Serological Diagnosis of Feline Toxoplasmosis: ELISA, IFAT and ICT

Diagnosing Toxoplasma gondii infection in cats can be challenging because the diagnostic value of a test depends on the infection stage, sample type, and purpose of testing. Serological methods remain important for feline toxoplasmosis because they can detect antibodies and are particularly useful for screening. Among the available approaches, enzyme-linked immunosorbent assay (ELISA), indirect fluorescent antibody test (IFAT), and immunochromatographic tests (ICT) offer different advantages in terms of sensitivity, standardization, speed, equipment requirements, and field applicability. 

For routine veterinary practice, understanding these differences helps clinicians select an appropriate initial test and determine when additional confirmation may be warranted. 

ELISA: A Practical Screening Approach 

ELISA detects specific antibodies through an antigen–antibody reaction and can be adapted for large-scale screening. SAG1-based ELISA has demonstrated excellent diagnostic performance for detecting T. gondii antibodies in cats. Its relatively simple protocol, ease of standardization, and high sensitivity make it practical for widespread screening applications¹. 

This makes ELISA particularly useful when a veterinarian needs to evaluate a larger number of cats rather than relying on a method requiring more specialized procedures. However, quantitative interpretation may still require laboratory-based analysis, even though strongly positive reactions may sometimes be visually assessed for qualitative interpretation². 

Where IFAT Fits In 

Although IFAT is no longer considered the primary screening approach, it retains an important role in feline toxoplasmosis diagnosis. It is described as a gold-standard method and can provide valuable secondary verification following initial screening. 

A practical approach is therefore to use ELISA for preliminary screening and IFAT when additional verification is required. This sequential strategy can improve diagnostic efficiency while potentially reducing the overall cost compared with relying on IFAT alone¹. 

The choice should also take into account the practical requirements of IFAT. Feline blood samples need to be collected and transported to a laboratory with appropriate testing equipment and trained personnel, making the method less suitable for field-based applications^1,3. 

ICT: Rapid Testing in Clinical and Field Settings 

Immunochromatographic tests provide a different advantage: speed and simplicity. ICTs use antigen–antibody binding and generally require only a few drops of sample, with results available in approximately 30 minutes. This makes them particularly attractive for point-of-care and field diagnostics. 

In cats, ICTs may detect T. gondii-specific antigens or antibodies, including IgM or IgG. A visible colour change on the test strip indicates the detection reaction. These rapid tests have demonstrated diagnostic performance comparable to established serological methods such as in-house IFAT^1,4. 

However, early infection can present a diagnostic challenge because antibody production may be insufficient for detection, creating a risk of false-negative results⁵. ICTs are also primarily qualitative rather than quantitative. 

Practical Clinical Insights 

For everyday veterinary practice, the choice can be approached according to the diagnostic requirement: 

• Large-scale preliminary screening: SAG1-based ELISA offers sensitivity, simplicity, and easier standardization. 

• Secondary verification: IFAT can provide additional confirmation when serological findings require verification. 

• Rapid or field-based assessment: ICT provides a convenient option when immediate qualitative results are required. 

• Early infection: A negative antibody-based result should be interpreted cautiously because antibody levels may remain below the detection threshold. 

No single serological method addresses every diagnostic situation. Combining methods according to the clinical requirement can provide a more efficient diagnostic pathway while accounting for differences in speed, laboratory requirements, and diagnostic limitations. 

References 

1. Zhao D, Liao Y, Liu H, Wang J, Liang R, Zhou R, Ding J, Zhang S, Tang X. Comprehensive diagnostic approaches to feline toxoplasmosis: Bridging traditional methods and emerging technologies. Virulence. 2025 Dec 31;16(1):2563766. https://www.tandfonline.com/doi/pdf/10.1080/21505594.2025.2563766 

2. Huang M, Huang H, Cao X, Yin Y, Shi K, Wang D, Hu D, Song X. Seroprevalence of Toxoplasma gondii in water buffaloes and cats in Guangxi, China. Parasitology Research. 2024 Jan;123(1):18. https://link.springer.com/content/pdf/10.1007/s00436-023-08055-3.pdf 

3. Oshiro LM, Motta-Castro AR, Freitas SZ, Cunha RC, Dittrich RL, Meirelles AC, Andreotti R. Neospora caninum and Toxoplasma gondii serodiagnosis in human immunodeficiency virus carriers. Revista da Sociedade Brasileira de Medicina Tropical. 2015;48(5):568-72. https://www.scielo.br/j/rsbmt/a/JSyZhFpp4W8X6hBtPd7dhhf/?format=pdf&lang=en 

4. Villanueva‐Saz S, Martínez M, Giner J, Pérez MD, Tobajas AP, Yzuel A, Verde MT, Lacasta D, Fernández A, Marteles D, Ruíz H. Evaluation of an immunochromatographic serologic test to detect the presence of anti‐Toxoplasma gondii antibodies in cats. Veterinary Clinical Pathology. 2023 Jun;52(2):284-7. https://onlinelibrary.wiley.com/doi/pdf/10.1111/vcp.13230 

5. Kim MJ, Park SJ, Park H. Trend in serological and molecular diagnostic methods for Toxoplasma gondii infection. European journal of medical research. 2024 Oct 28;29(1):520. https://link.springer.com/content/pdf/10.1186/s40001-024-02055-4.pdf