Feline Toxoplasmosis: Etiology, Transmission, Clinical Signs, and Zoonotic Risk

Etiology and Parasite Biology

Feline toxoplasmosis is caused by the obligate intracellular apicomplexan protozoan Toxoplasma gondii. This parasite exhibits a heteroxenous life cycle in which felids serve as the only definitive hosts, while virtually all warm-blooded vertebrates, including humans, act as intermediate hosts [1, 2]. The genus Toxoplasma contains a single species, T. gondii, which is classified within the phylum Apicomplexa, family Sarcocystidae [3]. The parasite exists in three infectious stages: tachyzoites (rapidly dividing forms), bradyzoites (slowly dividing forms contained within tissue cysts), and sporozoites (contained within sporulated oocysts) [1, 4]. Tachyzoites are crescent-shaped cells approximately 2 by 6 micrometers that replicate within any nucleated host cell via endodyogeny, a process of internal budding that produces two daughter cells [4]. Bradyzoites are morphologically similar but exhibit a slower replication rate and are encased within a thick, elastic cyst wall that can persist for years in tissues such as skeletal muscle, myocardium, and brain [4, 5]. Oocysts are spherical structures measuring 10 to 12 micrometers that are shed exclusively in feline feces and undergo sporulation in the environment to become infectious [1, 2].

The parasite genome is approximately 65 megabases in size and encodes a suite of secretory organelles, including micronemes, rhoptries, and dense granules, which mediate host cell invasion and immune evasion [6, 7]. Micronemal proteins, such as MIC17A, are critical for gliding motility and host cell attachment, and their expression is upregulated in the merozoite stage that develops during the enteroepithelial cycle in the feline definitive host [6, 8]. This stage-specific expression has important implications for diagnostic antigen selection, as antigens highly expressed in merozoites (e.g., MIC17A) demonstrate superior reactivity with feline sera compared to tachyzoite-stage antigens such as GRA1 or MIC3 [6, 8].

Transmission and Life Cycle

The life cycle of T. gondii in cats begins with the ingestion of tissue cysts containing bradyzoites from infected intermediate hosts, typically rodents or birds [1, 4]. Following ingestion, the cyst wall is digested by gastric and intestinal proteases, releasing bradyzoites that invade the epithelial cells of the small intestine [4, 5]. Within the feline intestinal epithelium, bradyzoites undergo multiple rounds of asexual replication (schizogony or merogony), producing merozoites that subsequently differentiate into male and female gametes (gametogony) [1, 4]. Fertilization results in the formation of unsporulated oocysts, which are shed into the intestinal lumen and excreted in feces [1, 2]. The prepatent period, defined as the interval between infection and the onset of oocyst shedding, ranges from 3 to 10 days following ingestion of tissue cysts, but may extend to 18 days or longer after ingestion of oocysts [4, 5]. Oocyst shedding typically lasts for 1 to 3 weeks, during which a single cat can excrete millions of oocysts [1, 2].

The presence of toxoplasmosis in cat poop is the primary mechanism for environmental contamination. Unsporulated oocysts are non-infectious when freshly excreted but undergo sporulation in the environment within 1 to 5 days under favorable conditions of temperature (15 to 30 degrees Celsius) and humidity [1, 2]. Sporulated oocysts are highly resistant to environmental degradation, surviving for months to years in soil, water, and on surfaces [1, 2]. Intermediate hosts, including humans, become infected through ingestion of sporulated oocysts from contaminated food, water, or litter, or through ingestion of tissue cysts in undercooked meat [2, 9]. Vertical transmission via transplacental passage of tachyzoites can occur in both cats and intermediate hosts, although this route is less common in felids than in humans or sheep [2, 4].

Epidemiology and Risk Factors

Seroprevalence of T. gondii in domestic cat populations varies widely by geographic region, management practices, and diagnostic methodology. A study in Greece reported an overall seroprevalence of 21.8% using a rapid immunochromatographic test for IgG antibodies, with significantly higher seropositivity in rural cats, cats with outdoor access, and hunting cats [10]. In Kuwait, a seroprevalence study using immunochromatographic assays detected IgG antibodies in 60% of sampled cats and IgM antibodies in 31.7% [11]. In Brazil, seroprevalence rates have been reported at 57.14% among cats presenting with ocular signs [12], while studies in northeastern Brazil have identified coinfection with feline immunodeficiency virus (FIV) and feline leukemia virus (FeLV) as significant associated factors [13]. In Finland, seroprevalence was lower, reflecting differences in climate and husbandry [14]. A survey of domiciled and stray cats in the United States found seroprevalence rates of 37.5% in adult domiciled cats and 57.9% in adult stray cats [15].

Risk factors consistently associated with seropositivity include outdoor access, hunting behavior, raw meat feeding, and increasing age [10, 16, 15]. Stray and feral cats exhibit higher seroprevalence than owned cats due to greater exposure to infected prey [10, 15]. Coinfection with immunosuppressive retroviruses, particularly FIV and FeLV, is associated with higher T. gondii seroprevalence and increased risk of clinical disease [17, 18, 13, 30]. Immunosuppressive therapy, such as the administration of oclacitinib for feline atopic skin syndrome, has been reported to precipitate fatal disseminated toxoplasmosis in FIV-positive cats [17].

Clinical Signs and Pathology

Clinical toxoplasmosis in cats is less common than seroprevalence data might suggest, with most infections remaining subclinical [3, 19]. When clinical disease occurs, it most frequently manifests as fever, anorexia, lethargy, and weight loss [19, 20, 18]. The organ systems most commonly affected include the respiratory tract, eyes, liver, pancreas, and central nervous system [3, 19, 21].

Respiratory signs include tachypnea, dyspnea, and cough, often associated with interstitial pneumonia [19, 18]. Ocular toxoplasmosis typically presents as anterior uveitis, characterized by aqueous flare, keratic precipitates, and miosis, but may also involve the posterior segment with retinochoroiditis [12, 19]. In a study of 60 seropositive cats with ocular signs, 63.33% had anterior uveitis, 20% had posterior segment involvement, and 8.33% had anterior chamber abnormalities [12]. Neurological signs, including seizures, ataxia, circling, and behavioral changes, result from focal or multifocal necrotizing encephalomyelitis [19, 21, 30]. Cerebral toxoplasmosis has been documented in cats coinfected with FeLV and feline infectious peritonitis virus, highlighting the role of immunosuppression in disease expression [30].

Hepatic involvement may cause icterus and elevated liver enzyme activities, particularly alanine aminotransferase (ALT) and aspartate aminotransferase (AST) [22, 21]. Pancreatitis can occur concurrently with hepatic disease [3]. Hematologic abnormalities include regenerative anemia in immunocompetent cats and non-regenerative anemia in immunocompromised individuals, along with lymphopenia and neutrophilia [22, 18]. Fatal disseminated toxoplasmosis, characterized by widespread necrosis in multiple organs, has been reported in immunocompromised cats [17, 30].

Pathogenesis and Immune Response

Following oral infection, bradyzoites released from tissue cysts invade intestinal epithelial cells and undergo the enteroepithelial cycle, leading to oocyst production [4, 5]. Concurrently, some parasites penetrate the intestinal wall and disseminate as tachyzoites via the lymphatic and hematogenous routes to reach extraintestinal tissues [4]. Tachyzoites invade nucleated cells by active penetration, forming a parasitophorous vacuole that resists fusion with host lysosomes [1]. Intracellular replication continues until host cell lysis, releasing tachyzoites that infect adjacent cells [4]. The host immune response, particularly cell-mediated immunity involving CD4+ and CD8+ T lymphocytes, macrophages, and natural killer cells, controls tachyzoite proliferation and induces conversion to the bradyzoite stage, resulting in tissue cyst formation [23, 24]. Tumor necrosis factor and nitric oxide are key effector molecules in this process [23]. Corticosteroid administration can suppress this immunity, leading to reactivation of latent infection and recrudescence of clinical disease [24].

Diagnostic Approaches

Diagnosis of feline toxoplasmosis requires integration of serological, molecular, and clinical findings, as no single test provides definitive diagnosis in all cases [1, 25]. The diagnostic approach is summarized in the following decision tree.

graph TD
    A[Cat with suspected toxoplasmosis], > B{Clinical signs consistent?}
    B, >|Yes| C[Perform serology: IgG and IgM ELISA or IFA]
    B, >|No| D[Monitor; no further testing]
    C, > E{IgM positive or rising IgG?}
    E, >|Yes| F[Perform PCR on blood, CSF, or aqueous humor]
    E, >|No| G[Consider latent infection; no active disease]
    F, > H{PCR positive?}
    H, >|Yes| I[Diagnose active toxoplasmosis]
    H, >|No| J[Consider other differentials; repeat serology in 2-4 weeks]
    I, > K[Initiate treatment: clindamycin or sulfonamide-based therapy]
    K, > L[Monitor clinical response and serology]

Serological Methods

Serological detection of anti-T. gondii antibodies is the most commonly employed diagnostic approach [1, 25]. The modified agglutination test (MAT), indirect fluorescent antibody (IFA) test, and enzyme-linked immunosorbent assay (ELISA) are widely used for detection of IgG and IgM antibodies [1, 7, 25]. A fourfold rise in IgG titers over 2 to 4 weeks or the presence of IgM antibodies suggests recent or active infection [25]. However, IgM antibodies can persist for months, limiting their utility for distinguishing acute from chronic infection [25]. Commercial ELISA kits using tachyzoite lysate antigen (TLA) demonstrate high sensitivity and specificity, with concordance rates exceeding 94% compared to latex agglutination tests [7]. Recombinant antigens, including combinations of surface antigen 2 (SAG2) and dense granule proteins (GRA2, GRA6, GRA7, GRA15), offer standardized, safer alternatives to TLA with comparable diagnostic performance [7]. More recently, the micronemal protein MIC17A, which is highly expressed in merozoites during the enteroepithelial cycle, has been identified as a superior diagnostic marker for feline toxoplasmosis, showing better reactivity with feline IgG than tachyzoite-stage antigens [6, 8]. Rapid immunochromatographic assays are available for point-of-care testing and have been used in seroprevalence studies [11, 10]. Carbon immunoassay has also been described as a simple and rapid serodiagnostic test [26].

Molecular Methods

Polymerase chain reaction (PCR) assays targeting conserved genetic elements, such as the B1 gene or the 529-base pair repetitive element, enable direct detection of T. gondii DNA in blood, cerebrospinal fluid, aqueous humor, bronchoalveolar lavage fluid, and tissue biopsies [1]. PCR is particularly valuable for confirming active infection in seropositive cats with compatible clinical signs [1, 25]. Quantitative real-time PCR provides additional information on parasite burden [1]. Emerging technologies, including nanomaterial-enhanced biosensors and artificial intelligence-driven diagnostic algorithms, are being developed to improve sensitivity and enable stage-specific detection [1].

Other Diagnostic Methods

Oocyst detection in feces by microscopic examination (e.g., fecal flotation) is insensitive due to intermittent shedding and morphological similarity to other coccidian oocysts [1, 15]. Bioassay in mice remains a gold standard for detecting viable parasites but is impractical for routine clinical use [1, 15]. Histopathological examination of biopsy or necropsy tissues can identify tachyzoites and tissue cysts, often accompanied by necrotizing inflammation [21, 30]. Immunohistochemistry using anti-T. gondii antibodies enhances detection in tissue sections [30].

Treatment

The primary therapeutic agent for feline toxoplasmosis is clindamycin, administered at a dosage of 10 to 12 mg/kg orally every 12 hours for 4 weeks [3, 12, 20]. Alternative treatments include sulfonamide-based combinations, such as sulfadiazine combined with pyrimethamine, although these may cause adverse effects including bone marrow suppression and anorexia [3, 20]. In a study of 60 cats with ocular toxoplasmosis treated with clindamycin and topical corticosteroids, 46.7% showed complete response, 41.7% showed partial response, and 11.6% showed poor response [12]. Immunocompetent cats generally respond favorably to treatment, whereas immunocompromised cats, particularly those with retroviral coinfection, may have a poorer prognosis [18]. Supportive care, including fluid therapy, nutritional support, and management of secondary infections, is essential in severe cases [3]. The immunomodulator polyprenyl phosphate (Phosprenyl) has been used as adjunctive therapy in cats with concurrent coronavirus infection and toxoplasmosis, with reported normalization of hematologic and biochemical parameters after 2 to 4 months of treatment [22].

Control and Prevention

Control of feline toxoplasmosis focuses on reducing environmental contamination with oocysts and preventing infection in cats [1, 2, 31]. Key preventive measures include:

• Feeding cats only cooked or commercially processed food to prevent ingestion of tissue cysts [3, 2].
• Preventing hunting behavior by keeping cats indoors [10, 3].
• Daily removal and proper disposal of feces from litter boxes to prevent sporulation of oocysts [1, 2].
• Cleaning litter boxes with hot water (above 70 degrees Celsius) to inactivate oocysts [2].
• Covering sandboxes and garden areas to prevent defecation by stray cats [2].
• Wearing gloves and practicing hand hygiene when handling litter boxes or gardening [2].

Vaccination of cats against T. gondii is not currently available in most regions [3]. Serological screening of cats is not recommended for public health purposes, as seropositive cats have already shed oocysts and are unlikely to re-shed [3, 25].

Zoonotic Risk

Cats are the only definitive hosts of T. gondii and thus the sole source of oocysts that contaminate the environment [1, 2]. The zoonotic risk associated with feline toxoplasmosis arises primarily from accidental ingestion of sporulated oocysts shed in cat feces [2, 9]. Humans can also become infected by consuming undercooked meat containing tissue cysts, but this route is not directly related to feline transmission [2]. Pregnant women and immunocompromised individuals are at highest risk for severe toxoplasmosis, which can cause congenital infection, encephalitis, and disseminated disease [1, 2]. Direct contact with cats is not considered a significant risk factor for human infection, as cats typically shed oocysts for only a short period and oocysts require 1 to 5 days to sporulate and become infectious [1, 2]. The primary preventive measures for at-risk humans include avoiding contact with cat feces, wearing gloves when gardening, and thoroughly washing fruits and vegetables [2]. Comprehensive reviews of feline toxoplasmosis and its zoonotic implications are available in related articles on this portal, including discussions of Feline Toxoplasmosis: Zoonotic Risks, Clinical Signs, and Management in Cats and Toxoplasmosis in Cats: Zoonotic Risks and Fecal Transmission.

References

[1] Dan Zhao, Yanzhen Liao, Hao Liu et al. Comprehensive diagnostic approaches to feline toxoplasmosis: Bridging traditional methods and emerging technologies. Virulence, 2025. URL: https://www.semanticscholar.org/paper/2e757aa2ddd10827f80fab2653e009ad1d5e71da

[2] K. Bresciani, A. L. Galvão, A. L. D. Vasconcellos et al. EPIDEMIOLOGICAL ASPECTS OF FELINE TOXOPLASMOSIS. 2016. URL: https://www.semanticscholar.org/paper/6b9f21d91d91295b742aef77632d2525479580ce

[3] S. Caney. Update on feline toxoplasmosis. 2021. URL: https://www.semanticscholar.org/paper/04388a62c4aa2e740b6e424bd443e7a1366da8e9

[4] J. P. Dubey, J. K. Frenkel. Feline toxoplasmosis from acutely infected mice and the development of Toxoplasma cysts. The Journal of Protozoology, 1976. URL: https://www.semanticscholar.org/paper/8baf73ffc19ee8034b04927ed5eb249b3daa9e18

[5] Y. Omata, H. Oikawa, M. Kanda et al. Experimental feline toxoplasmosis: humoral immune responses of cats inoculated orally with Toxoplasma gondii cysts and oocysts. Nihon Juigaku Zasshi. The Japanese Journal of Veterinary Science, 1990. URL: https://www.semanticscholar.org/paper/b51652947e0dfa715c4d08df67cd76221708e926

[6] Jinling Chen, Lilan Xue, Hongxia Hu et al. MIC17A is a novel diagnostic marker for feline toxoplasmosis. Animal Diseases, 2022. URL: https://www.semanticscholar.org/paper/409a1c77abdc8102e0ee7ad5b4a222f8efdd3e71

[7] A. Abdelbaset, Hend Alhasan, Doaa Salman et al. Evaluation of recombinant antigens in combination and single formula for diagnosis of feline toxoplasmosis. Experimental Parasitology, 2017. URL: https://www.semanticscholar.org/paper/1fd2239858e29ce336d8d273dd32e3c1ed51f467

[8] Özlem Günay-Esiyok, Ecem Su Koçkaya, Rana Yılmaz et al. The Potential of MIC17A both as an Entero-epithelial and Chronic Stage Marker for Detection of Feline Toxoplasmosis. Current Microbiology, 2026. URL: https://www.semanticscholar.org/paper/170f603aa98f7dd6b215d4a83f5f76f844c9e861

[9] MacKnight Kt, Robinson Hw. Epidemiologic studies on human and feline toxoplasmosis. 1992. URL: https://www.semanticscholar.org/paper/1f00a302a974f5cfc4ddec180eb6d4e0e52698df

[10] G. Sioutas, I. Symeonidou, A. Gelasakis et al. Feline Toxoplasmosis in Greece: A Countrywide Seroprevalence Study and Associated Risk Factors. Pathogens, 2022. URL: https://www.semanticscholar.org/paper/7a38aa6feab78b556c75fca605f426eadd8440c6

[11] S. M. Ibrahim, I. Mohamed, S. E. Suliman et al. Seroprevalence of Feline Toxoplasmosis in Kuwait State. International Journal of Current Microbiology and Applied Sciences, 2025. URL: https://www.semanticscholar.org/paper/f05f6b05b012c996ed244740c03869e7e95c3f5b

[12] K. M. Ali, A. M. Abu-Seida, M. Abuowarda. Feline ocular toxoplasmosis: seroprevalence, diagnosis and treatment outcome of 60 clinical cases. Polish Journal of Veterinary Sciences, 2021. URL: https://www.semanticscholar.org/paper/0d19fc80bf1bde129d6b11a3e14b5e0b7dc7192c

[13] A. Munhoz, Samir Batista Hage, R. Cruz et al. Toxoplasmosis in cats in northeastern Brazil: Frequency, associated factors and coinfection with Neospora caninum, feline immunodeficiency virus and feline leukemia virus. *Veterinary Parasitology:

[14] P. Jokelainen, O. Simola, E. Rantanen et al. Feline toxoplasmosis in Finland. Journal of Veterinary Diagnostic Investigation, 2012. URL: https://www.semanticscholar.org/paper/6a4fbacb25ee047707fcc3cc04c3c600dc321216

[15] J. Dubey. Feline toxoplasmosis and coccidiosis: a survey of domiciled and stray cats. Journal of the American Veterinary Medical Association, 1973. URL: https://www.semanticscholar.org/paper/ccb8d11ca9bbfc79aec59433b8a97df0159136cd

[16] A. Brennan. Risk factors in feline toxoplasmosis. 2018. URL: https://www.semanticscholar.org/paper/f488897999e2e89707722c6d17b4288f318c47a5

[17] A. Moore, A. Burrows, R. Malik et al. Fatal disseminated toxoplasmosis in a feline immunodeficiency virus-positive cat receiving oclacitinib for feline atopic skin syndrome. Veterinary Dermatology, 2022. URL: https://www.semanticscholar.org/paper/c931267329077e9f22210c3439872dddff053542

[18] E. A. Okewole, M. O. Akpan. Clinical feline toxoplasmosis: parasitological, haematological and serological findings in retroviral infected and uninfected cats. 2002. URL: https://www.semanticscholar.org/paper/2e27851bb71e0ccb0409282c54991b0237af3d91

[19] M. Lappin, C. Greene, S. Winston et al. Clinical Feline Toxoplasmosis. 1989. URL: https://www.semanticscholar.org/paper/1ae2d5c4420cf0ff662e99d9964dc6c34a70036b

[20] M. Lappin, C. Greene, S. Winston et al. Clinical feline toxoplasmosis. Serologic diagnosis and therapeutic management of 15 cases. Journal of Veterinary Internal Medicine, 1989. URL: https://www.semanticscholar.org/paper/188e9c3e7b48fcd3dd733c4dadbdd3bf4a2c1dbf

[21] R. Hirth, S. W. Nielsen. Pathology of feline toxoplasmosis. Journal of Small Animal Practice, 1969. URL: https://www.semanticscholar.org/paper/70ea7aed2b1cd57cf4238050c1395234eaa98dde

[22] S. Savoyskaya, A. Sanin, I. Ogorodnikova et al. Phosprenyl usage as part of the complex therapy of feline chronic coronavirus infection complicated by toxoplasmosis. Russian Veterinary Journal, 2022. URL: https://www.semanticscholar.org/paper/d2205256d3e3592683ec54c4e85a619eac09add0

[23] J. L. Faria, C. Couto, Sheron L Wierzynski et al. Feline Toxoplasmosis: Tumor Necrosis Factor, Nitric Oxide, and Free Radicals in Seropositive Cats. Journal of Parasitology, 2018. URL: https://www.semanticscholar.org/paper/6949c3df0034df3c74f48a543ed58094bbdee2c85

[24] J. Dubey, J. K. Frenkel. Immunity to Feline Toxoplasmosis: Modification by Administration of Corticosteroids. Veterinary Pathology-Supplement, 1974. URL: https://www.semanticscholar.org/paper/7d02cb1226cf6abea11de16a6dd851250bc20241

[25] M. R. Lappin. Feline toxoplasmosis: interpretation of diagnostic test results. Seminars in Veterinary Medicine and Surgery, 1996. URL: https://www.semanticscholar.org/paper/0ebd1635960c3958e8d9abd8d2646094a8be6fd8

[26] S. P. Pakes, W. Lai. Carbon immunoassay-a simple and rapid serodiagnostic test for feline toxoplasmosis. Laboratory Animal Science, 1985. URL: https://www.semanticscholar.org/paper/080360020a814df09eb4c1d8667765bf1f9c6448

[27] J. Teixeira, Jéssica S Oliveira, D. Almeida et al. Seroprevalence of feline toxoplasmosis in Teresina, Piauí, Brazil. 2016. URL: https://www.semanticscholar.org/paper/b5a75e8076e477c63e0ad3cf290165adcea31440

[28] M. Petrak, J. Carpenter. FELINE TOXOPLASMOSIS. Journal of the American Veterinary Medical Association, 1965. URL: https://www.semanticscholar.org/paper/b716bfc934da864c863f7e5d8c3624e2dcc7c73e

[29] J. Dubey. Feline toxoplasmosis and its nematode transmission. 1968. URL: https://www.semanticscholar.org/paper/9739e9c0ebc5b4da8a73d718ce795f65831c9bd1