Detection of Toxoplasma gondii in lambs via PCR screening and serological follow-up

Toxoplasma gondii in sheep is important as a cause of lambing losses and as a food hazard. We aimed to assess the prevalence of infection in lambs via development of a standardised PCR technique applied to neonates together with follow-up serology at age 4 months. We measured the sensitivity of PCR targeting the T. gondii sequences B1, SAG1, 5'SAG2, 3'SAG2 and SAG3 in the presence of abundant sheep DNA. B1-PCR was the most sensitive protocol, achieving 50% positivity when 0.02 parasite genome copies were present in an assay testing 10 ng of template. Standardised B1-PCR, and serological follow-up using the modified agglutination test (MAT), were used to estimate infection prevalence in lambs from two flocks in Northern England. Neonatal prevalence detected by PCR on umbilical cord did not differ significantly between viable Charollais (16/243 (6.6%)) and viable Swaledale (30/264 (11.4%)). In contrast, at age 4 months seroprevalence was higher (P<0.001, OR=4.42) in Charollais (50/411 (12.2%)) than in Swaledale (10/335 (3.0%)). There was no evidence of a relationship between the results of PCR and those of serology. In addition, prenatal exposure was not associated with mortality: among non-viable lambs, 3/54 Charollais but 0/16 Swaledale were PCR positive, and 1/26 Charollais and 1/14 Swaledale were seropositive. These results indicate that both standardised B1-PCR, and serology, can be used to detect T. gondii in lambs. Frequent prenatal exposure was detected without mortality and sometimes without an IgG response. Some lambs, without PCR evidence of prenatal exposure, seroconverted early.