Relative Efficiency of Biological Transmission ofAnaplasma marginale(Rickettsiales: Anaplasmataceae) byDermacentor andersoni(Acari: Ixodidae) Compared with Mechanical Transmission byStomoxys calcitrans(Diptera: Muscidae)

Anaplasma marginale Theiler is a tick-borne intraerythrocytic rickettsial pathogen of cattle that also can be mechanically transmitted by biting flies. Rickettsemia during the acute phase of infection may reach as high as 109 infected erythrocytes (IEs) per milliliter of blood. Animals that survive acute infection develop a life-long persistent infection that cycles between 102.5 and 107 IE/ml of blood. We compared stable fly Stomoxys calcitrans (L.)-borne mechanical transmission during acute infection with Rocky Mountain wood tick, Dermacentor andersoni Stiles-borne biological transmission in the persistent phase of infection to demonstrate quantitatively that biological transmission by ticks is considerably more efficient than mechanical transmission by biting flies. Stable flies that partially fed on an acutely infected calf and were immediately transferred to susceptible calves to complete their bloodmeals failed to transmit A. marginale. Ticks that fed on the original acquisition host after it reached the persistent phase of infection (>300-fold lower rickettsemia) successfully transmitted A. marginale after transfer to the same calves that failed to acquire infection after fly feeding. Failure of fly-borne mechanical transmission at a rickettsemia >300-fold higher than that from which ticks transmit with 100% efficiency demonstrates that tick-borne biological transmission is at least 2 orders of magnitude more efficient than direct stable fly-borne mechanical transmission.