Yellowstone bison (Bison bison) are chronically infected with brucellosis (Brucella abortus), which raises concerns about possible transmission to cattle when they migrate to winter ranges outside Yellowstone National Park. We monitored bison from April to mid-June during 2004-2007 to estimate the timing and location of parturition events that may shed tissues infected by B. abortus. Observed abortions (n = 29) occurred during January through May 19, while peak calving (80% of births) occurred during April 25 to May 26 and calving was finished by June 5. Observed parturition events (n = 115) occurred in the Park and on the Horse Butte peninsula in Montana, where cattle were not present during any time of year. Allowing bison to occupy public lands outside the Park where cattle are never present (e.g., Horse Butte peninsula) until most bison calving is completed (late May or early June) is not expected to significantly increase the risk of brucellosis transmission from bison to cattle because (1) bison parturition is essentially completed weeks before cattle occupy nearby ranges, (2) female bison meticulously consume birthing tissues, (3) ultraviolet light and heat degrade Brucella on tissues, vegetation, and soil, (4) scavengers remove fetuses and remaining birth tissues, and (5) management maintains separation between bison and cattle on nearby ranges. Allowing bison to occupy public lands outside the Park through their calving season will help conserve bison migratory behavior and reduce stress on pregnant females and their newborn calves, while still minimizing the risk of brucellosis transmission to cattle.
Key words: Birth, bison, brucellosis, cattle, disease, transmission, Yellowstone