Optimisation of cut-off titres in Toxoplasma gondii specific ELISA and IFAT in dog sera using immunoreactivity to SAG-1 antigen as a molecular marker of infection

摘要

Toxoplasma gondii is an Apicomplexan parasite that infects a broad range of hosts, including man and domestic animals. T. gondii infection in dogs is distributed worldwide, with prevalence rates ranging from 20 percent to 91 percent in the different countries (Dubey, 1985; Uggla et al., 1990; Guimaraes et al., 1992). Clinical toxoplasmosis is less common, occurring usually in young dogs, and associated with immunosuppressive conditions or concomitant canine distemper virus (CDV) infection (Dubey et al., 1989). Clinical signs can vary considerably and include neuromuscular, respiratory or gastrointestinal disorders and, the diagnosis of toxoplasmosis in dogs commonly needs to be supported by serological methods. However, the criteria adopted for serum samples to be considered positive differ between the different studies. Dilutions of 1:16, 1:20 or 1:64 have been used as cut off titres in the indirect fluorescence antibody test (IFAT) (Ahmed et al., 1983; Maldonado et al., 1986; Guimaraes et al., 1992) and 1:100 in the ELISA (Uggla et al., 1990). Recently, we considered a titre of 1/64 to be positive in the IFAT, indirect haemagglutination (IHA) and ELISA (Silva et al., 1997), but we did not know whether titres lower than this are specific for T. gondii.