Toxoplasmosis in Cats: Risks During Pregnancy and Transmission to Infants

Etiology and Life Cycle of Toxoplasma gondii

Toxoplasma gondii is an obligate intracellular apicomplexan parasite with a heteroxenous life cycle that culminates in sexual reproduction exclusively within the intestinal epithelium of felids, the definitive hosts [1, 2]. The parasite exists in three infectious stages: tachyzoites (rapidly dividing), bradyzoites (slowly dividing within tissue cysts), and sporozoites (within sporulated oocysts) [3, 4]. Cats become infected after ingesting tissue cysts from intermediate hosts (e.g., rodents, birds) or, less commonly, after ingesting sporulated oocysts from the environment [5, 6]. Following ingestion, bradyzoites excyst in the small intestine, invade enterocytes, and undergo multiple rounds of asexual multiplication (merogony) before differentiating into gametocytes [1, 7]. Fertilization yields unsporulated oocysts that are shed in feces, typically beginning 3 to 10 days post-infection and continuing for 1 to 3 weeks [8, 9]. A single cat can shed millions of oocysts, which sporulate in the environment within 1 to 5 days under favorable conditions of temperature and humidity, becoming infectious to a wide range of warm-blooded animals, including humans [10, 11].

The sexual development of T. gondii in the feline gut has been characterized at single-cell resolution, revealing distinct transcriptional programs for merozoites, gametocytes, and oocyst wall formation [1]. MicroRNA expression in the feline small intestine is dynamically altered during infection, with specific miRNAs implicated in host-parasite interactions and immune modulation [7]. Recent advances have enabled the differentiation of pre-sexual and sexual stages using retinal epithelial cells and intestinal organoids, providing in vitro models to study feline-specific development [2].

Epidemiology and Seroprevalence in Cats

Feline toxoplasmosis is distributed globally, with seroprevalence varying widely by geographic region, management practices, and diagnostic methods [5, 6, 11, 9]. In Hong Kong, seroprevalence in privately-owned cats and community cats was reported at 12.3% and 18.7%, respectively, using commercial ELISA kits [5]. In Jordan, a study combining serological and molecular detection found an overall seroprevalence of 41.2% in domestic cats, with risk factors including outdoor access, raw meat feeding, and lack of veterinary care [6]. In Brazil, stray cats in Northwestern São Paulo showed a seroprevalence of 28.4%, with spatial clustering in areas of high human density [9]. Similarly, in Salvador, Brazil, seroprevalence in companion animals from informal settlements reached 35.2%, highlighting the role of environmental contamination [11]. In Poland, wild felids (e.g., lynx, wildcats) exhibited seroprevalences ranging from 15% to 60%, indicating sylvatic cycles [12].

Molecular detection of T. gondii DNA in feline feces has been reported in stray cats in Bangkok, Thailand, with a prevalence of 8.7% by PCR targeting the B1 gene [8]. In Jordan, molecular detection in blood and feces yielded 15.3% positivity [6]. These data underscore that even seropositive cats may intermittently shed oocysts, posing a risk for environmental contamination [8, 9].

Clinical Signs and Pathology in Cats

Most immunocompetent cats infected with T. gondii remain asymptomatic [13, 14]. Clinical disease is more common in kittens, immunosuppressed cats (e.g., those co-infected with feline leukemia virus, FeLV), or cats infected with highly virulent strains [14]. Common clinical signs include fever, lethargy, anorexia, dyspnea (pneumonitis), icterus (hepatic involvement), and neurological deficits such as ataxia, seizures, and cranial nerve deficits [14, 15]. Ocular toxoplasmosis may present as uveitis, chorioretinitis, or retinal detachment [16, 15]. A case report described a mouse-virulent recombinant type I/III strain isolated from an immunosuppressed cat with FeLV-C subgroup infection, presenting with severe hepatic cytology abnormalities [14].

Pathologically, acute toxoplasmosis is characterized by multifocal necrosis in the liver, lungs, lymph nodes, and central nervous system, with abundant tachyzoites visible in tissue sections [17, 14]. Chronic infection is marked by tissue cysts (bradyzoites) in the brain, skeletal muscle, and myocardium, typically without significant inflammation [18, 15]. The MIC17A antigen has been identified as a potential marker for both entero-epithelial and chronic stages in feline toxoplasmosis, offering a target for stage-specific diagnostics [18].

Diagnostics

Diagnosis of feline toxoplasmosis relies on a combination of serological, molecular, and histopathological methods [15]. Serological detection of anti-T. gondii IgG and IgM antibodies is the most common approach, using commercial ELISA kits or indirect immunofluorescence assays [5, 6, 15]. A double-antigen sandwich colloidal gold immunochromatographic strip has been developed and field-validated for detection of antibodies in multiple host species, including cats, offering rapid point-of-care testing [19]. Similarly, a SAG1-based colloidal gold strip has been developed for swine but may be adapted for feline use [20].

Molecular diagnostics, particularly PCR targeting the B1 gene or 529 bp repeat element, are used to detect parasite DNA in blood, feces, or tissues [8, 21]. An antisense PCR assay has been developed specifically for domestic cats, improving sensitivity by targeting the highly expressed SAG1 mRNA [21]. Real-time PCR allows quantification of parasite burden and differentiation of acute versus chronic infection [15].

Histopathological examination of biopsy or necropsy tissues can reveal tachyzoites or tissue cysts, often confirmed by immunohistochemistry [17, 14]. A comprehensive diagnostic algorithm is presented in Figure 1.

flowchart TD
    A[Cat with suspected toxoplasmosis], > B{Clinical signs?}
    B, >|Yes| C[Serology: IgG/IgM ELISA]
    B, >|No| D[Routine screening?]
    D, >|Yes| C
    D, >|No| E[No further action]
    C, > F{IgM positive?}
    F, >|Yes| G[Acute infection suspected]
    F, >|No| H{IgG positive?}
    H, >|Yes| I[Chronic/latent infection]
    H, >|No| J[Seronegative]
    G, > K[Confirm with PCR on blood/feces]
    I, > L[PCR optional; assess shedding risk]
    K, > M{Feces PCR positive?}
    M, >|Yes| N[Active oocyst shedding]
    M, >|No| O[Tissue infection only]
    N, > P[Implement hygiene measures]
    O, > Q[Monitor clinical signs]
    P, > R[Consider treatment if clinical]
    Q, > R
    R, > S[Follow-up serology/PCR]

Figure 1. Diagnostic decision tree for feline toxoplasmosis. Adapted from Zhao et al. [15] and Li et al. [21].

Treatment

Treatment of clinical feline toxoplasmosis typically involves a combination of clindamycin (10-12 mg/kg orally or intramuscularly every 12 hours for 2-4 weeks) and supportive care [15]. Alternative regimens include trimethoprim-sulfonamide combinations or pyrimethamine with a sulfonamide, though these are less commonly used in cats due to potential adverse effects [15]. Treatment is most effective when initiated early in the course of acute disease. For asymptomatic cats, treatment is generally not recommended, as it does not eliminate tissue cysts and may select for resistance [3, 4].

Recent advances in vaccine development include gene-edited live-attenuated vaccines that show promise in preventing oocyst shedding and reducing tissue cyst burden in cats [3, 4]. An inactivated vaccine has been evaluated in captive wildlife, demonstrating reduced mortality associated with toxoplasmosis [22]. However, no commercial vaccine is currently licensed for cats [4].

Risks During Pregnancy: Feline and Zoonotic Perspectives

Feline Vertical Transmission

Vertical transmission of T. gondii from a pregnant queen to her kittens can occur if the queen acquires a primary infection during gestation [23, 24]. Tachyzoites cross the placenta and infect fetal tissues, leading to abortion, stillbirth, or congenital toxoplasmosis in kittens [23]. In a study of reproductive tissues from companion animals in a neutering program, T. gondii DNA was detected in ovarian and uterine tissues, confirming the potential for vertical transmission [23]. In other species, such as goats and sheep, vertical transmission is a well-documented cause of abortion [17, 25]. In horses, molecular detection of T. gondii in an aborted equine fetus has been reported [26]. The risk of vertical transmission in cats is highest when primary infection occurs during the first two-thirds of gestation [24].

Zoonotic Transmission to Pregnant Women and Infants

The primary public health concern regarding feline toxoplasmosis is the risk of zoonotic transmission to pregnant women and subsequent congenital infection of the fetus [10, 24]. Humans typically acquire infection by ingesting sporulated oocysts from contaminated soil, water, or food, or by consuming undercooked meat containing tissue cysts [27, 10]. Direct contact with cats is not a major risk factor, as cats shed oocysts only for a short period and oocysts require sporulation to become infectious [10, 9]. However, handling cat litter boxes or gardening in soil contaminated with cat feces poses a risk [10, 28].

Seroprevalence studies in pregnant women reveal significant variation. In Abidjan, Côte d'Ivoire, knowledge and practices regarding toxoplasmosis among pregnant women were found to be poor, with only 30% aware of the link between cats and infection [27]. In Kars, Turkey, anti-T. gondii antibodies were detected in 38.5% of women with a history of abortion or stillbirth, and seropositivity was associated with contact with cats and consumption of raw meat [29]. In Mexico, veterinary medicine professionals and students showed a seroprevalence of 15.2%, indicating occupational exposure [30]. In Burundi, the burden of congenital toxoplasmosis was estimated at 1.2 cases per 1,000 live births, with significant associated morbidity [24].

The term "cat toxoplasmosis baby" is often used in lay contexts to refer to the risk of congenital toxoplasmosis from maternal infection acquired via feline-derived oocysts. It is critical to communicate that the risk is not from the cat itself but from environmental contamination with oocysts [10]. Pregnant women who are seronegative should avoid cleaning litter boxes, wear gloves when gardening, and ensure that cats are kept indoors and fed cooked or commercial food to prevent hunting [10, 28].

Transmission to Infants: Congenital Toxoplasmosis

Congenital toxoplasmosis occurs when a pregnant woman acquires a primary T. gondii infection and the parasite crosses the placenta to infect the fetus [24]. The risk and severity of fetal infection depend on the gestational age at maternal seroconversion. Infection in early pregnancy is less likely to be transmitted but more likely to cause severe disease, including hydrocephalus, intracranial calcifications, and chorioretinitis [16, 24]. Infection in late pregnancy is more frequently transmitted but often results in subclinical infection at birth, with potential for late-onset ocular or neurological sequelae [16, 24].

Diagnosis of congenital toxoplasmosis in infants relies on detection of specific IgM and IgA antibodies in neonatal serum, as well as PCR on amniotic fluid or neonatal blood [15, 24]. Treatment of infected infants with pyrimethamine and sulfadiazine reduces the risk of long-term sequelae [24]. Prevention strategies include serological screening of pregnant women, health education regarding avoidance of oocyst exposure, and, in some countries, systematic screening programs [27, 10].

Control and Prevention

Control of feline toxoplasmosis and reduction of zoonotic risk require a multi-pronged approach. For individual cat owners, the following measures are recommended:

• Keep cats indoors to prevent hunting and ingestion of intermediate hosts [5, 10].
• Feed cats only commercial cooked or canned food; avoid raw meat diets [6, 10].
• Clean litter boxes daily (oocysts require >24 hours to sporulate) and dispose of feces in sealed bags [10].
• Pregnant women and immunocompromised individuals should avoid handling cat litter [10, 28].
• Cover children's sandboxes when not in use to prevent cat defecation [28].

At the population level, stray cat management, public education, and environmental decontamination are important [8, 9]. Vaccination of cats with live-attenuated or gene-edited vaccines could reduce oocyst shedding and environmental contamination, though no such vaccine is yet commercially available [3, 4]. Multi-omics studies have shown that T. gondii alters the gut microbiota and systemic metabolism in cats, which may have implications for shedding dynamics and potential probiotic interventions [31].

Conclusion

Feline toxoplasmosis is a complex parasitic disease with significant implications for both feline health and public health, particularly regarding the risk of congenital toxoplasmosis in infants. Understanding the life cycle, epidemiology, and diagnostic options is essential for veterinary practitioners. While the risk of direct transmission from a pet cat to a pregnant woman is low when basic hygiene measures are followed, environmental contamination with oocysts remains a major concern. Continued research into vaccines, diagnostics, and host-parasite interactions will further refine prevention and control strategies.

References

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