Abstract
Pneumocystis carinii pneumonia is an oftentimes fatal infection for hosts in an immunocompromised state. The disease occurs in a wide variety of mammals, but the etiologic agent of this disease has been referred to as P. carinii regardless of the host species. However, even within a single host species, such as laboratory rats, distinct varieties of P. carinii have been identified from differences in the electrophoretic migration of chromosomes in agarose gels. Here we present evidence indicating that some laboratory rats can contain two different genetic variants of P. carinii that differ not only in electrophoretic karyotype but also in the presence of a particular repeated DNA sequence, in the presence of an intron in the 18S ribosomal RNA gene, and in the sequence of part of the 18S rRNA gene. Most of the rat colonies studied were infected with P. carinii that contained the repeated DNA and the 18S rRNA gene intron. The other type of rat-derived P. carinii, which lacked the repeated DNA and the intron in the 18S rRNA gene, was found as a coinfection with the first. Parasite populations from different coinfected rats contained the two variants in different proportions.