Among critically ill patients, infections are a leading cause of death. In recent years, the incidence and mortality rates of infections have remained high due to the emergence of new pathogens, the proliferation of drug-resistant strains, and the increasing population of immunosuppressed individuals. Severe infections exhibit a rapid onset and progression and involve a diverse array of pathogens. Rapid identification of causative microorganisms is therefore crucial.

Over the past two decades, advancements in high-throughput sequencing and computational biology have led to the development of transformative diagnostic tools, enabling significant progress in infection detection. Metagenomic Next-Generation Sequencing (mNGS) is a high-throughput technology capable of simultaneously processing numerous DNA/RNA fragments from blood and cerebrospinal fluid (CSF) samples, generating extensive genomic datasets for comprehensive analysis of pathogen species and genetic information within samples (1, 2). mNGS does not rely on traditional microbial cultures, allowing rapid and unbiased testing of clinical samples for a wide range of pathogenic microorganisms without the need for specific amplification. It is particularly suitable for the critical and challenging diagnosis of infections. Molecular diagnostic approaches, including polymerase chain reaction (PCR) and mNGS, enable earlier pathogen detection compared to conventional culture-based methods. PCR is a targeted technique used to detect specific gene sequences of known pathogens. In contrast, the unbiased detection capability of mNGS allows it to identify a broader range of unknown pathogens. For fever of unknown origin or infectious diseases, mNGS can serve as a comprehensive screening tool to help identify potential novel pathogens or mixed infections. Using mNGS, neurologists and other subspecialists in internal medicine, infectious diseases, critical care, and rheumatology can determine whether a latent infection exists (3). One study demonstrated that mNGS of bronchoalveolar lavage fluid (BALF) efficiently identifies causative pathogens in pediatric pneumonia (4). Another research team developed a cell-free DNA-based mNGS assay for body fluids, which demonstrated superior sensitivity and specificity compared to microbiological testing using culture and PCR in their comparative analysis (5).

This case report describes a 55-year-old female patient with Rickettsia felis (R. felis) infection, highlighting the clinical utility of mNGS in identifying the etiology of her febrile illness.

mNGS of patient serum samples in this study was performed using the “RealPathogen” platform. This platform analyzes both microbial cellular nucleic acids and cell-free DNA/RNA fragments, employing high-throughput sequencing on Illumina NextSeq 550 systems. Pathogen identification was achieved by alignment with a curated microbial reference database, which includes 9,757 bacterial species (excluding 169 Mycobacteria and 126 Mycoplasma/Chlamydia species), 6,874 viruses, 1,563 fungi, and 297 parasites with known genomic sequences, followed by proprietary bioinformatic algorithms. The standard reporting time was 24–48 h for DNA detection and 48–72 h for RNA analysis. Further inquiries may be directed to the corresponding author.

To date, no cases of AS infection by R. felis have been reported in humans. The patient’s fever of unknown origin (FUO) resolved following appropriate treatment. R. felis is an obligate intracellular alpha-proteobacterium with broad ecological versatility, capable of infecting diverse vectors and hosts. The cat flea (Ctenocephalides felis) is recognized as its primary transmission vector and reservoir host (6, 7). Accumulating evidence also confirms its presence in other arthropods, including ticks, mites, lice, and mosquitoes. Concurrently, the host range of R. felis continues to expand, with confirmed infections reported globally in humans, domestic and wild small mammals, and other species (8). It was first discovered in the USA in 1990, with the first human case reported there in 1994 (9). Since then, hundreds of cases have been documented across more than 20 countries on five continents. This disease has no obvious regional preference and has the potential for global transmission (10). In China, cases of R. felis infection have been reported in multiple provinces. The initial detection via PCR occurred in Jiangsu Province in 2014 (8), followed by subsequent reports in Shandong, Guangdong, Taiwan, and other regions. Clinical manifestations of R. felis infection include central nervous system and respiratory involvement. A retrospective study in China reported that rickettsial infections accounted for 2.14% of patients with unexplained fever (11), although mortality data specific to R. felis are not available.

Studies have shown that host factors significantly influence the severity of rickettsial infections. Significant risk factors include advanced age, glucose-6-phosphate dehydrogenase (G6PD) deficiency, diabetes mellitus, and male sex (12). The risk of R. felis infection is also elevated by several behavioral factors, including ownership of domestic animals (particularly absence of routine veterinary surveillance in free-roaming animals), exposure to Ctenocephalides felis vectors (whether through pet contact or environmental exposure, further increases transmission likelihood), and outdoor activities such as hiking in forested endemic areas, which increase contact with infected arthropod vectors. The main symptoms of R. felis infection include fever, pain, and rash or eschar-based skin manifestations. In some cases, patients may also present with elevated liver enzymes and other indicators of liver and kidney dysfunction (13). A comprehensive literature search was conducted across multiple databases, including PubMed, Embase, Web of Science, and the Cochrane Library, without language restrictions. The search strategy employed the following key terms: Rickettsia felis, Rickettsiales, Rickettsiaceae, Rickettsieae, R. felis, and Rickettsia. The search was restricted to “case reports” published from January 2014 to July 2025. Table 2 presents the clinical characteristics of eight patients in eight studies. The cohort included five men and five women, with a mean age of 34.8 years. Fever was absent as the first symptom in only one patient, while six patients experienced headaches. Six patients reported a history of contact with dogs and cats, and three had a history of arthropod bites. Most CSF or blood samples were tested for trichinosis using mNGS. Clinical improvement was observed in most patients after 5–14 days of treatment. Given the wide range of reported symptoms, some studies suggest that clinical manifestations may be influenced by the patient’s geographic location (13). Patients from tropical regions typically present with fever, while those from Europe or the Americas often exhibit fever accompanied by skin manifestations. In parts of Asia, such as Thailand and South Korea, symptoms may include fever along with myalgia, chest pain, and joint pain. R. felis infections are generally responsive to doxycycline; however, diagnosis remains challenging. Inadequate or inappropriate treatment can result in severe complications, including hepatomegaly, myocarditis, meningoencephalitis, and other organ involvement, potentially leading to poor prognosis or even death.

AS is a chronic inflammatory disorder characterized by inflammation of the vertebral and sacroiliac joints (14), often with an insidious early onset. Fever in AS patients typically indicates inflammatory activity and is frequently accompanied by elevated CRP, ESR, and local or systemic inflammatory responses. Infection is an important contributor to the pathogenesis of AS and can influence disease progression. In the present study, most infectious foci in AS patients were located in the respiratory tract, gastrointestinal tract, genitourinary tract, and eyes, manifesting as pneumonia, inflammatory bowel disease, herpes zoster, and uveitis. Elevated body temperature in AS patients—or in otherwise healthy individuals without a significant increase in inflammatory markers—poses a diagnostic challenge for physicians. In this case, the patient lived with a cat and presented after admission with symptoms consistent with rickettsial infection, including persistent fever and systemic pain. She had a history of AS for more than 7 years and long-term use of immunosuppressive drugs. Additionally, she had a history of cervical intraepithelial neoplasia and vulvar condyloma acuminatum. These factors are likely to affect the patient’s immune function, which may explain why her temperature continued to fluctuate significantly despite levofloxacin treatment. Autoimmune diseases, including AS, increase the likelihood of secondary infections due to the use of medications such as hormones and immunosuppressants, making it crucial to identify the underlying cause of infection. mNGS, as a culture-independent diagnostic method, plays an important role in clarifying the etiology of such infections (1). Here, mNGS of the patient’s serum sample identified infection with R. felis, enabling the medical team to formulate an individualized and targeted treatment plan. Following doxycycline therapy against R. felis, the patient’s temperature returned to normal, with no further abnormal fluctuations. Therefore, when broad-spectrum antibiotic therapy is not effective, clinicians should consider the possibility of atypical pathogen infections and implement corresponding individualized treatment strategies to prevent disease progression.

At present, information on the clinical manifestations of R. felis infection remains limited. It is difficult to determine whether the patient’s asymptomatic urinary tract infection is specifically related to cat-associated rickettsia, beyond her age and medical history. Although our understanding of R. felis has advanced considerably in recent years, many aspects of its ecology and epidemiology remain unclear. For patients with autoimmune diseases, it is important to minimize exposure to potential sources of infection, such as reducing contact with cats and dogs and maintaining good environmental hygiene. For clinicians, mNGS can substantially improve the early diagnosis of infectious diseases due to its fundamental advantages, including culture-independent, unbiased pathogen detection directly from clinical specimens and a rapid turnaround time (typically 24–48 h) (15). However, the clinical adoption of mNGS remains limited by its high cost, as it is typically offered as a laboratory-developed test (LDT) by specialized clinical or commercial laboratories. Rather than replacing conventional diagnostic methods, mNGS should be regarded as a complementary tool. This limitation continues to be actively investigated, and further evidence is required to establish standardized clinical implementation guidelines for mNGS (2). From a public health perspective, the identification of human infections in individuals presenting with FUO or rash, particularly those with a history of animal contact, necessitates ongoing surveillance and investigation of indigenous reservoirs and vectors of R. felis within the community.