Deer Management: Deer Diseases
Deer Diseases in Michigan
White-tailed deer can carry and are susceptible to a number of diseases. Several of these diseases cause concern for human health as well as for other animal species both domestic and free ranging. Diseases with potential to impact deer populations and deer management in Michigan include Epizootic Hemorrhagic Disease (EHD), Eastern Equine Encephalitis (EEE), Lyme disease, Bovine Tuberculosis (bTB), and Chronic Wasting Disease (CWD).
Epizootic Hemorrhagic Disease (EHD) is an infectious, often fatal viral disease that is transmitted by biting flies. Characterized by extensive hemorrhages, EHD has been responsible for significant die-offs of deer in the northern United States and southern Canada. Outbreaks of EHD in white-tailed deer in Michigan occurred in 1955, 1974, 2006, and 2008 (T. Cooley, Michigan DNR, pers. comm.). Total mortality in these events ranged between 50 and 200 deer. Because of its very high mortality rate, EHD can have a significant effect on the deer population in a given area, reducing deer numbers drastically. There is no know treatment for the disease and there is no evidence that the virus can infect humans. For more information on the 2012 EHD Outbreak please click here.
Eastern Equine Encephalitis (EEE) is a fatal viral disease of horses that can infect a variety of avian and mammalian species but seldom causes clinical disease. In rare cases, white-tailed deer can be infected and the disease is fatal in affected animals. There have been single reports of mortality in deer in Georgia (Tate et al. 2005) and Wisconsin (J. Langenberg, Wisconsin DNR, pers. comm.) and multiple cases in Michigan (Schmitt et al. 2007). The die-off in Michigan occurred in 2005 in the southwestern portion of the state. Seven deer mortalities were documented in this outbreak. Eastern Equine Encephalitis can have a significant effect on the deer population in a given area, but because it rarely occurs, it is not an important mortality factor to the state as a whole. Although it occurs rarely, humans are susceptible to this disease and it can be fatal.
Lyme disease is an illness caused by a spirochete bacterium (Borrelia burgdorferi). This disease is transmitted to humans and animals primarily by the bite of an infected tick, Ixodes scapularis. The white-tailed deer is a host for the adult stage of this tick and therefore can be involved in exposing humans to the tick, and consequently, to the bacterium. White-tailed deer do not develop the disease when infected with Borrelia burgdorferi, and therefore this disease is not an important mortality factor for deer (Brown and Burgess 2001). This disease has human health significance as the bacterium can affect the cardiovascular system and the neurological system and cause severe arthritis.
Bovine tuberculosis (bTB), caused by Mycobacterium bovis, was first diagnosed in free- ranging Michigan white-tailed deer in November 1975 (Schmitt et al. 1997). Since that time, the extent and characteristics of the outbreak, as well as its ongoing management by the DNR, have been extensively described (de Lisle et al. 2002, Hickling 2002, O'Brien et al. 2002, O'Brien et al. 2006, Schmitt et al. 2002). Bovine tuberculosis is primarily of concern because of its ability to infect a wide variety of species (Oreilly 1995), including humans (Wilkins et al. 2003, Wilkins et al. 2008), and the resulting economic costs of infection for the livestock industry due to herd condemnations and closure of markets (Morris et al. 1994). After more than 13 years of surveillance and research, white-tailed deer remain the only proven reservoir of infection for cattle besides other cattle (Corner 2006).
Surveillance and control activities for bTB in free-ranging Michigan white-tailed deer have now been underway for over a decade. Significant progress has been made, lowering apparent prevalence of bTB in deer in the core area by greater than 60 percent, primarily by reducing deer densities through hunting, and placing restrictions on public feeding and baiting of deer. The DNR's broad disease control strategies, implemented with the cooperation of Michigan deer hunters, reduced the deer population by 50 percent in the portion of the northeast LP known as the 'TB area'. However, as hunters see fewer deer, their willingness to sustain aggressive harvests has waned (Van Deelen et al. 2003) and public resentment of control measures has grown (O'Brien et al. 2006).
During the past four years, apparent prevalence in core area deer has held approximately steady just below two percent. After bottoming out in 2004, deer numbers have since rebounded by about 30 percent. Public compliance with baiting and feeding restrictions has noticeably worsened. Some hunters in the core area do not perceive bTB as a major problem, in spite of 13 years of DNR outreach.
Some current initiatives justify cautious optimism. Promising research to support eventual vaccination of wild deer continues (Nol et al. 2008, O'Brien et al. 2009, Palmer et al. 2007). Some hunters and landowners have begun to recognize the costs of high deer densities and supplemental feeding. A peninsula-wide ban on baiting and feeding was enacted in 2008 due to the discovery of chronic wasting disease. Some cattle producers have begun work to change long-held opinions among their peers about farm biosecurity. Yet formidable challenges remain, and evidence suggests that eradication of bTB, if it can be achieved, will take decades, and will require greater public and political resolve than has been demonstrated thus far.
Chronic Wasting Disease (CWD) is a Transmissible Spongiform Encephalopathy (TSE), caused by mutant cellular protein, that affects four species of North American cervids (Sigurdson 2008, Williams 2005, Williams et al. 2002), including white-tailed deer. Both simulation modeling (Gross and Miller 2001, Miller et al. 2000) and field research (Miller et al. 2008) suggest that once established, CWD can build to high prevalence in infected deer populations, resulting in marked decreases in survival of infected deer and likely causing substantial population declines over decades. Where the disease has become established, no management agency has been able to control its spread, let alone eradicate it. Consequently, only efforts to prevent the disease from becoming established in free- ranging or captive deer are likely to be successful.
Following confirmed diagnosis of Michigan's first case of CWD in a captive white-tailed deer in a Kent County facility in August 2008, the DNR's intensified surveillance was implemented per the CWD Surveillance and Response Plan (MDNR/MDA 2002). In 2008, 9,151 free-ranging deer were tested for CWD statewide, including 1,523 from a nine township area surrounding the infected captive facility. All were negative. Since 1998, over 31,000 wild white-tailed deer have been tested statewide, and all have been negative. In accordance with the CWD Surveillance and Response Plan, heightened surveillance testing in southwest Michigan will continue until 2011.
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