Photomicrographs of lung smears and diseased lungs can be accessed via http://timm.main.teikyo-u.ac.jp/pfdb/diseases/P_C_Pneumonia.html

Pneumocystis carinii pneumonitis is a serious disease frequently encountered among individuals who are immunosuppressed due to infection with AIDS, chemotherapy, or dietary protein deprivation. Approximately 80% of individuals infected with AIDS will present to the hospital with the labored breathing characteristic of Pneumocystis carinii pneumonitis, and 95% will die from it. A more detailed look at the clinical problems associated with Pneumocystis carinii can be accessed via http://medlib.med.utah.edu/WebPath/TUTORIAL/AIDS/AIDS.html . Originally discovered in the lungs of gerbils in the early 1900's, it was originally considered a protist with taxonomic affinity to the Apicomplexa, which includes Toxoplasma and Plasmodium, the causative agent for malaria. More recent sequencing of the ribosomal RNA (r-RNA) suggests a different affinity for the organism:

From: http://www.umu.se/myconet/M9b.html
M Y C O N E T
ISSN 1403-1418
VOLUME 9
JANUARY 28, 2003
Notes on ascomycete systematics: Nos 3580-3623
3594. Pneumocystidomycetes O.E. Erikss. & Winka

Smulian (2001: 145) reviewed new information on genetic diversity and cell biology in the genus Pneumocystis P. Delano‘ & Delano‘. A single species, P. carinii P. Delano‘ & Delano‘, with several formae speciales in alveoles of different mammals is currently recognized. Smulian stated that there are differences between these f. spp. in the DNA sequences of all genes studied and that agreement was reached at the International Workshop on Opportunistic Protists (2001) that they should be treated as separate species. More dramatic differences between these taxa were seen in the gene organisation and structure of two gene families, the MSG (major surface glycoprotein) and protease gene families. MSG are surface antigens and there are many in each host specific type of Pneumocystis and there was only 54-86% resemblance between the MSG genes from rat-, human-, mouse- and ferret-derived Pneumocystis. It had been demonstrated earlier that there are two different Pneumocystis taxa in rats. The nomenclature of Pneumocystis taxa was discussed by Eriksson (1994). Typification and valid description of the Pneumocystis spp. will probably be published in the near future (see Smulian 2001: 148).

Studies on a signature sequence in SSU rDNA in Ascomycota (see Note 3592, Orbiliomycetes) indicate that there are large differences in this signature region between different strains of P. carinii, giving further support for recognizing Pneumocystis on different hosts as separate species. ). - (O.E. Eriksson: 2003-01-07).

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From; http://www.nal.usda.gov/indexing/indnotes.htm#no.%2024

No. 24, March 1993

SUBJECT: Pneumocystis carinii

The genus Pneumocystis was originally classified as a protozoan. However, according to more recent taxonomic opinion, it is a fungus. Currently, the CAB Thesaurus carries no Broader Term for PNEUMOCYSTIS but does carry the related term PROTOZOA, and AGRICOLA indexers have used Category Code L823, Pests of Animals (Protozoa), to index this organism.

Effective immediately, use instead Category Code L831, Animal Diseases (Fungal), to index Pneumocystis.

However, remember that Pneumocystis is only in scope for AGRICOLA when it occurs naturally in domestic or laboratory animals. It is not in scope as a human disease, nor when infections in laboratory animals are used as models for human disease. Since it is not known to be an important pathogen of domestic animals, do not select for indexing general articles on its biochemistry, genetics, ultrastructure, etc. This organism is extensively covered in the National Library of Medicine's MEDLINE database.