Application of fluorescent in situ hybridization for specific diagnosis ofPneumocystis carinii pneumonia in foals and pigs

Fluorescent in situ hybridization, immunohistochemistry, and Grocott's meth enamine-silver nitrate staining were compared as diagnostic methods for Pne umocystis carinii pneumonia in formalin-fixed lung tissue from foals and pi gs. An oligonucleotide probe targeting 18S ribosomal RNA of P, carinii was designed for in situ hybridization, and a commercially available monoclonal antibody was used for immunohistochemistry. Samples from six foals and 10 pigs with P. carinii pneumonia, as verified by Grocott's methenamine-silver nitrate staining:, were examined concurrently with samples from seven anim als with pneumonia caused by other pathogens. Fluorescent in situ hybridiza tion showed distinctive positive reactions for P. carinii in all test sampl es. The immunohistochemical procedure, however, only revealed P. carinii in the foals. The number of P. carinii organisms observed by fluorescent in s itu hybridization and immunohistochemistry far exceeded the number of organ isms stained by Grocott's methenamine-silver nitrate staining. The results show that fluorescent in situ hybridization targeting ribosomal RNA can pro vide a specific diagnosis of P. carinii pneumonia in foals and pigs.