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 US. By far, the greatest impact is on residents of impoverished tropical areas with poor sanitation, but parasitic infections are encountered in developed countries among immigrants and travelers returning from endemic regions and, on occasion, even among residents who have not traveled, particularly those with AIDS 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 US and other developed 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 are single-celled organisms that multiply by simple binary division (see Extraintestinal Protozoa Overview of Free-Living Amebas Free-living amebas are protozoa that live independently in soil or water and do not require a human or animal host. They rarely cause disease, in contrast to the parasitic ameba Entamoeba... read more and Intestinal Protozoa and Microsporidia Overview of Intestinal Protozoan and Microsporidia Infections Protozoa is a loose term for certain nucleated, unicellular organisms (eukaryotes) that lack a cell wall and are neither animals, plants, nor fungi. The most important intestinal protozoan pathogens... read more ). Protozoa can multiply in their human hosts, increasing in number to cause overwhelming infection. With rare exceptions, protozoan infections do not cause eosinophilia.
Microsporidia Microsporidiosis Microsporidiosis is infection with microsporidia. Symptomatic disease develops predominantly in patients with AIDS and includes chronic diarrhea, disseminated infection, and corneal disease... read more 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 AIDS or another severe immunocompromising condition. The clinical manifestations depend on the infecting species and include gastroenteritis, involvement of the eyes, or disseminated infection.
Helminths are multicellular and have complex organ systems. Helminths can be further divided into
Flatworms (platyhelminthes), which include tapeworms (cestodes Overview of Tapeworm Infections Tapeworms (cestodes) are flat, parasitic worms. The four main intestinal cestode pathogens of humans are Taenia saginata ( beef tapeworm) Taenia solium ( pork tapeworm) Hymenolepis... read more ) and flukes (trematodes Introduction to Trematodes (Flukes) Flukes are parasitic flatworms that infect the blood vessels, gastrointestinal tract, lungs, or liver. They are often categorized according to the principle organ system they invade: Clonorchis... read more )
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 Strongyloidiasis Strongyloidiasis is infection with Strongyloides stercoralis. Findings include abdominal pain and diarrhea, rash, pulmonary symptoms (including cough and wheezing), and eosinophilia.... read more , 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 Ascariasis Ascariasis is infection with Ascaris lumbricoides or occasionally Ascaris suum (a closely related parasite of pigs). Light infections may be asymptomatic. Early symptoms are pulmonary... read more causes life-threatening acute pancreatitis Acute Pancreatitis Acute pancreatitis is acute inflammation of the pancreas (and, sometimes, adjacent tissues). The most common triggers are gallstones and alcohol intake. The severity of acute pancreatitis is... read more 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 Ascariasis Ascariasis is infection with Ascaris lumbricoides or occasionally Ascaris suum (a closely related parasite of pigs). Light infections may be asymptomatic. Early symptoms are pulmonary... read more ), hookworms Hookworm Infection Ancylostomiasis is infection with the hookworm Ancylostoma duodenale or Necator americanus. Symptoms include rash at the site of larval entry and sometimes abdominal pain or other... read more , Trichuris (trichuriasis Trichuriasis Trichuriasis is infection with Trichuris trichiura. Symptoms may include abdominal pain, diarrhea, and, in heavy infections, anemia and undernutrition. Diagnosis is by finding eggs in... read more ), and Strongyloides (strongyloidiasis Strongyloidiasis Strongyloidiasis is infection with Strongyloides stercoralis. Findings include abdominal pain and diarrhea, rash, pulmonary symptoms (including cough and wheezing), and eosinophilia.... read more ).
Cestodes (tapeworms Overview of Tapeworm Infections Tapeworms (cestodes) are flat, parasitic worms. The four main intestinal cestode pathogens of humans are Taenia saginata ( beef tapeworm) Taenia solium ( pork tapeworm) Hymenolepis... read more ) 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 Diphyllobothriasis (Fish Tapeworm Infection) Diphyllobothriasis is infection with intestinal tapeworms of the family Diphyllobothriidae. They are acquired by eating raw or undercooked freshwater fish. Treatment is with praziquantel or... read more (Diphyllobothrium latum), beef tapeworm Taenia Saginata (Beef Tapeworm) Infection Infection with the beef tapeworm, Taenia saginata, may cause mild gastrointestinal upset or passage of a motile segment in the stool. It is treated with praziquantel or niclosamide. Cattle... read more (Taenia saginata), and pork tapeworm Taenia Solium (Pork Tapeworm) Infection and Cysticercosis Taenia solium infection (taeniasis) is an intestinal infection with adult tapeworms that follows ingestion of contaminated pork. Adult worms may cause mild gastrointestinal symptoms or passage... read more (Taenia solium).
Trematodes (flukes Introduction to Trematodes (Flukes) Flukes are parasitic flatworms that infect the blood vessels, gastrointestinal tract, lungs, or liver. They are often categorized according to the principle organ system they invade: Clonorchis... read more ) 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 Schistosomiasis Schistosomiasis is infection with blood flukes of the genus Schistosoma, which are acquired transcutaneously by swimming or wading in contaminated freshwater. The organisms infect the... read more ), liver flukes including Fasciola hepatica (fascioliasis Fascioliasis Fascioliasis is infection with the liver fluke Fasciola hepatica, which is acquired by eating contaminated watercress or other water plants. Clinical manifestations include abdominal... read more ) and Clonorchis sinensis (clonorchiasis Clonorchiasis Clonorchiasis is infection with the liver fluke Clonorchis sinensis. Infection is usually acquired by eating undercooked freshwater fish. Most infections are asymptomatic, but when present... read more ), and lung flukes including certain Paragonimus species (paragonimiasis Paragonimiasis Paragonimiasis is infection with the lung fluke Paragonimus westermani and related species. Humans are infected by eating raw, pickled, or poorly cooked freshwater crustaceans. Most infections... read more ).
Diagnosis of Parasitic Infections
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 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. Experience indicates that people who have immigrated from endemic areas to developed countries and who return home to visit friends and relatives are at particular risk. They frequently do not seek or cannot afford pretravel vaccines, medications, and 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 locally acquired parasitic infection must also be considered in residents of developed countries who present with suggestive clinical syndromes, even if they have not traveled; some parasites are endemic in developed 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.
Physicians 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 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 Strongyloidiasis Strongyloidiasis is infection with Strongyloides stercoralis. Findings include abdominal pain and diarrhea, rash, pulmonary symptoms (including cough and wheezing), and eosinophilia.... read more 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 Amebiasis Amebiasis is infection with Entamoeba histolytica. It is acquired by fecal-oral transmission. Infection is commonly asymptomatic, but symptoms ranging from mild diarrhea to severe dysentery... read more 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 Cryptosporidiosis Cryptosporidiosis is infection with the protozoan Cryptosporidium. The primary symptom is watery diarrhea, often with other signs of gastrointestinal distress. Illness is typically self-limited... read more and microsporidiosis Microsporidiosis Microsporidiosis is infection with microsporidia. Symptomatic disease develops predominantly in patients with AIDS and includes chronic diarrhea, disseminated infection, and corneal disease... read more .
Serologic testing for parasitic infections
Some parasites can be detected by serologic tests (see table Serologic and Molecular Tests for Parasitic Infections Serologic and Molecular Tests for Parasitic Infections ).
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, no vaccines are yet available for prevention of human parasitic infections. Prevention is based on avoidance strategies.
Transmission of most intestinal parasites can be prevented by
Sanitary disposal of feces
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 Prevention Malaria is infection with Plasmodium species. Symptoms and signs include fever (which may be periodic), chills, rigors, sweating, diarrhea, abdominal pain, respiratory distress, confusion... read more and many other vector-borne diseases involves
Wearing long-sleeved shirts and pants
Applying diethyltoluamide (DEET)-containing insect repellants to exposed skin and permethrin to clothing
Using window screens, air-conditioning, and bed nets impregnated with permethrin or other insecticides
For residents of nonendemic areas who travel in regions where malaria is transmitted, taking prophylactic antimalarial drugs
Travelers to rural Latin America should not sleep in adobe dwellings where reduviid bugs can transmit Chagas disease Prevention Chagas disease is infection with Trypanosoma cruzi, transmitted by Triatominae bug bites or, less commonly, via ingestion of sugar cane juice or foods contaminated with infected Triatominae... read more . 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 Prevention African trypanosomiasis is infection with protozoa of the species Trypanosoma brucei, transmitted by the bite of a tsetse fly. Symptoms include characteristic skin lesions, intermittent... read more 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 2020.
The following are some English-language resources from the Centers for Disease Control and Prevention (CDC) that may be useful. Please note that THE MANUAL is not responsible for the content of these resources.