Submitted to: Journal of Medical Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/24/2005
Publication Date: 7/1/2005
Citation: Scoles, G.A., Lysyk, T.J., Broce, A.B., Palmer, G.H. 2005. Relative Efficiency of Biological transmission of Anaplasma marginale (Rickettsiales: Anaplasmataceae) by Dermacentor andersoni Stiles (Acari: Ixodidae) Compared to Mechanical Transmission by Stomoxys calcitrans (L.)(Diptera: Muscidae). Journal of Medical Entomology. 42(4):668-675. Interpretive Summary: The pathogen which causes Anaplasmosis in cattle, Anaplasma marginale, infects the red blood cells. Numbers of infected red cells may reach very high levels during acute infection. Acute infection may result in anemia, weight loss, abortion and in some cases death. If animals survive the acute infection they develope a life long persistent infection. Persistently infected cattle show no obvious signs of disease but they can be a reservoir of infection for vector ticks. A. marginale is normally transmitted biologically by ticks. In Biological transmission the vector ticks become infected with the pathogen, resulting in amplification of the pathogen within the tick; vector ticks are capable of transmitting A. marginale even from persistently infected animals with very low levels of infection. Biting flies have been shown to be able to mechanically transmit A. marginale during acute infection, and fly-borne mechanical transmission has long been assumed to play an important role in the natural cycle of maintenance and transmission of the pathogen. In this study we compared the efficiency of mechanical transmission of A. marginale by stable flies during the acute phase of infection with biological transmission by the rocky mountain wood tick during the persistent phase of infection. Flies fed on a calf with a very high level of acute infection failed to transmit the pathogen to 4 calves, whereas, ticks were able to transmitted to all 4 cattle with 100% efficiency from a persistently infected host with at least 300 fold lower level of infection. Although this work does not rule out the possibility of fly-borne mechanical transmission of Anaplasma, we have clearly demonstrated that mechanical transmission by direct transfer of flies from an infected to a susceptible host is at least two orders of magnitude less efficient than tick-borne biological transmission. Because fly-borne mechanical transmission of A. marginale is generally believed to be epidemiologically important in some areas, additional studies will be necessary to clarify the conditions under which mechanical transmission occurs, and the role that it plays in the epidemiology of A. marginale.
Technical Abstract: Anaplasma marginale Theiler is a tick-borne intra-erythrocytic rickettsial pathogen of cattle that can also be mechanically transmitted by biting flies. Rickettsemia during the acute phase of infection may reach as high as 10^9 infected erythrocytes (IE) per ml of blood. Animals that survive acute infection develop a life long persistent infection that cycles between 10^2.5 and 10^7 IE/ml of blood. We compared stable fly-borne mechanical transmission during acute infection with Dermacentor andersoni-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 blood-meals 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 more than 300 fold higher than that from which ticks transmit with 100% efficiency demonstrates that tick-borne biological transmission is at least two orders of magnitude more efficient than direct stable fly-borne mechanical transmission.