Antigen for the indirect fluorescent antibody test (IFAT) was routinely prepared from infected erythrocytes from horses experimentally infected with Babesia equi and Babesia caballi. With the successful
establishment of in vitro cultures of B. equi and B. caballi, it is now possible to employ culture-
derived antigens in this test. In this study, in vitro-propagated B. equi- and B. caballi-infected
erythrocytes were used as antigen in the IFAT. Various modifications to an established protocol had
to be implemented to allow repeatable results. Cultures with 3-4% parasitized erythrocytes were
found to be most suitable. As cross-reactions of control sera on heterologous antigen were observed
at serum dilutions of up to 1/40, a reciprocal titre of 80 was considered to be positive. In positive
samples, specific fluorescence of Babesia parasites and/or erythrocyte membranes was observed.
Fifteen sera from Babesia-free horses from Japan all tested negative in the IFAT. One hundred and
ten field-horse sera from Central Mongolia were investigated in this study. The results indicate that
both B. equi and B. caballi are endemic in horses in Central Mongolia, with 88,2% and 84,5% of
horses being seropositive to B. equi and B. caballi, respectively.