Natural infection of rats with SV is generally asymptomatic, with only minor effects on reproduction and growth of pups. Natural infections of mice present as enzootic or epizootic infections. Enzootic infections are those endemic to a colony, where the constant supply of susceptible animals maintains the infection. Mice are infected shortly after weaning as maternal antibody levels wane, and they show few clinical signs. Since there is no carrier state, cessation of breeding eventually results in elimination of the infection, although antibody titers remain in previously infected animals.
Epizootic infections occur upon first introduction of the virus to a colony.
Clinical signs may include teeth chattering, dyspnea, prolonged gestation, poor growth, and death of young mice. Where breeding is occurring, the enzootic pattern eventually takes over. SV contains HN protein, with hemagglutinating and neuraminidase activities, and F glycoprotein, with cell fusion, cell entry, and hemolytic activities. Conversion of the F glycoprotein to the active form is dependent on host proteases and is inhibited by pulmonary surfactant. However, there are considerable differences in susceptibility to SV among both rat and mouse strains.