Etiology:  Ectromelia virus, the agent of mousepox, is an enveloped DNA poxvirus of the vaccinia subgroup.

Incidence:  Incidence of disease is rare, with sporadic epizootics usually resulting from passage of infected cells or other biological material into naive mice.

Transmission:  Natural infections occur via the fecal-oral route, urine contamination or by direct contact.  Skin abrasions are thought to provide the main route of entry.  Inoculation of mice with poxvirus-infected tumor cells or serum products has also caused disease outbreaks.

Clinical Signs:  Clinical signs are dependent on the mouse strain. Strains A, C3H and BALB/c mice are considered susceptible strains, and C57BL/6 is considered a resistant strain.  In acute disease, there is high morbidity and high mortality with affected animals exhibiting hunched posture, conjunctivitis and facial swelling.  Subacute to chronically infected animals develop a cutaneous vesicular body rash which often progresses to swelling, necrosis and sloughing of the extremities.  Deaths are sporadic.

Pathology: Histologic lesions include massive splenic, lymph node, thymic and hepatic necrosis, small intestinal mucosal erosions, and cytoplasmic inclusions in the skin and liver. Although clinical and gross lesions are suggestive of the disease, histological demonstration of intracytoplasmic inclusion bodies in epithelial cells surrounding vesicular skin ulcers in small intestinal and pancreatic cells is helpful to confirm the diagnosis.

Diagnosis:  Serologic testing using MFI and IFA help screen mouse colonies for the presence of infection.  PCR can be performed on skin lesions and spleen.