Evaluation of the indirect fluorescent antibody test for diagnosis of Babesia gibsoni infections in dogs

The extent of the serological cross-reactivity was determined in the indirect fluorescent antibody (IFA) test for B. gibsoni, and the optimal cut-off titre for seropositivity. The lowest titre to B. gibsoni detected in a dog with naturally acquired clinical babesiosis was 1280 and 7 of 12 dogs had titres between 10 240 and 20 480. Two experimentally infected normosplenic dogs developed high titres (40 960 to 81 920) to B. gibsoni, and the same sera reacted in IFA tests for B. canis (titre ≤ 640), Toxoplasma gondii (titre ≤ 2560), and Neospora caninum (titre ≤ 10 240). Dogs that were experimentally infected with B. canis and T. gondii had titre ≤ 160 to B. gibsoni. Dogs that were experimentally infected with N. caninum had titre (80 to 10 240) to N. caninum, but failed to have serological reactivity to B. gibsoni. Serological titre to healthy dogs from Australia, a country where B. gibsoni is not known to exist, was ≤ 160 to B. gibsoni. On the basis of these findings, a cut-off titre of 320 was considered to be appropriate for serodiagnosis of B. gibsoni in dogs with clinical signs of babesiosis. A more conservative titre of 1280 was established as the cut-off titre for seroepidemiology studies based on relative costs and benefits of false-positive results and failure to isolate B. gibsoni parasites after splenectomy and immuno-suppression from a clinically normal dog with B. gibsoni titre of 640. It is concluded that the results indicate the importance of establishing optimum cut-off titre for the B. gibsoni IFA test that takes into consideration the purpose of the test, seroreactivity to antigenically related parasites, and factors that contribute to interlaboratory test variation.