Transmission and Colonization of Pneumocystis jirovecii

Abstract

Pneumocystis spp. was discovered in 1909 and was classified as a fungus in 1988. The species that infects humans is called P. jirovecii and important characteristics of its genome have recently been discovered. Important advances have been made to understand P. jirovecii, including aspects of its biology, evolution, lifecycle, and pathogenesis; it is now considered that the main route of transmission is airborne and that the infectious form is the asci (cyst), but it is unclear whether there is transmission by direct contact or droplet spread. On the other hand, P. jirovecii has been detected in respiratory secretions of hosts without causing disease, which has been termed asymptomatic carrier status or colonization (frequency in immunocompetent patients: 0-65%, pregnancy: 15.5%, children: 0-100%, HIV-positive patients: 20-69%, cystic fibrosis: 1-22%, and COPD: 16-55%). This article briefly describes the history of its discovery and the nomenclature of Pneumocystis spp., recently uncovered characteristics of its genome, and what research has been done on the transmission and colonization of P. jirovecii. Based on the literature, the authors of this review propose a hypothetical natural history of P. jirovecii infection in humans.

Keywords: Pneumocystis jirovecii; colonization; epidemiology; natural history; transmission.

Figure 1

Chronological illustration of the most important events in the discovery of Pneumocystis jirovecii from 1908–1999.

Figure 2

Hypothetical representation of the natural history of Pneumocystis jirovecii. Some of the references that support the definitions used in this figure are: route of transmission [38,39,40,41,57,58], infective form [45], incubation period and period of transmission of an ill patient [55]. The question marks in some parts of the figure mean that there are several aspects that are not clear and need further research. We used the definitions of the “natural history and spectrum of disease”, as well as the “chain of infection” in “epidemiology of infectious diseases” from the Centers for Disease Control and Prevention. Based on those definitions, and the literature reviewed, we summarized that the reservoir of P. jirovecii is humans and that the transmission is from humans to humans. The source of infection can be asymptomatic carriers or a person with P. jirovecii pneumonia. It has been described that there are carriers of P. jirovecii, both asymptomatic carriers and people with underlying conditions that are colonized by P. jirovecii. However, it is unclear for how long these carriers can be colonized and whether there is a transitory state (it is unclear if the person can be colonized by days, weeks, months, or even years). The portal of entry and exit is the respiratory tract, and the most plausible route of transmission in P. jirovecii is airborne.