Blood and Tissue Protozoa

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Blood protozoa of major clinical significance include members of genera Trypanosoma (T. brucei and T. cruzi); Leishmania (L. donovani, L. tropica and L. braziliensis); Plasmodium (P. falciparum, P. ovale, P. malariae and P. vivax); Toxoplasma gondii; and Babesia (B. microti).

TRYPANOSOMIASIS

African trypanosomiasis (Sleeping sickness)

Etiology

There are two clinical forms of African trypanosomiasis: 1) a slowly developing disease caused by Trypanosoma brucei gambiense and 2) a rapidly progressing disease caused by T. brucei rhodesiense.

Epidemiology

T. b. gambiense is predominant in the western and central regions of Africa, whereas T. b. rhodesiense is restricted to the eastern third of the continent. 6,000 to 10,000 human cases are documented annually. 35 million people and 25 million cattle are at risk. Regional epidemics of the disease are cause of major health and economic disasters.

Morphology

T. b. gambiense and T. b. rhodesiense are similar in appearance: The organism measures 10-30 micrometers x 1-3 micrometers. It has a single central nucleus and a single flagellum originating at the kinetoplast and joined to the body by an undulating membrane. The outer surface of the organism is densely coated with a layer of glycoprotein, the variable surface glycoprotein (VSG).

Life cycle

The infective, metacyclic form of the trypanosome is injected into the primary host during a bite by the vector, the tsetse fly. The organism transforms into a dividing trypanosomal (trypomastigote) blood form as it enters the draining lymphatic and blood stream. The trypanosomal form enters the vector during the blood meal and travels through the alimentary canal to the salivary gland where it proliferates as the crithidial form (epimastigote) and matures to infectious metacyclic forms. Trypomastigotes can traverse the walls of blood and lymph capillaries into the connective tissues and, at a later stage, cross the choroid plexus into the brain and cerebrospinal fluid. The organism can be transmitted through blood transfusion.

Symptoms

The clinical features of Gambian and Rhodesian disease are the same, however they vary in severity and duration. Rhodesian disease progresses more rapidly and the symptoms are often more pronounced. The symptoms of the two diseases are also more pronounced in Caucasians than in the local African population. Classically, the progression of African trypanosomiasis can be divided into three stages: the bite reaction (chancre), parasitemia (blood and lymphoid tissues), and CNS stage.

Bite reaction: A non-pustular, painful, itchy chancre forms 1-3 weeks after the bite and lasts 1-2 weeks. It leaves no scar.

Parasitemia: Parasitemia and lymph node invasion is marked by attacks of fever which starts 2-3 weeks after the bite and is accompanied by malaise, lassitude, insomnia headache and lymphadenopathy and edema. Painful sensitivity of palms and ulnar region to pressure (Kerandel's sign) may develop in some Caucasians. Very characteristic of Gambian disease is visible enlargement of the glands of the posterior cervical region (Winterbottom's sign). Febrile episodes may last few months as in Rhodesian disease or several years as in Gambian disease. Parasitemia is more prominent during the acute stage than during the recurrence episodes.

CNS Stage: The late or CNS stage is marked by changes in character and personality. They include lack of interest and disinclination to work, avoidance of acquaintances, morose and melancholic attitude alternating with exaltation, mental retardation and lethargy, low and tremulous speech, tremors of tongue and limbs, slow and shuffling gait, altered reflexes, etc. Males become impotent. There is a slow progressive involvement of cardiac tissue. The later stages are characterized by drowsiness and uncontrollable urge to sleep. The terminal stage is marked by wasting and emaciation. Death results from coma, intercurrent infection or cardiac failure

The clinical features of Rhodesian disease are similar but briefer and more acute. The acuteness and severity of disease do not allow typical sleeping sickness. Death is due to cardiac failure within 6-9 months.

Pathology and Immunology

An exact pathogenesis of sleeping sickness is not known, although immune complexes and inflammation have been suspected to be the mechanism of damage to tissues. The immune response against the organism does help to eliminate the parasite but it is not protective, since the parasite has a unique ability of altering its antigens, the VSG. Consequently, there is a cyclic fluctuation in the number of parasites in blood and lymphatic fluids and each wave of parasite represents a different antigenic variant. The parasite causes polyclonal expansion of B lymphocytes and plasma cells and an increase in total IgM concentration. It stimulates the reticuloendothelial function. It also causes severe depression of cell mediated and humoral immunity to other antigens.

Diagnosis

Detection of parasite in the bloodstream, lymph secretions and enlarged lymph node aspirate provides a definitive diagnosis in early (acute) stages. The parasite in blood can be concentrated by centrifugation or by the use of anionic support media. Cerebrospinal fluid must always be examined for organisms. Immuno-serology (enzyme-linked immune assay, immunofluorescence) may be indicative but does not provide definite diagnosis.

Treatment and Control

The blood stage of African trypanosomiasis can be treated with reasonable success with Pentamidine isethionate or Suramin. These drugs have been reported also to be effective in prophylaxis although they may mask early infection and thus increase the risk of CNS disease. Cases with CNS involvement should be treated with Melarsoprol, an organic arsenic compound.

The most effective means of prevention is to avoid contact with tsetse flies. Vector eradication is impractical due to the vast area involved. Immunization has not been effective due to antigenic variation.

American trypanosomiasis (Chagas' disease)

Etiology

Chagas' disease is caused by the protozoan hemoflagellate, Trypanosoma cruzi.

Epidemiology

American trypanosomiasis, also known as Chagas' disease, is scattered irregularly in Central and South America, stretching from parts of Mexico to Argentina. Rare cases have been reported in Texas, California and Maryland. It is estimated that 16-18 million people are infected by the parasite and 50 million are at risk. About 50,000 people die each year from the disease.

Morphology

Depending on its host environment, the organism occurs in three different forms. The trypanosomal (trypomastigote) form, found in mammalian blood, is 15 to 20 microns long and morphologically similar to African trypanosomes. The crithidial (epimastigote) form is found in the insect intestine. The leishmanial (amastigote) form, found intracellularly or in pseudocysts in mammalian viscera (particularly in myocardium and brain), is round or oval in shape, measures 2-4 microns and lacks a prominent flagellum.

Life cycle

The organism is transmitted to mammalian host by many species of kissing (riduvid) bug, most prominently by Triatoma infestans, T. sordida, Panstrongylus megistus and Rhodnius prolixus. Transmission takes place during the feeding of the bug which normally bites in the facial area (hence the name, kissing bug) and has the habit of defecating during feeding. The metacyclic trypamastigotes, contained in the fecal material, gain access to the mammalian tissue through the wound which is often rubbed by the individual that is bitten. Subsequently, they enter various cells, including macrophages, where they differentiate into amastigotes and multiply by binary fission. The amastigotes differentiate into non-replicating trypomastigotes and the cells rupture to release them into the bloodstream. Additional host cells, of a variety of types, can become infected and the trypomastigotes once again form amastigotes inside these cells. Uninfected insect vectors acquire the organism when they feed on infected animals or people containing trypomastigotes circulating in their blood. Inside the alimentary tract of the insect vector, the trypomastigotes differentiate to form epimastigotes and divide longitudinally in the mid and hindgut of the insect where they develop into infective metacyclic trypomastigotes. Transmission may also occur from man to man by blood transfusion and by the transplacental route.

More than one hundred mammalian species of wild and domestic animals including cattle, pigs, cats, dogs, rats, armadillo, raccoon and opossum are naturally infected by T. cruzi and serve as a reservoir.

Symptoms

Chagas' disease can be divided into three stages: the primary lesion, the acute stage, and the chronic stage. The primary lesion, chagoma, appearing at the site of infection, within a few hours of a bite, consists of a slightly raised, flat non-purulent erythematous plaque surrounded by a variable area of hard edema. It is usually found on the face, eyelids, cheek, lips or the conjunctiva, but may occur on the abdomen or limbs. When the primary chagoma is on the face, there is an enlargement of the pre- and post- auricular and the submaxillary glands on the side of the bite. Infection in the eyelid, resulting in a unilateral conjunctivitis and orbital edema (Ramana's sign), is the commonest finding.

Acute Stage: The acute stage appears 7-14 days after infection. It is characterized by restlessness, sleeplessness, malaise, increasing exhaustion, chills, fever and bone and muscle pains. Other manifestations of the acute phase are cervical, axillary and iliac adenitis, hepatomegaly, erythematous rash and acute myocarditis. There is a general edematous reaction associated with lymphadenopathy. Diffuse myocarditis, sometimes accompanied by serious pericarditis and endocarditis, is very frequent during the initial stage of the disease. In children, Chagas' disease may cause meningo-encephalitis and coma. Death occurs in 5-10 percent of infants. Hematologic examination reveals lymphocytosis and parasitemia.

Chronic Stage: The acute stage is usually not recognized and often resolves with little or no immediate damage and the infected host remains an asymptomatic carrier. An unknown proportion (guessed at 10-20%) of victims develop a chronic disease. They alternate between asymptomatic remission periods and relapses characterized by symptoms seen in the acute phase. Cardiac arrhythmia is common. The chronic disease results in an abnormal function of the hollow organs, particularly the heart, esophagus and colon. The cardiac changes include myocardial insufficiency, cardiomegaly, disturbances of atrio-ventricular conduction and the Adams-Stoke syndrome. Disturbances of peristalsis lead to megaesophagus and megacolon.

Pathology and Immunology

The pathological effects of acute phase Chagas' disease largely result from direct damage to infected cells. In later stages, the destruction of the autonomic nerve ganglions may be of significance. Immune mechanisms, both cell mediated and humoral, involving reaction to the organism and to autologous tissues have been implicated in pathogenesis.

T. cruzi stimulates both humoral and cell mediated immune responses. Antibody has been shown to lyze the organism, but rarely causes eradication of the organism, perhaps due to its intracellular localization. Cell mediated immunity may be of significant value. While normal macrophages are targeted by the organism for growth, activated macrophages can kill the organism. Unlike T. brucei, T. cruzi does not alter its antigenic coat. Antibodies directed against heart and muscle cells have also been detected in infected patients leading to the supposition that there is an element of autoimmune reaction in the pathogenesis of Chagas' disease. The infection causes severe depression of both cell mediated and humoral immune responses. Immunosuppression may be due to induction of suppressor T-cells and/or overstimulation of macrophages.

Diagnosis

Clinical diagnosis is usually easy among children in endemic areas. Cardiac dilation, megacolon and megaesophagus in individuals from endemic areas indicate present or former infection. Definitive diagnosis requires the demonstration of trypanosomes by microscopy or biological tests (in the insect or mice). Antibodies are often detectable by complement fixation or immunofluorescence and provide presumptive diagnosis.

Treatment and Control

There is no curative therapy available. Most drugs are either ineffective or highly toxic. Recently two experimental drugs, Benznidazol and Nifurtimox have been used with promising results in the acute stage of the disease, however their side effects limit their prolonged use in chronic cases.

Control measures are limited to those that reduce contact between the vectors and man. Attempts to develop a vaccine have not been very successful, although they may be feasible.

LEISHMANIASIS

Etiology

Several species of Leishmania are pathogenic for man: L. donovani causes visceral leishmaniasis (Kala-azar, black disease, dumdum fever); L. tropica (L. t. major, L. t. minor and L. ethiopica) cause cutaneous leishmaniasis (oriental sore, Delhi ulcer, Aleppo, Delhi or Baghdad boil); and L. braziliensis (also, L. mexicana and L. peruviana) are etiologic agents of mucocutaneous leishmaniasis (espundia, Uta, chiclero ulcer).

Epidemiology

Leishmaniasis is prevalent world wide: ranging from south east Asia, Indo-Pakistan, Mediterranean, north and central Africa, and south and central America.

Morphology

Amastigote (leishmanial form) is oval and measures 2-5 microns by 1 - 3 microns (figure 10A-D), whereas the leptomonad measures 14 - 20 microns by 1.5 - 4 microns, a similar size to trypanosomes

Life cycle

The organism is transmitted by the bite of several species of blood-feeding sand flies (Phlebotomus) which carry the promastigote in the anterior gut and pharynx. The parasites gain access to mononuclear phagocytes where they transform into amastigotes and divide until the infected cell ruptures. The released organisms infect other cells. The sandfly acquires the organisms during the blood meal; the amastigotes transform into flagellate promastigotes and multiply in the gut until the anterior gut and pharynx are packed. Dogs and rodents are common reservoirs.

Symptoms

Visceral leishmaniasis (kala-azar, dumdum fever): L. donovani organisms in visceral leishmaniasis are rapidly eliminated from the site of infection, hence there is rarely a local lesion, although minute papules have been described in children. They are localized and multiply in the mononuclear phagocytic cells of spleen, liver, lymph nodes, bone marrow, intestinal mucosa and other organs. One to four months after infection, there is occurrence of fever, with a daily rise to 102-104 degrees F, accompanied by chills and sweating. The spleen and liver progressively become enlarged. With progression of the diseases, skin develops hyperpigmented granulomatous areas (kala-azar means black disease). Chronic disease renders patients susceptible to other infections. Untreated disease results in death.

Cutaneous leishmaniasis (Oriental sore, Delhi ulcer, Baghdad boil): In cutaneous leishmaniasis, the organism (L. tropica) multiplies locally, producing of a papule, 1-2 weeks (or as long as 1-2 months) after the bite. The papule gradually grows to form a relatively painless ulcer. The center of the ulcer encrusts while satellite papules develop at the periphery. The ulcer heals in 2-10 months, even if untreated but leaves a disfiguring scar. The disease may disseminate in the case of depressed immune function.

Mucocutaneous leishmaniasis (espundia, Uta, chiclero): The initial symptoms of mucocutaneous leishmaniasis are the same as those of cutaneous leishmaniasis, except that in this disease the organism can metastasize and the lesions spread to mucoid (oral, pharyngeal and nasal) tissues and lead to their destruction and hence sever deformity. The organisms responsible are L. braziliensis, L. mexicana and L. peruviana.

Pathology

Pathogenesis of leishmaniasis is due to an immune reaction to the organism, particularly cell mediated immunity. Laboratory examination reveals a marked leukopenia with relative monocytosis and lymphocytosis, anemia and thrombocytopenia. IgM and IgG levels are extremely elevated due to both specific antibodies and polyclonal activation.

Diagnosis

Diagnosis is based on a history of exposure to sandfies, symptoms and isolation of the organisms from the lesion aspirate or biopsy, by direct examination or culture. A skin test (delayed hypersensitivity: Montenegro test) and detection of anti-leishmanial antibodies by immuno-fluorescence are indicative of exposure.

Treatment and Control

Sodium stibogluconate (Pentostam) is the drug of choice. Pentamidine isethionate is used as an alternative. Control measures involve vector control and avoidance. Immunization has not been effective.

MALARIA

Etiology

Four Plasmodium species are responsible for human malaria These are P. falciparum, P. vivax, P. ovale and P. malariae.

Epidemiology

There are an estimated 200 million global cases of malaria leading a mortality of more than one million people per year. P. falciparum (malignant tertian malaria) and P. malariae (quartan malaria) are the most common species of malarial parasite and are found in Asia and Africa. P. vivax (benign tertian malaria) predominates in Latin America, India and Pakistan, whereas, P. ovale (ovale tertian malaria) is almost exclusively found in Africa.

Morphology

Malarial parasite trophozoites are generally ring shaped, 1-2 microns in size, although other forms (ameboid and band) may also exist. The sexual forms of the parasite (gametocytes) are much larger and 7-14 microns in size. P. falciparum is the largest and is banana shaped while others are smaller and round. P. vivax causes stippling of infected red cells

Life cycle

Malarial parasites are transmitted by the infected female anopheline mosquito which injects sporozoites present in the saliva of the insect. Sporozoites infect the liver parenchymal cells where they may remain dormant (hypnozoites) or undergo stages of schizogony to produce schizonts and merogony to produce merozoites (meronts). When parenchymal cells rupture, thousands of meronts are released into blood and infect the red cells. P. ovale and P. vivax infect immature red blood cells whereas P. malariae infects mature red cells. P. falciparum infects both. In red cells, the parasites mature into trophozoites. These trophozoites undergo schizogony and merogony in red cells which ultimately burst and release daughter merozoites. Some of the merozoites transform into male and female gametocytes while others enter red cells to continue the erythrocytic cycle. The gametocytes are ingested by the female mosquito, the female gametocyte transforms into ookinete, is fertilized, and forms an oocyst in the gut. The oocyte produces sporozoites (sporogony) which migrate to the salivary gland and are ready to infect another host. The liver (extraerythrocytic) cycle takes 5-15 days whereas the erythrocytic cycle takes 48 hours or 72 hours (P. malariae). Malaria can be transmitted by transfusion and transplacentally.

Symptoms

The symptomatology of malaria depends on the parasitemia, the presence of the organism in different organs and the parasite burden. The incubation period varies generally between 10-30 days. As the parasite load becomes significant, the patient develops headache, lassitude, vague pains in the bones and joints, chilly sensations and fever. As the disease progresses, the chills and fever become more prominent. The chill and fever follow a cyclic pattern (paroxysm) with the symptomatic period lasting 8-12 hours. In between the symptomatic periods, there is a period of relative normalcy, the duration of which depends upon the species of the infecting parasite. This interval is about 34-36 hours in the case of P. vivax and P. ovale (tertian malaria), and 58-60 hours in the case of P. malariae (quartan malaria). Classical tertian paroxysm is rarely seen in P. falciparum and persistent spiking or a daily paroxysm is more usual.

The malarial paroxysm is most dramatic and frightening. It begins with a chilly sensation that progresses to teeth chattering, overtly shaking chill and peripheral vasoconstriction resulting in cyanotic lips and nails (cold stage). This lasts for about an hour. At the end of this period, the body temperature begins to climb and reaches 103-106 degrees F (39- 41degrees C). Fever is associated with severe headache, nausea (vomiting) and convulsions. The patient experiences euphoria, and profuse perspiration and the temperature begins to drop. Within a few hours the patient feels exhausted but symptom-less and remains symptomatic until the next paroxysm. Each paroxysm is due to the rupture of infected erythrocytes and release of parasites.

Without treatment, all species of human malaria may ultimately result in spontaneous cure except with P. falciparum which becomes more severe progressively and results in death. This organism causes sequestration of capillary vasculature in the brain, gastrointestinal and renal tissues. Chronic malaria results in splenomegaly, hepatomegaly and nephritic syndromes.

Pathology and immunology

Symptoms of malaria are due to the release of massive number of merozoites into the circulation. Infection results in the production of antibodies which are effective in containing the parasite load. These antibodies are against merozoites and schizonts. The infection also results in the activation of the reticuloendothelial system (phagocytes). The activated macrophages help in the destruction of infected (modified) erythrocytes and antibody-coated merozoites. Cell mediated immunity also may develop and help in the elimination of infected erythrocytes. Malarial infection is associated with immunosuppression.

Diagnosis

Diagnosis is based on symptoms and detection of parasite in Giemsa stained blood smears. There are also antibody tests.

Treatment and Control

Treatment is effective with various quinine derivatives (quinine sulphate, chloroquine, meflaquine and primaquine, etc.). Drug resistance, particularly in P. falciparum and to some extent in P. vivax is a major problem. Control measures are eradication of infected anopheline mosquitos. Vaccines are being developed and tried but none is available yet for routine use.

BABESIOSIS

Etiology

Babesia microti is the only member of the genus that infects man.

Morphology

The trophozoite is very similar to the ring form of the Plasmodium species

Life cycle

The organism (sporozoite) is transmitted by a tick and enters the red cell where it undergoes mitosis and the organisms (merozoite) are released to infect other red cells. Ticks acquire the organism during feeding on an infected individual. In the tick, the organism divides sexually in the gut and migrates into the salivary gland.

Symptoms

Babesiosis is associated with hemolytic anemia, jaundice, fever and hepatomegaly, usually 1-2 weeks after infection.

Diagnosis

Diagnosis is based on symptoms, patient history and detection of intraerythrocytic parasite in the patient or transfer of blood in normal hamsters which can be heavily parasitized.

Treatment and Control

Drugs of choice are clindamycin combined with quinine. The patient may recover spontaneously. One should avoid tick exposure and, if bitten, remove the tick from the skin immediately.

TOXOPLASMOSIS

Etiology

Toxoplasma gondii is the organism responsible for toxoplasmosis

Epidemiology

Toxoplasma has worldwide distribution and 20%-75% of the population is seropositive without any symptomatic episode. However, the infection poses a serious threat in immunosuppressed individuals and pregnant females.

Morphology

The intracellular parasites (tachyzoite) are 3x6 microns, pear-shaped organisms that are enclosed in a parasite membrane to form a cyst measuring 10-100 microns in size. Cysts in cat feces (oocysts) are 10-13 microns in diameter.

Life cycle

The natural life cycle of T. gondii occurs in cats and small rodents, although the parasite can grow in the organs (brain, eye, skeletal muscle, etc.) of any mammal or birds. Cats gets infected by ingestion of cysts in flesh. Decystation occurs in the small intestine, and the organisms penetrate the submucosal epithelial cells where they undergo several generations of mitosis, finally resulting in the development of micro- (male) and macro- (female) gametocytes. Fertilized macro-gametocytes develop into oocysts that are discharged into the gut lumen and excreted. Oocysts sporulate in the warm environment and are infectious to a variety of animals including rodents and man. Sporozoites released from the oocyst in the small intestine penetrate the intestinal mucosa and find their way into macrophages where they divide very rapidly (hence the name tachyzoites) and form a cyst which may occupy the whole cell. The infected cells ultimately burst and release the tachyzoites to enter other cells, including muscle and nerve cells, where they are protected from the host immune system and multiply slowly (bradyzoites). These cysts are infectious to carnivores (including man). Unless man is eaten by a cat, it is a dead-end host.

Symptoms

Although Toxoplasma infection is common, it rarely produces symptoms in normal individuals. Its serious consequences are limited to pregnant women and immunodeficient hosts. Congenital infections occur in about 1-5 per 1000 pregnancies of which 5-10% result in miscarriage and 8-10% result in serious brain and eye damage to the fetus. 10-13% of the babies will have visual handicaps. Although 58-70% of infected women will give birth to a normal offspring, a small proportion of babies will develop active retino-chorditis or mental retardation in childhood or young adulthood. In immunocompetent adults, toxoplasmosis, may produce flu-like symptoms, sometimes associated with lymphadenopathy. In immunocompromised individuals, infection results in generalized parasitemia involvement of brain, liver lung and other organs, and often death.

Immunology

Both humoral and cell mediated immune responses are stimulated in normal individuals. Cell-mediated immunity is protective and humoral response is of diagnostic value.

Diagnosis

Suspected toxoplasmosis can be confirmed by isolation of the organism from tonsil or lymph gland biopsy.

Treatment

Acute infections benefit from pyrimethamine or sulphadiazine. Spiramycin is a successful alternative. Pregnant women are advised to avoid cat litter and to handle uncooked and undercooked meat carefully.

PNEUMOCYSTIS PNEUMONIA

Pneumocystis jiroveci (formerly known as Pneumocystis carinii)

Pneumocystis jiroveci was formerly thought to be a protozoan but is now known to be a fungus. It is included here because pneumocystis pneumonia is often described as an opportunistic parasitic disease.

Pneumocystis pneumonia is an infection of immunosuppressed individuals and is particularly seen in AIDS patients. The organism is pleomorphic, exhibiting, at various stages of its life cycle: 1-2 micron sporozoites, 4-5 micron trophozoites and 6-8 micron cysts. It spreads from person to person in cough droplets. Infection in immunosuppressed individuals results in interstitial pneumonia characterized by thickened alveolar septum infiltrated with lymphocytes and plasma cells. Pneumonia is associated with fever, tachypnea, hypoxia, cyanosis and asphyxia. Diagnosis is based on isolation of organisms from affected lungs. Trimethoprim-sulphamethoxazole is the treatment of choice

FACULTATIVE PARASITIC PROTOZOA

These are free-living amebae that occasionally cause serious human disease. They are of particular significance in immunocompromised hosts.

Negleria fowleri

This organism is a flagellate that may inhabit warm waters (spas, warm springs, heated swimming pools, etc.) and gain access via the nasal passage to the brain and cause encephalitis

Acanthemeba

Several species of free-living Acanthemeba are pathogenic to man. They normally reside in soil and can infect children who swallow dirt while playing on the ground. In normal individuals, the infection may cause mild disease (pharyngitis) or remain asymptomatic, but in immunodeficient individuals, the organism may penetrate the esophageal mucosa and reach the brain where it causes granulomatous encephalitis.