Parasitic infections are caused by parasites. Parasites are organisms that live on or inside another organism (the host) and depend on the host for nutrients and survival.
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Parasitic infections are more common in areas with limited access to clean water and inadequate sanitation systems.
In areas with adequate sanitation systems, these infections may occur in people who have traveled from areas lacking adequate sanitation systems or in people with a weakened immune system.
Parasites usually enter the body through the mouth or skin.
Parasitic infections can affect various body systems and can cause a range of symptoms, from mild discomfort to severe illness, depending on the type of parasite, the specific organ systems involved, and the host's immunity.
Doctors diagnose the infection by taking samples of blood, stool, urine, sputum, or other infected tissue and examining or sending them to a laboratory for analysis.
Medications are available to treat most parasitic infections.
Travelers to areas where food, drink, and water may be contaminated are advised to cook it, boil it, peel it, or forget it.
There are 3 types of human parasites:
Protozoa, which are microscopic, consist of only one cell, and live inside the host
Worms (helminths), which are larger, consist of many cells, have internal organs, and live inside the host
External parasites (ectoparasites), which are small organisms, have internal organs, and live in or on the skin of the host
Protozoa reproduce by cell division (cells divide and create more cells) and can multiply inside people. The protozoa include a wide range of single-cell organisms such as Giardia and Entamoeba histolytica, which infect the intestine, and Plasmodium, which travel in the blood stream and cause malaria.
Most worms, in contrast, produce eggs or larvae that develop in the environment before they become capable of infecting and living inside of people. Development in the environment may involve an animal (an intermediate host). Worms include roundworms, such as hookworms, and flatworms, such as tapeworms and flukes.
Ectoparasites also produce eggs but they lay them on or in the skin of the host. Ectoparasites include lice, ticks (which can cause Lyme disease and Rocky Mountain Spotted Fever), mites (which can cause scabies), bedbugs, and fleas (which can cause cat-scratch disease and plague).
Parasitic infections are more common in tropical and subtropical areas, and intestinal parasites that infect the intestines, such as some protozoa and worms, are often linked to areas with inadequate sanitation. A person who visits such an area can unknowingly acquire a parasitic infection, and a doctor may not readily diagnose the infection when the person returns home.
In the United States and other resource-rich countries, parasitic infections tend to affect mainly immigrants, international travelers, and people with a weakened immune system (such as those who have HIV infection or who take medications that suppress the immune system). Parasitic infections also may occur in places with poor sanitation and unhygienic practices.
Some parasites are common in the United States and other resource-rich countries. Examples are pinworms, the protozoa that cause trichomoniasis (a sexually transmitted infection) and toxoplasmosis, and intestinal infections such as giardiasis and cryptosporidiosis.
Transmission of Parasites
Parasites usually enter the body through the:
Mouth
Skin
Parasites that enter through the mouth are swallowed and can remain in the intestine or burrow through the intestinal wall and invade other organs. Often parasites enter the mouth through fecal-oral transmission.
Some parasites can enter directly through the skin. Others are transmitted by insect bites.
Rarely, parasites are spread through blood transfusions, in various transplants including bone marrow and organ transplants, through injections with a needle previously used by an infected person, or from pregnant person to fetus.
Fecal-oral transmission of parasites
Fecal-oral transmission is a common way to acquire a parasite. "Fecal" refers to feces or stool, and "oral" refers to the mouth, including things taken into the mouth. Infection that is spread through the fecal-oral route is acquired when a person somehow ingests something that is contaminated by stool from an infected person or feces from an infected animal, such as a dog or cat. Many parasites invade or live in a person's digestive tract, so the parasites or their eggs are often present in stool.
Infected people often spread their infection when they do not wash their hands adequately after using the toilet. Because their hands are contaminated, anything they touch afterward may be contaminated with parasites. If people with contaminated hands touch food—in restaurants, grocery stores, or homes—the food may become contaminated. Then, anyone who eats that food may get the infection.
Ingestion does not have to involve food. For example, if a person with contaminated hands touches an object, such as a restroom door, the door can become contaminated. Other people who touch the contaminated door and then touch their fingers to their mouth can be infected through the fecal-oral route.
Other ways infection can be spread through the fecal-oral route include the following:
Drinking water contaminated with raw sewage (in areas with poor sanitation)
Eating raw shellfish (such as oysters and clams) that have been cultivated in contaminated water
Eating raw fruits or vegetables washed in contaminated water or grown in contaminated soil
Eating undercooked, contaminated meat
Engaging in sexual activity that involves mouth-to-anus contact
Swimming in pools that have not been adequately disinfected or in lakes or parts of the ocean that are contaminated with sewage
Skin transmission of parasites
Some parasites live inside the body and enter through the skin. They may:
Bore (burrow) directly through the skin
Be introduced by the bite of an infected insect
Some parasites, such as hookworms, penetrate the skin on the soles of the feet when a person walks barefoot on contaminated soil. Others, such as schistosomes, which are flukes, enter through the skin when a person swims or bathes in water containing the parasites.
Insects that carry and transmit organisms that cause disease are called vectors. For example, infected female mosquitoes are vectors that transmit the parasites that cause malaria and infected female blackflies are the vectors that transmit the parasites that cause river blindness.
Ectoparasites are transmitted by having close physical contact with an infected person or sometimes with their belongings.
Diagnosis of Parasitic Infections
Laboratory analysis of samples of blood, stool, urine, sputum (phlegm), or other bodily fluids or of skin samples
Doctors suspect a parasitic infection in people who have typical symptoms and who live in or have traveled to an area where sanitation is poor or where such an infection is known to occur.
Laboratory analysis of specimens, including special tests to identify proteins released by the parasite (antigen testing) or genetic material (DNA) from the parasite, may be needed. Samples of blood, stool, urine, skin, fluid, or sputum may be used, depending on which parasite doctors are looking for.
Doctors may test blood samples for antibodies to the parasite. Antibodies are proteins produced by the immune system to help defend the body against a particular attack, including by parasites.
Doctors may also take a sample of tissue that may contain the parasite. For example, a biopsy may be done to obtain a sample of intestinal or other infected tissue. A sample of skin may be snipped. Several samples and repeated examinations may be necessary to find the parasite.
Identifying parasites in the intestinal tract
If parasites live in the intestinal tract, the parasite or its eggs or cysts (a dormant and hardy form of the parasite) may be found in the person’s stool when a sample is examined under a microscope. Parasites may also be identified by testing the stool for proteins released by the parasite or genetic materials from the parasite.
Antibiotics, laxatives, and antacids should not be used until after the stool sample has been collected. These medications can make it difficult for a doctor to see eggs or parasites in a stool sample.
Treatment of Parasitic Infections
Sometimes antiparasitic medications
For some parasitic infections, no treatment is needed because the infection disappears on its own.
Antiparasitic medications are designed to eliminate parasites or, in the case of some worm infections, reduce the number of worms enough so that symptoms clear up. Also, certain antibiotics and antifungal medications are effective against some parasitic infections.
No single medication is effective against all parasites. For some parasitic infections, no medication is effective.
Prevention of Parasitic Infections
Malaria is the only human parasitic infection that has vaccines (see World Health Organization: Malaria vaccines (RTS,S and R21)). Substantial research efforts are ongoing to determine whether vaccines are effective in other parasitic infections such as leishmaniasis, schistosomiasis, and hookworms.
Immigrants returning to their home countries are at greater risk of endemic infections. This is because they have waning immunity, are less likely to seek pre-travel health precautions, and tend to visit areas with higher disease transmission than tourists in resort settings.
Otherwise, prevention is usually centered around protective measures. In general, these measures involve the following:
Good personal hygiene
Sanitary disposal of stool and animal waste
Avoiding insect bites
Avoiding contact with contaminated food, water, or soil
Many preventive measures are sensible everywhere, but some are more important in specific areas. Information about precautions needed in specific areas is available from the Centers for Disease Control and Prevention's Travelers' Health page.
Preventing parasites acquired by mouth
People need to be particularly careful when they travel to areas where sanitation methods are questionable. In addition, people should think about what they are eating and drinking before they consume it and make sure food is adequately cooked and water is not contaminated. For example, people should avoid drinking from lakes and streams and should avoid swallowing water when using swimming pools or water parks. Even water that looks fresh and clean can contain parasites, so people should not use the appearance of water to judge its safety for drinking.
In areas of the world where food, drink, and water may be contaminated with parasites, wise advice for travelers is to
Avoid drinking tap water
"Cook it, boil it, peel it, or forget it"
This advice means that travelers should avoid the following: eating meat, fish, shellfish, and eggs that are not fully cooked; eating uncooked fruit or vegetables; and eating or drinking unpasteurized dairy products or fruit juices. An exception to guidance is that fruit or vegetables with a thick peel (for example, bananas) are usually considered safe to eat raw if the peel is washed before it is removed.
Because some parasites survive freezing, ice cubes can sometimes transmit disease unless the cubes are made from purified water.
Thorough handwashing using soap and water is very important. People who prepare food for others (for example, restaurant workers) must be particularly careful to wash their hands thoroughly because they can spread infection to many people. Handwashing is important in the following situations:
After using the toilet
After changing a child's diapers or cleaning a child who has used the toilet
Before, during, and after preparing food
Before eating food
Before and after caring for a person who is sick
Before and after treating a cut or wound
After touching an animal or animal waste
Preventing parasites acquired through the skin
For precautions to take in specific countries, people should check the Centers for Disease Control and Prevention's Travelers' Health page.
Measures that help protect against insect bites include
Using insecticide (permethrin or pyrethrum) sprays in homes and outbuildings
Placing screens on doors and windows and using air-conditioning
Using permethrin- or pyrethrum-saturated mosquito netting over beds
Applying insect repellents containing DEET (diethyltoluamide) on exposed skin
Wearing long pants and long-sleeved shirts, particularly between dusk and dawn, to protect against insect bites, and applying permethrin to clothing
Taking antimalarial medications before travel to regions where malaria is transmitted
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.
