Etiology:  Mouse encephalomyelitis virus is a member of the picornaviridae family.  It is a non-enveloped, single-stranded positive-sense RNA virus.  There are several strains of TMEV that infect mice including GDVII, FA, BeAn and DA. TMEV can be divided into 2 groups, virulent strains such as GDVII, which cause acute encephalitis and do not persist in immune competent mice; and less virulent strains such as DA which experimentally cause a chronic demyelinating disease.

Incidence:  The incidence of infection is low to moderate.

Transmission: The virus is spread horizontally by contact with feces and urine (dirty bedding).  In experimental infections with GDVII, virus was excreted for up to 154 days, even in the presence of circulating antibody.

Clinical Signs:  Most infected mice do not express clinical signs of disease. SCID mice may express clinical signs from naturally occurring disease and rarely develop neurologic signs, lethargy and loss of body weight. Experimental intracerebral inoculation with GDVII can cause acute encephalitis, while inoculation with DA causes persistent infections and demyelinating disease. Mice with the demyelinating disease exhibit paresis and eventually paralysis.

Pathology: Most infected mice have no gross or histologic lesions.  The virus can cause transient and acute encephalitis with gliosis and perivascular and meningeal infiltrates.  In prolonged infections, an immune-mediated demyelinating disease develops. The leptomeningeal and white matter of the spinal cord are infiltrated with lymphocytes and spongiform lesions occur in white matter of the brain stem and spinal cord.

Diagnosis:  Serologic detection of circulating antibody can be used.  PCR can also be used to identify virus in feces.  Virus can be isolated from feces and cultured on BHK-21 cells.