Submitted to: Journal of Medical Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/15/2007
Publication Date: 1/3/2008
Citation: Scoles, G.A., Miller, J.A., Foil, L.D. 2008. Comparison of the Efficiency of Biological Transmission of Anaplasma marginale (Rickettsiales: Anaplasmataceae) by Dermacentor andersoni Stiles (Acari: Ixodidae) with Mechanical Transmission by the Horse Fly, Tabanus fuscicostatus Hine (Diptera: Muscidae). Journal of Medical Entomology. 45(1):109-114.
Interpretive Summary: In some parts of the Unites States horse flies are thought to contribute significantly to the transmission of the bacterium which causes anaplasmosis in cattle. Anaplasma is normally transmitted by ticks, and since the bacterium also replicates in ticks (biological transmission), very low levels of infection are necessary for transmission. On the other hand, horse flies are not biological vectors of anaplasma; since the bacterium does not replicate in flies, transmission occurs purely mechanically, i.e. by movement of blood from an infected to an uninfected host. We compared the efficiency of mechanical transmission of anaplasma by horse flies during the acute infection (a high level of infected red blood cells) with biological transmission by the Rocky Mountain Wood Tick during the persistent phase of infection (a very low level of infected red blood cells). Ticks were much more efficient vectors than biting flies were. When horse flies were partially fed on a calf with a very high level of infection, and then immediately transferred to susceptible calves to complete their blood meal, transmission of anaplasma did not occur. However, ticks that were fed on the donor host after its infection had dropped to a very low level were able to transmit anaplasma. Failure of fly-borne mechanical transmission at a level of infection that was more than 240 times higher than than the level at which ticks transmitted with 100% efficiency demonstrated that tick-borne biological transmission is much more efficient than mechanical transmission by horse flies.
Technical Abstract: Mechanical transmission of Anaplasma marginale by horse flies (Tabanidae) is thought to be epidemiologically significant in some areas of the US. We compared the relative efficiencies of mechanical transmission of Anaplasma marginale by the horse fly, Tabanus fuscicostatus Hine during acute infection (approx.10^7 to 10^9 infected erythrocytes (IE)/ml blood) with biological transmission by Dermacentor andersoni Stiles in the persistent phase of infection (approx. 10^2.5 to 10^6 IE/ml). Biological transmission by ticks was considerably more efficient than mechanical transmission by biting flies. When horse flies were partially fed on an acutely infected donor calf and immediately transferred to susceptible calves to complete their blood meal, transmission of A. marginale was not observed. Ticks that were acquisition fed on the donor host after it reached the persistent phase of infection successfully transmitted A. marginale when transferred to the same recipient calves that failed to acquire infection after fly feeding. Failure of fly-borne mechanical transmission at a rickettsemia more than 240 fold higher than that from which ticks transmitted with 100% efficiency demonstrates that tick-borne biological transmission is at least 2 orders of magnitude more efficient than mechanical transmission by horse flies.