Pneumocystis Colonization and Pulmonary Infection in Immunocompetent Adults

Abstract & Commentary

By Dean L. Winslow, MD, FACP, FIDSA, Chief, Division of AIDS Medicine, Santa Clara Valley Medical Center; Clinical Professor, Stanford University School of Medicine, is Associate Editor for Infectious Disease Alert.

Dr. Winslow serves as a consultant for Siemens Diagnostics, and is on the speaker's bureau for GSK and Cubist.

Synopsis: The lungs of people who died of trauma and nonviolent sudden death, who underwent autopsy, were examined for Pneumocystis by nested polymerase chain reaction (PCR). P. jirovecii was identified in 50 (65%) of 77 individuals overall and in 15 (79%) of 19 individuals who died of nonviolent causes.

Source: Ponce CA, et al. Pneumocystis colonization is highly prevalent in the autopsied lungs of the general population. Clin Infect Dis. 2010;50: 347-53.

Nested PCR using primers to amplify the mito chondrial large subunit of P. jirovecii was performed on lungs from patients who underwent autopsy at a large urban medical examiner's office in Chile. Lung-tissue concentration methods were used to analyze approximately 3% of the RUL lung weight, and PCR-negative samples from those in the violent death group were reanalyzed using 6%-7% of RUL lung weight to confirm the initial results. P. jirovecii DNA was detected by nested PCR in 50 (65%) of 77 subjects overall and in 15 (79%) of 19 subjects who died of medical conditions. Immunofluorescent microscopy was used to examine the lungs of all 55 subjects who died of trauma. Pneumocystis forms (which were few in number) were visualized in all 34 subjects who tested positive and in none of those who tested negative by PCR. No sex difference in PCR positivity was noted. P. jirovecii did appear to be more common in those < 20 and > 60 years of age.

Commentary

Pneumocystosis is a fascinating disease, and our understanding of it, while still evolving, has changed tremendously over the 38 years that I have been in medicine. While Pneumocystis carinii pneumonia (PCP) was first described in malnourished orphaned infants, it first came to be recognized as an important clinical entity in iatrogenically immunocompromised patients in the late 1960s. I saw a handful of PCP cases each year in this group and, in the 1980s, as HIV infection spread in the United States, I often saw several new cases of PCP each week. It has been appreciated for many years that pneumocystis infection was almost universal in young children and generally subclinical in healthy children. It was initially assumed that symptomatic pneumocystosis usually occurred as a result of reactivation of "latent" infection (analogous to toxoplasma encephalitis) in immunocompromised patients. However, different Pneumocystis genotypes were later demonstrated in HIV-infected patients who experienced two episodes of pneumonia occurring > 6 months apart, thereby casting doubt on the reactivation hypothesis.1 In addition, clusters of PCP have been described in wards of immunocompromised patients, and the lack of PCP reactivation in CD4 T-cell-deficient mice makes latency unlikely. Lastly, the presence of cyst and trophozoite forms seen on immunofluorescent microscopy in the present study, and the presence of viable, actively replicating Pneumocystis organisms in immunocompetent animal models, all point to the common occurrence of recurrent Pneumocystis infections throughout life in healthy individuals, and suggests PCP in immunocompromised patients is due to reinfection.

In the same issue of Clinical Infectious Diseases, the same group report on the results of their study of 110 older adults who had oropharyngeal wash and nasal swab specimens examined by the same nested PCR technique used in the autopsy study.2 P. jirovecii DNA was detected in 21.5% of paired specimens, demonstrating that upper respiratory tract colonization by P. jirovecii is common in older healthy adults.

References:

  1. Keely SP, et al. Genetic variation among Pneumocystis carinii hominis isolates in recurrent pneumocystis. J Infect Dis. 1995;172:595-598.
  2. Vargas SL, et al. Pneumocystis colonization in older adults and diagnostic yield of single versus paired noninvasive respiratory sampling. Clin Infect Dis. 2010;50:e19-21.