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Yersinia pseudotuberculosis is a gram-negative bacteria known to cause a variety of disease conditions in many species and is a major cause of mortality in farm and captive wild ungulates (Obwolo 1976, Mackintosh and Henderson 1984). It has not been as well documented in wild populations. Yersinia pseudotuberculosis was first confirmed in wild muskoxen on Banks Island in 1986 (Blake et al 1991). A l l age and sexes suffered Yersinia mortalities but bulls accounted for at least 75 % of the mortalities in 1986 (Blake et al 1989). The objectives of this study in 1987-90 were: 1) Locate carcasses and determine the cause of death of all muskoxen in the study area. 2) Estimate total numbers of muskoxen in the study area and classify a sample of the age and sex structure. Muskox carcasses and skeletal remains were located by aerial and ground surveys in the study area in July or August. Age, sex if discernible, location, and the position (sternal, side) of the carcass were noted. A full necropsy was done using standard field technique on intact carcasses, to assess physical condition at time of death and possible cause of mortality. Any evidence of rauma prior to death was noted. Suspected Yersinia mortalities generally had good fat stores, died in the summer, and if recent, had obvious enteritis, enlarged lymph nodes or septicemia. A loop of intestine, a mesenteric lymph node and intestinal swabs (placed in Phosphate Buffered Saline (PBS)) were taken for subsequent culturing for the presence of bacteria. Overwinter mortalities were determined by the absence of a solid marrow core, position (sternal, fetal, spread out) of the carcass and the state of decomposition in some cases. Any new carcasses since the previous year's survey were examined as .potential Yersinia mortalities which occurred after we left the study area and were added to the yearly totals. Suspected Yersinia mortalities accounted for 88 % of 72 carcasses found in 1987 but only 23 % of 109 carcasses found in 1989 (Table 1). The estimated Yersinia mortality rate was 8-10 % of the 1987 summer population, while in 1989 and 1990 the rate was only approximately 2-4 % of the summer population. The data for 1987¬ 1990 suggests that adult males are still the age/ sex class most affected by Yersinia while starvation affected all age classes but predominantly young and old age classes (Table 2). The expression of this disease in muskoxen may be an indication of stress associated with a high density population and the high contamination levels in some years. Densities of muskoxen in the area varied from 2.7 to 3.2 muskoxen per k m 2 (excluding calves) in the summer. Adult bulls are the most susceptible to infection

Yersiniosis in muskoxen on Banks Island, N.W.T., 1987-1990