History and pathology of an enzootic Pneumocystis carinii pneumonia in athymic Han:RNU and Han:NZNU rats.

A report is given on the history and pathology of an enzootic outbreak of spontaneous Pneumocystis carinii pneumonia (PCP) that occurred in a colony of male and female athymic Han:RNU rnu/rnu and Han:NZNU rnuN/rnuN rats maintained in a longevity study from weaning up to their natural death. The rats were obtained from specified pathogen free (SPF) breeding colonies and were kept in a barrier type animal quarter with standardized housing conditions. Groups of heterozygous, euthymic littermates in the same room remained unaffected. First cases of PCP were observed simultaneously in both stocks and both sexes when the rats were 15 to 18 months old. During the following 18 months, most of the animals developed pneumonia. Shortly prior to death the animals showed dyspnea and cachexia. Gross examination revealed focal or diffuse pulmonary consolidation. Histologically, characteristic lung lesions consisted of an interstitial pneumonia with perivascular and peribronchiolar infiltrations and with clusters of foamy macrophages within single or groups of alveoli. Most prominent in moderate and severe infections was an amorphous, honey-combed eosinophilic material within enlarged alveolar spaces. The diagnosis was based on the histological identification of Pneumocystis carinii (Pc) cysts in lung sections stained with Grocott's methenamine silver and was confirmed immunohistochemically. The evaluation of all evidence leads to the conclusion that Pc was introduced into the experimental colony by care-takers. The high incidence of PCP and its resemblance to human PCP indicate the suitability of athymic Han:RNU and Han:NZNU rats as natural models for this disease.