The Role of White-tailed Deer (Odocoileus virginianus) in the Epidemiology of Bovine Viral Diarrhea Virus
Type of Degreedissertation
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Bovine viral diarrhea virus (BVDV) is the prototypic member of the genus Pestivirus in the family Flaviviridae. Infections with BVDV cause substantial economic losses to the cattle industries, prompting the introduction of organized control programs in several countries. In North America, these control programs are mainly focused on the identification and removal of persistently infected (PI) cattle, the main reservoir for BVDV. Further measures include the enhancement of BVDV-specific immunity through vaccination and the implementation of biosecure farming practices. For the successful execution of these control measures, knowledge of the epidemiology of BVDV must be complete, including the recognition of other potential sources of the virus. Bovine viral diarrhea virus does not possess strict host-specificity and infections have been demonstrated in over 50 species in the mammalian order Artiodactyla. In North America, the white-tailed deer (Odocoileus virginianus) is the most abundant free-ranging ruminant, and evidence of BVDV infections was described in serological surveys and a single experimental inoculation of fawns. However, susceptibility of white-tailed deer to BVDV infection does not alone imply or prove a role in the epidemiology of the virus or prove the existence of a wildlife reservoir. Additional criteria including shedding and maintenance of BVDV and sufficient contact to cause spill-back infections to cattle must be met to cause concern for developing BVDV control measures. The overall goal of this research was to examine whether white-tailed deer could be a wildlife reservoir for BVDV, and studies were developed on the basis of the necessary criteria for wildlife reservoirs. For each experiment, free-ranging white-tailed deer were captured by cannon-net or dart-gun and translocated to a captive research facility. Upon capture, all animals were determined to be free from BVDV and antibodies against BVDV. Persistently infected cattle are the most efficient source of BVDV and constitute the major source of transmission of the virus within and among cattle herds. If the phenomenon of persistent infection could occur in white-tailed deer, these PI animals could shed and maintain BVDV, and pose a potential threat to BVDV control and eradication programs. To assess the outcome of fetal infection in white-tailed deer, in our first experiment, nine pregnant does were inoculated intranasally with 106 CCID50 each of a BVDV 1 (BJ) and BVDV 2 (PA131) strain. In this study, only one doe gave birth and delivered a live fawn and a mummified fetus. The live fawn was determined to be PI with BVDV 2 by serial virus isolation from serum, buffy coat, and nasal swabs; immunohistochemistry on skin samples; and RT-PCR on serum. Analogous to PI cattle, the fawn continuously shed BVDV, as indicated by BVDV positive nasal swab samples. Central to the development of a wildlife reservoir is interspecific contact and transmission of BVDV, which were the focus of our second experiment. At approximately 50 days of pregnancy, seven female and one male white-tailed deer were cohabitated with two PI cattle. In a pen of approximately two acres, both species shared feed and water sources for a period of 60 days. Cohabitation of species resulted in the transmission of BVDV from PI cattle to deer, as indicated by seroconversion in all adult deer. Transplacental transmission of BVDV was demonstrated in four of seven pregnancies, as two stillborn twin fetuses and three live singletons were infected with BVDV at birth. All singlet fawns were demonstrated to be PI by immunohistochemistry and ELISA on skin samples, and RT-PCR and virus isolation procedures on blood and tissues samples. These findings demonstrate that BVDV may efficiently cross the species barrier to provide a potential alternative niche to ensure viral survival and propagation. Shedding of BVDV as demonstrated in the PI fawns may result in maintenance of the virus within populations of white-tailed deer, and this would be central to perpetuation of a wildlife reservoir. To evaluate the potential for intraspecific transmission of BVDV and the potential for BVDV maintenance in white-tailed deer, six pregnant does were cohabitated with a PI fawn during the first trimester of pregnancy. All does gave birth to live fawns and no reproductive losses were observed. At birth, evidence of BVDV infection was identified in two singlet fawns, of which one was determined to be PI by repeated serum RT-nPCR, whole blood virus isolation and immunohistochemistry. The birth of a PI fawn in this study strengthens the possibility of continuous intrapopulational propagation of BVDV and the potential for indefinite maintenance of the virus in deer. While the experiment was designed to emulate a wild population, the size of the captive deer facility limits possible extrapolations to free-ranging populations in which various factors may influence the transmission and maintenance of BVDV. We therefore studied free-ranging white-tailed deer in Alabama for evidence of BVDV infection by assessing serum and skin samples collected from hunter-harvested deer. Upon site visits to deer processing units, 165 serum samples and 406 skin biopsies were collected and analyzed by virus neutralization and immunohistochemistry, respectively. Two serum samples contained virus neutralizing antibodies, and one skin sample was positive on immunohistochemistry. The viral antigen distribution in the skin sample was similar to that of PI cattle, indicating the existence of free-ranging PI white-tailed deer in Alabama. A central question to control of BVDV in US cattle populations is whether the virus can be transmitted from PI white-tailed deer to susceptible cattle. Our research indicated that survival of PI white-tailed deer appears to be reduced, prompting the research question whether clinically ill or dead PI deer may be a source of BVDV and hamper current control programs. We therefore evaluated the transmission of BVDV to cattle exposed to carcasses of PI white-tailed deer. In two trials, steers were exposed to the carcass of PI fawn A (BVDV 2) or PI fawn B (BVDV 1). One steer from each of two groups was separated into a pen with the carcass. We chose to expose only one steer from each group to the carcass to simulate the effects of bovine contact networks and social hierarchies on the transmission of BVDV. Following 8 hours, the single steer from each trial was commingled with four other steers for 28 days. Subsequently, one steer was inoculated intranasally with spleen homogenate from fawn A, and two steers were inoculated intranasally or intravenously with spleen homogenate from fawn B as infection controls. Steers in both trials made contact with the carcass, but BVDV transmission did not occur. The intranasally inoculated control for trial A and the intravenously inoculated control for trial B became viremic and seroconverted. Although both PI fawn carcasses were potentially infectious, this study indicated exposure of a single inquisitive bovid to a PI fawn carcass did not result in transmission of BVDV. The results of our research indicate that BVDV infections in white-tailed deer share many features with infections in cattle, indicating a potential for the development of a wildlife reservoir in this species. This development, however, would be influenced by various endogenous and exogenous factors and further research is needed to aid in the successful control of BVDV.