Probable causes of increasing brucellosis in free-ranging elkof the Greater Yellowstone Ecosystem

Cross, P.; Cole, E.; Dobson, A.; Edwards, W.; Hamlin, K.; Luikark, G.; Middleton, A.; Scurlock, B.; White, P.

While many wildlife species are threatened, some populations have recoveredfrom previous overexploitation, and data linking these population increases with diseasedynamics are limited. We present data suggesting that free-ranging elk (Cervus elaphus) are amaintenance host for Brucella abortus in new areas of the Greater Yellowstone Ecosystem(GYE). Brucellosis seroprevalence in free-ranging elk increased from 0–7% in 1991–1992 to 8–20% in 2006–2007 in four of six herd units around the GYE. These levels of brucellosis arecomparable to some herd units where elk are artificially aggregated on supplemental feedinggrounds. There are several possible mechanisms for this increase that we evaluated usingstatistical and population modeling approaches. Simulations of an age-structured populationmodel suggest that the observed levels of seroprevalence are unlikely to be sustained bydispersal from supplemental feeding areas with relatively high seroprevalence or an older agestructure. Increases in brucellosis seroprevalence and the total elk population size in areas withfeeding grounds have not been statistically detectable. Meanwhile, the rate of seroprevalenceincrease outside the feeding grounds was related to the population size and density of eachherd unit. Therefore, the data suggest that enhanced elk-to-elk transmission in free-rangingpopulations may be occurring due to larger winter elk aggregations. Elk populations insideand outside of the GYE that traditionally did not maintain brucellosis may now be at risk dueto recent population increases. In particular, some neighboring populations of Montana elkwere 5–9 times larger in 2007 than in the 1970s, with some aggregations comparable to theWyoming feeding-ground populations. Addressing the unintended consequences of theseincreasing populations is complicated by limited hunter access to private lands, which placesmany ungulate populations out of administrative control. Agency–landowner hunting accesspartnerships and the protection of large predators are two management strategies that may beused to target high ungulate densities in private refuges and reduce the current and futureburden of disease.

Probable causes of increasing brucellosis in free-ranging elkof the Greater Yellowstone Ecosystem

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