Mousepox detected in a research facility: case report and failure of mouse antibody production testing to identify Ectromelia virus in contaminated mouse serum.

An outbreak of mousepox in a research institution was caused by Ectromelia-contaminated mouse serum that had been used for bone marrow cell culture and the cells subsequently injected into the footpads of mice. The disease initially was diagnosed by identification of gross and microscopic lesions typical for Ectromelia infection, including foci of necrosis in the liver and spleen and eosinophilic intracytoplasmic inclusion bodies in the skin. The source of infection was determined by PCR analysis to be serum obtained from a commercial vendor. To determine whether viral growth in tissue culture was required to induce viral infection, 36 mice (BALB/cJ, C57BL/6J) were experimentally exposed intraperitoneally, intradermally (footpad), or intranasally to contaminated serum or bone marrow cell cultures using the contaminated serum in the culture medium. Mice were euthanized when clinical signs developed or after 12 wk. Necropsy, PCR of spleen, and serum ELISA were performed on all mice. Mice injected with cell cultures and their cage contacts developed mousepox, antibodies to Ectromelia, and lesions, whereas mice injected with serum without cells did not. Mouse antibody production, a tool commonly used to screen biologic materials for viral contamination, failed to detect active Ectromelia contamination in mouse serum.

Specific detection of mousepox virus by polymerase chain reaction.

Polymerase chain reaction was applied to the rapid identification and detection of mousepox virus. This was accomplished by selection of primers targeting the A-type inclusion body protein gene. By investigating 20 strains belonging to five different species of the genus Orthopoxvirus, amplification was achieved only with the seven mousepox virus strains examined. The size of the resulting DNA fragment accounted for 116 base pairs and contained a recognition site for the restriction enzyme HindII, thus confirming its viral origin. Amplification of mousepox virus specific sequences was also possible from infected mouse lung tissue and serum.

Mousepox outbreak in a laboratory mouse colony.

Mousepox was diagnosed in and eradicated from a laboratory mouse colony at the Naval Medical Research Institute. The outbreak began with increased mortality in a single room; subsequently, small numbers of animals in separate cages in other rooms were involved. Signs of disease were often mild, and overall mortality was low; BALB/cByJ mice were more severely affected, and many of them died spontaneously. Conjunctivitis was the most common clinical sign of disease in addition to occasional small, crusty scabs on sparsely haired or hairless areas of skin. Necropsy findings included conjunctivitis, enlarged spleen, and pale liver. Hemorrhage into the pyloric region of the stomach and proximal portion of the small intestine was observed in experimentally infected animals. In immune competent and immune deficient mice, the most common histologic finding was multifocal to coalescing splenic necrosis; necrosis was seen less frequently in liver, lymph nodes, and Peyer's patches. Necrosis was rarely observed in ovary, vagina, uterus, colon, or lung. Splenic necrosis often involved over 50% of the examined tissue, including white and red pulp. Hepatic necrosis was evident as either 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 epizootic by electron microscopy, immunohistochemistry, animal inoculations, serologic testing, virus isolation, and polymerase chain reaction. Serologic testing was of little value in the initial stages of the outbreak, although 6 weeks later, orthopoxvirus-specific antibody was detected in colony mice by indirect fluorescent antibody and enzyme-linked immunosorbent 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.

The inflammatory and immune response to mousepox (infectious ectromelia) virus.

The ectromelia virus (EV) has been recognized as the etiological agent of a relatively common infection in laboratory mouse colonies around the world, i.e., Europe (including Poland), USA and Asia. Due to widespread use of mice in biomedical research, it is important to study the biology of strains characteristic for a given country. This is particularly significant for the diagnosis, prevention and control ectromelia. In severe epizootics, approximately 90% morbidity is observed within colonies and mortality rate exceeding 70% is observed within 4 to 20 days from the appearance of clinical symptoms. The resistance to lethal infection is mouse strain-dependent. Several inbred strains of mice, including C57BL/6 and AKR are resistant to the lethal effects of EV infection, while others, such as A and BALB/c are susceptible. Recent studies indicate that (1) T lymphocytes, NK cells and interferon (IFN)-dependent host defenses must operate for the expression of resistance, (2) virus-specific T-cell precursors appear earlier in regional lymph nodes of resistant than susceptible mice, and (3) resistance mechanisms are expressed during early stages of infection. Over the past several years, (1) induction of anti-EV cytotoxic CD8+ T lymphocytes (CTL) responses in vivo in the absence of CD4+ (T helper) cells, (2) importance of some cytokines e.g., IFN-gamma in EV clearance at all stages of infection, and (3) induction of nitric oxide (NO) synthase, which is necessary for a substantial antiviral activity of IFN-gamma, have been demonstrated.(ABSTRACT TRUNCATED AT 250 WORDS)

Clinical, pathologic, and serologic features of an epizootic of mousepox in Minnesota.

An epizootic of mousepox in two colonies of mice was described. Clinical signs, morbidity and mortality rates, and necropsy lesions were substantially influenced by husbandry practices, intercurrent diseases (Sendai pneumonia, mouse hepatitis virus), and total body x-irradiation. In one colony, 54 mice were necropsied; 19 mice had cutaneous or visceral lesions of mousepox although none had combined visceral and cutaneous lesions. In the other colony, 20 mice were necropsied, and 10 mice had cutaneous lesions; none had visceral lesions. Sera from 24 mice with the cutaneous form of the disease had no demonstrable HI antibody titer. Experimental mice injected with spleen/liver homogenates from infected colony mice developed typical visceral lesions with numerous poxvirus profiles on electron micrographs and positive HI titers. Mice immunized with vaccinia virus were not susceptible to the disease.

Control of mousepox epizootics in St Louis and Chicago.

In September 1980, mousepox was diagnosed in mice from the Jewish Hospital in St Louis, Missouri. The disease was eliminated by vaccination with vaccinia virus. In October 1980, mousepox was suspected in mice located in an off-campus research facility of the University of Chicago. Based on history, clinical signs, serology, and characteristic pathological lesions, mousepox was diagnosed. The disease was eliminated by killing or moving all mice from the facility. During serological surveillance of mice from other University of Chicago campus facilities, mice from an on-campus facility were found to have positive serological titers and pathological lesions suggestive of ectromelia infection. Some mice in this facility originated from a non-commercial source in which mousepox recently was discovered.

Clinical manifestations of mousepox in an experimental animal holding room.

A study of the clinical aspects of mousepox was conducted during the 1979-80 outbreak at the National Institutes of Health. The disease was detected serologically in a room located adjacent to the index room. The index room received animals prior to this outbreak from a noncommercial colony which later was found to be infected with mousepox. The infection was present in the room for at least 6 weeks prior to the completion of the study. The paucity of clinical signs and low mortality were striking when compared to previous descriptions of mousepox in the United States. Only 27 of the 939 mice in the room were infected, and only one of these had typical skin lesions. A few of the mice had non-specific signs such as ruffled hair coat and hunched appearance. Minimal spread of the disease was evidenced by clustering of infected cages on one of five animal racks in the room.