminants, such as pesticides, may contribute to ocular alterations, as reported in Eurasian otters (Williams et al., 2000). However, more detailed studies, including clinical evaluation and a complete ophthalmic examination are necessary to identify the prevalence and etiology of ocular pathologies in giant otters.

This study represents the first documented series of ocular disorder cases in free-ranging giant otters. Although limited by the absence of clinical diagnosis confirmed through detailed ophthalmic examinations, these findings presented here expand current knowledge of health issues affecting this endangered species. Furthermore, these conditions may be linked to multiple causes, including trauma, infections, parasitism, and exposure to environmental contaminants such as organochlorine insecticides. These observations highlight the need for comprehensive clinical and ecotoxicological investigations, especially considering the increasing anthropogenic pressures in the Pantanal. Future studies should aim to develop more effective capture techniques for this species, which would allow the safe collection of biological samples and enable accurate clinical and laboratory diagnoses, thereby improving understanding of the causes and prevalence of ocular and systemic disorders in giant otters.