Transovarial transmission efficiency of Babesia bovis tick stages acquired by Rhipicephalus (Boophilus) microplus during acute infection.

The protozoan parasite Babesia bovis, a reemerging threat to U.S. cattle, is acquired by adult female ticks of the subgenus Boophilus and is transovarially transmitted as the kinete stage to developing larval offspring. Sporozoites develop within larvae and are transmitted during larval feeding on a bovine host. This study evaluated the efficiency of B. bovis infection within Rhipicephalus (Boophilus) microplus following acquisition feeding on acutely parasitemic cattle. Parasite levels were quantified in blood from experimentally infected cattle and within hemolymph and larvae derived from acquisition-fed female B. microplus. There was a positive correlation between blood parasite levels in acutely parasitemic cattle and kinete levels in the hemolymph of adult female Boophilus ticks following acquisition feeding; however, there was no relationship between kinete levels in females and infection rates of larval progeny. Boophilus microplus females that acquisition fed produced larval progeny with infection rates of 12% to 48%. Importantly, larvae derived from replete females with very low levels of kinete infection, as demonstrated by microscopy and PCR, had infection rates of 22% to 30% and transmitted B. bovis during transmission feeding. These data demonstrate that although hemolymph infection may be undetectable, transmission to larval progeny occurs at a level which ensures transmission to the bovine host.