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

Incidence:  Incidence of infection is uncommon in laboratory guinea pigs.  Predisposing factors include pregnancy, environmental temperature changes, shipping, subclinical vitamin C deficiency, negligent husbandry practices, and stressful experimental procedures.

Transmission:  Transmission occurs primarily by direct contact and aerosol.  Rats and humans can serve as carriers.

Clinical Signs:  Often, animals may carry virulent pneumococci in the upper respiratory tract without clinical disease.  Respiratory signs may include nasal discharge, rhinitis, conjunctivitis, torticollis (see photo), dyspnea, and acute death.  Guinea pigs may be lethargic, anorexic, and have a ruffled hair coat.  Pregnant dams may abort. During epizootics, mortality rates are very high in naive animals.

Pathology:  Commonly a fibrinopurulent bronchopneumonia (A.), with or without fibrinous pleuritis and pericarditis is seen on necropsy.  Peritonitis and meningitis are occasionally seen.  Thrombosis occurs in acute cases.

Diagnosis:  A tentative diagnosis can be made during the necropsy exam because the fibrinopurulent exudate is characteristic of infection with S. pneumoniae.  Diagnosis is based upon demonstration of paired, Gram-positive cocci (diplococci) on impression smears (B.).  Culture of affected tissues on blood agar and incubation in a 5% CO₂, 37^o C environment allows for identification of alpha-hemolytic cocci which are sensitive to the optochin reagent.  PCR is a sensitive diagnostic tool.