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Emerg Infect Dis. 2003 February; 9(2): 276–277.
PMCID: PMC2901955

Pneumocystis carinii versus Pneumocystis jiroveci: Another Misnomer (Response to Stringer et al.)

To the Editor: The proposal by Stringer et al. to change the name of Pneumocystis carinii found in humans to Pneumocystis jirovec requires critical consideration (1). First, their rationale for the choice of Jírovec is not compelling. Principle III of the International Code of Botanical Nomenclature (ICBN) states: “the nomenclature of a taxonomic group is based upon priority of publication” (2). Jírovec’s publication in 1952 was not the first to report P. carinii infection in human lungs. In 1942, two Dutch investigators, van der Meer and Brug, described P. carinii as the infecting organism in a 3-month-old infant with congenital heart disease and in 2 of 104 autopsy cases (a 4-month-old infant and a 21-year-old adult) (3). Their description, photomicrographs, and drawings of P. carinii are unequivocal. They also described the typical “honeycomb” patterns in alveoli. In 1951, Dr. Josef Vanek at Karls-Universität in Praha, Czechoslovakia, reported his study of lung sections from 16 children with interstitial pneumonia and demonstrated that the disease was caused by P. carinii (4). Vanek notes in his report, “In man the parasite was for the first time established as a cause of pneumonia in a child by G. Meer and S. L. Brug (1942).” In 1952, Jírovec reported P. carinii as the cause of interstitial plasmacellular pneumonia in neonates (5). A year later, in a coauthored publication, Vanek, Jírovec, and J. Lukes acknowledged and referenced the earlier reports of van der Meer and Brug and Vanek (6). If principle III is to be followed, as well as fairness to the investigators, both van der Meer and Brug and Vanek hold priority over Jírovec, assuming the designation of the species name should be based on the name of the first person to discover P. carinii in humans.

The nomenclature of P. carinii has actually been fraught with errors from the beginning. In the earliest publications, Carlos Chagas and Antonio Carini mistook the organism for stages in the life cycle of trypanosomes. Chagas placed it in a new genus, Schizotrypanum (7,8). In 1912, Delanoë and Delanoë at the Pasteur Institute in Paris published the first description of the organism as a new entity unrelated to trypanosomes (9). They proposed the name “Pneumocystis carinii” as a tribute to Carini. The Delanoë paper has remained unchallenged as the original description of P. carinii. Both Chagas and Carini later acknowledged their errors and the validity of the Delanoës’ conclusion. By current ICBN principles, P. carinii is acceptable nomenclature because the authors of the first publication proposed the name of Carini, rather than their own.

In addition, changing the name to P. jiroveci will create confusion in clinical medicine where the name P. carinii has served physicians and microbiologists well for over half a century. I was moved to write this letter because of a call from a knowledgeable oncologist asking for information on “the new strain of P. carinii that has just been reported from the Centers for Disease Control and Prevention,” referring to the report by Stringer et al (1).

AIDS patients are well informed about P. carinii pneumonia and avidly monitor medical news about their disease. Without doubt, the name change will cause confusion and undue anxiety among the many thousands of HIV-infected patients who attend clinics. Health-care workers will have an added burden of explaining why the name was changed, but the organism and infection are unchanged. Also, versions of the pronunciation of jiroveci (yee row vet zee) by American patients, physicians, and health-care workers will be interesting to hear.

The tone of the article by Stringer et al. implies that the change of P. carinii to P. jiroveci is final, which is not the case. The nomenclature of fungi is governed by ICBN under the auspices of the International Botanical Congress and is not based solely on molecular genetics. Neither P. carinii nor P. jiroveci have been submitted for ICBN scrutiny. In another paper, Stringer et al. outline the mechanics for submission, but indicate that no application has been submitted for their proposal (10). In fact, P. carinii has not been acknowledged as a fungus by ICBN or any other authoritative taxonomic system. Only when nomenclature is registered in ICBN, can a name be referred to as “formally accepted.” In the meantime, the workable terminology proposed earlier by Stringer et al. in 1994 (11) will suffice for clinical use.


1. Stringer JR, Beard CB, Miller RF, Wakefield AE A new name (Pneumocystis jiroveci) for Pneumocystis from humans. Emerg Infect Dis 2002;8:891–6 [PMC free article] [PubMed]
2. Editorial Committee, International Botanical Congress. International Code of Botanical Nomenclature (St. Louis Code). Konigstein, Germany: Koeltiz Scientific Books; 2000
3. van der Meer MG, Brug SL Infection à Pneumocystis chez l’homme et chez les animaux. Ann Soc Belg Med Trop 1942;22:301–9
4. Vanek J. Atypicka (interstitiálni) pneumonie detí vyvolaná Pneumocystis carinii (atypical interstitial pneumonia of infants produced by Pneumocystis carinii). Casop lék cesk 1951;90:1121–4. [PubMed]
5. Jírovec O.Pneumocystis carinii puvodce t. zv intertitialnich plasmocelularnich pneumonii kojencw (Pneumocystis carinii, the cause of interstitial plasmacellular pneumonia in neonates) Csl. Hyg epid mikrob 1952;1:141.
6. Vanek J, Jírovec O, Lukes J Interstitial plasma cell pneumonia in infants. Ann Paediatr 1953;180:1–21 [PubMed]
7. Chagas C Uber eine neue Trypoanosomiasis des Menschen. Mem Inst Oswaldo Cruz 1909;3:1–218
8. Carini A Formas des eschizogonia do Trypanosoma lewisi. Soc Med Cir São Paulo 1910;38:8
9. Delanoë P, Delanoë M. Sur les rapports des kystes de Carini du poumon des rats avec le Trypanosoma lewisi Comptes rendus de “l’Academie des sciences. 1912;155:658–61.
10. Stringer JR, Cushion MT, Wakefield AE New nomenclature for the genus Pneumocystis. J Eukaryot Microbiol 2001;Suppl:184–9 10.1111/j.1550-7408.2001.tb00512.x [PubMed] [Cross Ref]
11. Stringer J and the Pneumocystis Workshop Revised nomenclature for Pneumocystis carinii. J Eukaryot Microbiol 1994;41:121–2 [PubMed]
Emerg Infect Dis. 2003 February; 9(2): 276–277.

In Response

We appreciate Dr. Hughes’ letter of concern regarding our article endorsing the name Pneumocystis jiroveci (1). When working with well-known disease agents and syndromes, these types of changes are more difficult to adopt because of the effect they have on daily communication, patient care, record keeping, and other important routines of health-care providers. However, in this case, new information and understanding dictate that a change be made.

For some time, scientists have known that humans are infected by a particular species of Pneumocystis and that this species does not infect other host species. In recognition of these facts, Frenkel named the human pathogen Pneumocystis jiroveci, using the procedure prescribed by the International Code of Botanical Nomenclature (ICBN) (2). Although Dr. Hughes raised a number of issues, none justifies rejecting the new, valid name.

Dr. Hughes suggested that the name P. jiroveci is incorrect on the basis of principal III of ICBN, which holds that “the nomenclature of a taxonomic group is based upon priority of publication.” He indicated that Jírovec was not the first investigator to report Pneumocystis in humans. Although this situation may be the case, principal III has not been violated because “priority of publication” refers to the time when a name is validly published, not to the time when an organism is first described. The name P. jiroveci was validly published in 1999, and this name therefore has priority. To be valid, all of the following steps must be completed: a name must be published in a scientific journal, the name must be a binary Latin name, the organism must be described in Latin, the rank of the organism must be indicated, and the new species must be called by the term “typus or holotypus,”and the specimen or microscope slides must be placed in a public holding (details are available from: URL: Dr. Frenkel was the first to fulfill these requirements in his 1999 publication (2).

The 1912 publication by Delanoë and Delanoë does not have priority in naming Pneumocystis from humans because the organism studied by the Delanoës was from the rat. The rat-derived Pneumocystis organism continues to be known as P. carinii, in keeping with principal III. As an additional historical note, Dr. Frenkel was the first investigator to understand the clear differences between human and rat-derived Pneumocystis, which were described in a landmark publication in 1976 (3). He proposed a change in nomenclature in which the name Pneumocystis jiroveci n. sp. applied to the human organisms and Pneumocystis carinii was retained for the rat organism. However, Frenkel did not attempt to follow ICBN procedures because at the time Pneumocystis was thought to be a protozoan. Nevertheless, this early paper established the idea of naming human Pneumocystis, P. jiroveci.

Dr. Hughes stated a concern over the possibility that the name change may cause “confusion and undue anxiety among the many thousands of HIV-infected patients who attend clinics.” Such concern is understandable. However, patients will have guidance in understanding the significance of the name change. Health-care providers will allay any fears that might be elicited by the application of the new name. The level of anxiety experienced by persons at risk of acquiring Pneumocystis carinii pneumonia (PCP) is more likely to decline than to increase. People may be relieved to learn that they are not going to catch PCP from a pet, for example.

Dr. Hughes suggested that the name P. jiroveci is unofficial because it has not yet been sanctioned by a body of experts that scrutinizes proposed name changes and has the power to either accept or reject them. This situation is not the case. The process by which new names are validated does not directly involve a body of experts. The International Botanical Congress does not evaluate names. Instead, the congress has established ICBN, which sets forth the procedures authors must follow to publish a valid new name. The scientific basis for the new name is included in the publication. In the case of P. jiroveci, abundant evidence shows that P. carinii and P. jíroveci are different species. This evidence, which also indicates that the genus Pneumocystis contains many additional species, has been reviewed extensively (4,5). Dr. Hughes gave the impression that that this evidence is exclusively molecular genetic data. In fact, the molecular genetic evidence is mirrored by clear biologic differences, the most dramatic being host species specificity.

As our knowledge of the biology and genetics of disease-causing microorganisms grows, openness to changes in the taxonomy and classification is needed. Given the impact such changes can have outside of the realm of basic science, the decision to accept the proposed changes in the nomenclature used for Pneumocystis has not been made frivolously. This decision is the result of a long and deliberate process that began almost 10 years ago, when data demonstrating that different mammalian hosts harbor different Pneumocystis species first began to appear. In 1994 and 2001, nomenclature issues were discussed at international meetings of the Pneumocystis community, with both physicians and research investigators present. In 1994, the data supporting new species were relatively limited. Consequently, a provisional tripartite nomenclature was adopted in lieu of recognizing new species (6). By 2001, however, the existence of multiple species and the necessity of assigning new species names were accepted by consensus. Because Frenkel had already published the name P. jiroveci, the suitability and validity of this name were also discussed. The new name was approved by consensus (4). We recognize that the results of these meetings do not necessarily reflect all opinions on the matter of Pneumocystis nomenclature, but we know of no better way to assess the majority opinion.

In endorsing the name Pneumocystis jiroveci, we hope to foster scientific understanding and communication. The tripartite name formerly used to denote the distinctness of this organism is not only cumbersome, it is inadequate because its meaning is not apparent and must be defined every time it is used. The arcane nature of the tripartite name tended to deprive the broad audience of persons interested in PCP of vital information, namely, a unique species of Pneumocyctis infects humans. By contrast, the new species name clearly states the uniqueness of P. jiroveci; a distinction is needed when assessing the significance of findings obtained by studies on other members of the genus. Recognition of this uniqueness will undoubtedly stimulate more research on this species. Communication will best be served by uniformity in nomenclature.

Frenkel has assigned a valid name to the Pneumocystis species found in humans. Ignoring this name on the grounds of inconvenience is not only unjustified, it is impractical. If names published in accordance with ICBN are not accepted, the field will have no recognized mechanism for conferring names, fostering the use of idiosyncratic, inadequate, and misleading names. Communication and progress will suffer as a result. Furthermore, if we choose to ignore critical changes in taxonomy for subjective reasons, when these changes are mandated by new and indisputable information, then taxonomy itself becomes meaningless and as a consequence, practitioners of public health, clinical medicine, and biomedical research might as well call any disease agent by whatever name they choose.


1. Stringer JR, Beard CB, Miller RF, Wakefield AE A new name (Pneumocystis jiroveci) for Pneumocystis from humans. Emerg Infect Dis 2002;8:891–6 [PMC free article] [PubMed]
2. Frenkel JK Pneumocystis pneumonia, an immunodeficiency-dependent disease (IDD): a critical historical overview. J Eukaryot Microbiol 1999;46:89S–92 [PubMed]
3. Frenkel JK Pneumocystis jiroveci n. sp. from man: morphology, physiology, and immunology in relation to pathology. Natl Cancer Inst Monogr 1976;43:13–30 [PubMed]
4. Stringer JR, Cushion MT, Wakefield AE New nomenclature for the genus Pneumocystis. Supplement: Proceedings of the Seventh International Workshops on Opportunistic Protists, Cincinnati, Ohio, June 13–16, 2001. J Eukaryot Microbiol 2001;Suppl:184s–9 10.1111/j.1550-7408.2001.tb00512.x [PubMed] [Cross Ref]
5. Wakefield AE, Stringer JR, Tamburrini E, Dei-Cas E Genetics, metabolism and host specificity of Pneumocystis carinii. Med Mycol 1998;36:183–93 [PubMed]
6. Revised nomenclature for Pneumocystis carinii. The Pneumocystis Workshop. J Eukaryot Microbiol 1994;41:121S–2 [PubMed]

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