Articles on Medical Diseases and Conditions

Tag: Medical Parasitology

  • Microsporidia

    Microsporidia are intracellular-living spore-forming protozoa, classified into more than 50 genera and over 600 species. They infect various animals and nonvertebrates. Some nonvertebrate hosts are mosquitos, honey bees, fish, and grasshoppers. In humans, clinical disease has only recently been noticed, and only in persons with AIDS. Here, the predominant species is Enterocytozoon bieneusi, which primarily infects epithelial cells of the small intestine (producing diarrhea), but also epithelial cells from the biliary tract, colon, pancreas, liver, eye, and probably other tissues and organs. The organism produces spores that are gram-positive, are 1.5 by 0.9 microns in size, and can be found inside or outside infected epithelial cells. About 50% of patients with AIDS develop chronic diarrhea; about 50% of these cases have no known cause; and about 30% of those with previously unknown etiology are now thought to be caused by microsporidia. Those patients with Microsporidium-induced diarrhea usually have CD4 lymphocyte counts less than 200. One study was able to find intracellular microsporidia in some AIDS patients without diarrhea.

    Diagnosis can be made by electron micrography of infected tissue epithelial cells; this technique also identifies microsporidial spores. Gram stain of tissues may show the spores. One report described a modification of the standard trichrome stain used for O&P permanent slides that can be applied to nonconcentrated stool specimens or duodenal aspirates.

  • Strongyloides Stercoralis

    Strongyloides is a small roundworm that infects the small intestine. Many cases are asymptomatic, but some infections produce diarrhea. In immunocompromised persons, especially those with AIDS, infection may spread to organs or tissues outside the intestine and become fatal. About 70%-80% of infected persons are said to have eosinophilia, but immunocompromised persons with systemic larval dissemination (“hyperinfection syndrome”) often have normal numbers of eosinophils. Diagnosis is usually made through stool examination, which contains rhabditiform larvae rather than ova. Single nonconcentrated stool examinations are said to be positive in only 30%-40% of cases, so that concentrating the specimen and (if negative) obtaining one or two more specimens on different days should be done if results of the first are negative. EIA for antibodies is available in some reference laboratories.

  • Trichomonas Vaginalis

    This motile flagellated protozoan infests the vagina and labial area in the female and the prostate and urethra of the male. Estrogen effect on vaginal epithelial cells is a predisposing factor toward infestation in females, so that trichomoniasis is uncommon before puberty or after menopause. The infection is usually transmitted during sexual intercourse, although some cases are not. In one study, 70% of men examined less than 48 hours after intercourse with an infected woman were found to also be infected. The incubation period before clinical symptoms is 4-28 days. There is a concurrent significantly higher incidence of other sexually transmitted diseases. Symptoms in the female most often consist of vaginal discharge, with the differential diagnosis including infection by Trichomonas, Gardnerella vaginalis, and Candida albicans. The disease is usually asymptomatic in males. However, one study found that 15%-20% of male urethritis resistant to standard therapy was due to Trichomonas. Also, 25%-50% of culture-positive female patients are asymptomatic, although there is some question as to whether these are true infections or the equivalent of bacterial colonization without infection.

    Culture has been considered the gold standard for diagnosis. However, culture needs special media and takes 2-7 days. Some reports indicate that culture sensitivity is 86%-97%. One report found that only 60% of male urethral cultures were positive. A wet preparation collected by swab from the posterior vaginal fornix and placed into a drop of 0.9% saline is the most simple and rapid method of diagnosis. The specimen should not be taken from the endocervix, since this area is rarely involved. If the wet preparation cannot be made and read immediately, the swab can be inoculated into a standard bacteriologic transport medium. Compared to culture, the wet preparation detects about 50%-70% (range, 15%-89%) of patients but correlates much better with symptomatic infection than does culture. One study from the Communicable Disease Center found detection rates on wet mount varied from 62%-92% from different technologists. The organism can also be detected with stained smears using either Wright’s stain or Papanicolaou stain. Sensitivity of the Papanicolaou smear compared to culture is about 50%-60% (range, 34%-70%), in addition to a considerable false positive rate in some laboratories. The diagnosis sometimes is made by accident from microscopic examination of urine sediment during a urinalysis if the organisms are still motile. Nonmotile Trichomonas organisms round up and become very difficult to differentiate from white blood cells or small epithelial cells in urine or wet-mount preparations. If the organisms appear in the urine of female patients, the urine probably was contaminated by vaginal or labial contact. Fluorescent immunoassay kits that can be applied to prepared slides are now commercially available with sensitivities compared to culture of 82%-86%. There also are commercial available enzyme immunoassay (EIA) methods applied directly to prepared slides, with sensitivity stated as 81%-82%. LA tests are recently available, but little independent evaluation has appeared. A nucleic acid probe has also been reported, with a sensitivity in one study being 83% versus culture.

  • Trichinella Spiralis

    The larvae of this small roundworm are usually ingested with raw or insufficiently cooked pork or insufficiently cooked meat products contaminated by infected pork. During the first week after ingestion, symptoms consist of nausea and diarrhea; these may be minimal or absent. About 7-8 days after ingestion there is onset of severe muscle pain, which sometimes begins in the face. Bilateral periorbital edema often develops. Eosinophilia may begin as early as 10 days after ingestion and, with muscle pain and periorbital edema, forms a very suggestive triad. Eosinophilia is present in about 90% of patients and reaches its peak during the third week. Most patients have a slight or mild leukocytosis. Most patients display elevation of creatine phosphokinase (CPK) enzyme of varying degree due to involvement of skeletal muscle. Hypoalbuminemia is also frequent. The most helpful laboratory procedures are bentonite flocculation (BF) or LA slide tests. Bentonite flocculation takes about 3 weeks after infection for results to become positive; peak titers develop in about 3 months, and titers remain elevated for several years. Reports indicate 80%-95% sensitivity by the time of maximal titer. The LA slide test detects antibody earlier than bentonite, becoming detectable in about 20%-30% of cases by day 7 after onset of symptoms and 80%-92% in 4-5 weeks. False positive results have been recorded in polyarteritis nodosa (also in tuberculosis, typhoid, and infectious mononucleosis, but these are not ordinarily considered in the differential diagnosis). ELISA is also available; peak sensitivity is about 88%. In one study, all three methods detected significantly more patients (BF 52%, LA 36%, and ELISA 16%) when repeated 3 weeks after the initial testing.

    Muscle biopsy is occasionally useful. It is considered best to wait until at least 3 weeks after infection to do this procedure in order to allow the larvae time to encyst. A painful area of a skeletal muscle has been recommended as the preferred site for biopsy.

  • Tapeworm

    The fish tapeworm Diphyllobothrium latum (Dibothriocephalus latus) is only rarely a problem in the United States. The organisms are ingested with raw pike fish from the Great Lakes area. Usually very few symptoms are produced, but occasionally the syndrome of megaloblastic anemia may result from ingestion of dietary vitamin B12 by the parasite. Diagnosis consists of stool examination for ova or detached tapeworm proglottid segments. The beef tapeworm (Taenia saginata) and the pork tapeworm (Taenia solium) also are diagnosed by stool examination for ova or proglottids.

    Tapeworm infestation in a human ordinarily occurs when the intermediate host (animal or fish) ingests tapeworm eggs or prolarvae, the egg (or prolarva) evolves to a larval form within the intermediate host, the person eats flesh from the intermediate host that contains the larva, and the larva develops into an adult worm in the lumen of the individual’s intestine. If someone ingests ova or prolarva rather than the larva, larvae may develop within the person’s intestine, proceed through the intestinal wall, and reach the bloodstream, whence they are capable of producing abnormality in various organs or tissues.

    The most publicized tapeworm larval diseases are Echinococcus or hydatid cyst infection (Echinococcus granulosus, the larva of the dog tapeworm), Cysticercus cellulosa (infection by the larva of T. solium, the swine tapeworm), and sparganosis (infection by the larva of a dog or cat tapeworm of the Spirometra genus related to the fish tapeworm D. latum). No good laboratory method for diagnosis of sparganosis is available. Enzyme immunoassays have been reported for IgM and IgG antibodies to cysticercosis, but these are available only in some reference laboratories. The laboratory may be helpful in Echinococcus (hydatid) disease, although this condition is rare in the United States. The primary host is the dog. The dog sheds ova in the feces, and material contaminated by dog feces is ingested by humans or sheep (or other animals) who act as intermediate hosts. Larvae emerge from the ova, penetrate the intestinal wall, and travel to the liver. Cystic structures (hydatid cysts) containing brood capsules filled with scolices grow in the liver (75% of cases) but may appear in the lungs or other locations. Diagnostic aids include imaging procedures such as the radioisotope liver scan, ultrasound, or computerized tomography; a skin test known as the Casoni test; a hemagglutination test; and immunofluorescent procedures. Results of the Casoni test and the hemagglutination tests are said to be positive in 90% of those patients with hepatic lesions but abnormal in less than one half of patients with cysts elsewhere. False positive results in the Casoni test are said to occur with some frequency. Therefore the Casoni test has mostly been replaced by serologic tests, such as immunofluorescence and enzyme immunoassay. These are available in reference laboratories.

  • Hookworm

    Hookworm is a problem in some areas of the southern United States and occasionally may be the cause of an iron-deficiency anemia in children. Routine stool examination for ova and parasites is usually adequate for diagnosis.

  • Pinworm (Enterobius Vermicularis or Oxyuris Vermicularis)

    Infestation with pinworms is fairly common in children. The female worm lays her eggs at night around the anal region. The best diagnostic procedure, therefore, is some method to swab the anal region thoroughly with an adhesive substance such as transparent celluloid tape (Scotch tape). The sticky surface with the eggs can then be directly applied to a glass slide and later examined microscopically for the characteristic pinworm ova. Such slides can also be sent through the mail, if necessary. Swabs lightly coated with vaseline are also useful to obtain specimens. The best time for obtaining the parasite ova is early in the morning, before the child gets up. Stool samples are less satisfactory for diagnosis of enterobiasis. Since the worms do not lay eggs every night, repeated specimens may be necessary.

  • Schistosoma Mansoni

    Schistosoma mansoni is sometimes encountered in the United States because it is endemic in Puerto Rico. Routine stools for parasite ova are often not sufficient, because the adult lays its eggs in the venous system, and the ova must penetrate the intestinal mucosa to appear in the stool. In difficult cases, proctoscopic rectal biopsy with a fresh unstained crush preparation of the biopsy specimen has been advocated. Serologic tests have been developed; the tests most commonly mentioned in the literature are complement fixation (CF) and immunofluorescence. There is some disagreement on how much help the tests can provide. The major problem is differentiation between past exposure and current active infection.

  • Leishmaniasis

    Leishmaniasis is caused by a protozoan of the genus Leishmania that includes many species, some of which cause more than one clinical syndrome. Leishmaniasis is best known in the Middle East but also occurs in the Far East (except Japan), various areas of Africa, Central and South America, and occasionally in the European side of the Mediterranean and in some islands of the Caribbean. There are three fairly well-defined syndromes: visceral (kala-azar), cutaneous (localized or widespread), and mucosal. Kala-azar is a chronic systemic disease most commonly associated with Leishmania donovani, Leishmania infantum, and Leishmania chagasi. Symptoms are fever, hepatosplenomegaly, normocytic-normochromic anemia, leukopenia, sometimes thrombocytopenia, hypergammaglobulinemia, and loss of cell-mediated immunity (delayed hypersensitivity). The reservoir of disease is dogs, wild canine species, rodents, and humans; the vector is the sandfly (Phlebotomus species in most areas but other species in South America). The organism infects mononuclear cells of the reticuloendothelial system. Incubation varies from 2 weeks to over 2 years, but most often is 3-8 months.

    Diagnosis can be made by aspiration of commonly infected organs with Giemsa or Wright’s stain of an aspirate smear to detect organisms within monocyte cytoplasm. Sensitivity in one report was said to be 98% from spleen, 54%-86% from bone marrow, and 60% from liver or lymph nodes. Various serologic tests are available in large reference laboratories or public health laboratories. ELISA or IFA (fluorescent antibody) are most frequently mentioned. There is considerable variation between these tests because the antibodies are not raised against a standard antigen preparation. The various tests cross-react to greater or lesser degree with trypanosomiasis, schistosomiasis, malaria, leprosy, and cutaneous leishmaniasis.

    Cutaneous leishmaniasis occurs in most areas that host kala-azar, and is usually subdivided into Old World and New World types. In the Americas, the major reservoir is forest small animals. A cutaneous ulcer develops at the site of the sandfly vector bite. A widespread form also exists. Mucosal leishmaniasis is the least common syndrome, occurs in Central and South America, and follows cutaneous leishmaniasis, involving the nose or mouth area. Diagnosis can be made from biopsy (with special stains for the organism); aspiration and stained smears; or serologic tests (similar to visceral leishmaniasis). Test sensitivity is said to be about 80%-90% (range, 62%-96%).

  • Cryptosporidium

    Cryptosporidium is another sporozoan organism with some similarities to Toxoplasma. It was originally found in cattle with diarrhea, where it caused diarrhea in calves (predominantly 7-14 days old but sometimes up to 30 days old). Other animals and some birds (including turkeys and chickens) also can become infected. Cryptosporidium was next reported to cause diarrhea in humans who were immunocompromised, particularly those with AIDS. Then, it was discovered that Cryptosporidium-associated diarrhea occurred in nonimmunocompromised persons, most often children, with a frequency in Western countries of 0.6%-7.3% and in developing countries of 5%-30% of patients with diarrhea. This incidence is similar to that of Giardia and the major bacterial GI pathogens. Cryptosporidium infection is also found in nonimmunocompromised persons in the cattle industry, male homosexuals, travelers in various parts of the world, and in day-care centers. The organism is rarely found in humans without diarrhea. The type found in cattle and humans (C. parvum) lives predominantly in the small intestine from which oocysts pass in the feces to act as sources of infection. Transmission is most frequently through contaminated water, although person-to-person spread has been reported in families, hospital personnel, and care centers. Cryptosporidium cysts (oocysts) are environmentally resistant and also resistant to standard water chlorination. The average incubation period is said to be about 7 days (range, 1-12 days). In nonimmunocompromised persons, illness and oocyst shedding are nearly always finished by 31 days after exposure. The most severe and persistent infections occur in human immunodeficiency virus 1 infections, particularly in AIDS and AIDS-related illnesses (about 6% of patients; range, 3%-28%). These patients have long-standing watery diarrhea, anorexia, abdominal pain, weight loss, and low-grade fever.

    Diagnosis. Diagnosis is most commonly made through stool examination. Although the organisms can be seen in standard concentrated stool preparations, they are hard to identify, are about the same size as yeast cells (4-6 µm), and are most often overlooked. Also, cyst shedding varies from day to day, and there is some correlation between the number of fecal cysts and the presence and severity of diarrhea. Permanent stained slide preparations stained with a special modification of the mycobacterial acid-fast stain has been the most common reasonably effective approach. In two large proficiency test studies, detection rates varied between 75%-96% (in specimens where the participants were instructed to look for cryptosporidia). One study using a standard stool concentration method found that detection needed 5 times as many cysts in formed stools than in liquid stools. Also, various noncryptosporidial objects or organisms in stool specimens may appear acid-fast, requiring observer experience for accurate results. Fluorescent auramine-rhodamine staining has been reported by some (but not others) to be superior to acid-fast slide stain for screening patients. One commercial company markets a kit based on fluorescent monoclonal antibody against Cryptosporidium cyst antigen contained in smears of concentrated fecal specimens on glass slides. In three evaluations to date, this method detected 91% (range, 83%-100%) of cases while Ziehl-Neelsen acid-fast stain detected 85% (range, 76%-93%) of the same cases. Specimens cannot be fixed in PVA or microimmunofluorescent (MF or MIF) stool fixatives. Two companies have recently marketed very similar ELISA kits for combined Giardia and cryptosporidia that use small wells in plastic slides and is read by visual color change. PVA stool fixative cannot be used. In the only two evaluations to date, 93%-97% of cases were detected (in the study with 93% detection, the specimens were not concentrated). One additional company has a somewhat similar kit for cryptosporidia alone; the only published full evaluation to date reported 97% sensitivity.