The earliest report of this genus appears to have been that of
Carlos Chagas in 1909, who discovered it in experimental animals, but confused it with part of the lifecycle of
Trypanosoma cruzi (causal agent of
Chagas disease) and later called both organisms
Schizotrypanum cruzi, a form of
trypanosome infecting humans. The rediscovery of
Pneumocystis cysts was reported by
Antonio Carini in 1910, also in
Brazil. The genus was again discovered in 1912 by Delanoë and Delanoë, this time at the
Pasteur Institute in Paris, who found it in rats and proposed the genus and species name
Pneumocystis carinii after Carini.
Pneumocystis was redescribed as a human pathogen in 1942 by two Dutch investigators, van der Meer and Brug, who found it in three new cases: a 3-month-old infant with
congenital heart disease and in two of 104
autopsy cases – a 4-month-old infant and a 21-year-old adult. There being only one described
species in the
genus, they considered the human parasite to be
P. carinii. Nine years later (1951), Dr. Josef Vanek at
Charles University in Prague, Czechoslovakia, showed in a study of lung sections from 16 children that the organism labelled "
P. carinii" was the causative agent of pneumonia in these children. The following year, Czech scientist
Otto Jírovec reported "
P. carinii" as the cause of
interstitial pneumonia in
neonates. Following the realization that
Pneumocystis from humans could not infect experimental animals such as rats, and that the rat form of
Pneumocystis differed physiologically and had different
antigenic properties, Frenkel was the first to recognize the human pathogen as a distinct species. He named it "
Pneumocystis jiroveci" (corrected to
P. jirovecii - see nomenclature above). Controversy existed over the relabeling of
P. carinii in humans as
P. jirovecii, which is why both names still appear in publications. However, only the name
P. jirovecii is used exclusively for the human pathogen, whereas the name
P. carinii has had a broader application to many species. Frenkel and those before him believed that all
Pneumocystis were
protozoans, but soon afterwards evidence began accumulating that
Pneumocystis was a fungal genus. Recent studies show it to be an unusual, in some ways a primitive genus of
Ascomycota, related to a group of
yeasts. Currently, only five species have been formally named:
P. jirovecii from humans,
P. carinii as originally named from rats,
P. murina from mice,
P. wakefieldiae also from rats, and
P. oryctolagi from rabbits. Historical and even recent reports of
P. carinii from humans are based upon older classifications (still used by many, or those still debating the recognition of distinct species in the genus
Pneumocystis) which does not mean that the true
P. carinii from rats actually infects humans. In an intermediate classification system, the various
taxa in different mammals have been called
formae speciales or forms. For example, the human "form" was called
Pneumocystis carinii f. [or f. sp.]
hominis, while the original rat infecting form was called
Pneumocystis carinii f. [or f. sp.]
carinii. This terminology is still used by some researchers. The species of
Pneumocystis originally seen by Chagas have not yet been named as distinct species. Currently, they are cryptic taxa. ==References==