Human parasites are organisms that live on or in a person and derive nutrients from that person (its host). There are 3 types of parasites:
Single-cell organisms (protozoa, microsporidia)
Multicellular helminths (worms)
Ectoparasites such as scabies and lice
Parasitic infections due to protozoa and helminths 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 United States. By far, the greatest impact is on residents of resource-limited tropical areas with poor sanitation, but parasitic infections are encountered in resource-rich countries with adequate sanitation systems among immigrants and travelers returning from endemic regions and, on occasion, even among residents who have not traveled, particularly those with HIV infection or other conditions that cause immunodeficiency.
Some parasites have adapted to living in the lumen of the intestine or vagina where conditions are anaerobic; others reside in blood or tissues in aerobic conditions.
Many intestinal parasitic infections are spread through fecal contamination of food or water. They are most frequent in areas where sanitation and hygiene are poor. Some parasites, such as hookworms, can enter the skin during contact with contaminated dirt or, in the case of schistosomes, with freshwater. Others, such as malaria, are transmitted by arthropod vectors. Rarely, parasites are transmitted via blood transfusions or shared needles or congenitally from mother to fetus.
Some parasites are endemic in the United States and other resource-rich countries. Examples are the pinworm Enterobius vermicularis, Trichomonas vaginalis, Toxoplasma gondii, and enteric parasites such as Giardia intestinalis (also known as G. duodenalis or G. lamblia) and Cryptosporidium species.
The characteristics of protozoan and helminthic infections vary in important ways.
Protozoa
Protozoa are single-celled organisms that multiply by simple binary division (see Extraintestinal Protozoa and Intestinal Protozoa and Microsporidia). Protozoa can multiply in their human hosts, increasing in number to cause overwhelming infection. With rare exceptions, protozoan infections do not cause eosinophilia.
Microsporidia
Microsporidia are intracellular spore-forming organisms that used to be classified as protozoa, but genetic analysis indicates that they are fungi or closely related to them. Human disease is mainly limited to people who have HIV infection or another severe immunocompromising condition. The clinical manifestations depend on the infecting species and include gastroenteritis, involvement of the eyes, or disseminated infection.
Helminths
Helminths are multicellular and have complex organ systems. Helminths can be further divided into
Roundworms (nematodes)
Flatworms (platyhelminthes), which include tapeworms (cestodes) and flukes (trematodes)
In contrast to protozoa, 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. A few, including Strongyloides stercoralis, Capillaria philippinensis, and Hymenolepis nana, can increase in number because of autoinfection (offspring reinfect the same host rather than being shed to infect another host). In strongyloidiasis, autoinfection can result in life-threatening, disseminated hyperinfection in immunosuppressed people, particularly those taking corticosteroids.
The severity of helminthic infections usually correlates with the worm burden, but there are exceptions as when a single ascaris causes life-threatening acute pancreatitis by migrating into and obstructing 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 in humans, many species have life spans of only a few years or less.
Nematodes are nonsegmented cylindric worms ranging from 1 mm to 1 m in length. Nematodes have a body cavity, distinguishing them from tapeworms and flukes. Depending on the species, different stages in the life cycle are infectious to humans. Hundreds of millions of humans are infected with nematodes that live in the intestines and are transmitted by ova or larvae in feces; the most common are Ascaris (ascariasis), hookworms, Trichuris (trichuriasis), and Strongyloides (strongyloidiasis).
Cestodes (tapeworms) as adults are multisegmented flatworms that lack a digestive tract and absorb nutrients directly from the host’s small bowel. In the host’s digestive tract, adult tapeworms can become large, up to 40 m for one species. Tapeworms that infect humans include the fish tapeworm (Diphyllobothrium latum), beef tapeworm (Taenia saginata), and pork tapeworm (Taenia solium).
Trematodes (flukes) are nonsegmented flatworms that infect the blood vessels, liver, lungs, or gastrointestinal tract. They are usually no more than a few centimeters in length; however, some are only 1 mm, and some are as large as 7 cm. In humans, most fluke infections are caused by Schistosoma species (schistosomiasis), liver flukes including Fasciola hepatica (fascioliasis) and Clonorchis sinensis (clonorchiasis), and lung flukes including certain Paragonimus species (paragonimiasis).
Diagnosis of Parasitic Infections
Microscopic examination
Antigen and DNA tests
Methods used to diagnose specific parasitic diseases are summarized in the table Collecting and Handling Specimens for Microscopic Diagnosis of Parasitic Infections.
Parasitic infections should be considered in the differential diagnosis of clinical syndromes in residents of or travelers to areas where sanitation and hygiene are poor or where vector-borne diseases are endemic. For example, fever in a traveler returning from an endemic area suggests the possibility of malaria. People who have immigrated from endemic areas to nonendemic countries and who return home to visit friends and relatives are at particularly increased risk. They frequently do not seek pretravel vaccines, medications, and advice on disease prevention and are more likely to spend time in high-risk areas compared with tourists who stay at resort facilities (1).
Although less frequent, the possibility of locally acquired parasitic infection must also be considered in residents of countries with modern sanitation systems who present with suggestive clinical syndromes, even if they have not traveled; some parasites are endemic in these countries and others (mainly those transmissible by the fecal-oral route) may be acquired from infected travelers.
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.
The diagnosis of parasitic infections was once based on the identification of ova, larvae, or adult parasites in stool, blood, tissue or other samples or the presence of antibodies in serum, but diagnosis is being increasingly based on identification of parasite antigens or molecular tests for parasite DNA.
Clinicians with expertise in parasitic infections and tropical medicine are available for consultation at many major medical centers, travel clinics, and public health facilities.
For detailed descriptions of diagnostic methods, see the Centers for Disease Control and Prevention (CDC) Laboratory Identification of Parasites of Public Health Concern.
Gastrointestinal tract parasites
Various stages of protozoa and helminths that infect the gastrointestinal tract are typically shed in the stool. Routine detection requires examination of stool specimens, preferably 3 collected on different days, because shedding can vary. Sensitivity of stool examination for ova and parasites is low enough that when clinical suspicion is strong, empirical treatment should be considered. Sensitive and specific assays are now available to detect antigens of Giardia, Cryptosporidium, and Entamoeba histolytica in stool. Although expensive, molecular tests also are available for Giardia, Cryptosporidium, E. histolytica, and Cyclospora. Tests for one or more of these organisms are typically included in multiplex polymerase chain reaction (PCR)-based screens for enteric bacterial, viral, and parasitic pathogens in stool samples (see table Serologic and Molecular Tests for Parasitic Infections).
Freshly passed stools uncontaminated with urine, water, dirt, or disinfectants should be sent to the laboratory within 1 hour; unformed or watery stools are most likely to contain motile trophozoites. If not examined immediately, stools should be refrigerated, but not frozen. Portions of fresh stools should also be emulsified in fixative to preserve gastrointestinal protozoa. Concentration techniques can be used to improve sensitivity. Anal cellophane tape or swabs may collect pinworm or tapeworm eggs. If strongyloidiasis is suspected, one or more specialized stool tests should be done if larvae are not seen on direct examination of fresh stool. Antibiotics, x-ray contrast material, purgatives, and antacids can hinder detection of ova and parasites for several weeks.
Sigmoidoscopy or colonoscopy should be considered when routine stool examinations are negative and amebiasis is suspected in patients with persistent gastrointestinal symptoms. 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 biopsy specimens may be necessary for diagnosis of such infections as cryptosporidiosis and microsporidiosis.
Serologic testing for parasitic infections
Some parasites can be detected by serologic tests (see table Serologic and Molecular Tests for Parasitic Infections).
Reference
1. Bacaner N, Stauffer B, Boulware DR, et al: Travel medicine considerations for North American immigrants visiting friends and relatives. JAMA 291(23):2856-2864, 2004. doi:10.1001/jama.291.23.2856
Treatment of Parasitic Infections
Various treatments, depending on the specific infection
See under specific infections in THE MANUAL.
Advice for treating parasitic infections also is available from experts at major medical and public health centers and travel clinics, at the Centers for Disease Control and Prevention (CDC) web site, in textbooks of infectious diseases and tropical medicine, and in summary form from The Medical Letter on Drugs and Therapeutics.
Some drugs that are not approved by the US Food and Drug Administration for parasitic infections can be obtained from the CDC Drug Service.
Prevention of Parasitic Infections
Despite substantial investment and research, only one vaccine is currently available for prevention of human parasitic infections, and that is for malaria (see WHO recommends groundbreaking malaria vaccine for children at risk). Otherwise, prevention is largely based on avoidance strategies.
Transmission of most intestinal parasites can be prevented by
Sanitary disposal of feces
Handwashing
Adequate cooking of food
Provision of purified water
For the international traveler, the best advice is “cook it, boil it, peel it, or forget it.” When followed, these measures reduce but do not eliminate the risk of intestinal parasitic infections as well as the risk of bacterial and viral gastroenteritis. Handwashing is very important after use of bathrooms and latrines and prior to food preparation. Meat, particularly pork, and fish, especially freshwater varieties, should be thoroughly cooked before ingestion. Other safety measures include removing cat 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 or sit bare-bottom in areas where hookworms are found.
Prevention of malaria and many other vector-borne diseases involves
Wearing long-sleeved shirts and pants
For residents of nonendemic areas who travel in regions where malaria is transmitted, taking prophylactic antimalarial medications
Travelers to rural Latin America should not sleep in adobe dwellings where reduviid bugs can transmit Chagas disease. In Africa, travelers should avoid bright-colored clothing and wear long-sleeved shirts and pants to avoid tsetse flies in regions where African sleeping sickness occurs.
Country-specific recommendations for travel are available from the Centers for Disease Control and Prevention (CDC): Travelers' Health and from the CDC Yellow Book.
More Information
The following English-language resources from the World Health Organization (WHO) and Centers for Disease Control and Prevention (CDC) may be useful. Please note that THE MANUAL is not responsible for the content of these resources.
