Outbreak of Orthoreovirus-induced meningoencephalomyelitis in baboons.

Departments of 1Physiology and Medicine, 2Laboratory Animal Medicine, and 3Virology and Immunology, Southwest Foundation for Biomedical Research, San Antonio, TX; and 4Department of Microbiology, Tulane Regional Primate Research Center, Covington, LA. Spontaneous viral encephalitis is seldom diagnosed in baboons (1, 2). Yet from January, 1993 to January, 1994, eight juvenile baboons (Papio cynocephalus spp.) housed at the Southwest Foundation for Biomedical Research (SFBR) developed clinical signs of disease that were consistent with acute, progressive encephalitis. The eight animals were juveniles housed in three 20 by 30-ft or one 20 by 50-ft concrete-floored, one-inch galvanized chain-link gang cages that were widely separated from each other by other identical gang cages that did not house clinically affected baboons. These gang cages were in two buildings of essentially identical gang cages, which housed baboons of mixed age and sex, with most of them being juveniles. Each cage housed 25 or fewer baboons at any time (Fig. 1). Virus isolation from brain tissues from the affected baboons yielded a novel syncytium-inducing orthoreovirus (3). Viruses of this genus are usually nonpathogenic, except in rodents and poultry (4). Orthoreoviruses have been studied intensively with respect to CNS infections in mice (5). Sabin observed that after experimentally induced infection by intracranial inoculation, monkeys developed meningitis, but not encephalitis (6). There have been reports of reovirus-associated encephalitis in humans (7), but data are limited. In rare instances, reovirus has been implicated as a primary etiologic agent of respiratory tract disease in nonhuman primates but, to the authors’ knowledge, has not be reported in the baboon (8). There also has been a speculative association with hepatitis and extrabiliary atresia in the macaque (9). Under natural conditions, reoviruses enter the host through the respiratory and gastrointestinal tracts then spread either hematogenously (5) or in retrograde fashion through peripheral nerves to sensory and motor ganglia (10-12). Variants with as little as a single amino-acid substitution can have altered patterns of CNS tropism (11). Reoviruses are characterized by a double-stranded RNA of 10 linear segments with individual molecular mass between 0.5 to 3.0 x 106 Da. The G:C ratio is approximately 43:100; buoyant density in CsCl is 1.36 g/cm2. Virions are isometric, nonenveloped, and 60 to 80 nm in diameter. Reoviruses are resistant to treatment with ether and acid. Mammalian-type reoviruses agglutinate human type-O red blood cells. Reoviruses 1 and 2 are uniform, whereas reovirus 3 has multiple subtypes (13). We report an outbreak of viral encephalitis in group-housed juvenile baboons at the SFBR. Because this report follows the isolation and identification of the viral agent, the emphasis is on clinical signs of disease, pathologic changes, and management leading to identification of the etiologic agent, treatment, and epidemiology of the first orthoreovirus to be isolated from baboons.