Impacts of Disease on Wolves in Yellowstone National Park

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In 1995 and 1996, wolves were reintroduced into the Northern Rockies where they have since established and spread. Within Yellowstone National Park, one of the core protected release sites, the unmanaged population steadily increased to high densities, producing a large wolf population susceptible to infections such as canine parvovirus (CPV), canine distemper virus (CDV) and sarcoptic mange.

Much of the wolf research to date has focused on predator-prey interactions, population dynamics and social behavior. The USGS NOROCK effort, in collaboration with the Yellowstone Wolf Project, has been focused on assessing the impacts of disease on the wolf population.

Infectious disease in wildlife populations is often difficult to study for numerous reasons, such as inability to measure an infection through time, too short of a study duration to quantify dynamics, and unknown contact patterns that correlate with transmission events. The Yellowstone National Park (YNP) wolf population is an ideal subject for investigating the effects of social structure on disease. A rich, long-term data set (1995-present) and intensive disease monitoring provide an unparalleled research opportunity. In addition, disease may be a regulating factor in the YNP wolf population. 5 of the 6 years with population growth rates <1 are years with known infections in the population. In total, four major disease outbreaks, caused by two different pathogens, have caused large population declines  Although YNP wolves host a suite of pathogens, we have focused on canine distemper virus (CDV) and mange (Sarcoptes scabiei) because they are known to cause morbidity and mortality.

CDV (Canine morbillivirus) is an RNA virus related to measles. This generalist virus causes a wide range of symptoms, typically starting as an upper respiratory tract infection, and then moving to the gastrointestinal tract, urogenital tract, and eventually the central nervous system. CDV causes significant host mortality, specifically in pups, and induces life-long immunity. YNP wolves have experiences three major CDV outbreaks in the years 1999, 2005, and 2008 and resulting pup survival averaged ~28%. CDV also infects animals in the families Canidae, Felidae, Mustelidae, and Procyonidae. Thus outbreaks in wolves are correlated with outbreaks in other species.

In addition to acute CDV infections, YNP wolves also suffer from chronic Sarcoptic mange infections. Sarcoptes scabiei is a mite that burrows under the epidermis where it lays eggs and ingests host tissue. This causes an allergic, inflammatory reaction to which the wolves scratch and bite themselves, leading to hair loss, alopecia, and consequently heat loss and increased susceptibility to secondary infections. Sarcoptic mange entered the YNP wolf population in January 2007 and is now endemic. Nearly every wolf pack since the initial introduction has experienced a mange infection.