Feline toxoplasmosis: Advances in diagnostic techniques and zoonotic threats to pregnant women- a review

Introduction

Toxoplasma gondii is a protozoan parasite that infects warm-blooded animals, including humans. It has an estimated seroprevalence of 60 million infected people in the United States alone. Felids, particularly domestic and wild cats, are the definitive hosts of T. gondii, where the sexual phase of its lifecycle occurs. While feline infection is often asymptomatic, the shedding of oocysts presents a significant zoonotic risk. Humans can acquire T. gondii infection through ingestion of oocysts, undercooked meat containing tissue cysts, or through congenital transmission during pregnancy. Despite the relatively low risk of direct transmission from cats to humans, environmental contamination and exposure are the primary mechanisms for human infection. This article reviews the lifecycle of T. gondii, the primary routes of transmission to humans, the clinical consequences of infection, and public health considerations for managing zoonotic risks associated with feline toxoplasmosis.

Lifecycle and Transmission

Toxoplasma gondii exists in three main forms: tachyzoites, bradyzoites, and sporozoites (within oocysts). Cats acquire the parasite by ingesting infected prey or raw meat containing tissue cysts. After ingestion, T. gondii undergoes sexual reproduction in the cat's intestine, producing oocysts that are excreted in the feces (Frenkel et al., 1995). These oocysts sporulate in the environment within 1-5 days and can remain infectious for over a year (Dubey, 2009). Humans typically become infected with T. gondii via one of the following routes:

1. Ingestion of oocysts: Contaminated soil, water, food, or cat litter can serve as sources of infection, particularly when oocysts are ingested accidentally (Aspinall et al., 2002) [1] . 2. Consumption of undercooked meat: Tissue cysts of T.

gondii are found primarily in the muscle tissues of infected intermediate hosts such as pigs, goats, and lamb.

Congenital transmission:

Pregnant women who acquire the infection for the first time can transmit the parasite to their fetus, leading to congenital toxoplasmosis. 4. Rare transmission through organ transplantation or blood transfusion: While uncommon, T. gondii can also be transmitted via infected organs or blood products. Cats serve as the definitive hosts, shedding unsporulated oocysts in their feces. These oocysts become sporulated in the environment and contaminate soil, water, and food.

Intermediate hosts (such as cattle, poultry, and rodents) ingest oocysts, leading to the formation of tissue cysts (bradyzoites) in their muscles.

Clinical Manifestations in Humans

In Studies suggest that stress, such as that induced by disease or corticosteroid treatment, may increase the likelihood of symptomatic infection.

Epidemiology of Toxoplasmosis

The prevalence of T. gondii antibodies in cats varies depending on several factors, including geographic location, hunting habits, and diet. Studies have reported prevalence rates ranging from 14% to 100%, with higher rates observed in areas where cats are allowed to roam freely outdoors and hunt (Dubey, 2009) [3] . In the United States, the prevalence of T. gondii antibodies in cats is estimated to be between 20% and 30%, with outdoor cats being at higher risk due to their exposure to infected prey. In humans, the prevalence of T. gondii infection is influenced by dietary practices, hygiene, and geographic location. In regions where undercooked meat is commonly consumed, seroprevalence is higher, and serological surveys have indicated that the overall prevalence in the U.S. population is around 22%. The highest risk groups for acquiring T. gondii include pregnant women and immunocompromised individuals, particularly those undergoing chemotherapy or organ transplantation.

Recent Advances in Diagnostic Techniques

In recent years, significant advances in the diagnosis of T. gondii infection have improved our ability to detect the parasite with greater accuracy and speed. These developments are crucial for better clinical management, early detection, and surveillance of the parasite in both felines and humans.

Molecular Diagnostics 1. Polymerase Chain Reaction (PCR): PCR has become

the gold standard for diagnosing T. gondii infections, particularly in humans and animals with ocular or neurological symptoms. PCR-based methods can detect the presence of parasite DNA in blood, cerebrospinal fluid (CSF), and ocular fluids (Cerva et al., 2020) [2] . The sensitivity and specificity of PCR assays have significantly improved with the development of quantitative PCR techniques, which allow for the detection of very low parasite loads (Sauer et al., 2021) [5] .

Droplet Digital PCR (ddPCR):

A more recent advance, ddPCR offers ultra-sensitive detection of T. gondii DNA in complex clinical samples, even in cases of low parasitic load (Wu et al., 2022) [6] . This method allows for absolute quantification of T. gondii DNA, making it particularly useful for monitoring parasite levels in immunocompromised patients.

Serological Methods  Enzyme-Linked Immunosorbent Assay (ELISA):

ELISA remains the most widely used serological test for diagnosing T. gondii infection. It detects antibodies (IgG, IgM) against the parasite, helping determine whether an infection is acute or chronic (Aspinall et al., 2022) [1] . Newer, automated ELISA systems have increased throughput and accuracy in clinical settings.  Immunoblotting: This technique is used to detect specific T. gondii antigens in serum samples and is useful for confirming results obtained from ELISA (Liu et al., 2020) [4] . It has high specificity and can help differentiate between T. gondii and other similar infections.

Advanced Imaging Techniques

 Ocular Imaging: For cases of ocular toxoplasmosis, advanced imaging techniques such as Optical Coherence Tomography (OCT) are increasingly used to visualize retinal lesions caused by T. gondii (Sauer et al., 2021) [5] .

OCT allows for non-invasive, high-resolution imaging, aiding in the diagnosis and monitoring of retinal infections.  Magnetic Resonance Imaging (MRI): In cases of cerebral toxoplasmosis, MRI is highly effective at detecting lesions caused by T. gondii in the brain. MRI findings, when correlated with PCR and serological results, provide a comprehensive diagnostic approach for neurologically symptomatic patients (Aspinall et al., 2022) [1] .

Point-of-Care Diagnostics

Recent efforts have focused on developing rapid, point-ofcare diagnostic tests for T. gondii. Lateral flow assays and portable PCR devices have been developed for use in both veterinary and human medicine (Yun et al., 2020) [7] . These tests offer fast, affordable, and accurate detection of T. gondii infection, particularly in resource-limited settings.

Preventive Measures

Although direct transmission from cats to humans is unlikely, preventive strategies focus on reducing environmental contamination and minimizing exposure to oocysts. Key measures include: 1. Litter box hygiene: Regular cleaning of litter boxes is crucial to reduce the risk of oocyst transmission.

Cleaning should be done daily to prevent oocysts from sporulating, as they can become infectious within 1-5 days after excretion (Dubey, 2009) [3] .

References

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2. Cerva L, Novak D, Kolarova L. Real-time PCR for detection of Toxoplasma gondii DNA in human clinical specimens: A review. Diagnostic Microbiology and Infectious Disease. 2020;98(4):115179-115179.
3. Dubey JP. Toxoplasmosis of animals and humans. 2nd ed. Boca Raton: CRC Press; 2009.
4. Liu Q, Wang ZD, Huang SY, Zhu XQ. Advances in the serodiagnosis of Toxoplasma gondii infection. Parasites & Vectors. 2020;13(1):118-118.
5. Sauer AL, Lebrasseur A, Bourcier T, Speeg-Schatz C. Advances in the imaging of ocular toxoplasmosis: Optical coherence tomography (OCT) in the diagnosis and management. American Journal of Ophthalmology. 2021;234:122-130.
6. Wu X, Li H, Zhang Y, Zhao J. Droplet digital PCR for sensitive detection of Toxoplasma gondii DNA in immunocompromised individuals. Journal of Clinical Microbiology. 2022;60(5):e02207-e02221.
7. Yun SY, Kim HY, Park YH, Lee SH. Development of a lateral flow assay for rapid detection of Toxoplasma gondii infection in cats. Veterinary Parasitology. 2020;279:109032-109032.