498 Journal of Zoo and Wildlife Medicine 37(4): 498–512, 2006 Copyright 2006 by American Association of Zoo Veterinarians MYCOBACTERIUM AVIUM IN PYGMY RABBITS (BRACHYLAGUS IDAHOENSIS): 28 CASES Lisa A. Harrenstien, D.V.M., Dipl. A.C.Z.M., Mitchell V. Finnegan, D.V.M., Nina L. Woodford, D.V.M., M.P.H., Dipl. A.C.L.A.M., Kristin G. Mansfield, D.V.M., M.P.V.M., W. Ray Waters, D.V.M., Ph.D., John P. Bannantine, Ph.D., Michael L. Paustian, Ph.D., Michael M. Garner, D.V.M., Dipl. A.C.V.P., Antony C. Bakke, Ph.D., Charles A. Peloquin, Pharm.D., and Terry M. Phillips, Ph.D., D.Sc. Abstract: The Columbia basin subpopulation of pygmy rabbit Brachylagus idahoensis was listed as endangered by the United States Fish and Wildlife Service in November 2001, and no pygmy rabbits have been seen in the wild since spring 2002. Captive propagation efforts have attempted to increase population size in preparation for reintroduction of animals into central Washington. Disseminated mycobacteriosis due to Mycobacterium avium has been the most common cause of death of adult captive pygmy rabbits. Between June 2002 and September 2004, mycobacteriosis was diagnosed in 28 captive adult pygmy rabbits (representing 29% of the captive population), in contrast to 18 adult pygmy rabbits dying of all other causes in the same time period. Antemortem and postmortem medical records were evaluated retrospectively to describe the clinical course of mycobacteriosis in pygmy rabbits, physical examination findings, and diagnostic test results in the diagnosis of mycobacteriosis in pygmy rabbits. Various treatment protocols, possible risk factors for mortality, and recommendations for prevention of mycobacteriosis were evaluated also. Com- promised cell-mediated immunity appears to be the best explanation at this time for the observed high morbidity and mortality from mycobacterial infections in pygmy rabbits. Key words: Brachylagus idahoensis, cell-mediated immunity, mycobacteriosis, Mycobacterium avium complex, pygmy rabbit. INTRODUCTION The pygmy rabbit (Brachylagus idahoensis) is the only species in the genus Brachylagus and has been found in isolated populations in southeastern Washington, southern Idaho, southwestern Mon- tana, western Wyoming, western Utah, northern Nevada, northeastern California, and eastern Oregon. 9,14,26 The population in Washington, con- From the Oregon Zoo, 4001 Southwest Canyon Road, Portland, Oregon 97221, USA (Harrenstien, Finnegan); Office of the Campus Veterinarian, Washington State Uni- versity College of Veterinary Medicine, Pullman, Wash- ington 99164, USA (Woodford); Washington Department of Fish and Wildlife, 2315 North Discovery Place, Spo- kane Valley, Washington 99216, USA (Mansfield); Bac- terial Diseases of Livestock Research Unit, National An- imal Disease Center, Agricultural Research Service, Unit- ed States Department of Agriculture, 2300 Dayton Ave- nue, Ames, Iowa 50010, USA (Waters, Bannantine, Paustian); Northwest ZooPath, 654 West Main Street, Monroe, Washington 98272, USA (Garner); Department of Pathology and Medicine, Oregon Health and Science University, Portland, Oregon 97239, USA (Bakke); De- partment of Medicine, National Jewish Medical and Re- search Center, and University of Colorado Schools of Pharmacy and Medicine, Denver, Colorado 80206, USA (Peloquin); and the National Institutes of Health, 9000 Rockville Pike, Bethesda, Maryland 20892, USA (Phil- lips). Correspondence should be directed to Dr. Harren- stien. fined to the basin of the Columbia River, is esti- mated to have been isolated geographically from other populations of the species for at least 10,000 yr and exhibits less genetic diversity than do other populations. 21,22,42,43 Adult pygmy rabbits weigh ap- proximately 400 g and display several dietary and behavioral traits that distinguish them from cotton- tails (Sylvilagus spp.), jackrabbits (Lepus spp.), and Old World rabbits (Oryctolagus sp.). In the last decade, pygmy rabbit populations have declined in Washington and other states where sagebrush habitat has been burned, has been con- verted to agriculture, or has been cleared from large areas and replaced with bunch grasses that increase livestock forage opportunities. The wild Columbia Basin pygmy rabbit (CBPR) population declined precipitously from about 150 rabbits in 1995 to fewer than 30 in 2001. 16,42 Fearing extinction of the CBPR, the Washington Department of Fish and Wildlife initiated a captive breeding program in co- operation with the Washington State University College of Veterinary Medicine (WSU), Oregon Zoo (OZ), and Northwest Trek Zoological Park (NWT). In December 2000, four wild pygmy rab- bits from a nonendangered population in the Lemhi Valley in Idaho were brought to OZ for the purpose of developing pygmy rabbit husbandry protocols. Sixteen endangered CBPR were captured from 2001–2002 to serve as founders for captive breed-