Wednesday, 17 November 2004 - 2:36 PM
1081

Biological transmission of Anaplasma marginale by the Rocky Mountain wood tick, Dermacentor andersoni (Acari: Ixodidae) is at least 300 times more efficient than mechanical transmission by the stable fly, Stomoxys calcitrans (Diptera: Muscidae)

Glen A Scoles, scoles@vetmed.wsu.edu, USDA, ARS, Animal Disease Research Unit, 3003 ADBF, Washington State University, PO Box 646630, Pullman, WA, Alberto Broce, abroce@oznet.ksu.edu, Kansas State University, Department of Entomology, 123 West Waters Hall, Manhattan, KS, Timothy J. Lysyk, lysyk@agr.gc.ca, Agriculture and Agri-Food Canada, 5403 First Avenue South, Lethbridge, AB, Canada, and Guy Palmer, gpalmer@vetmed.wsu.edu, Washington State University, Department of Veterinary Microbiology and Pathology, Bustad Hall 402, Pullman, WA.

Anaplasma marginale is a tick-borne intra-erythrocytic rickettsial pathogen of cattle which is also mechanically transmitted by biting flies. During acute infection rickettsemia may reach as high as 109 infected erythrocytes/ml of blood (i.e./ml). Surviving animals develop a life long persistent rickettsemia which cycles between 102 and 107 i.e./ml, a level below the threshold for detection on stained blood smears. Although the relative importance of tick-borne vs. fly-borne transmission has not been studied quantitatively, it has been suggested that biological transmission is important during persistent infection, whereas mechanical transmission is more important during acute infection. We have used quantitative PCR to demonstrate that biological transmission by ticks is at least 2 orders of magnitude more efficient than mechanical transmission by stable flies. Stable flies were partially fed on an acutely infected calf (5.8 x 108 i.e./ml) and immediately transferred to susceptible calves. Transmission was attempted with three flies and with 30 flies for each of two calves; none of the 4 stable fly transmission calves acquired infection. After the original acquisition host became persistently infected (1.84 x 106 i.e./ml) the experiment was repeated using ticks as biological vectors. The same 4 calves that failed to acquire infection after stable fly feeding, did become infected after transmission feedings with three or 30 ticks that had been acquisition-fed on the persistently infected host. The rickettsemia at the time of stable fly acquisition feeding was more than 300 times higher than the rickettsemia at the time of tick feeding, demonstrating that biological transmission is at least 300 times more efficient than mechanical transmission.


Species 1: Diptera Muscidae Stomoxys calcitrans (stable fly, dog-fly)
Species 2: Acari Ixodidae Dermacentor andersoni (Rocky Mountain wood tick)
Keywords: mechanical transmission, vector capacity

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