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Parasitic infections are responsible for substantial morbidity and mortality worldwide. They are prevalent in Central and South America, Africa, and Asia. They are much less common in Australia, Canada, Europe, Japan, New Zealand, and the US. By far the greatest impact is on residents of developing areas, but parasitic infections are encountered in industrialized countries in immigrants and travelers returning from endemic regions and, on occasion, even in residents who have not traveled, particularly those with AIDS or other causes of immunodeficiency.
Many parasitic infections are spread through fecal contamination of food or water. They are most frequent in impoverished areas where sanitation and hygiene are poor. Some parasites, like the hookworm, can enter the skin during contact with infected dirt or, in the case of schistosomes, with freshwater. Others, like malaria, are transmitted by arthropod vectors. On rare occasions, parasites may be transmitted via blood transfusions or shared needles or congenitally from mother to fetus.
Some parasites are endemic in the US and other industrialized countries. Examples include the pinworm, Enterobius vermicularis
, Trichomonas vaginalis
, toxoplasmosis, and enteric parasites such as Giardia
lamblia and Cryptosporidium spp.
Taxonomically, parasites can be divided into 2 major groups: protozoa, which are single-celled organisms that multiply by simple binary division (see Intestinal Protozoa; see Extraintestinal Protozoa), and helminths, or worms, that are multicellular and have complex organ systems. The helminths can be further divided into the roundworms (nematodes—see Nematodes (Roundworms)) and the flatworms (platyhelminthes), which include tapeworms (cestodes—see Cestodes (Tapeworms)) and flukes (trematodes—see Trematodes (Flukes)). Some parasites have adapted to living in the lumen of the intestine where conditions are anaerobic; others reside in the blood or tissues.
The characteristics of protozoan and helminthic infections vary in important ways. Protozoa can multiply in their human hosts, increasing in number to produce overwhelming infection. With rare exceptions, protozoan infections do not cause eosinophilia.
In contrast, helminths do not multiply in humans but can elicit eosinophilic responses when they migrate through tissue. Most helminths have complex life cycles that involve substantial time outside their human hosts. Exceptions are Strongyloides
stercoralis
, Capillaria philippinensis, and Hymenolepis nana, which can increase in number due to autoinfection. In strongyloidiasis, autoinfection can result in life-threatening, disseminated hyperinfections in immunosuppressed people, particularly those taking corticosteroids.
The severity of helminthic infections usually correlates with the worm burden, but exceptions exist such as when a single migrating Ascaris produces life-threatening pancreatitis by occluding the pancreatic duct. The worm burden depends on the degree of environmental exposure, parasite factors, and the host's genetically determined immune responses. If a person moves from an endemic area, the number of adult worms diminishes over time. Although a few parasites (eg, Clonorchis sinensis) can survive for decades, many species have life spans of only a few years or less.
Diagnosis
Parasitic infections should be considered in the differential diagnosis of clinical syndromes arising in residents of or travelers to areas where sanitation and hygiene are poor or where vector-borne diseases are endemic. For example, fever in the returning traveler may suggest the possibility of malaria. Recent experience indicates that immigrants from developing areas to industrialized countries who return home to visit friends and relatives are at particular risk. They frequently do not seek or cannot afford pretravel advice on disease prevention and are more likely to enter high-risk settings than tourists who stay at resort facilities. Although less frequent, the possibility of an endemic or imported parasitic infection must also be considered in residents of industrialized countries who present with suggestive clinical syndromes, even if they have not traveled.
Physicians with expertise in parasitic infections and tropical medicine are available for consultation at many major medical centers, travel clinics, and public health facilities. Historical information, physical findings, and laboratory data may also suggest specific parasitic infections. For example, eosinophilia is common when helminths migrate through tissue and suggests a parasitic infection in an immigrant or returning traveler.
Methods for the diagnosis of specific parasitic infections are discussed in the chapters to follow and are summarized in
Table 1: Approach to Parasitic Infections: Collecting and Handling Specimens for Microscopic Diagnosis of Parasitic Infections  . “Laboratory Identification of Parasites of Public Health Concern” provides detailed descriptions of diagnostic methods and is available from the Centers for Disease Control and Prevention (CDC) (www.dpd.cdc.gov/dpdx). Ova or parasites of protozoa and helminths that infect the GI tract typically are shed in the stool. Routine detection requires examination of stool specimens, preferably 3 collected every other day or on 3 consecutive days, because shedding can be sporadic.
Freshly passed stools uncontaminated with urine, water, dirt, or disinfectants should be sent to the laboratory within 1 h; unformed or watery stools are most likely to contain motile trophozoites. Stools should be refrigerated, but not frozen, if not examined immediately. Portions of fresh stools should also be emulsified in fixative to preserve GI protozoa. Concentration techniques can be used to improve sensitivity. Anal cellophane tape or swabs may demonstrate pinworm or tapeworm eggs. If strongyloidiasis is suspected, fresh stool should be smeared on an agar plate to identify larvae. Antibiotics, x-ray contrast material, purgatives, and antacids can hinder detection of ova and parasites for several weeks. Serologic assays, antigen detection tests (eg, Giardia lamblia or Cryptosporidium), or PCR testing may aid diagnosis (see Table 2: Approach to Parasitic Infections: Serologic and Molecular Tests for Parasitic Infections). Nonetheless, laboratory testing has sensitivity that is low enough that, when clinical suspicion is strong, empirical treatment may be given.
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Table 2
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Serologic
and Molecular Tests for
Parasitic Infections
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Parasite, Infection
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Antibody
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Antigen or DNA/RNA
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Protozoans
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African sleeping sickness
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CATT
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Amebiasis
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EIA
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EIA, PCR
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Babesiosis
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IFA
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PCR
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Chagas' disease
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IFA, EIA
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Cryptosporidiosis
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IFA, EIA, DFA, PCR
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Cyclosporiasis
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PCR
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Giardiasis
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EIA, DFA, PCR
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Leishmaniasis
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IFA, EIA
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Malaria (all species)
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IFA
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PCR
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Microsporidiosis
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PCR, IIF
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Toxoplasmosis
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IFA, EIA (IgG and IgM)
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Roundworms
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Filariasis
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EIA, PCR
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Strongyloidiasis
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EIA
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Trichinellosis
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EIA
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Toxocariasis
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EIA
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Flukes
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Paragonimiasis
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IB, EIA
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Schistosomiasis
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EIA, IB
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Tapeworms
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Cysticercosis
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IB, EIA
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Echinococcosis
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IB, EIA, IHA
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CATT = card agglutination trypanosomiasis test for T. b. gambiense; DFA = direct fluorescent antibody; EIA = enzyme immunoassay; IB = immunoblot; IFA = indirect fluorescent antibody test; IHA = indirect hemagglutination assay; IIF = immunofluorescence assay; PCR = polymerase chain reaction.
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Note: Some antigen and parasite detection kits are available commercially. Others are available at the CDC or other reference laboratories.
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Based on CDC's “Laboratory Identification of Parasites of Public Health Concern” (www.dpd.cdc.gov/dpdx).
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Sigmoidoscopy or colonoscopy should be considered when routine stool examinations are negative in patients with persistent GI symptoms who are suspected of having amebiasis. Sigmoidoscopic specimens should be collected with a curet or spoon (cotton swabs are not suitable) and processed immediately for microscopy. Duodenal aspirates or small-bowel biopsies may be necessary for the diagnosis of such infections as cryptosporidiosis and microsporidiosis.
Treatment
Advice for treating parasitic infections is available from experts at major medical and public health centers and travel clinics, in textbooks of infectious diseases and tropical medicine, and in summary form from The Medical
Letter on Drugs and Therapeutics (www.medletter.org, August 2004 issue). Drugs for unusual parasitic infections can be obtained from the manufacturer or from the CDC Drug Service.
Prevention
Despite substantial investment and research, no vaccines are yet available for prevention of human parasitic infections. Prevention is based on avoidance strategies.
Sanitary disposal of feces and provision of purified water can prevent transmission of most intestinal parasites. For the international traveler the best advice is “cook it, boil it, peel it, or forget it.” When followed, these measures substantially reduce the risk of intestinal parasitic infections as well as bacterial and viral gastroenteritis. Meat, particularly pork, and fish, particularly freshwater varieties, should be thoroughly cooked before ingestion. Other safety measures include removing litter boxes from areas where food is prepared to prevent toxoplasmosis. People should not swim in freshwater lakes, streams, or rivers in areas where schistosomiasis is endemic or walk barefoot in areas where hookworms are found.
The risk of malaria and many other vector-borne diseases can be decreased by wearing long-sleeved shirts and pants and applying N, N-diethylmetatoluamide (DEET)–containing insect repellants to exposed skin and permethrin to clothing. Window screens, air conditioning, and mosquito nets impregnated with permethrin or other insecticides provide further protection. In addition, prophylactic antimalarial drugs should be taken by those traveling in endemic regions.
Travelers to rural Latin America should not sleep in adobe dwellings where reduviid bugs can transmit Chagas' disease. In Africa, travelers should avoid bright colors and wear long-sleeved shirts and pants to avoid tsetse flies in regions where African sleeping sickness is found.
Specific recommendations for travel are provided by the CDC, in hardcopy and on the web (www.cdc.gov/travel).
Last full review/revision November 2005
Content last modified November 2005
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