Objective.—To review and update the literature on current trends with regard to Pneumocystis carinii (jiroveci ) diagnosis, treatment modalities, and its role in human disease processes.
Data Sources.—Bibliographic databases (PubMed and Ovid) were searched for material and data between 1980 and September 2003 relevant to the review. Indexing terms used were “Pneumocystis carinii pneumonia,” and “Pneumocystis jiroveci,” with the English language as a constraint. Other sources were the PhD thesis of one of the authors (J.F.W., London University, 1993) and the library at the Arabian Gulf University in the Kingdom of Bahrain.
Study Selection.—Acquired immunodeficiency syndrome and organ transplant cases with Pneumocystis carinii pneumonia.
Data Extraction.—Independent extraction by 2 observers.
Data Synthesis.—We reviewed the major characteristics of P carinii (jiroveci ) with special emphasis on the more recently acquired data including the presence of a round pore in the cyst wall, which appears to be used for the release of sporozoites, supporting the hypothesis of sexual reproduction in P carinii (jiroveci ).
Conclusions.—Opportunistic infection with P carinii (jiroveci ) remains a significant cause of morbidity and mortality in human immunodeficiency virus and non-human immunodeficiency virus–associated immunosuppressed patients. Diagnosis may be achieved in the majority of cases by routine cytochemical stains and specialized techniques such as immunocytochemistry and polymerase chain reaction. The incidence of P carinii pneumonia can significantly be reduced with effective use of prophylaxis and early detection of cases at high risk. Immunization for P carinii pneumonia is in the early stages and presents a challenging area for research.
Pneumocystis carinii is a eukaryotic microorganism that is found worldwide. Its host range is wide and includes humans and other mammals such as rabbits,1 dogs,2 goats,3 swine,4 cats,5 chimpanzees,6 owl monkeys,7 and horses.8 ,Pneumocystis carinii was first described as a developmental stage in the life cycle of Trypanosoma cruzi by Chagas in 19099 but was later recognized as a separate entity.10 It was first shown to cause human disease in 1951. The Pneumocystis organism causing disease in human beings is named Pneumocystis jiroveci.11
In Europe, an epidemic of interstitial plasma cell pneumonia occurred in premature and malnourished children, particularly those in orphanages in the late 1930s and early 1940s.12 A new epidemic of P. carinii pneumonia was described in the 1960s in children with congenital defects of the immune system and in children and adults with acquired defects secondary to malignancy or its treatment. The relationship of P. carinii pneumonia with immunosuppression was established in organ transplant cases. In the 1980s, P. carinii pneumonia was detected in previously healthy men, which led to a search for an underlying cause of immunosuppression, and the pandemic known as acquired immunodeficiency syndrome (AIDS) was defined. Those most at risk from P carinii pneumonia were identified on the basis of the degree of human immunodeficiency virus (HIV)–induced immunosuppression as measured by a CD4 T lymphocyte count less than 200. In the United States, Australia, and Europe, P carinii pneumonia in HIV-infected patients is seen largely in those unaware of their HIV serostatus at presentation or in those noncompliant with or intolerant of prophylaxis and antiretroviral therapy.13 In sub-Saharan Africa, recent illuminating autopsy studies have shown that P carinii pneumonia contributes significantly to the overall death toll in the HIV-infected population, especially children.14,15 In the non-HIV–associated immunosuppressed patients, P carinii infection remains a significant cause of morbidity and mortality.16–18
The taxonomic position of P carinii is still uncertain. Pneumocystis carinii is sometimes called an orphan organism because its phylogenetic position has not yet been established, perhaps because of lack of generally accepted criteria for the classification of fungi and protozoa.19 It has been suggested that it should be classified as fungus, because the ribosomal RNA is homologous to that found in fungi.20,21 A comparative study of small subunits of ribosomal RNA (16S-like rRNA) of P carinii and the fungus Saccharomyces cerevisiae was carried out. The similarities between P carinii and S cerevisiae were consistent with a close evolutionary linkage between Pneumocystis and fungi.20 These small subunits of ribosomal RNA are used to establish phylogenetic relationships, because they have the same functions in all organisms.22 Furthermore, P carinii organisms share staining characteristics of fungi by showing affinity for Grocott's methenamine silver stain.
In contrast, some investigators think that Pneumocystis is more closely related to protozoa than fungi. Chagas9 and Carinii23 thought P carinii to be a protozoan in view of its association with Trypanosoma cruzi and Trypanosoma lewisi. However, ultrastructural studies of P carinii failed to demonstrate the cytoskeletal elements and complex organelle systems, peculiar to protozoa.24 In addition P carinii shows surface antigenic variation, which occurs in protozoa and bacteria, which has not been described in fungi. The major antigen of P carinii is major surface glycoprotein (MSG), including gpA and gp120.25 The MSG is encoded by a family of approximately 100 genes, each of which expresses a different isoform of MSG.26 The antigenic variation may be important for host immune evasion.
Some workers are of the opinion that P carinii should be recognized by the name of genus of the host species from which it is isolated; for example, P carinii derived from human beings is identified as Pneumocystis carinii hominis and from rats as Pneumocystis carinii rattus.27 The host specificity is further emphasized by the fact that cross-infection between different host species is not possible and that P carinii infection in man is not from other animals.28,29
The morphological characteristics of P carinii are quite constant in all mammalian species. Generally, 4 morphological forms are identified: trophozoites, cysts, precysts, and sporozoites (also known as intracystic bodies). The trophozoites are pleomorphic and measure 2 to 4 μm. Under electron microscopy, trophozoites appear as unicellular structures with a thin wall or pellicle and sometimes have 2 or more nuclei.
The cyst is the diagnostic form of P carinii and stains with Giemsa, Papanicolaou, and Grocott methenamine silver nitrate stains and immunocytochemical techniques using monoclonal antibodies. Giemsa- and Papanicolaou- stained smears show an indirect evidence of P carinii infection by the demonstration of foamy exudate in the form of alveolar casts as shown in Figures 1 and 2. This makes the Grocott methenamine silver nitrate and immunocytochemical stains mandatory to confirm the cysts of P carinii (Figures 3 through 5).
In the light microscope, the cyst appears as a spherical, cup-shaped, or crescent-shaped object measuring 4 to 8 μm in diameter.30 Some cysts are empty and collapsed; others contain dark bodies or dots in silver-stained preparations as shown in Figure 3. The dark bodies or dots are focal thickenings of the cyst wall (Figure 6). The dark bodies acquire different positions in relation to the cyst depending on the angle of visualization. Hence, dark bodies or dots should not be confused with sporozoites. Serial ultrathin sectioning revealed an irregular round pore in the thickened area of the cyst wall of P carinii, which is thought to be used for the release of sporozoites into external environment (Figure 6).31 Phase contrast and electron microscopy have revealed up to 8 sporozoites in the cyst.24,32 In transmission electron microscopy, the cysts exhibit various shapes depending on whether or not they contain sporozoites (Figure 7). The cysts with sporozoites in them have a spherical structure, with a pellicle (70–140 nm) consisting of 3 layers: the inner plasmalemma, an electron lucent middle layer, and electron dense outer layer (Figure 8). Empty cysts are crescent- or cup-shaped (Figure 7). In contrast to trophozoites, cysts have a smooth surface. However, the cysts might have attached membranotubular extensions from the neighboring trophozoites because of their intimate relationship.
CLINICAL DISEASE AND PRESENTATION
Clinical disease was thought to be due to reactivation of childhood latent infection in immunosuppressed cases. In the recent past, studies have shown a de novo infection.33 This is supported by the fact that P carinii infection may be transmitted among animals and human beings by airborne route, as P carinii DNA has been detected in the air.34,35 Using internal transcribed spacers (ITS) genotyping, different ITS types of P carinii were identified in most episodes of P carinii pneumonia. The same ITS types were observed in patients immunosuppressed as a result of HIV infection or malignancies and who were sharing the same room or who had prolonged close ward contact.36 However, there are as yet insufficient data to suggest isolation of P carinii pneumonia cases from susceptible patients.
Pneumocystis carinii causes a potentially severe and fatal pneumonia in patients with compromised immune status caused by HIV infection, chemotherapeutic regimes for malignancies, immunosuppressed therapy in organ transplant cases,37 and congenital immune diseases.38,39
Clinically, patients with P carinii pneumonia present with several weeks' history of dry cough, fever with sweats, and difficulty in taking a deep breath.40 Haemoptysis has also been recorded as a presenting feature.41 Clinical examination often reveals an increased respiratory rate, tachycardia, cyanosis, and fine crackles on auscultation of the chest.42 Reduced transfer factor and vital capacity and reduction in total lung capacity have been described.43 Arterial blood gas measurement is mandatory in order to determine the presence and indicate extent of respiratory failure.44 A chest radiograph shows diffuse bilateral infiltrates. However, the chest x-ray occasionally shows multiple nodules.45
Bronchoalveolar lavage (BAL) and induced sputum samples are frequently used for the investigation of P carinii pneumonia.49,50 However, open lung biopsy is considered as a gold standard, but is rarely performed in the current clinical practice.
The routine cytological staining techniques (Giemsa, Papanicolaou, and Grocott's methenamine silver nitrate) are used on BAL and induced sputum samples for the detection of P carinii infection. The specialized techniques such as immunocytochemistry and polymerase chain reaction (PCR) are also used with slightly more promising results. However, these techniques are laborious and not cost-effective. Thus, most of the routine clinical laboratories would resort to such specialized techniques for patients having negative results on routine stained smears and showing high index of suspicion for P carinii pneumonia on clinical grounds. Wazir et al51 carried out a comparative study of the routine cytological techniques and immunocytochemical technique using 3F6 monoclonal antibody for the detection of P carinii in BAL and induced sputum samples. Their specificity, sensitivity, and predictive values were determined. All 4 techniques were highly specific for the detection of P carinii pneumonia. However, the immunocytochemical technique was 2.5% more sensitive than Grocott and 7.6% more sensitive than Giemsa stain.51 A similar study by Tuncer et al52 was performed by comparing the sensitivity of routine cytological staining techniques with immunocytochemistry and PCR using induced sputum samples from 30 immunosuppressed patients. They found P carinii infection in 8 patients using immunocytochemistry and PCR, of whom 4 were also detected by routine staining methods.
PNEUMOCYSTIS CARINII PNEUMONIA
Co-trimoxazole used in full dose is a first-line treatment of P carinii pneumonia.54 This drug is routinely used for 21 days for the treatment of P carinii pneumonia in HIV-infected cases. However, 14 to 17 days of treatment is recommended for non-HIV–associated P carinii pneumonia. The incidence of adverse drug reaction to co-trimoxazole, such as rash, fever, and abnormalities of liver function tests, is more common in HIV-associated cases compared with non-HIV–associated cases. Patients with solid organ transplantation such as kidney and liver transplants are more likely to develop nephrotoxicity with co-trimoxazole.54 In such cases, second-line treatment with intravenous administration of pentamidine or clindamycine with primaquine or of dapsone with trimethoprim is recommended.54 Atovaquone and trimetrexate with folinic acid may be used as a third line of treatment for P carinii pneumonia.
Some are of the opinion that addition of glucocorticosteroids to the treatment regimes of cases with HIV-associated P carinii pneumonia has shown to reduce the risk of respiratory failure and subsequent need for mechanical ventilation followed by death.3–5 However, Pareja et al55 suggested that all the treatment regimes for P carinii pneumonia must be supplemented by glucocorticosteroid. Such cases treated for P carinii pneumonia by routine drugs in conjunction with glucocorticosteroids have shown shorter duration of mechanical ventilation, length of stay on the intensive care unit, and overall hospitalization compared with patients without glucocorticosteroids in the treatment regimes. Based on these findings, it is suggested that glucocorticosteroids may accelerate recovery in adults with severe non-HIV–associated cases of P carinii pneumonia.55
The mortality rate among individuals with non-HIV– associated P carinii pneumonia is high, ranging from 34% to 49%, in comparison with patients with HIV-associated Pneumocystis infection.16–18 This may either be due to continuous and effective monitoring of CD4 T-lymphocyte count in HIV-infected cases or the continuous use of glucocorticosteroids in non-HIV–associated cases.
Immunosuppressed patients due to HIV infection or other non-HIV causes of immunosuppression who are at high risk of developing P. carinii pneumonia should be started on prophylactic treatment. Patients with HIV infection showing a CD4 lymphocyte cell count of less than 200 should receive prophylaxis. Sepkowitz and colleagues37,56 suggested that immunosuppression induced by chemotherapy or radiotherapy, or patients with inflammatory diseases receiving glucocorticosteroids 20 mg/day or more for 4 weeks or more should receive prophylaxis.
Co-trimoxazole is the drug of first choice for prophylaxis in daily or intermittent doses.57 In the event of adverse reactions to co-trimoxazole, alternative agents such as dapsone with or without pyrimethamine and nebulized pyrimidine may be used as prophylaxis.58
The possibility of immunization of patients at high risk of P carinii pneumonia may be used in the future as prophylaxis. This is supported by the fact that successful attempts have been made to elicit protective antibody responses to major surface glycoprotein immunization in an immunosuppressed rat model.59 The immunization with major surface glycoprotein may protect patients at high risk of P carinii pneumonia.59 The candidates for immunization include newly diagnosed cases of malignancy, pre–organ transplant cases, or cases with HIV infection having a CD4 lymphocyte count still in the normal range.59
The incidence of P carinii pneumonia can significantly be reduced with effective use of prophylaxis and early detection of cases at high risk of P carinii infection. The experimental work on immunization for P carinii infection is still in the early stages, which may be taken as a research challenge for further investigation into this problem.
The authors have no relevant financial interest in the products or companies described in this article.