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The rat model of Pneumocystis carinii pneumonia is frequently used to study human P. carinii infection, but there are many differences between the rat and human infections. We studied naturally acquired P. carinii in wild rats to examine the relevance of the rat model for human infection. P. carinii DNA was detected in 47 of 51 wild rats and in 10 of 12 nonimmunosuppressed laboratory rats. Evidence for three novel formae speciales of rat-derived P. carinii was found, and these were provisionally named Pneumocystis carinii f. sp. rattus-secundi, Pneumocystis carinii f. sp. rattus-tertii, and Pneumocystis carinii f. sp. rattus-quarti. Our data suggest that low-level carriage of P. carinii in wild rats and nonimmunosuppressed laboratory rats is common and that wild rats are frequently coinfected with more than one forma specialis of P. carinii. We also examined the diversity in the internally transcribed spacer (ITS) regions of the nuclear rRNA operon of Pneumocystis carinii f. sp. carinii by using samples from wild rats and laboratory rats and spore trap samples. We report a lack of variation in the ITS1 and ITS2 regions that is consistent with an evolutionary bottleneck in the P. carinii f. sp. carinii population. This study shows that human- and rat-derived P. carinii organisms are very different, not only in genetic composition but also in population structure and natural history.

Genotyping at the internal transcribed spacer (ITS) regions of the nuclear rRNA operon was performed on isolates of P. carinii sp. f. hominis from three clusters of P. carinii pneumonia among eight patients with haematological malignancies and six with HIV infection. Nine different ITS sequence types of P. carinii sp. f. hominis were identified in the samples from the patients with haematological malignancies, suggesting that this cluster of cases of P. carinii pneumonia was unlikely to have resulted from nosocomial transmission. A common ITS sequence type was observed in two of the patients with haematological malignancies who shared a hospital room, and also in two of the patients with HIV infection who had prolonged close contact on the ward. In contrast, different ITS sequence types were detected in samples from an HIV-infected homosexual couple who shared the same household. These data suggest that person-to-person transmission of P. carinii sp. f. hominis may occur from infected to susceptible immunosuppressed patients with close contact within hospital environments. However direct transmission between patients did not account for the majority of cases within the clusters, suggesting that person-to-person transmission of P. carinii sp. f. hominis infection may be a relatively infrequent event and does not constitute the major route of transmission in man.

One hundred eighty-three toluidine blue O-stained necropsy lung imprint smears from different avian species were examined microscopically for Pneumocystis carinii. No cyst forms of the organism could be identified. Seventy-eight serum samples from a total of 155 chickens were examined by a competition enzyme-linked immunosorbent assay (ELISA) for antibodies to P. carinii; 53 serum samples were from individual chickens, and 25 samples were pools of sera from two to five chickens. Diluted 1:50, the 78 serum samples showed a specific ELISA-inhibition of 4% to 56% (the 95% confidence limit being 25% to 30% inhibition). Diluted 1:50, nine serum pools representing 34 chickens and 17 of the 53 individual serum samples (32.1%) showed an inhibition greater than 30%. No specific pneumocyst DNA could be detected in serum from 13 of the 53 chickens using polymerase chain reaction and dihydrofolate reductase gene as a specific probe. Specific antibodies to a 116,000-molecular-weight antigen of rat pneumocysts were shown in two (13.3%) of 15 individual chicken serum samples. The results indicate that P. carinii organisms do not commonly reside in the lungs of birds, although some birds may be exposed to external sources of organisms.

Sera from 1) 8 AIDS patients with Pneumocystitis carinii pneumonia, 2) 13 HIV antibody-positive patients, 9 with AIDS and 4 with AIDS-related symptoms, all 13 without Pneumocystis carinii pneumonia, and 3) 36 healthy male blood donors, all HIV antibody negative, were examined for antibodies against Pneumocystis carinii using the indirect immunofluorescence antibody technique. Seven (87%) out of eight sera from group 1, 12 (92%) out of 13 sera from group 2, and 35 (97%) out of 36 sera from group 3 were positive. There was no significant difference between group 1 and 2, and group 2 and 3. The difference between group 1 and group 3 was significant (p = 0.03).

Stained imprints made from autopsy lung biopsies from hares, foxes, badgers, stone martens, one hedgehog, roe deer from Denmark and from caribou from Greenland were examined microscopically for Pneumocystis carinii. Pneumocysts were found in the lungs of 3 out of 28 hares, 3 out of 18 foxes and 1 out of 6 roe deer. This is the first report of Pneumocystis carinii in hares, foxes, and roe deer in Denmark. It seems to be the first report of Pneumocystis carinii in roe deer.

Pneumocystis carinii has been found in 17 of 90 (19%) wild living trapped brown rats (Rattus norvegicus). The positive brown rats originated from about 25% of the trapping localities examined. Among brown rats of two breeding stocks, H and S, pneumocysts were found in 43.5% of the H rats, but in none of the S rats. By keeping brown H rats on a low protein diet for 8 weeks, it was possible to augment the prevalence to 53.8%. Pneumocystis carinii has furthermore been found in one of eight (12.5%) wild living long-tailed field mice (Apodemus sylvaticus), in one of 16 (6.3%) wild living house mice (Mus musculus), and in one of ten roof rats (Rattus rattus) of the Danish Pest Infestation Laboratory's breeding stock. Other rodent species, yellow-necked field mouse (Apodemus flavicollis), water vole (Arvicola terrestris), bank vole (Clethrionomys glareolus), field vole (Microtus agrestis), and an insectivor species, the common shrew (Sorex araneus) were all examined in small numbers, and were all negative for pneumocysts. Among rodents in Denmark. Rattus norvegicus seems to be the most important reservoir host for Pneumocystis carinii.

To get some information about the prevalence of Pneumocystis carinii in the Danish population we have examined autopsy lung biopsies from patients at 2 large hospitals in Copenhagen, Righospitalet (RH) and Finseninstituttet (F), in a 5-year period from 1979 to 1984. Imprints made from the biopsies were stained with the toluidine blue O method and examined microscopically for pneumocysts. A total of 1762 patients were examined and pneumocysts found in 83 (4.7%); at RH 54 (4.9%) of 1106 patients and at F29 (4.4%) of 656 patients. A possible influence of certain climatic factors on the prevalence of pneumocysts in humans was shown: low temperature, low vapour pressure and low relative humidity seem to be correlated with low prevalence of pneumocysts. The general use of trimethoprim-sulfamethoxazole in the hospitals may possibly influence the prevalence of pneumocysts: the amount requested in the period of the investigation is negatively correlated with the prevalence of pneumocysts.