Etiology:  Streptococcus pneumoniae is an alpha-hemolytic, Gram-positive diplococcus.

Incidence:  Incidence of infection is uncommon in research rats.

Transmission:  Direct contact is the known route of infection.

Clinical Signs:  Subclinical carriers are common.  In rats which display signs, serous to mucopurulent nasal discharge, rhinitis, sinusitis, and conjunctivitis may be observed.  Younger age groups are particularly affected.

Pathology:  Fibrinopurulent pleuritis (A.) and pericarditis, fibrinous lobar pneumonia, consolidation of lung lobes, frothy, serosanguinous fluid in trachea, and otitis media are common necropsy findings.  The fibrinopurulent nature of the exudate is characteristic of the bacterial infection. Histopathologic examination of the lungs reveals a fibrinopurulent bronchopneumonia.

Diagnosis:  Diagnostic tests include gross pathology, Gram-stain of tissue impression smear revealing Gram-positive lancet-shaped bacteria in pairs (B.), histopathologic examination of lungs revealing fibrinopurulent bronchopneumonia; and culture of affected tissues on blood agar with observation of growth inhibition in the presence of optochin discs.