A widespread hair-loss syndrome (HLS) has affected Columbian black-tailed deer (Odocoileus hemionus columbianus) in western Oregon and Washington (USA) since 1996. In order to better characterize the condition, 21 HLS-affected black-tailed deer (BTD) were necropsied, and body condition, parasite burdens, and significant lesions were noted. All deer were in poor body condition, and at least 17 had severe internal parasite burdens. A consistent finding was the presence of large numbers of chewing lice, identified as an indeterminate species of Damalinia (Cervicola). Four animals were infested with intrafollicular Demodex sp., the first report of this genus of mites in BTD. We postulate that the hair loss is largely due to ectoparasitism by a species of chewing louse that represents a new pathogen for BTD. This loss of pelage also may be an important contributor to the poor body condition of these animals.

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