An encephalomyocarditis virus epizootic in a baboon colony.

Approximately 80 baboon deaths were caused by encephalomyocarditis virus (EMCV) infection in a 3060 member research and production colony. The epizootic extended over a 9-month period and occurred in baboons ranging from 1 day to 22 years of age. Acute death was the most common history. When clinical disease was detected, it was characterized by labored respiration associated with acute congestive heart failure. The salient necropsy findings were pulmonary congestion and edema, hydropericardium, hydrothorax, ascites, lymph node and splenic hypertrophy, and pale white-to-tan mottled hearts. The most significant histologic lesion was nonsuppurative necrotizing myocarditis. Placental infection with fetal loss occurred. Diagnosis was confirmed by light microscopy, transmission electron microscopy, virus culture, and serology. Rarely, EMCV-induced antibody persisted in surviving baboons for more than 24 months. EMCV-infected feral rats were the probable source of the virus and their control stopped the epizootic. No EMCV neutralizing antibody was detected in colony support personnel or chimpanzees.