Abstract
Among the micro-organisms that may affect the respiratory apparatus are the protozoa. The diseases they may give rise to constitute a relatively uncommon group of respiratory ailments with, in the majority of cases, an underlying clinical situation corresponding to states of suppressed immunity (AIDS, transplants, malign haemopathies, corticotherapy, etc.). Other factors, such as visits to endemic areas and immigration, also have to be taken into account.
In view of the probable increase in the number of cases and the appearance of new emerging diseases, it is the intention of the present work to review the publications available, in different fields of medicine, that refer to the principal kinds of protozoa (Entamoeba, Acanthamoeba, Balamuthia, Leishmania, Trypanosoma, Trichomonas, Lophomonas, Cryptosporidium, Cyclospora, Toxoplasma, Plasmodium, Babesia, Encephalitozoon, Enterocytozoon and Balantidium) and, at the same time, detail and comment on the latest findings on this subject.
Protozoa are unicellular eukaryotic micro-organisms. Their morphology is varied, and their physiology and metabolism are adapted to their needs; nutrition is heterotrophic in the parasitic forms and autotrophic in the free-living ones, they have more or less complex life cycles, both free-living and parasitic, and in many cases, a vegetative form (trophozoite) and another resistant form (cyst).
Although at present there exists a tendency towards systematic unification on the basis of molecular phylogens of the ribosomic fraction of 18S 1, the established classification of the protozoa of greatest interest 2 consists of five phyla: Sarcomastigophora, Apicomplexa, Microspora, Mixozoa and Ciliophora.
The effects of protozoa on the respiratory apparatus, as opposed to other localisations (intestine, liver, vagina, urethra, etc.), constitute a group of rare diseases 3, with the existence, in most cases, of an underlying clinical situation corresponding to certain states of suppressed immunity (AIDS, transplants, malign haemopathies, corticotherapy etc.), although other factors, such as visits to endemic areas and immigration, also have to be taken into account 4.
The mechanisms by means of which the protozoa affect the respiratory apparatus may be: direct damage to the parenchyma (e.g. toxoplasmosis); through a systemic inflammatory response by haematogenous dissemination (e.g. malaria); and contiguity with an adjacent lesion (e.g. amoebiasis).
Although many of the protozoa that are of medical interest may be cultivated, this technique is little used for diagnostic purposes, enhancing the importance of microscopic visualisation. This may be achieved either by fresh examination or by the employment of specific stains (Wheatley's trichromic, Giemsa, Heidenhain, Lugol, etc.). Furthermore, together with the electron microscope, there exist more specific diagnostic methods based on immunological analysis or molecular biology tests.
It is the intention of the present work to review the publications available, in different fields of medicine, that refer to the principal kinds of protozoa that may affect the respiratory apparatus, presenting them in a single monographic paper, and at the same time detail and comment on the latest findings on this subject, one which may well, in the relatively near future, open up new fields of research in this area of respiratory pathology.
The literature review was identified through electronic databases, such as MEDLINE, EMBASE and the Cochrane Database of Systematic Reviews. The search was performed using the keywords “protozoa”, “lung” and “pulmonary infection”, and the names of each kind of protozoon implicated in the study. Peer-reviewed publications in English, French and Spanish languages, along with English-language abstracts of non-English papers identified in the present research, were included. Inclusion or exclusion of each paper was decided mainly by considering the relevance of the article, journal impact factor, number of patients included in the work and pathological findings.
PATHOGENIC AGENTS AND CLINICAL SITUATIONS
Table 1⇓ shows the different types of protozoa that may cause pathology in the respiratory apparatus. Preferred route and duration of therapy for each organism are summarised in table 2⇓.
Protozoa involved in pathology of respiratory apparatus
The most relevant first-choice drug for protozoa in respiratory pathology
Phylum Sarcomastigophora
This phylum includes the subphylum Sarcodina (amoebae) and Mastigophora (flagellates).
The general characteristic of the Sarcodina subphylum is that it presents mobile trophozoites, inconstant in shape, in the form of pseudopods, although some may present temporary flagellae. However, in the Mastigophora subphylum, the trophozoites are constant in shape and have one or two flagellae, occasionally associated with other organelles, both at the base and as undulating membranes.
Genus Entamoeba
Within this genus, the only species traditionally recognised as a pathogen for humans, who are its principal host and reservoir, is Entamoeba histolytica. This protozoon is distributed worldwide, although it is more frequent in countries where health conditions are deficient. Malnutrition, advanced age, pregnancy, immune suppression states, alcoholism and certain sexual practices are risk factors favouring its development. In the present context, visits to endemic areas (Africa, Asia and Central America) and immigration are two additional factors to be taken into account.
This is the agent responsible for clinical situations such as traveller's diarrhoea, amoebic diarrhoea and amoebic dysentery, all of which are consequences of infection of the large intestine, as well as for extraintestinal amoebiases due to trophozoites (fig. 1⇓) acceding to the blood vessels and then being carried in the bloodstream to various organs, such as the liver, lung, brain, heart, skin, etc.
Entamoeba histolytica: two trophozoites in faecal iodine-stained wet mount. Scale bar = 15 μm.
Pleuropulmonary infection by E. histolytica is, after that of the liver, the most frequent form of extraintestinal amoebiasis and occurs, in most cases, as a complication of a hepatic abscess (fig. 2a⇓ and b) due to fistulisation 6–8, with the lower lobule of the right lung being the area most frequently affected. However, the haematogenous path has also been described as a cause of pulmonary lesions due to E. histolytica 9, 10.
Amoebic hepatic abscess. a) Echographic image showing multiple cavities. b) Computed tomography scan shows that the lesion is located at the top of the right hepatic lobule, very near to the diaphragmatic cupola.
The clinical situation largely depends on the associated pulmonary pathology, frequent symptoms being fever, cough, haemoptysis and pleuritic pain 11, 12. On occasion, a hepatic abscess may provoke venous compression and be the cause of respiratory distress and alterations in arterial gasometry 13. The appearance of a purulent fluid of a chocolate-like appearance (often described as anchovy paste) following the puncture of an abscess, discharge, or through vomiting, are highly suggestive signs 14.
Radiographic findings are variable, ranging from fine basal reticular patterns, with focal areas of atelectasis, to extensive infiltrates. Elevation of the right diaphragm (fig. 3⇓), hepatobronchial fistula, lung abscess, pleural effusion or discharge and bronchopleural fistula with pyopneumothorax are other lesions described 15–17.
Thoracic amoebiasis. Thorax radiograph showing elevation of the right diaphragmatic cupola, and extensive infiltrates.
Procedures such as sputum cytology or fine-needle puncture aspiration have proved their usefulness in the diagnosis of pulmonary lesions in which the diagnosis of pulmonary amoebiasis was not initially suspected 18, 19. Moreover, in cases of pulmonary amoebiasis without data indicating intestinal and/or hepatic repercussions, which have to be distinguished from other processes, such as bacterial abscesses, neoplasias, empyema and tuberculosis 20, the combination of trophozoite visualisation through cytological sputum examination with DNA extraction techniques has proved to be of great utility 21. It is important to note that, in sputum samples, a nonpathogenic species, Entamoeba gingivalis, may appear, which is present in the oral cavity 22 and may be confused with E. histolytica 23, 24. Complementary laboratory tests, such as serological detection of antibodies, also help establish a definitive diagnosis.
Genera Acanthamoeba and Balamuthia
A series of protozoa ubiquitously present in nature (they may be isolated from earth, air and water) and which may be pathogenic for humans, whether their condition is normal or immunosuppressed, have been grouped under the denomination “free-living amoebae” 25. Knowledge of these protozoa is important for two reasons: on the one hand, they cause pathologies in various organs (brain, cornea, skin, lung, etc.), while on the other, they may harbour and transmit pathogenic bacteria 26–29, some of which, such as Legionella pneumophilia 30, 31, mycobacterias 32, 33, Chlamydia pneumoniae 34 and Francisella tularensis 35, have important repercussions on the respiratory system. This last factor may have clinical relevance, as a series of micro-organisms associated with amoebae are mentioned as agents causing nosocomial pneumonia 36.
Of the genera known at present, only four are associated with human pathology: Acanthamoeba, Balamuthia, Naegleria and Sappinia. Studies carried out on experimental animals have shown that certain species, such as Naegleria fowleri, Acanthamoeba castellanii and Acanthamoeba polyphaga may cause direct damage to the pulmonary parenchyma in the form of pneumonitis, with thickening of the alveolar walls, mononuclear inflammatory infiltrate and development of hyaline membranes in the alveolar spaces 37–40. In the case of humans, only Acanthamoeba spp. and Balamuthia mandrillaris have been related with pulmonary pathology 41–43.
Pneumonitis is the most frequent lesion, with the presence of cystic forms and trophozoites in the alveolar spaces. Radiological examination reveals diffuse areas of alveolar consolidation. The identification of trophozoites in bronchoalveolar lavage samples and their subsequent cultivation help establish a diagnosis 44, 45. In other types of samples, such as nasal exudates, the presence of amoebae (Naegleria spp. and Acanthamoeba spp.) has been described 46.
Genera Trichomonas, Tritrichomonas and Tetratrichomonas
The literature reviewed shows that the three species of Trichomonas that are pathogenic in humans (Trichomonas vaginalis, T. tenax and T. hominis) are implicated in infectious processes that affect the respiratory apparatus.
Pulmonary infection by T. vaginalis (fig. 4⇓) may appear in newborn babies 47, 48, adults presenting suppressed immunity 49, 50 and in patients with acute respiratory distress syndrome 51, 52. In the case of newborn babies there exists the antecedent of vaginal birth by mothers infected with the protozoon 53, 54. Complications may be immediate, in the form of respiratory difficulty 55, or long term 56. Detection of the presence of mobile forms of the micro-organism in secretions of the respiratory tract, and their subsequent cultivation in an appropriate medium, are procedures used in the diagnosis.
Trichomonas vaginalis: two Giemsa-stained trophozoites. Scale bar = 15 μm.
With regard to T. tenax, present in the oral cavity and frequently found in dental plaque, this is the species most frequently described in the literature reviewed. The appearance of this micro-organism is the most frequent cause of the development of a pulmonary pathology 57; factors that predispose towards its development are poor dental hygiene, malnutrition, alcoholism and prior debilitating or pulmonary diseases (carcinoma, abscess, bronchiectasis, etc.) 58, 59. In these cases, pleural discharge is the most common complication 60–63, and the presence of mobile forms is observed in fresh preparations of the fluids obtained. Giemsa staining is also quite useful. In other kinds of samples, such as bronchoalveolar lavage, its presence has been described, together with numerous eosinophils, in a patient with a history of asthma 64. Identification of the species is possible using molecular biology techniques 65, 66.
T. hominis, also known as Pentratrichomonas hominis, is the species that appears least often in the literature 67, 68. As it is a micro-organism located in the intestine, it is supposed that it may reach the respiratory apparatus by aspiration or through a bronchoenteral fistula. A necrotising pulmonary abscess and pleural effusions are the pathological situations described.
In the genera Tritrichomonas and Tetratrichomonas there are many species that may be pathogenic for various animals (sheep, pigs, cats, birds, etc.). Of these, Tritrichomonas foetus and Tetratrichomonas gallinarum are two of the most representative. It has recently been suggested that some of these species originating in animals may have become adapted to humans. Proof of this are two studies that, by means of biological tests, have demonstrated genetic sequences of these protozoa in samples from the human respiratory apparatus 69, 70. These findings have been recognised as opening up an important new field of research 3.
Genus Lophomonas
Protozoa belonging to this genus are found as symbionts in the intestine of certain arthropods, such as termites and cockroaches, contributing to the process of digestion of some materials, such as cellulose.
In the published literature, two studies from China are mentioned in which protozoa of the Hypermastigida order are related to pulmonary pathology, responding to metronidazole therapy 71, 72. Furthermore, the species Lophomonas blattarum is implicated in four cases of patients undergoing renal transplant who developed pulmonary pathology; in all these cases, therapeutic response with metronidazole was also demonstrated 73.
Protozoa forms not so far catalogued but which, from their morphological and staining characteristics, are presumed to be flagellated protozoa (fig. 5a⇓ and b), have been described in the sputum of asthmatic patients 74, nasal secretions of patients with allergic rhinitis 75 and sputa of immunodepressed patients, especially those with AIDS 76, 77.
Multiflagellated protozoa on sputum smears from AIDS patients (Papanicolaou stain). Scale bars = 20 μm.
A striking fact is the finding of similar flagellated protozoa in intestinal extracts of dust mites and cockroaches, establishing a possible nexus between these micro-organisms and respiratory allergy 78–80.
Genus Leishmania
This genus includes a great number of species, noteworthy among which are L. donovani, L. tropica, L. major and L. infantum (present in the Mediterranean basin), with a biological cycle integrated by an extracellular form or promastigote in the intestinal tract of phlebotomes (small insects similar to mosquitoes), and an intracellular form or amastigote that mainly attacks the cells of the reticuloendothelial system. Leishmaniasis is endemic in regions of Asia, Africa, Central and South America and the Mediterranean area of Europe 81.
There are three forms of leishmaniasis: cutaneous, mucocutaneous and visceral (kala-azar). Many of the forms that affect the viscera (lungs, larynx, gastrointestinal tract, etc.) are considered opportunistic infections in cases of AIDS 82, 83.
In animal experimentation models, the pulmonary lesion caused by Leishmania spp. is characterised by chronic diffuse interstitial pneumonitis with thickening of the alveolar septa due to the depositing of collagen and a cellular exudates formed principally by macrophages, lymphocytes and plasmatic cells 84. A similar pulmonary pathology involving foci of septal fibrosis and interstitial pneumonitis with a predominance of mononuclear cells has also been described in humans 85. Other forms in which the disease presents itself are granulomatous inflammation of the bronchial mucosa and mediastinal lymph nodes 86, and the development of pleural effusions with the presence of the intracellular protozoon in macrophages 87, 88. In other types of samples of the respiratory apparatus, such as transbronchial biopsy 89 and bronchoalveolar lavage 90, 91, the presence of Leishmania amastigotes has also been detected.
Infection of the mucosa of the larynx is another complication of leishmaniasis. This infection is frequently encountered in AIDS patients 92, 93, although cases have also been described in immunocompetent patients 94. In these latter cases, differential diagnosis with respect to neoplasic processes has been proposed 95; apart from the histological findings, the demonstration of specific DNA sequences in the tissues affected by means of molecular biology techniques is also very useful 96.
Direct observation of the parasite (fig. 6⇓) is one of the best methods of diagnosis and, in the case of visceral leishmaniasis, is usually carried out in bone marrow aspirates by means of haematological stains. Nowadays, molecular biology techniques are an alternative that should also be considered.
Leishmania infantum: Giemsa-stained amastigotes in a smear from the bone marrow. Scale bar = 7 μm.
The use of medicines such as pentavalent antimonial derivatives is one of the treatments of choice in leishmaniasis, although other substances such as paromomycine and liposomal amphoterycine B are effective alternatives 97.
Genus Trypanosoma
The Trypanosoma genus comprises various species of haemoflagellated protozoa transmitted to man by the bite of flies and bedbugs. Their names reflect the region of the planet they inhabit: T. brucei rhodesiense and T. brucei gambiense (Africa), responsible for sleeping sickness or African trypanosomiasis; and T. (Schizotrypanum) cruzi (South America), responsible for Chagas' disease or trypanosomiasis Americana. It is the latter that may affect the respiratory apparatus, either in organs such as the heart and oesophagus 98, or during pregnancy through the placenta (the congenital form of the disease).
In a wide study of post mortem cases both myocarditis and megaoesophagus were the cause of a series of pulmonary complications, such as pleural effusion, thromboembolism, aspiration pneumonia, pulmonary abscess, bronchiectasis and tuberculosis 99.
In experimental animals it has been demonstrated that parasitaemia due to T. cruzi is the cause of pneumonitis 100, with changes in the pulmonary parenchyma, such as thickening of the alveolar walls due to proliferation of type II pneumocytes, macrophages and mononuclear inflammatory infiltration with oedema. The alveolar spaces contain liquid, fibrin, hyaline membranes and erythrocytes. The presence of micro-organisms was observed in the walls of the bronchia accompanied by an inflammatory reaction. Micro-organisms also appeared in the walls of the large vessels.
In cases of the congenital form of the disease, pneumonitis is the most frequent lesion, with the presence of amastigotes in lungs, placenta membranes and the umbilical cord 101.
During the acute phase of Chagas' disease, numerous parasites that can be detected by direct parasitological tests appear in peripheral blood. Microscopic observation of fresh blood can easily reveal the presence of the parasite, thanks to its motility.
Blood smears in fine thin film and thick drop, appropriately stained, allow the typical morphological characteristics of the parasite to be observed (fig. 7⇓). If the degree of parasitaemia is low, it is vital to use concentration methods such as the microhaematocritic. Xenodiagnosis (visualisation of the parasite in the faeces of the vector) and haemocultivation are the established indirect methods, the sensitivity of which depends on the degree of patient parasitaemia. Moreover, there is widespread use of serological and molecular methods.
Trypanosoma brucei gambiense: Giemsa-stained trypomatigotes in a thin blood film. Scale bar = 14 μm.
Nifurtimox, a derived from nitrofurane, and benznidazole (a nitroimidazole) are practically the only medicines used to treat Chagas' disease.
Phylum Apicomplexa (Sporozoa)
These protozoa are intracellular parasites with one exclusive characteristic, the presence of what is known as the “apical complex”, an intracytoplasmic structure related with the adhesion to and invasion of the host cells. Various genera of this phylum may act on the respiratory apparatus.
Genus Cyclospora
Of the various species belonging to this genus, only Cyclospora cayetanensis is pathogenic for man; the micro-organism is acquired following the ingestion of sporulated oocysts through contaminated food and water 102. In immunocompetent individuals, depending on their immune state, infection may be asymptomatic or provoke a self-limited diarrhoea episode (cases frequently occur following visits to tropical countries). In immunodepressed patients, especially in AIDS cases, the intestinal episode is more serious and tends to become chronic.
Although effects of Cyclospora on the respiratory apparatus have not so far been identified, two cases have been documented of the presence in sputum of oocysts, corresponding to the species C. cayetanensis, with concomitant infection by tuberculosis in both cases 103, 104.
Microscopic identification of Cyclospora oocysts (round structures 7–10 μm in diameter and with a visible wall) can be carried out either in fresh samples or using special staining techniques (trichromic, modified Ziehl–Neelsen, safranine, calcofluor white, etc.). Detection methods based on the PCR technique also exist. The treatment of choice is the combination of trimethoprim plus sulfamethoxazole.
Genus Cryptosporidium
The genus Cryptosporidium comprises 13 species of intracellular enteric protozoa widely distributed in animals, especially birds, cattle and sheep. In humans, the species most often detected are C. parvum, C. hominis and C. meleagridis 105. The infection is acquired by the ingestion of oocysts (fig. 8⇓) through water and food contaminated with faecal matter, contact with animals, or from person to person.
Cryptosporidium sp. oocysts (arrowhead) and Cyclospora cayetanensis oocysts (arrow) in red in a modified acid-fast stained faecal smear. Scale bar = 10 μm.
The principal clinical expression of cryptosporidiosis is watery diarrhoea of variable duration, the process being self-limiting in immunocompetent persons, and tending to become chronic in immunodepressed patients, among whom it may prove fatal. It is among this latter group that clinical forms of extraintestinal repercussions may be observed 106.
All the documented cases of invasion of the respiratory apparatus by Cryptosporidium correspond to immunodepressed patients, the principal cause being the appearance of AIDS 107–116. Other processes also exist, such as those corresponding to malign haematological pathologies 117 and bone marrow transplant recipients 118, 119. Although the mechanism by which Cryptosporidium colonises the respiratory apparatus is not clear, aspiration of gastric content and haematogenous dissemination facilitated by macrophages originating in an intestinal focal point have been proposed as possible routes 120.
Both clinical observation and radiography tend to be imprecise, the symptoms most often referred being persistent productive cough, fever, dyspnoea and tachypnoea, in some cases requiring mechanical ventilation due to respiratory failure and hypoxaemia. On chest radiographs, the most commonly observed pattern is that of interstitial pneumonia.
Studies in experimental animals subjected to immunosuppression have demonstrated that Cryptosporidium may directly damage the respiratory epithelium 121–123, especially noteworthy being the presence of tracheitis with scaly metaplasia and submucous lymphocyte infiltration, peribronchial glandular hyperplasia, loss of cilia and both nuclear and cytoplasmatic changes. The occupation by a purulent exudate in the bronchial cavities, with numerous micro-organisms adhering to the epithelial surface, is also described. Thickening of the intra-alveolar walls with an inflammatory infiltrate based on macrophages and lymphocytes was observed and also, in the alveolar spaces and even in macrophages, free micro-organisms.
These same alterations have been described in post mortem findings from patients who died of respiratory failure; data also exist regarding diffuse alveolar damage, especially hyperplasia of type-II pneumocytes and interstitial fibrosis.
Despite this, even when the autopsy confirms the presence of Cryptosporidium in lung tissue, it is sometimes difficult to confirm its implication in death owing to the fact that other pathogens are also present in the bronchial tree 124.
The presence both of noncystic forms of the protozoon (sporozoites and merozoites), free or in macrophages, and of oocysts (with a wall, spherical in shape and having a diameter of 4–6 μm) can be demonstrated in samples such as sputum, tracheal aspirate and bronchoalveolar lavage 125. Staining methods such as the modifications of Kinyoun and Ziehl–Neelsen and staining with auramine fluorescence are useful here. Molecular biology techniques help in determining the species of the parasite 126.
Paromomycin and azithromycin are the drugs of choice for the treatment of this protozoosis 127, with inhalation a route to be borne in mind 128.
Genus Toxoplasma
Toxoplasma gondii, an obligate intracellular protozoon, is the agent causing toxoplasmosis, an infectious disease found worldwide that affects humans and many animal species alike, cats being the definitive host of the parasite. In its life cycle, T. gondii adopts three forms: oocyst, found in the intestine of the cat and excreted to the exterior in its faeces; tachyzoite, the mature form responsible for parasitaemia; and bradyzoite, which forms dormant intracellular cystic aggregates and is responsible for the immune state of the individual.
In healthy persons, toxoplasmosis causes asymptomatic infection or subclinical forms (fever and lymphadenopathies). The most serious form is congenital toxoplasmosis (contracted by the mother during pregnancy), which affects immunodepressed women and is responsible for systemic infections.
Although pulmonary toxoplasmosis may develop in immunocompetent humans 129, there is in most cases a severe deficiency in the immune system. It is the second or third most frequent systemic infection among immunodepressed persons after brain and cardiac locations 130, 131. Apart from AIDS 132–136, other processes, such as malign haematological pathologies and organ transplants, are also important causes of pulmonary toxoplasmosis 137–143. In the majority of these cases, the most widely accepted hypothesis regarding the development of pulmonary toxoplasmosis is the reactivation of a previously latent infection 144, 145, the levels of interferon-γ and the activity of alveolar macrophages being important causative factors. Furthermore, experimental studies on animals have revealed changes in the levels of interleukins segregated by T-helper type-1 lymphocytes in the lung in cases of reactivation of the infection by T. gondii 146.
The most frequent clinical symptoms are fever, cough, laboured breathing, tachypnoea and dyspnoea, with this latter, in some cases, leading to hypoxaemia with acute respiratory distress syndrome, which is one of the main causes of death among the patients 147–149.
Radiological examination usually reveals two patterns: the typical bilateral diffuse infiltrate (fig. 9⇓), indistinguishable from other processes such as pneumonia caused by Pneumocystisis jiroveci; and a thick nodular bilateral infiltrate, which may be of use in differential diagnosis 150. In general, these effects reveal interstitial pnemonitis and alveolar condensation. The existence of concomitant pleural effusions is another of the effects described. The anatomopathological examinations carried out, either in pulmonary biopsies or during necropsies, show lesions relating to the state and intensity of the infection 135. Macroscopically, the lungs appear congested, with petechial haemorrhages and areas of consolidation. Histopathology shows interstitial pneumonitis with inflammatory lymphocyte infiltrate, diffuse alveolar damage with fibrinous alveolar exudates and the formation of alveolar hyaline membranes. There are usually numerous alveolar macrophages, which contain cysts of the parasite. Necrotising pneumonia appears as a more advanced lesion, characterised by extensive areas of parenchymatose necrosis and the presence of numerous tachyzoites, both extracellular and intracellular. In the cytological smears prepared from pleural effusions and stained with the May–Grünwald–Giemsa technique, the visualisation of many tachyzoites located extracellularly and intracellularly of polymorphonuclear leukocytes and macrophages is described 151, 152.
Pulmonary toxoplasmosis in a bone marrow transplanted patient, showing bilateral diffuse infiltrates.
Various diagnostic procedures permit visualisation of the parasite in samples from the respiratory apparatus, one of the most significant being bronchoalveolar lavage 153–156 with Giemsa staining of the samples. In the sputum T. gondii also appears, this being an alternative diagnostic route if invasive techniques cannot be carried out 157. In cases in which infection is suspected, or in which the micro-organisms are not observed, techniques such as PCR may be of great use 158, 159.
The most appropriate therapeutic measures are the combination of pyrimethamine and sulfadiazine administered to prevent haematological complications, or the substitution of sulfadiazine by clindamycin. Alternatives to these combinations are azithromycin and doxycycline. Apart from therapy, in immunodepressed patients primary prevention and appropriate prophylaxis against T. gondii may significantly mitigate pulmonary repercussions 160.
Genus Plasmodium
This genus comprises the intracellular protozoa that are responsible for malaria. The disease is endemic in >90 countries, affects 200–300 million people each year and, each year, is responsible for the death of ∼3 million. It is, therefore, one of the diseases with highest morbidity and mortality. Four of the species belonging to this genus affect humans: P. falciparum, P. malariae, P. ovale and P. vivax. The disease is transmitted by the bite of the female Anopheles mosquito, and the Plasmodium has a life cycle consisting of an asexual stage in human hosts, with extraerythrocytic and intraerythrocytic phases, and a sexual stage that develops in the mosquito.
In the current authors’ environment, malaria is one of the principal imported acute diseases with pulmonary manifestations 161, 162. P. falciparum and P. vivax (fig. 10⇓) are the species that most frequently affect the lungs, while P. ovale and P. malariae are the rarest 163. The groups principally affected are children, pregnant women and travellers to countries where the disease is endemic 164.
Plasmodium vivax: infected red blood cell containing a stained trophozoite showing the typically amoeboid appearance in a Giemsa thin blood film. Scale bar = 7 μm.
Pulmonary oedema is the principal manifestation of the effects of malaria on the lung 165, especially in cases involving P. falciparum and, to a lesser extent, P. vivax 166. The increased permeability of the alveolar capillaries appears to be the principal mechanism by which the plasmatic liquid fills the alveolar spaces 167. In vitro experimental models have demonstrated that the adherence of the infected erythrocytes to the vascular epithelium, through various molecules (intercellular adhesion molecule-1, E-selectin, vascular cell adhesion molecule-1, etc.), is one of the main factors provoking capillary damage with subsequent increase in permeability 168, 169. Another frequent pulmonary complication of malaria is acute respiratory distress syndrome, in which both P. falciparum 170–172 and P. vivax 173–179 are implicated.
The clinical impact of malaria on the lung may range from mild, with fever, cough, dyspnoea, expectoration and thoracic pain 180, 181, to serious respiratory insufficiency requiring admittance to an intensive care unit 182.
Radiological examination reveals the presence of small diffuse and bilateral infiltrates, with a marked increase of the vascular net and areas of lobar consolidation 183. Associated pleural discharge may also be observed.
In the anatomopathological examination, macroscopically, the lungs appear congested and oedematose, with numerous haemorrhagic focal areas. The coexistence of pleural and/or pericardial discharge may be observed. In histological sections, pleural oedema, capillary congestion, hyaline membranes and thickening of the alveolar septa are evident. The presence of a brown pigment (haemozoin, or malarial pigment) may be observed in the interior of the alveolar macrophages. A case of obliterating bronchiolitis with organisational pneumonia associated to P. vivax is described in the literature, with therapeutic response to corticoids 184.
Visualisation of the protozoon through microscopic examination of the blood is the diagnostic method of choice, although there may be recourse to other diagnostic techniques, including PCR, for a correct identification of the species 185.
As well as depending on the appropriate support treatment to control haemodynamic and respiratory function, antimalarial drug therapy depends on a series of factors, such as the type of species, clinical state of the patient and the susceptibility of the parasite to the drugs, especially in relation to the area where the infection was acquired. One noteworthy aspect is the pulmonary toxicity that may be produced by the use of mefloquine, with the development of diffuse alveolar damage 186–188.
Genus Babesia
The Babesias are intraerythrocytic protozoa of which there exist various species, fundamentally B. divergens and B. microti. Their principal reservoirs are cattle and wild rodents. Humans acquire the infection (babesiosis), characterised fundamentally by the presence of fever and haemolysis, through tick bites. Among the factors of risk for systemic infection are immunosuppression, advanced age and antecedents of splenectomy. Babesiosis is quite a rare disease, in which the effect on the lung is, as is the case with malaria, a consequence of a systemic inflammatory response 189, which, in some cases, may be secondary to the antiprotozoa therapy employed 190, 191. Among the clinical manifestations are fever, cough and laboured breathing, with noncardiogenic pulmonary oedema being the most frequent development 192. Among the possible causes of this pulmonary complication are the lack of deformability of the infected red blood cells as they pass through the pulmonary capillaries and an increase in their adhesiveness to the vessel walls.
In the literature there are three cases in which acute respiratory distress syndrome developed as a complication of the disease 193, 194, with mechanical ventilation being required due to respiratory failure and hypoxaemia. In the radiological examinations, bilateral infiltrates with an alveolar pattern and thickening of the septa are described 195. In one of the cases, the post mortem findings showed marked congestion and pulmonary consolidation; in the histological sections, hyperplasia of type II-pneumocytes, interstitial oedema and foci of formation of hyaline membranes are observed.
The diagnosis of babesiosis is established with the observation of intraerythrocytary parasites with characteristic morphology (gemmated or “Maltese cross” shapes) in peripheral blood. Techniques such as PCR allow a more specific diagnosis. The treatment of choice is the joint administration of clindamycin and quinine sulphate.
Phylum Microspora
The protozoa belonging to this phylum are intracellular obligate parasites (they lack organs such as mitochondria and Golgi apparatus). Their life cycle consists of an infective extracellular phase and a multiplication phase in the host cell, with spores, made up of small encapsulated elements (1–4 μm in diameter, oval morphology), as the infecting forms 196. Of the eight kinds implicated in human pathology, the most frequent species are Enterocytozoon bieneusi, Encephalitozoon cuniculi, Encephalitozoon hellem and Encephalitozoon intestinalis.
In an immunocompetent person, microsporidiosis may be an asymptomatic infection or cause mild diarrhoeic episodes, while in immunodepressed patients the infection acquires particular importance, the most frequent form being gastrointestinal, although multiorganic repercussions also exist 197, 198. Oral, ocular, sexual and respiratory transmission routes are described, the latter being confirmed by the observation of micro-organisms in the sputum and in the tracheobronchial tree 199, 200.
All the documented cases of pulmonary microsporidiosis correspond to immunodepressed patients, the predisposing factors being AIDS 201–204 and bone marrow transplant recipients 205–207. Prominent among the possible mechanisms by which the micro-organisms colonise the respiratory apparatus are inhalation, regurgitation, orofaecal contamination and haematogenous dissemination from an intestinal focus 208.
Among the clinical manifestations of infection of the respiratory apparatus by microsporidia are rhinosinusitis 209, fever, persistent cough, dyspnoea and acute respiratory distress that, in some cases, developed before the patients' death due to cardiorespiratory failure.
Noteworthy in radiographic examinations is the presence of infiltrates and bilateral pulmonary opacities, along with the existence of areas of extensive consolidation.
The samples in which the micro-organism have been identified are nasal secretions, sputa, tracheobronchial aspirate and bronchoalveolar lavage (fig. 11⇓), in which it was possible to carry out staining (Weber's modified trichromic and calcofluor), immunofluorescence, electron microscopy and specific cell cultures 210, 211.
Encephalitozoon hellem: spores stained with a modified trichrome stain in bronchoalveolar lavage. Scale bar = 6 μm.
In those cases in which an anatomopathological examination was performed, the biopsies show bronchiolitis with epithelial inflammatory infiltrate due to lymphocytes. In the necropsies, the lungs presented multiple areas of abscess, especially in the subpleural area.
Although therapy depends on the species involved and the immunological state of the patient, albendazole and fumagaline are two of the compounds habitually used.
Phylum Ciliophora (ciliates)
The fundamental feature of these protozoa is the presence of cilia, along the whole of the cellular membrane or at specific locations, which are used both for movement and for food capture. Until now, the only species known to produce pathology in humans is Balantidium coli; the only species in this genus. This is one of the largest protozoa (50–300 μm in length), with an ovoid body covered with cilia (fig. 12⇓). Its principal natural habitat and reservoir is the large intestine of the pig, although it also inhabits that of humans. Although there is controversy as to the way this protozoon is acquired by humans, an important factor would seem to be some relation with pigs. Even so, person-to-person transmission should also be considered. Infection in humans fundamentally affects the colon, provoking clinical situations of variable intensity, ranging from asymptomatic forms to severe diarrhoea with ulceration of the mucosa.
Balantidium coli: trophozoite in wet mount stained with iodine. Scale bar = 200 μm.
Pulmonary infection by B. coli is quite rare, with few cases described in the literature. Among the predisposing factors are, fundamentally, immunosuppression 212, 213 and contact with pigs 214, 215, although a case has also been described of a patient with antecedents of asthma who was not immunodepressed 216. Although the mechanism by which the lungs are affected is not completely clear, haematogenous and lymphatic dissemination by ulceration of the intestine and also, in the case of perforation, through the diaphragm, seems probable. In the majority of cases, the following are among the clinical manifestations: fever, nonproductive and persistent cough, dyspnoea of variable intensity (one of the patients required assisted respiration) and thoracic pain. Noteworthy in the radiological examinations was the presence of diffuse and bilateral interstitial infiltrates. In the examination of fresh samples, whether obtained by aspiration or by bronchoalveolar lavage, the presence of numerous trophozoites is described.
Metronidazole or tetracycline therapy seems to be effective.
Statement of interest
None declared.
Acknowledgments
The authors would like to thank J. Mosquera-Osés, radiologist of the Hospital Juan Canalejo (La Coruña, Spain), for his help with radiographic images.
- Received February 12, 2008.
- Accepted May 23, 2008.
- © ERS Journals Ltd
References
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
- ↵
