A parasite is an organism that lives on or inside another organism (the host) and benefits (for example, by getting nutrients) from the host at the host's expense. Although this definition actually applies to many microbes, including bacteria, fungi, and viruses, doctors use the term "parasites" to refer to
Protozoa (such as amebas), which consist of only one cell
Worms (helminths), which are larger and consist of many cells and have internal organs
Protozoa reproduce by cell division and can multiply inside people. The protozoa include a wide range of single-cell organisms such as Giardia, 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 people. Development in the environment may involve another animal (an intermediate host). Worms include roundworms, such as hookworms, and flatworms, such as tapeworms and flukes.
Parasitic infections are more common in areas with 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.
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.
Travelers to areas where food, drink, and water may be contaminated are advised to cook it, boil it, peel it, or forget it.
Medications are available to treat most parasitic infections.
Most parasitic infections are more common in tropical and subtropical areas, and intestinal parasites 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 industrialized countries, parasitic infections tend to affect mainly immigrants, international travelers, and people with a weakened immune system (such as those who have AIDS or who take medications that suppress the immune system—called immunosuppressants). Parasitic infections may occur in places with poor sanitation and unhygienic practices (as occurs in some psychiatric hospitals and day care centers).
Some parasites are common in the United States and other industrialized countries. Examples are pinworms and the protozoa that cause trichomoniasis (a sexually transmitted infection), 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 transplanted organs, through injections with a needle previously used by an infected person, or from a pregnant woman to her fetus.
Some other infectious organisms, such as certain viruses and bacteria, also are transmitted by these same methods.
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 feces from an infected person or animal, such as a dog or cat. Many parasites invade or live in people's digestive tract. Thus, parasites or their eggs are often present in people's feces.
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 (or with bacteria or viruses that cause digestive tract disorders). 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 finger to their mouth can be infected through the fecal-oral route.
Other ways infection can be spread through the fecal-oral route include
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
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 directly through the skin
Be introduced by the bite of an infected insect
Some parasites, such as hookworms, enter through 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. Some insect vectors transmit parasites called protozoa (such as those that cause malaria) and some helminths (such as those that cause river blindness). Many of these parasites have very complex life cycles.
Insects (for example, lice) and mites (for example, scabies) that live on or burrow into skin are known as ectoparasites. They are transmitted by having close contact with an infected person or their belongings.
Diagnosis of Parasitic Infections
Laboratory analysis of samples of blood, stool, urine, skin, or sputum (phlegm)
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, 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 that 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. Or parasites may 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 reduce the number of parasites enough to make seeing the parasites in a stool sample difficult or impossible.
Treatment of Parasitic Infections
Antiparasitic medications
For some parasitic infections, no treatment is needed because the infection disappears on its own.
Some medications (antiparasitic medications) are designed particularly 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
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).
In general, measures that help prevent infection by parasites involve
Good personal hygiene
Avoiding insect bites
Avoiding contact with contaminated 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 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"
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
To know precautions to take in specific countries, people should check the Centers for Disease Control and Prevention 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
Using permethrin- or pyrethrum-saturated mosquito netting over beds
Applying insect repellents containing DEET (diethyltoluamide) on exposed areas of the skin
Wearing long pants and long-sleeved shirts, particularly between dusk and dawn, to protect against insect bites
If insect exposure is likely to be long or involve many insects, applying permethrin to clothing before it is worn
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.
