Submitted to: Veterinaria Italiana
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/12/2004
Publication Date: 2/1/2005
Citation: White, D.M., Mecham, J.O. 2005. Lack of detectable bluetongue virus in skin of seropositive cattle: implications for vertebrate overwintering of bluetongue virus. Veterinaria Italiana.40:513-519.
Interpretive Summary: Bluetongue virus (BTV) is an arthropod-borne virus that is transmitted to sheep, cattle, and wild ruminants by Culicoides spp. biting insects. BTV may be maintained in year-round transmission cycles in tropical parts of the world, and possibly even in areas with mild temperate winters. However, in more temperate regions, virus transmission ceases during the winter months and begins again with re-emergence of the insect vector. Three principal hypotheses have been proposed for BTV trans-seasonal maintenance: 1) yearly high altitude, air-current based, reintroduction of infected insects from year-round endemic regions to temperate regions, 2) vertical transmission and maintenance from the infected insect to its progeny, and 3) survival in a vertebrate host, either through persistent infection of adults or transplacental transmission to fetuses. The purpose of this study was to examine whether cattle infected with BTV can serve as a reservoir for maintaining the virus through the winter. Naïve colonized Culicoides sonorensis insects were fed on cattle that had been naturally infected with BTV the previous summer and fall, as determined by the presence of serum antibody. Both the blood fed insects and skin biopsies from the cattle at the sites where the insects had fed were tested for the presence of BTV by both virus isolation and viral nucleic acid detection. The lack of virus isolation and/or nucleic acid detection suggests that cattle probably do not serve as a significant reservoir for overwintering of BTV in temperate climates.
Technical Abstract: The overwintering mechanism of bluetongue virus (BTV) has eluded researchers for many years. It was recently proposed that ovine gamma-delta T-cells may become persistently infected with BTV, and serve as a reservoir for infection of naïve vectors in the next transmission season (Takamatsu et al., Journal of General Virology 2003, v. 84 pp. 227-35). Since cattle are more numerous than sheep in the western United States (where BTV is endemic), this hypothesis was tested in bovines. In the winter of 2002, 56 cattle from an endemic site in northern Colorado were age-selected such that possible BTV exposure must have occurred in the summer of 2002, and were tested for the presence of anti-BTV antibody by ELISA; 55 were seropositive, and one was seronegative. Naïve colony insects were fed on skin sites at day 0, then sequentially on separate sites for 4 days. Virus isolation and RT-nested-PCR from engorged insects and 6mm skin biopsy samples were performed for detection of viable BTV or BTV nucleic acid. Results will be presented and discussed.