Inapparent infections and carrier states are common, and very few infected animals will ever show disease. Despite its periodic detection in large breeding colonies, no outbreaks of S. pneumoniae disease have been reported in rats in 35 years, nor in guinea pigs in 20 years, raising the possibility that previous “outbreaks” were the result of mixed infections of
S. pneumoniae together with additional infectious agents. When it occurs, disease is usually seen in young animals, especially after perturbation of host defense mechanisms, such as concurrent infection, experimental manipulation, or a change in environment. Clinical signs can include general ill rodent signs, such as hunched posture, ruffled fur, inappetance, or death with no premonitory signs, or specific signs such as nasal discharge, conjunctivitis, and vestibular signs. In guinea pigs, stillbirths and abortions are part of the