Geographic clustering of Pneumocystis carinii pneumonia in patients with HIV infection

Pneumocystis jirovecii pneumonia in people living with HIV: a review

Pneumocystis jirovecii is a ubiquitous opportunistic fungus that can cause life-threatening pneumonia. People with HIV (PWH) who have low CD4 counts are one of the populations at the greatest risk of Pneumocystis jirovecii pneumonia (PCP). While guidelines have approached the diagnosis, prophylaxis, and management of PCP, the numerous studies of PCP in PWH are dominated by the 1980s and 1990s. As such, most studies have included younger male populations, despite PCP affecting both sexes and a broad age range. Many studies have been small and observational in nature, with an overall lack of randomized controlled trials. In many jurisdictions, and especially in low- and middle-income countries, the diagnosis can be challenging due to lack of access to advanced and/or invasive diagnostics. Worldwide, most patients will be treated with 21 days of high-dose trimethoprim sulfamethoxazole, although both the dose and the duration are primarily based on historical practice. Whether treatment with a lower dose is as effective and less toxic is gaining interest based on observational studies. Similarly, a 21-day tapering regimen of prednisone is used for patients with more severe disease, yet other doses, other steroids, or shorter durations of treatment with corticosteroids have not been evaluated. Now with the widespread availability of antiretroviral therapy, improved and less invasive PCP diagnostic techniques, and interest in novel treatment strategies, this review consolidates the scientific body of literature on the diagnosis and management of PCP in PWH, as well as identifies areas in need of more study and thoughtfully designed clinical trials.

No Change of Pneumocystis jirovecii Pneumonia after the COVID-19 Pandemic: Multicenter Time-Series Analyses

Consolidated infection control measures imposed by the government and hospitals during COVID-19 pandemic resulted in a sharp decline of respiratory viruses. Based on the issue of whether Pneumocystis jirovecii could be transmitted by airborne and acquired from the environment, we assessed changes in P. jirovecii pneumonia (PCP) cases in a hospital setting before and after COVID-19. We retrospectively collected data of PCP-confirmed inpatients aged ≥18 years (N = 2922) in four university-affiliated hospitals between January 2015 and June 2021. The index and intervention dates were defined as the first time of P. jirovecii diagnosis and January 2020, respectively. We predicted PCP cases for post-COVID-19 and obtained the difference (residuals) between forecasted and observed cases using the autoregressive integrated moving average (ARIMA) and the Bayesian structural time-series (BSTS) models. Overall, the average of observed PCP cases per month in each year were 36.1 and 47.3 for pre- and post-COVID-19, respectively. The estimate for residuals in the ARIMA model was not significantly different in the total PCP-confirmed inpatients (7.4%, p = 0.765). The forecasted PCP cases by the BSTS model were not significantly different from the observed cases in the post-COVID-19 (−0.6%, 95% credible interval; −9.6~9.1%, p = 0.450). The unprecedented strict non-pharmacological interventions did not affect PCP cases.

Structural basis for the acetylation of histone H3K9 and H3K27 mediated by the histone chaperone Vps75 in Pneumocystis carinii

Rtt109 is a histone acetyltransferase (HAT) that is a potential therapeutic target in conditioned pathogenic fungi Pneumocystis carinii (P. carinii). The histone chaperone Vps75 can stimulate the Rtt109-dependent acetylation of several histone H3 lysines and preferentially acetylates H3K9 and H3K27 within canonical histone (H3-H4)2 tetramers. Vps75 shows two protein conformations assembled into dimeric and tetrameric forms, but the roles played by multimeric forms of Vps75 in Rtt109-mediated histone acetylation remain elusive. In P. carinii, we identified that Vps75 (PcVps75) dimers regulate H3K9 and H3K27 acetylation by directly interacting with histone (H3-H4)2 tetramers, rather than by forming a Vps75-Rtt109 complex. For PcVps75 tetramers, the major histone-binding surface is buried within a walnut-like structure in the absence of a histone cargo. Based on crystal structures of dimeric and tetrameric forms of PcVps75, as well as HAT assay data, we confirmed that residues 192E, 193D, 194E, 195E, and 196E and the disordered C-terminal tail (residues 224-250) of PcVps75 mediate interactions with histones and are important for the Rtt109 in P. carinii (PcRtt109)-mediated acetylation of H3K9 and H3K27, both in vitro and in yeast cells. Furthermore, expressing PcRtt109 alone or in combination with PcVps75 variants that cannot effectively bind histones could not fully restore cellular growth in the presence of genotoxic agents that block DNA replication owing to the absence of H3K9 and H3K27 acetylation. Together, these data indicate that the interaction between PcVps75 and histone (H3-H4)2 tetramers is a critical regulator of the Rtt109-mediated acetylation of H3K9 and H3K27.

Pneumocystosis as a Complication of H1N1 Influenza A Infection in an HIV-Positive Patient on Effective cART

H1N1 influenza A virus can affect the immune system, causing lymphopenia. This might be of great concern for HIV individuals undergoing effective antireroviral therapy (cART). We report the first confirmed case of H1N1-induced AIDS and Pneumocystis jiroveci pneumonia in an HIV-positive woman on effective cART since 2006.

The Relationship between Pneumocystis Infection in Animal and Human Hosts, and Climatological and Environmental Air Pollution Factors: A Systematic Review

Background:

Over the past decade, there has been rising interest in the interaction of Pneumocystis with the environment. This interest has arisen in part from the demonstration that environmental factors have important effects on the viability and transmission of microbes, including Pneumocystis. Environmental factors include climatological factors such as temperature, humidity, and precipitation, and air pollution factors including carbon monoxide, nitrogen dioxide, sulfur dioxide, and particulate matter.

Methods:

We undertook a systematic review in order to identify environmental factors associated with Pneumocystis infection or PCP, and their effects on human and animal hosts.

Results:

The systematic review found evidence of associations between Pneumocystis infection in animal and human hosts, and climatological and air pollution factors. Data from human studies infers that rather than a seasonal association, presentation with PCP appears to be highest when the average temperature is between 10 and 20°C. There was evidence of an association with hospitalization with PCP and ambient air pollution factors, as well as evidence of an effect of air pollution on both systemic and bronchoscopic lavage fluid humoral responses to Pneumocystis. Interpretation of human studies was confounded by possible genetically-determined predisposition to, or protection from infection.

Conclusions:

This systematic review provides evidence of associations between Pneumocystis infection in both animal and human hosts, and climatological and environmental air pollution factors. This information may lead to an improved understanding of the conditions involved in transmission of Pneumocystis in both animal and human hosts. Such knowledge is critical to efforts aimed at prevention.

A Molecular Window into the Biology and Epidemiology of Pneumocystis spp

Pneumocystis, a unique atypical fungus with an elusive lifestyle, has had an important medical history. It came to prominence as an opportunistic pathogen that not only can cause life-threatening pneumonia in patients with HIV infection and other immunodeficiencies but also can colonize the lungs of healthy individuals from a very early age. The genus Pneumocystis includes a group of closely related but heterogeneous organisms that have a worldwide distribution, have been detected in multiple mammalian species, are highly host species specific, inhabit the lungs almost exclusively, and have never convincingly been cultured in vitro, making Pneumocystis a fascinating but difficult-to-study organism. Improved molecular biologic methodologies have opened a new window into the biology and epidemiology of Pneumocystis. Advances include an improved taxonomic classification, identification of an extremely reduced genome and concomitant inability to metabolize and grow independent of the host lungs, insights into its transmission mode, recognition of its widespread colonization in both immunocompetent and immunodeficient hosts, and utilization of strain variation to study drug resistance, epidemiology, and outbreaks of infection among transplant patients. This review summarizes these advances and also identifies some major questions and challenges that need to be addressed to better understand Pneumocystis biology and its relevance to clinical care.

Pneumocystis jirovecii detection in asymptomatic patients: what does its natural history tell us?

Pneumocystis jirovecii is an unusual ascomycetous fungus that can be detected in the lungs of healthy individuals. Transmission from human to human is one of its main characteristics in comparison with other fungi responsible for invasive infections. P. jirovecii is transmitted through the air between healthy individuals, who are considered to be the natural reservoir, at least transiently. In immunocompromised patients, P. jirovecii multiplies, leading to subacute infections and acute life-threatening pneumonia, called Pneumocystis pneumonia [PCP]. PCP is caused by genotypically distinct mixtures of organisms in more than 90% of cases, reinforcing the hypothesis that there is constant inhalation of P. jirovecii from different contacts over time, although reactivation of latent organisms from previous exposures may be possible. Detection of P. jirovecii DNA without any symptoms or related radiological signs has been called “colonization”. This situation could be considered as the result of recent exposure to P. jirovecii that could evolve towards PCP, raising the issue of cotrimoxazole prophylaxis for at-risk quantitative polymerase chain reaction (qPCR)-positive immunocompromised patients. The more accurate way to diagnose PCP is the use of real-time quantitative PCR, which prevents amplicon contamination and allows determination of the fungal load that is mandatory to interpret the qPCR results and manage the patient appropriately. The detection of P. jirovecii in respiratory samples of immunocompromised patients should be considered for potential risk of developing PCP. Many challenges still need to be addressed, including a better description of transmission, characterization of organisms present at low level, and prevention of environmental exposure during immunodepression.

The ecology of pneumocystis: perspectives, personal recollections, and future research opportunities

I am honored to receive the second Lifetime Achievement Award by International Workshops on Opportunistic Protists and to give this lecture. My research involves Pneumocystis, an opportunistic pulmonary fungus that is a major cause of pneumonia ("PcP") in the immunocompromised host. I decided to focus on Pneumocystis ecology here because it has not attracted much interest. Pneumocystis infection is acquired by inhalation, and the cyst stage appears to be the infective form. Several fungal lung infections, such as coccidiomycosis, are not communicable, but occur by inhaling < 5 μm spores from environmental sources (buildings, parks), and can be affected by environmental factors. PcP risk factors include environmental constituents (temperature, humidity, SO2 , CO) and outdoor activities (camping). Clusters of PcP have occurred, but no environmental source has been found. Pneumocystis is communicable and outbreaks of PcP, especially in renal transplant patients, are an ongoing problem. Recent evidence suggests that most viable Pneumocystis organisms detected in the air are confined to a patient's room. Further efforts are needed to define the risk of Pneumocystis transmission in health care facilities; to develop more robust preventive measures; and to characterize the effects of climatological and air pollutant factors on Pneumocystis transmission in animal models similar to those used for respiratory viruses.

Pneumocystis jirovecii infection: an emerging threat to patients with rheumatoid arthritis

Accompanying the increased use of biologic and non-biologic antirheumatic agents, patients with RA have been exposed to an increased risk of Pneumocystis jirovecii infection, which causes acute fulminant P. jirovecii pneumonia (PCP). Mortality in this population is higher than in HIV-infected individuals. Several guidelines and recommendations for HIV-infected individuals are available; however, such guidelines for RA patients remain less clear. Between 2006 and 2008 we encountered a clustering event of P. jirovecii infection among RA outpatients. Through our experience with this outbreak and a review of the recent medical literature regarding asymptomatic colonization and its clinical significance, transmission modes of infection and prophylaxis of PCP, we have learned the following lessons: PCP outbreaks among RA patients can occur through person-to-person transmission in outpatient facilities; asymptomatic carriers serve as reservoirs and sources of infection; and short-term prophylaxis for eradication of P. jirovecii is effective in controlling PCP outbreaks among RA outpatients.

Environmental risk factors for Pneumocystis pneumonia hospitalizations in HIV patients

BACKGROUND

Pneumocystis pneumonia (PcP) is the second leading cause of morbidity and mortality in human immunodeficiency virus (HIV)-infected patients in the United States. Although the host risk factors for the development of PcP are well established, the environmental (climatological, air pollution) risk factors are poorly understood. The major goal of this study was to determine the environmental risk factors for admissions of HIV-positive patients with PcP to a single medical center.

METHODS

Between 1997 and 2008, 457 HIV-positive patients with microscopically confirmed PcP were admitted to the San Francisco General Hospital. A case-crossover design was applied to identify environmental risk factors for PcP hospitalizations. Climatological and air pollution data were collected from the Environmental Protection Agency and Weather Warehouse databases. Conditional logistic regression was used to evaluate the association of each environmental factor and PcP hospital admission.

RESULTS

Hospital admissions were significantly more common in the summer than in the other seasons. Increases in temperature and sulfur dioxide levels were independently associated with hospital admissions for PcP, but the effects of sulfur dioxide were modified by increasing carbon monoxide levels.

CONCLUSIONS

This study identifies both climatological and air pollution constituents as independent risk factors for hospitalization of HIV-positive patients with PcP in San Francisco. Thus, the environmental effects on PcP are more likely complex than previously thought. Further studies are needed to understand how these factors exert their effects and to determine if these factors are associated with PcP in other geographic locations.

Colonization by Pneumocystis jirovecii and its role in disease

Although the incidence of Pneumocystis pneumonia (PCP) has decreased since the introduction of combination antiretroviral therapy, it remains an important cause of disease in both HIV-infected and non-HIV-infected immunosuppressed populations. The epidemiology of PCP has shifted over the course of the HIV epidemic both from changes in HIV and PCP treatment and prevention and from changes in critical care medicine. Although less common in non-HIV-infected immunosuppressed patients, PCP is now more frequently seen due to the increasing numbers of organ transplants and development of novel immunotherapies. New diagnostic and treatment modalities are under investigation. The immune response is critical in preventing this disease but also results in lung damage, and future work may offer potential areas for vaccine development or immunomodulatory therapy. Colonization with Pneumocystis is an area of increasing clinical and research interest and may be important in development of lung diseases such as chronic obstructive pulmonary disease. In this review, we discuss current clinical and research topics in the study of Pneumocystis and highlight areas for future research.

Pneumocystis jirovecii Pneumonia

Pneumocystis jirovecii has gained attention during the last decade in the context of the AIDS epidemic and the increasing use of cytotoxic and immunosuppressive therapies. This article summarizes current knowledge on biology, pathophysiology, epidemiology, diagnosis, prevention, and treatment of pulmonary P jirovecii infection, with a particular focus on the evolving pathophysiology and epidemiology. Pneumocystis pneumonia still remains a severe opportunistic infection, associated with a high mortality rate.

Modeling of spatially referenced environmental and meteorological factors influencing the probability of Listeria species isolation from natural environments

Many pathogens have the ability to survive and multiply in abiotic environments, representing a possible reservoir and source of human and animal exposure. Our objective was to develop a methodological framework to study spatially explicit environmental and meteorological factors affecting the probability of pathogen isolation from a location. Isolation of Listeria spp. from the natural environment was used as a model system. Logistic regression and classification tree methods were applied, and their predictive performances were compared. Analyses revealed that precipitation and occurrence of alternating freezing and thawing temperatures prior to sample collection, loam soil, water storage to a soil depth of 50 cm, slope gradient, and cardinal direction to the north are key predictors for isolation of Listeria spp. from a spatial location. Different combinations of factors affected the probability of isolation of Listeria spp. from the soil, vegetation, and water layers of a location, indicating that the three layers represent different ecological niches for Listeria spp. The predictive power of classification trees was comparable to that of logistic regression. However, the former were easier to interpret, making them more appealing for field applications. Our study demonstrates how the analysis of a pathogen's spatial distribution improves understanding of the predictors of the pathogen's presence in a particular location and could be used to propose novel control strategies to reduce human and animal environmental exposure.

Incidence and nature of infectious disease in patients treated with anti-TNF agents

Tumor necrosis factor alpha (TNF-alpha) inhibitors offer a targeted therapeutic strategy that contrasts with the nonspecific immunosuppressive agents traditionally used to treat most inflammatory diseases. These biologic agents have had a significant impact in ameliorating the signs and symptoms of inflammatory rheumatoid disease and improving patient function. From the onset of clinical trials, a central concern of cytokine blockade has been a potential increase in susceptibility to infections. Not surprisingly, a variety of infections have been reported in association with the use of TNF-alpha inhibitor agents. In particular, there is evidence suggesting an increased rate of granulomatous infections in patients treated with monoclonal TNF-alpha inhibitors. This review provides the incidence and nature of infections in patients treated with TNF-alpha inhibitor agents and reminds the clinician of the required vigilance in monitoring patients.

On the use of ZIP codes and ZIP code tabulation areas (ZCTAs) for the spatial analysis of epidemiological data

BACKGROUND

While the use of spatially referenced data for the analysis of epidemiological data is growing, issues associated with selecting the appropriate geographic unit of analysis are also emerging. A particularly problematic unit is the ZIP code. Lacking standardization and highly dynamic in structure, the use of ZIP codes and ZIP code tabulation areas (ZCTA) for the spatial analysis of disease present a unique challenge to researchers. Problems associated with these units for detecting spatial patterns of disease are explored.

RESULTS

A brief review of ZIP codes and their spatial representation is conducted. Though frequently represented as polygons to facilitate analysis, ZIP codes are actually defined at a narrower spatial resolution reflecting the street addresses they serve. This research shows that their generalization as continuous regions is an imposed structure that can have serious implications in the interpretation of research results. ZIP codes areas and Census defined ZCTAs, two commonly used polygonal representations of ZIP code address ranges, are examined in an effort to identify the spatial statistical sensitivities that emerge given differences in how these representations are defined. Here, comparative analysis focuses on the detection of patterns of prostate cancer in New York State. Of particular interest for studies utilizing local, spatial statistical tests, is that differences in the topological structures of ZIP code areas and ZCTAs give rise to different spatial patterns of disease. These differences are related to the different methodologies used in the generalization of ZIP code information. Given the difficulty associated with generating ZIP code boundaries, both ZIP code areas and ZCTAs contain numerous representational errors which can have a significant impact on spatial analysis. While the use of ZIP code polygons for spatial analysis is relatively straightforward, ZCTA representations contain additional topological features (e.g. lakes and rivers) and contain fragmented polygons that can hinder spatial analysis.

CONCLUSION

Caution must be exercised when using spatially referenced data, particularly that which is attributed to ZIP codes and ZCTAs, for epidemiological analysis. Researchers should be cognizant of representational errors associated with both geographies and their resulting spatial mismatch, especially when comparing the results obtained using different topological representations. While ZCTAs can be problematic, topological corrections are easily implemented in a geographic information system to remedy erroneous aggregation effects.

Pneumocystis jiroveci infection in patients with hyper-immunoglobulin E syndrome

The hyper-immunoglobulin E syndrome is a primary immunodeficiency characterized by recurrent pyogenic skin and lung abscesses, dermatitis, and elevated serum immunoglobulin E levels. Pneumocystis jiroveci (formerly Pneumocystis carinii) is not typically associated with hyper-immunoglobulin E syndrome. We identified 7 patients with hyper-immunoglobulin E syndrome with P. jiroveci detected in respiratory or pulmonary pathology specimens. In 5 patients it was the sole pathogen, and in 2 other patients it contributed to a polymicrobial etiology. No consistent prophylaxis was given, and there have been no recurrences on long-term follow-up. Our experience suggests that P. jiroveci can cause pneumonia in patients with hyper-immunoglobulin E syndrome both with and without chronic lung disease.

Genotypic variation in Pneumocystis jirovecii isolates in Britain

BACKGROUND

Pneumocystis jirovecii is the cause of Pneumocystis pneumonia (PCP) in immunosuppressed humans. Asymptomatic colonisation with P jirovecii may occur in patients with minor immunosuppression or chronic lung disease. The aim of this study was to describe the molecular epidemiology of P jirovecii in Britain over a period of 12.5 years.

METHODS

Between January 1989 and July 2001 161 samples of P jirovecii were obtained from patients with PCP (n = 119), patients colonised by P jirovecii (n = 35), and from air spora (n = 6). Genotyping of samples was performed at the mitochondrial large subunit rRNA (mt LSU rRNA).

RESULTS

Genotype 1 (38%) was the most frequently identified genotype: genotypes 2 (26.6%), 3 (20.3%), and 4 (5%) were less common. Mixed infection (more than one genotype) was identified in 10% of samples. While genotype 1 was the most frequently detected type in both patients with PCP and those colonised by P jirovecii (38% and 42%, respectively), these groups differed in the relatively lower rate of detection of genotype 4 (2% v 17%) and the higher detection of mixed infection in those with PCP (13% v 3%). Detection of specific genotypes of P jirovecii was associated with the patient's place of residence (p = 0.02). There was no association between specific genotypes and severity of PCP as measured by arterial oxygen tension (p = 0.3).

CONCLUSIONS

The evidence of clustering of specific genotypes with patient's postcode of residence is consistent with the hypothesis of person to person transmission of P jirovecii via the airborne route. The lack of association between specific mt LSU rRNA genotypes and severity of PCP suggests that this locus is not implicated in the virulence of the organism.

Prevalence and clinical predictors of Pneumocystis colonization among HIV-infected men

BACKGROUND

The epidemiology and transmission of Pneumocystis are poorly understood. The incidence of colonization, or detection of organisms without signs of disease, has been debated, and risk factors for colonization are largely unknown.

OBJECTIVE

To determine the rate of Pneumocystis colonization among HIV-infected patients at autopsy and analyze associated clinical variables.

METHODS

Subjects were selected from the Multicenter AIDS Cohort Study. Subjects who died from causes other than Pneumocystis pneumonia and consented to autopsy were included in analysis. DNA was extracted from lung tissue, and nested PCR was performed to detect the presence of Pneumocystis. Clinical data were obtained from the Multicenter AIDS Cohort database. Univariate and multivariate analyses were performed to determine predictors of Pneumocystis colonization.

RESULTS

Pneumocystis DNA was detected in 42 of 91 (46%) subjects by nested PCR. Clinical variables such as CD4 cell count, use of Pneumocystis prophylaxis or antiretroviral drugs, and history of previous Pneumocystis pneumonia were not related to risk of colonization. Multivariate analysis demonstrated that cigarette smoking was related to an increased risk of colonization [odds ratio (OR), 4.5; 95% confidence interval (CI), 1.27-15.6; P = 0.02] and risk also varied by city of residence (OR, 0.12; 95% CI, 0.03-0.45; P = 0.002 for living in Los Angeles).

CONCLUSIONS

This study found a high rate of Pneumocystis colonization among HIV-infected patients. We also identified cigarette smoking and city of residence as novel, independent risk factors for colonization. The role of subclinical colonization in disease transmission and the effects of Pneumocystis colonization on the lung require further study.

Update on the epidemiology and transmission of Pneumocystis carinii

Although Pneumocystis carinii pneumonia is one of the leading causes of morbidity and mortality among patients with the acquired immunodeficiency syndrome, many questions about its epidemiology and transmission remain unanswered. Whereas traditional theory postulates that the disease results from reactivation of latent infection, recent data suggest that active acquisition of infection, either through environmental exposure or person-to-person transmission, may occur. This review summarizes the current state of knowledge about the epidemiology and transmission of P. carinii and reports on evolving techniques that may improve our understanding of this organism in the future.

Pneumocystis carinii f. sp. hominis DNA in immunocompetent health care workers in contact with patients with P. carinii pneumonia

The possible transmission of Pneumocystis carinii f. sp. hominis from patients with P. carinii pneumonia to asymptomatic health care workers (HCW), with or without occupational exposure to human immunodeficiency virus (HIV)-infected patients with P. carinii pneumonia, was examined. HCW in a specialist inpatient HIV-AIDS facility and a control group in the general medical-respiratory service in the same hospital provided induced sputum and/or nasal rinse samples, which were analyzed for the presence of P. carinii f. sp. hominis DNA by using DNA amplification (at the gene encoding the mitochondrial large subunit rRNA [mt LSU rRNA]). P. carinii f. sp. hominis DNA was detected in some HCW samples; those with the closest occupational contact were more likely to have detectable P. carinii DNA. P. carinii DNA was detected in one HCW who carried out bronchoscopy over a 2-year period. P. carinii-positive samples were genotyped by using DNA sequence variations at the internal transcribed spacer (ITS) regions of the nuclear rRNA operon, along with bronchoalveolar lavage samples from patients with P. carinii pneumonia hospitalized at the same time. Genotyping identified 31 different P. carinii f. sp. hominis ITS genotypes, 26 of which were found in the patient samples. Five of the eight ITS genotypes detected in HCW samples were not observed in the patient samples. The results suggested that HCW in close occupational contact with patients who had P. carinii pneumonia may have become colonized with P. carinii. Carriage was asymptomatic and did not result in the development of clinical disease.

Geographic distribution of human immunodeficiency virus-associated Pneumocystis carinii pneumonia in San Francisco

The epidemiology of Pneumocystis carinii pneumonia (PCP) and its geographic distribution are incompletely understood. We examined the influence of geographic location as a risk factor for PCP through a retrospective case-control study of HIV-infected persons evaluated for PCP at San Francisco General Hospital. Subjects had microscopically confirmed PCP diagnosed between January 1996 and June 1999. Control subjects had a presentation suggestive of PCP, but had bronchoalveolar lavage examination that did not reveal P. carinii. Medical chart review was performed to obtain demographic and clinical characteristics of the subjects as well as their addresses at time of PCP evaluation. Multivariate analyses were performed in order to identify variables associated with PCP. Lack of P. carinii prophylaxis and a CD4 cell count </= 50 cells/microl were independent predictors of an increased risk of PCP. Interestingly, subjects living in the 94103 zip code had a significantly lower risk of PCP than subjects in other areas (odds ratio = 0.2, 95% confidence interval 0.1-0.6, p < 0.01). This effect was present despite controlling for risk factors for PCP such as use of prophylaxis or antiretroviral therapy, CD4 cell count, and HIV RNA level. We conclude that residence in the 94103 zip code of San Francisco is associated with a substantially lower risk of PCP than residence in other areas of the city.