Engineered Multi-Epitope Protein Yields Reliable New Test for Feline Toxoplasmosis

A newly developed diagnostic test for toxoplasmosis in cats promises to close a stubborn gap in the fight against one of the world’s most widespread parasitic infections. Researchers have built an indirect enzyme-linked immunosorbent assay, or iELISA, around an artificial protein stitched together from the most immunologically powerful fragments of six different Toxoplasma gondii antigens. The result, described in a study published in Parasites & Vectors, is a screening tool that matched an established reference assay in nearly every sample tested while showing no cross-reactivity with antibodies raised against three common feline viruses. Because domestic cats are the only animals in which T. gondii can complete its sexual cycle, a dependable way to identify infected cats has consequences far beyond veterinary medicine, touching food safety, pregnancy care, and public health surveillance alike.

Toxoplasma gondii infects an estimated third of the human population, usually without symptoms, but it can cause severe disease in immunocompromised people and in fetuses when a woman acquires the parasite for the first time during pregnancy. The parasite’s life cycle depends on cats: after a cat eats tissue cysts in infected prey, the parasite reproduces sexually in the cat’s intestine and millions of resistant oocysts are shed in feces, contaminating soil, water, and food. Identifying which cats carry the parasite therefore sits at the hinge of prevention strategies. Yet existing serological tests for feline toxoplasmosis face a trade-off. Whole-parasite lysate antigens deliver sensitivity but are expensive and laborious to produce, requiring the propagation of live parasites, while single recombinant antigens are convenient but can miss infections because antibody responses vary between animals and across stages of infection.

The research team, led by scientists at Tarim University in Xinjiang and the Shanghai Veterinary Research Institute of the Chinese Academy of Agricultural Sciences, set out to combine the breadth of whole-parasite antigens with the convenience and consistency of recombinant proteins. Their strategy began with the rational selection of six T. gondii antigens that are well characterized in the literature: SAG1 and SAG2, surface proteins abundantly displayed on tachyzoites; ROP1, a component of the rhoptry organelles the parasite uses to invade host cells; GRA1, a dense granule protein involved in modifying the vacuole the parasite occupies inside cells; and two further candidates, LEA870 and LEA880. Each of these antigens is known to be recognized by antibodies during natural infection, making their epitopes logical building blocks for a synthetic diagnostic target.

Once the antigens were chosen, the team turned to bioinformatics to identify the dominant B-cell epitopes within them, the short linear stretches of amino acids most likely to be exposed on the protein surface and most likely to be bound by antibodies circulating in infected cats. Epitope prediction tools evaluate properties such as hydrophilicity, surface accessibility, flexibility, and sequence conservation to score each region. The strongest candidate peptides from all six antigens were then linked in tandem into a single open reading frame and cloned for expression, producing one recombinant fusion protein the authors named Polyantigenic Epitope Peptide 1, or PEP1. The logic is straightforward: by fusing epitopes from multiple antigens into one molecule, the assay gains multiple opportunities to capture antibodies from a serum sample, whatever the individual animal’s antigen preference or infection stage might be.

With PEP1 in hand, the researchers optimized every step of the indirect ELISA protocol. Using checkerboard titration, a method in which antigen concentrations and serum dilutions are varied simultaneously in a matrix, they identified the conditions that maximize the signal difference between positive and negative sera. The final recipe calls for coating microplate wells with PEP1 at 5 micrograms per milliliter overnight at 4 degrees Celsius for 12 hours, then blocking residual binding sites with 0.5 percent bovine serum albumin for 60 minutes at 37 degrees Celsius. Test sera are diluted 1:1,000 and incubated for 45 minutes, followed by an enzyme-conjugated secondary antibody diluted 1:2,500 for an hour. The colorimetric reaction is developed with the chromogenic substrate 3,3′,5,5′-tetramethylbenzidine, TMB, for 25 minutes at room temperature in the dark before the optical density is read. Each parameter was validated after the titration, ensuring the protocol is reproducible rather than a lucky configuration.

The performance figures reported for the assay are the heart of the study. In preliminary evaluation, PEP1-based iELISA showed no cross-reactivity with serum samples positive for three clinically important feline viruses: feline panleukopenia virus, feline herpesvirus, and feline calicivirus. Cross-reactivity is the perennial weakness of serology, since antibodies to unrelated pathogens can sometimes bind assay antigens and generate false positives; the absence of such reactions here speaks to the specificity of the chosen epitopes. The detection limit of the assay extended to a serum dilution of 1:6,400, meaning that even antibodies present at very low concentration in highly diluted samples were still detected, an indication of high analytical sensitivity. Reproducibility was assessed by repeating the assay within a single plate and across separate plates on different days; both the intra-assay and inter-assay coefficients of variation remained below 10 percent, a conventional benchmark for a robust immunoassay.

Perhaps most convincing is how the new assay fared against an established comparator. The researchers ran the PEP1 assay in parallel with an iELISA based on T. gondii lysate antigen, TLA-iELISA, on 78 feline serum samples. The two methods agreed on 97 percent of samples, suggesting that the artificial multi-epitope protein performs on par with an antigen preparation drawn from whole parasites. That is a meaningful result because TLA is widely regarded as a sensitive coating antigen but carries practical drawbacks: it must be prepared from parasite cultures, varies between batches, and raises biosafety and cost considerations. A single recombinant protein produced in bacteria, by contrast, can be manufactured in large, consistent quantities, lyophilized for stability, and shared between laboratories with minimal variation, making large-scale screening of cat populations far more feasible.

The authors frame the work not only as a diagnostic advance but as a platform for future research. A reliable serological test is a prerequisite for evaluating vaccine efficacy in cats, because any trial of a feline toxoplasmosis vaccine must distinguish vaccinated-and-protected animals from naturally infected ones, and must track antibody responses over time. The paper’s conclusions state that the PEP1-based indirect ELISA is a reliable tool for the serodiagnosis of feline toxoplasmosis and provides a solid technical platform for future studies of vaccine efficacy. Given that several candidate vaccines against T. gondii in cats have been explored without one reaching broad deployment, having a standardized, reproducible serological readout could accelerate that pipeline considerably.

The broader implications extend to public health, where cats occupy a unique and paradoxical position: beloved companions in tens of millions of households, yet the definitive host of a parasite that causes congenital infection worldwide. Routine, affordable screening could inform advice to pregnant women and immunocompromised individuals about the risk posed by their pets, guide removal of infected animals from catteries and shelters, and support surveillance in regions where the parasite is endemic. The study’s technical approach, epitope prediction followed by gene synthesis of a poly-epitope fusion protein, is also a template that could be adapted to other pathogens where multi-antigen coverage is desirable but whole-organism antigens are impractical. In that sense, the contribution is as much methodological as it is pathogen-specific.

The work was supported by the National Natural Science Foundation of China and the National Key Research and Development Program of China, and was conducted with approval from the Animal Administration and Ethics Committee of the Shanghai Veterinary Research Institute. Positive feline serum samples were generously provided by Professors Chunchun Meng and Quan Wang of the Shanghai Veterinary Research Institute, while clinically derived samples came from collaborating veterinary hospitals in Shanxi and Henan provinces. As the world continues to grapple with a parasite that infects humans and animals across every continent, a simple engineered protein that captures the antibody signature of infection in the parasite’s definitive host is a small molecule with an outsized role to play.

Subject of Research: Development of a recombinant multi-epitope protein-based serological assay for detecting Toxoplasma gondii antibodies in cats

Article Title: Development of a serological assay based on a recombinant protein containing polyantigenic epitope peptides derived from Toxoplasma gondii

Article References: Sun, L., Feng, W., Cao, J., Zhou, Y., Zhou, J., Wu, J., & Zhang, H. (2026). Development of a serological assay based on a recombinant protein containing polyantigenic epitope peptides derived from Toxoplasma gondii. Parasites & Vectors. https://doi.org/10.1186/s13071-026-07686-6

Image Credits: AI Generated

DOI: 10.1186/s13071-026-07686-6

Keywords: Toxoplasma gondii, feline toxoplasmosis, serological assay, ELISA, recombinant protein, polyantigenic epitope peptides, SAG1, B-cell epitopes, diagnostics, parasitology, cats, public health