Evaluation of an indirect ELISA using a tachyzoite surface antigen SAG1 for diagnosis of Toxoplasma gondii infection in cats

Abstract

Toxoplasma gondii infection is very common in cats throughout the world. Most cats are subclinically infected and potentially fatal clinical disease occurs in some of them. The aim of this study is to develop an indirect enzyme linked immunosorbent assay (ELISA) test using an affinity purified tachyzoite surface antigen (SAG1) to detect T. gondii infection in cats. Six sero-negative kittens were used in this study; four kittens received 10(4) T. gondii tachyzoites of NED strain (type III) and the remaining two were used as uninfected controls. Serum samples were collected within 41 days and were evaluated for anti-T. gondii antibodies using indirect fluorescent antibody test (IFAT) and ELISA method. IgG antibodies were detectable at least from eight days after tachyzoites inoculation and an increasing pattern in both serum ELISA indices (SIns) and IFAT titers were detected. SIns were significantly different in sera of cats presenting different IFAT titers. In order to evaluate the performance of ELISA to detect anti-T. gondii antibodies of naturally infected cats, serum samples were also collected from household and stray cats and evaluated in the same way. IFAT was regarded as the standard test and sensitivity and specificity of the ELISA to detect the infection in naturally infected kittens were analyzed using two-graph receiver operating characteristic (TG-ROC) analysis. An area under curve (AUC) of 0.996 revealed the test as a highly accurate test with relative sensitivity and specificity of 100% and 96% for a cut-off value of 0.10 for SIn.