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Title: PERSISTENCE OF BLUETONGUE VIRUS IN THE INSECT VECTOR AND ITS IMPLICATIONS FOR DISEASE CONTROL

Author
item Mecham, James
item White, David
item Drolet, Barbara
item Wilson, William

Submitted to: United States Animal Health Association Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 10/23/2004
Publication Date: 1/1/2005
Citation: Mecham, J.O., White, D.M., Drolet, B.S., Wilson, W.C. 2005. Persistence of bluetongue virus in the insect vector and its implications for disease control. United States Animal Health Association Proceedings. 108:100-107.

Interpretive Summary: Bluetongue virus (BTV) infects sheep, cattle and other ruminants and is transmitted by biting midges, Culicoides spp. The virus infection cycle is maintained by the insect vector taking a blood meal from an infected ruminant host and then transmitting the virus to an uninfected animal during subsequent feeding. Virus transmission is interrupted in temperate climates during winter months when the insect vector is no longer active. Seasonal bluetongue disease outbreaks coincide with the re-emergence of the insect vector. Possible mechanisms for over-wintering of BTV and the seasonal re-emergence cycles include 1) the spread from areas of year-round endemic activity to areas of epizootic seasonal activity via the movement of vectors and/or animals 2) prolonged persistence of virus in cattle or wild ruminants; and 3) virus persistence in the insect vector. In the latter, over-wintering of BTV in the insect vector could occur by infection and survival in the adult insect, vertical transmission of virus from the infected adult to its progeny, or a combination of these two events. There is evidence that adult Culicoides can survive relatively mild winters. If infected with BTV, these insects could infect susceptible hosts when they resume blood-feeding activity. In more temperate climates, experiencing harsher winters, adult insects probably do not survive, but recent evidence suggests BTV may persist via vertical transmission in Culicoides. We present an overview of the available evidence for persistence of BTV in the insect vector by these two mechanisms. If vertical transmission in the insect is a mechanism for maintaining BTV in nature, then modification of insect control strategies may be necessary to effectively control bluetongue disease.

Technical Abstract: Bluetongue virus (BTV) infects sheep, cattle and other ruminants and is transmitted by Culicoides spp. biting midges. The virus infection cycle is maintained by the insect vector taking a blood meal from an infected ruminant host and then transmitting the virus to an uninfected host during subsequent feeding. Virus transmission is interrupted in temperate climates during winter months when the insect vector is no longer active. Seasonal bluetongue disease outbreaks coincide with the re-emergence of the insect vector. Possible mechanisms for over wintering of BTV and the seasonal re-emergence cycles include 1) the spread from areas of year-round endemic activity to areas of epizootic seasonal activity; 2) prolonged persistence of virus in cattle or wild ruminants; and 3) persistence of virus in the insect vector. We present an overview of evidence for the latter mechanism. BTV can persist in the insect vector and this persistence may play an important role in over-wintering of the virus as well as explain the seasonal re-emergence of disease in susceptible ruminant hosts. Understanding the maintenance of BTV in nature, in the absence of apparent disease outbreaks, will help us develop more effective risk management and control strategies for bluetongue disease.