MOUSEPOX OUTBREAK IN A LABORATORY MOUSE COLONY

Mousepox was diagnosed in and eradicated from a laboratory mouse colon y at the Naval Medical Research Institute. The outbreak began with inc reased mortality in a single room; subsequently, small numbers of anim als in separate cages in other rooms were involved. Signs of disease w ere often mild, and overall mortality was low; BALB/cByJ mice were mor e severely affected, and many of them died spontaneously. Conjunctivit is was the most common clinical sign of disease in addition to occasio nal small, crusty scabs on sparsely haired or hairless areas of skin. Necropsy findings included conjunctivitis, enlarged spleen, and pale l iver. Hemorrhage into the pyloric region of the stomach and proximal p ortion of the small. intestine was observed in experimentally infected animals. In immune competent and immune deficient mice, the most comm on histologic finding was multifocal to coalescing splenic necrosis; n ecrosis was seen less frequently in liver, lymph nodes, and Peyer's pa tches. Necrosis was rarely observed in ovary, vagina, uterus, colon, o r lung. Splenic necrosis often involved over 50% of the examined tissu e, including white and red pulp. Hepatic necrosis was evident as eithe r large, well-demarcated areas of coagulative necrosis or as multiple, random, interlacing bands of necrosis. Intracytoplasmic eosinophilic inclusion bodies were seen in conjunctival mucosae and haired palpebra . Ectromelia virus was confirmed as the causative agent of the epizoot ic by electron microscopy, immunohistochemistry, animal inoculations, serologic testing, virus isolation, and polymerase chain reaction. Ser ologic testing was of little value in the initial stages of the outbre ak, although 6 weeks later, orthopoxvirus-specific antibody was detect ed in colony mice by indirect fluorescent antibody and enzyme-linked i mmunosorbent assay procedures. The outbreak originated from injection of mice with a contaminated, commercially produced, pooled mouse serum . The most relevant concern may be the unknown location of the source of the virus and the presence of a reservoir for this virus within the United States.