Role of insects in the transmission of bovine leukosis virus: potential for transmission by stable flies, horn flies, and tabanids.

The ability of stable flies (Stomoxys calcitrans), horn flies (Haematobia irritans), and tabanids (Diptera: Tabanidae) to transmit bovine leukosis virus (BLV) was investigated. Stable flies and horn flies were fed on blood collected from an infected cow, and the flies' mouthparts were immediately removed, placed in RPMI-1640 medium, ground, and inoculated into sheep and calves. Infection of sheep occurred with mouthparts from as few as 25 stable flies or 25 horn flies. However, sheep were not infected when removal of stable fly mouthparts was delayed greater than or equal to 1 hour after blood feeding. Infection of calves occurred after inoculation of mouthparts removed immediately after feeding from as few as 50 stable flies or 100 horn flies. Infected blood, applied by capillary action to the mouthparts (labella) of 15 deer flies (Chrysops sp) and a single horse fly (Tabanus atratus) caused infection in each of 2 sheep. Infection did not occur in 2 calves inoculated daily for 5 days with mouthparts from 50 horn flies collected after feeding on a BLV-infected steer. Four calves receiving bites from 75 stable flies interrupted from feeding on a BLV-positive cow also were not infected. Seronegative cattle held for 1 to 4 months in a screened enclosure with positive cattle in the presence of biting flies were not infected with BLV. The feeding behavior of each insect is discussed to assess their potential as vectors of BLV.