The role of rodents as a potential reservoir for Pasteurella multocida on the National Elk Refuge, Wyoming

Abstract

Hemorrhagic septicemia (HS) is a fatal disease affecting domestic and wild ruminants caused by the bacterium Pasteurella multocida. Although uncommon in the U.S, outbreaks of HS in elk (Cervus elaphus) occurred on the National Elk Refuge (NER) in the winters of 1986, 1987, 1993, 1995, 1999, and 2001. DNA fingerprinting of P. multocida from the 1987 and 1993 outbreaks (B:3,4 HhaI 036/HpaII 001) revealed the same organism was responsible for mortality in both years. However, testing has failed to find this genetic variant in healthy elk on the NER, suggesting reservoirs other than elk might play a role in HS epidemiology. I investigated the potential for rodents to serve as biological reservoirs for bacteria responsible for HS on the NER. Rodents are known to harbor P. multocida, may be carriers of variants capable of causing HS, and have been observed at sites where elk are fed during winter on the NER. I used mark-recapture techniques to determine densities of rodents on feedgrounds, feed-storage areas and other sites and removal trapping to collect tissues to determine prevalence of P. multocida in rodents on the NER.

Weather conditions, age-gender class, and feedground characteristics also were assessed as risk factors for HS. I captured 849 small mammals, 283 of which (mostly Peromyscus maniculatus) were sampled for P. multocida. None were positive for P. multocida. These data did not support the hypothesis that rodents serve as a reservoir for HS; however, my detection sensitivity was low due to small sample sizes. Snow depth was the only weather variable significantly associated with the incidence of HS on the NER. The positive association between snow and number of elk dying from HS may be due to increased survival of bacteria in the environment under wet conditions. Calves and cows were found to be at a higher risk than males greater than one year of age, but, with winter feeding, there is no evidence that cows and calves were more stressed from nutrient shortages or crowding than males. All analyses of feedground characteristics failed to find a relationship between these characteristics and HS.