Articles on Medical Diseases and Conditions

Category: Medical Parasitology

Medical Parasitology

  • Pneumocystis Carinii

    This organism is thought to be a sporozoan parasite with some similarities to Toxoplasma. Clinical infection is very frequent in patients who are immunocompromised and is rare otherwise. However, immunocompromise is selective; predominantly conditions that decrease T4 lymphocyte number or function (e.g., HIV-1 and HTLV-I infection, childhood acute lymphoblastic leukemia, and cyclosporin or corticosteroid therapy). About 70% of patients with AIDS eventually develop pneumocystis disease (range, 35%-80%), almost always confined to the lungs and with T4 lymphocyte counts below 200mm3. Infections present as pneumonia that rapidly becomes bilateral, with an x-ray pattern of the interstitial type with or without an alveolar component. At present, tissue culture is necessary for culture diagnosis. Currently, the most common means of diagnosis are nonimmunologic special stains performed on material from one of the following sources: lung transbronchial biopsies, with about 85% detected (range, 35%-98%); bronchial lavage, about 85%-90% detected (range, 59%-100%); bronchial brushing, about 40% detected (range, 12%-57%); aerosol-induced sputum, about 57% detected (range, 55%-79%); open lung biopsy, about 65% detected, and lung needle aspiration. Ordinary (noninduced) sputum is not considered an adequate source material. Sensitivity of these methods depends not only on the method but also on technique and the way the organisms are visualized (type of stain, antibody used, etc.). There are advocates for rapid staining techniques using touch preparations and stains such as toluidine blue O and also for the much more time-consuming silver stains such as methenamine silver. Each stain has been reported to detect as many as 75%-85% of cases. However, such statistics depend to a considerable extent on how the specimen was obtained and the experience of the institution. Toluidine blue O stain is much faster than silver strain but in general requires significantly more experience to interpret. Both toluidine blue O and silver methods stain pneumocystic cyst walls. Yeast cells are about the same size as pneumocystic cysts, and also stain with cyst wall stains. Differentiation between yeast and pneumocystic cysts may sometimes be difficult, especially if there are only a few organisms. Gram stain, Giemsa, or Wright’s stain used on smears can demonstrate pneumocystic trophozoites within cysts; the cyst wall does not stain. Trophozoite detection requires considerable experience and is much more time-consuming and less sensitive (in most laboratories) than cyst wall stains.

    Immunofluorescent commercial kits using either direct (applied to specimen or slides) or indirect (liquid specimen) are available, and several evaluations found their sensitivity equal to or somewhat better than various histologic stains (85%-90% sensitivity; range, 27%-97%). One study using nucleic acid probes with PCR amplification reported sensitivity of 70% compared to the IFA sensitivity of 52% on induced sputum. Another study reported 100% sensitivity.

  • Giardia Lamblia

    This protozoan lives in the duodenum and proximal jejunum and is said to be the most frequent intestinal parasite in the United States. An estimated 3%-7% of U.S. adults may have the disease. The organism is usually transmitted through fecal (sewage) contamination of water. Chloridation will not kill Giardia lamblia, but iodine will. Some reports suggest a high incidence in male homosexuals. G. lamblia can also be transmitted by fecal-oral contact, especially in day-care centers, where it has been estimated that 5%-15% of young children of diaper age become infected. Acute infection typically shows foul-smelling watery diarrhea, usually accompanied by greasy floating stools, considerable intestinal gas, and epigastric pain. The symptoms may last 3-4 days or 1-3 weeks or may become chronic. In severe cases, steatorrhea and intestinal malabsorption have been reported.

    Diagnosis. Diagnosis is made through the same type of stool examinations discussed under amebiasis. Overall detection rate of stool microscopy (direct examination plus stained slide) is usually considered to be 50%-70%. Cathartics do not increase detection rates. Permanent stain techniques detect about one-third more cases than wet mounts. Cyst excretion in the stool may be irregular; Giardia cysts may be passed at 1, 2-3, or even 7-8 day intervals. Although three specimens (one specimen collected every 2 days) are usually sufficient, more may be necessary. The first specimen detects about 75% of cases diagnosed through multiple specimens. Duodenal aspiration has been found to detect more infections (about 80% of cases) than repeated stool specimens. A commercial company has a string apparatus (Entero-Test) that can be swallowed instead of resorting to duodenal aspiration.

    Serologic tests. ELISA tests for antibody have not been widely used, as current tests do not differentiate between past or currently active infection. A commercial test (Pro Spec T Giardia) for antigen detection in stool is commercially available and in several evaluations was found to have a sensitivity of 92%-100% compared to the total positive patients by ELISA plus standard O& P examinations. A commercial indirect immunofluorescent test is also available, and the manufacturer claims 97% sensitivity. To date, I have not seen any independent evaluations.

  • Entamoeba Histolytica

    E. histolytica is a unicellular single-nucleus protozoan that is said to infect 10%-12% of the world’s population, the majority of these being in the tropics. In the United States, the population at greatest risk are travelers to third-world countries, immigrants or migrants from these areas, immunocompromised persons, and about 20%-32% of male homosexuals. The organism life cycle consists of a trophozoite free-living stage that occurs in the human colon. There are two subgroups (strains or possibly species) of E. histolytica (different by enzyme analysis). One strain can invade tissue (10% of E. histolytica infections) and causes diarrhea with or without mucosal ulceration, sometimes entering the bloodstream and producing abscesses in the liver (or occasionally other organs). The much more common, relatively nonpathogenic strain can be asymptomatic, cause mild nonspecific gastrointestinal (GI) symptoms, or cause bloodless diarrhea that usually is not severe. Both types have the same basic life cycle in which the trophozoite forms a cyst, develops four nuclei, and passes outside the body in the feces. The cyst is said to be fairly resistant to the environment and can survive up to 3 months under the right conditions. The cyst is the infective stage and produces infection after being ingested in contaminated water or on food that has been in contact with either contaminated water or soil or fertilizer contaminated with human feces. Once the cyst reaches the colon the trophozoite inside emerges and divides, eventually forming eight trophozoites. About 10% of persons infected have symptoms; of these, about 10% (range 2%-20%) develop extraintestinal amebic infection and the remainder have diarrhea or the more severe colon mucosal inflammation known as amebic dysentery. Interestingly, male homosexuals with or without HIV-1 virus infection usually do not develop the invasive type of amebiasis. Because of the two subtypes of E. histolytica, there is a wide clinical spectrum of infection: a severe acute colitis (amebic dysentery) that may resemble severe ulcerative colitis (sometimes with blood and mucus in the stool) or shigellosis; chronic diarrhea similar to milder ulcerative colitis; intermittent mild diarrhea; asymptomatic carriers; and even a group with constipation. Acutely ill patients are usually afebrile and have normal white blood cell counts and hemoglobin values; although patients with severe amebic colitis or hepatic abscess frequently have low-grade fever, leukocytosis between 10,000 and 20,000/cu mm, and mild anemia. E. histolytica usually does not produce eosinophilia.

    Stool examination. E. histolytica is more difficult to diagnose than most of the common intestinal parasites and requires special precautions. If the stool specimen is soft or liquid, it may contain motile trophozoites and should be sent to the laboratory immediately (with the time of collection noted) or placed into a special fixative, because trophozoites are fragile and quickly degenerate. Wet mounts should be done within 30 minutes after the specimen is obtained. Well-formed stools usually contain protozoan cysts rather than trophozoites and may be either temporarily refrigerated or placed into fixative. For collection procedure, three specimens, one specimen collected every other day, are more reliable than a single specimen. If results of the stool specimens on three alternate days are all negative, and if strong clinical suspicion is still present, a saline purge should be used. After a saline purge (e.g., Fleet Phosphosoda), the patient should pass liquid stools within a few hours. Oily laxatives (e.g., mineral oil or magnesia) make the stools useless for examination. Enema specimens are not advisable because they are too dilute to be of much value, and, in addition, the trophozoites may be destroyed. Barium, if present, also makes the preparation unfit to read. If stool specimens for amebae must be sent by mail, they should be placed in a preservative (one part specimen to three parts of 10% formalin). If possible, a second portion preserved in a special polyvinyl alcohol fixative (PVA) (in the same proportions) should be included along with the formalin-fixed portion. Formalin preserves ameba cysts and also eggs and larvae of other parasites. Polyvinyl alcohol fixative is used to make permanent stained preparations of protozoan trophozoites and cysts, which is not possible after formalin fixation (there is some disagreement about preservation of cysts with PVA). Stained slides considerably increase the chances of finding protozoan trophozoites and cysts and provide better cytologic detail for identification purposes.

    Serologic tests. Serologic tests for amebiasis are available in reference laboratories. The most widely used procedures are gel diffusion, IHA, and slide LA. Various ELISA methods have been reported, based on several purified or recombinant antigens from E. histolytica. In patients with intestinal amebiasis, these tests detect about 10% of those who are asymptomatic E. histolytica carriers, less than 50% of those with mild amebic diarrhea, and about 85%-90% of those with invasive amebiasis. The IHA, LA, and ELISA tests are slightly more sensitive than gel diffusion. Thus, results of the more severe cases are more likely to be positive. The IHA and LA antibody levels persist for several years, so a positive test result does not necessarily mean active infection. Gel diffusion antibodies may become undetectable in 6 months, although some have reported persistent elevation for 1-2 years. Nucleic acid probes for antigen in stools have been reported and may be available in some university medical centers or large reference laboratories.

    Extraintestinal amebiasis

    The preceding discussion was concerned with the usual type of amebiasis—amebiasis localized to the colon. Visceral amebiasis is not common. Liver involvement with abscess formation is seen in a majority of these cases. Clinical hepatic amebiasis is always associated with chronic rather than acute ameba infestation. Only 30%-50% of patients provide a history of diarrhea. Only about 25% of patients have amebae detectable in the stool. Patients with classic hepatic amebiasis have hepatomegaly, right upper quadrant pain, elevation of the right hemidiaphragm, leukocytosis as high as 20,000/mm3 (20 x 109 L), and fever. Surprisingly, the alkaline phosphatase level is normal in more than one half of patients. Liver scan is often very helpful, both for detection and localization of a lesion. Results of the various serologic tests for amebiasis are positive in 90%-95% of cases.

    Amebic encephalitis is a rare condition produced by free-living amebae of the Acanthamoeba and Naegleria species. Naegleria infection takes place in normal individuals, usually with a recent history of swimming in rivers, fresh-water lakes, or fresh-water swimming pools. Acanthamoeba infections occur in persons with decreased immunologic defenses, frequently without any history of swimming. Acanthamoeba keratitis is also being reported due to contamination of contact lenses. Diagnosis can be made through phase contrast examination of spinal fluid or permanent slides of centrifuged spinal fluid stained by Wright’s stain or trichrome stain. Centrifugation or refrigeration decreases motility of the organisms, which would hinder phase contrast examination but not permanent stained slide examination. Gram-stained smears are not recommended. Calcofluor white stain used for detection of fungus is reported to detect Acanthamoeba cysts as well (but not trophozoites).

  • Stool Examination for Ova and Parasites

    There are three standard methods of fecal examination for ova and parasites: direct examination, concentration methods followed by direct examination, and permanent stained slides prepared after concentration. Concentration techniques are useful on all types of stool specimens but especially on formed stools or soft stools. Concentration techniques detect larvae, ova, and protozoan cysts. Direct wet mounts can also be performed on all types of stools and also can detect larvae, ova, and protozoan cysts, but will not detect them as frequently as concentration techniques (in one study, only half as often). However, direct wet mount occasionally will detect some ova and protozoan cysts missed by concentration techniques. Permanent stained slide preparations are designed to detect protozoan cysts and trophozoites rather than ova or larvae, and should be performed on all very soft or liquid stools. Stained slides in addition to concentration plus direct examination are essential for optimal detection of E. histolytica and G. lamblia. In several reports, concentration techniques yielded about 50%-60% positive results for these two protozoans (literature range, 45%-74%), whereas stained smears yielded about 75%-85% positive results (literature range, 59%-95%). In another study, nonfixed direct wet mounts detected only 5% of trophozoites, whereas permanent stain detected about 60%.

  • Gastrointestinal Parasites

    Ascaris, hookworm, Strongyloides, the tapeworms, and the protozoans Giardia lamblia and Entamoeba histolytica form the majority of gastrointestinal parasites that have clinical significance in the United States. Of these, roundworms (e.g., Ascaris, Strongyloides, and Trichinella) typically are accompanied by peripheral blood eosinophilia. Diagnosis usually depends on examination of the feces for larvae or eggs. There is always a question regarding how many stool specimens should be obtained. One literature review citing studies before 1960 states that a single “ova and parasites” (O&P) specimen detected only 35%-50% of pathogens. Another in 1984 found 83% in the first specimen and 12% in the second. A study in 1989 and another in 1992 detected 90% in the first specimen. The results would be influenced by factors such as whether specimen concentration, stained permanent slides, purged or nonpurged specimens, and parasitologist or nonspecialist examination were performed. In most situations, standard recommendations state that three routine stool specimens, collected one every day and sent for ova and parasites examination is adequate (Giardia and E. histolytica may require more, if the initial specimen is negative). According to one report from a specialized parasitology laboratory, a single specimen obtained after use of magnesium sulfate cathartic detected about 80% of the most important parasites that were eventually detected using multiple specimens. The average laboratory would probably not obtain this degree of proficiency.

  • Malaria

    Malaria is a widespread cause of serious infection in Asia and Africa and may be acquired by travelers or military personnel. The standard diagnostic test is still the examination of thick and thin peripheral blood smears. It has been recommended that smears be collected just after episodes of chills and also 10 hours later. A more recent method is the QBC malaria test, based on a special tube containing patient blood and acridine orange dye, high-speed centrifugation, and examination under fluorescent light for malarial parasites that are collected just below or in the buffy coat layer.

    Babesiosis is an uncommon red blood cell infestation in the United States originally found on Nantucket island in New England. The organism responsible is a protozoan from the genus Babesia, which infects several species of animals (especially rodents) and has a tick vector. In humans, the infection is either asymptomatic or is manifested by a mild or moderate febrile illness, usually self-limited, which may be accompanied by mild hemolytic anemia. Splenectomized persons, however, develop severe illness. Diagnosis necessitates the same tests used in malaria.

  • Toxoplasmosis

    Toxoplasmosis is caused by a protozoan organism, Toxoplasma gondii. About 30%-50% (range, 3%-70%) of the U.S. population is reported to have serologic evidence of past infection. The disease is transmitted in some cases via raw or poorly cooked meat but in many cases by oocysts in feces of infected cats. The cats shed oocysts for 7-20 days after infection. The oocysts may remain infective in soil for over a year. There is also a possibility of infection from contact with cat litter box contents while the cat is shedding oocysts. Once ingested, the organisms encyst in various tissues, particularly in muscle, and remain dormant for many years or for the life of the host. Initial infection in children or adults produces clinical disease in about 10% (range, 10%-20%) of cases, usually in the form of lymphadenopathy of varying extent. There is a congenital form of clinical disease and an acquired form. The congenital form of toxoplasmosis occurs when T. gondii organisms are transmitted to the fetus through the placenta when the mother acquires active Toxoplasma infection near the time of conception or during pregnancy (infection several weeks or more before conception will not injure the fetus). Maternal acute infection during the first trimester infects 14% of fetuses; during the second trimester, 29%; and during the third trimester, 59%. One report found the highest incidence of severe fetal infections resulted from first trimester maternal infection and the least in third trimester infection. About 90% of mothers acutely infected during pregnancy are asymptomatic. Congenital toxoplasmosis is manifested most often by chorioretinitis (usually bilateral), which sometimes does not become manifest until teenage or young adulthood. Other frequent findings include brain damage (mental retardation, microcephalus or hydrocephalus, and convulsions) and intracerebral calcifications on x-ray film. Cerebrospinal fluid (CSF) is abnormal in about two thirds of patients, showing xanthochromia, mononuclear pleocytosis, and elevated protein levels (i.e., the findings usually associated with aseptic meningitis). Less frequently there is an acute neonatal disease with fever, hepatosplenomegaly, and cerebral symptoms that are clinically similar to bacterial septicemia. The more severe acute and chronic clinical disorders are more common when the mother is infected early in pregnancy, whereas infection later in pregnancy is more likely to result in symptoms (e.g., mental retardation) that are not manifest until after the neonatal period. Acquired toxoplasmosis is usually seen in older children or adults. The most common manifestations are either asymptomatic lymphadenopathy or a viral type of illness with lymphadenopathy, low-grade fever, malaise, and possibly other symptoms.

    There may (or may not) be some atypical lymphocytes, and the clinical picture may suggest “heterophil-negative” infectious mononucleosis, cytomegalovirus infection, other viral infections or even malignant lymphoma. Another type of acquired infection is associated with deficient immunologic defense mechanisms, due to either immunologic suppression therapy or a disease such as leukemia or acquired immunodeficiency syndrome (AIDS). This is actually a reactivation of previous latent Toxoplasma infestation. Cerebral infection occasionally is the first manifestation of AIDS. Overall, Toxoplasma infects the central nervous system (CNS) in 12%-31% of patients with AIDS and comprises 25%-80% of CNS infections in AIDS.

    Diagnosis

    Diagnosis requires isolation of the organisms or serologic tests for antibody formation. Culture has proved to be very difficult, and most laboratories are not equipped to do it. Tissue culture is positive in about 40% of cases. In one study, culture sensitivity was only 25%. Mouse inoculation is a form of culture method and is positive in up to 70% of cases. Lymph node biopsy frequently can suggest the diagnosis by showing small groups of histiocytes that involve germinal centers, although the pattern is not specific enough for definitive diagnosis. Histologic slides rarely show organisms in lymph nodes.

    Serologic tests form the backbone of diagnosis. The indirect fluorescent antibody (IFA) procedure is the most commonly used present-day serologic procedure. The IFA test detects the same antibody as the Sabin-Feldman methylene blue dye test, which was the original standard test for toxoplasmosis. The dye test, however, required working with live Toxoplasma organisms. The IFA procedure can be used to detect either IgM or IgG antibody. It is somewhat better in detection of IgG than IgM.

    The IFA-IgM antibody titer begins to rise about 7-10 days after infection, reaches a peak at about 8 weeks (range, 4-10 weeks) and usually becomes nondetectable 6-12 months after infection. In one study, 20% were still detectable at 12 months. High titers of IgG antibody may cause false negative IgM results. If the IgG can be separated from the IgM, the false negative reaction can be avoided. The presence of rheumatoid factor or antinuclear antibody may produce false positive results. False positive results may also be found on tests of some newborns with other congenital infections. If the IgG can be separated from the IgM, the false positive reaction can be eliminated.

    The IFA-IgG antibody titer begins to rise about 4-7 days after the IgM antibody; reaches a peak in about 8 weeks (range, 4-10 weeks), and begins to fall slowly in about 6 months. Low titers usually persist for many years. Antinuclear antibody may produce false positive results.

    Rise in titer of either IgM or IgG antibody to Toxoplasma is rapid, with considerable elevation occurring by the end of 1 week. Therefore, a low titer of IgM or IgG usually means old rather than recent infection in patients with maternal or congenital infection. However, when ocular infection is seen as an isolated finding in older children or adults, titers frequently are not high because the initial infection was congenital. Therefore, it may be difficult to tell whether the ocular disease is due to toxoplasmosis or to something else. This is made more difficult because exposure to Toxoplasma infection is very common.

    An indirect hemagglutination (IHA) test that detects only IgG antibody is used in some laboratories. It is used mainly to see if a newly pregnant woman has antibody against Toxoplasma, thus suggesting immunity to infection.

    Enzyme-linked immunosorbent assay (ELISA) methods detecting IgM, IgG, or both, as well as other methods such as latex agglutination (LA), are commercially available. Evaluations in general show 85%-100% sensitivity compared to IFA in older children and adults and 30%-77% in newborns. However, there is significant variation in different kits. Nucleic acid probes with polymerase chain reaction (PCR) amplification have also been reported.

    In immunosuppressed patients with serious toxoplasma infection, the infection often is reactivated from previous but dormant infection. In these cases, unfortunately, antibody does not increase in response to the reactivation.

    Interpretation of toxoplasma serologic results

    If a woman is pregnant and it is desirable to know if she is immune to Toxoplasma infection, a negative titer means that she is susceptible to infection. A low titer of IgG (<1:1000) or the presence of IHA antibodies usually means that she had the disease at some time in the past and is immune. Since there is a small chance that a very early infection could be present and the titer is just beginning to rise, an IFA-IgM test can then be done. If the IFA-IgM antibody titer is greatly elevated but the IFA-IgG antibody titer is still low, this indicates early acute infection with the IgG antibody just starting to rise. Alternatively, an additional specimen may be drawn 7-10 days later and retested for IFA-IgG antibody. A fourfold rising titer indicates acute infection. A stable low titer means no recent infection. If the original specimen contains a high titer of IFA-IgG or IHA antibodies, the problem is difficult. The infection could be either acute or recent (within 1-2 years) but not acute. Only the acute infection is dangerous to the fetus. A high IgM titer is suggestive of recent infection, but in some cases it may persist for several weeks or months, falsely suggesting a more recent infection. On the other hand, it may fall rapidly and thus can be negative in association with a high IFA-IgG titer, even though acute infection began only a few weeks before.

    In a newborn with possible congenital toxoplasmosis, most investigators believe that an elevated IgM level would support the diagnosis (although other infections can also produce IgM response). The IFA-IgG titer may be considerably elevated, but IgG antibody can cross the placenta; thus, maternal IgG antibody can appear in the fetal circulation if the mother has an elevated titer from either old or recent infestation. Therefore, to make a diagnosis of congenital toxoplasmosis, it is necessary to demonstrate a rising IgG titer in the infant. If no active infant infection is present and antibody is only passively acquired from the mother, the infant IgG titer, instead of rising, should fall by the sixth week as the maternally acquired antibody is gradually eliminated.