Parasitic Diseases

Parasitic Infections

What are parasitic diseases?

Parasitic diseases occur when an organism invades a person’s body and causes illness to the host. They are common in many rural parts of regions such as Africa, Asia and Latin America. People who travel to these areas are at an increased risk of contracting a parasitic disease. These diseases occur much less frequently in industrialized nations such as the United States. However, foodborne illness as a result of parasites is on the rise in the United States.

Parasites are organisms that gain protection and nourishment from living inside or on a host organism, such as humans. There are many parasitic diseases that can infect humans and cause illness. In most cases, parasites invade the body through a person’s mouth or skin.

Many parasites enter a person’s mouth because they are ingested when the person eats or drinks a contaminated substance. For example, a person can become infected when eating animal meat contaminated with a parasite originally ingested by the animal through contaminated feed. In other cases, people may become infected by drinking or swimming in contaminated water. People who hike and camp outdoors are at increased risk of contracting a parasitic infection if they drink from rivers or streams without first boiling the water.

Often, parasites may enter the body when people touch a contaminated object and then touch their own mouth or nose. Some parasites may enter the body when inhaled through a person’s nostrils. After these parasites enter a human host, they may remain in the intestine or be carried to other parts of the body.

Parasites that invade the body through the skin may bore directly through the skin, or may enter the body with the bite of infected insects, or through a cut or open wound. Parasites also enter the skin of people who bathe in contaminated water and through the soles of the feet of people who walk barefoot.

There are many different types of parasites, which may range from tiny, single-celled organisms (protozoa) to multicellular worms (helminths) that may be visually apparent without use of a microscope. Helminths include roundworms (nematodes) and tapeworms (cestodes).

Parasites have varying life cycles. Some reproduce only inside the host, particularly if they are single-celled. Others produce eggs or larvae in the environment or an insect before they become infectious and are transferred to humans.

In some cases, parasitic diseases may cause no symptoms in the host. Other parasitic diseases may cause mild symptoms that resemble the flu. While many illnesses associated with infection of parasites are mild, some can be severe and even life-threatening. In general, people with weakened immune systems have an increased risk of contracting parasitic diseases and developing severe to deadly symptoms as a result of infection. Many of the parasitic infections discussed in this patient guide have become more common since the early 1980s when AIDS was first identified. People with healthy immune systems can easily fight some of these infections, but an AIDS patient has weakened immunity and such cases a parasitic infection may be a serious or even fatal complication.

Types and differences of parasitic diseases

The Division of Parasitic Diseases at the U.S. Centers for Disease Control and Prevention (CDC) divides parasitic diseases into three major categories: parasites of the intestinal tract, bloodborne parasites and parasites found at other sites (e.g., skin). Many different parasitic diseases can cause illness in humans. Some of the better-known infections include:

African trypanosomiasis

Also known as African sleeping sickness, this illness is caused by two parasites that are carried by the tsetse fly, which is found only in Africa. The East African illness is acute, while West African (Gambian) is chronic. Both are fatal if left untreated. The disease is endemic to sub-Saharan Africa, but travelers to the region are not at high risk and few cases have been reported outside Africa.


Infection of humans or other animals by a nematode of the family Anisakidae. Humans typically are infected after eating undercooked infected marine fish (e.g., herring) such as often appears in sashimi, sushi and ceviche. The larvae burrow into the stomach wall and cause a mass to form. Symptoms include nausea and vomiting, diarrhea or abdominal pain within 12 hours of consuming the infected food.


Disease caused by a one-celled parasite known as Entamoeba histolytica. It most often is found in developing countries but sometimes is found in the United States in immigrants from areas where the disease is common. Symptoms usually are mild and include loose stools, stomach pain and stomach cramping. A severe form of this illness called amebic dysentery features stomach pain, bloody stools and fever.

American trypanosomiasis

Also known as Chagas disease, this parasitic disease is caused by the protozoan Trypanosoma cruz, which occurs mainly in Central and South America, although sometimes in the United States. It typically is transmitted to humans by insects called triatomine bugs that live in cracks and holes of substandard housing. Chagas disease has symptoms such as swelling or fever at the time of infection. In some people, it may enter a chronic stage and produce cardiac or digestive complications 20 to 30 years after the infection.


Infection by the intestinal roundworm, Ascaris lumbricoides, which looks like a common earthworm. These parasites can grow to about a foot long and become as thick as a pencil. Contact with infected feces (especially in areas with poor sanitation) spreads the disease. Up to 100 worms can infect a single person. The greater the number of worms in the body, the more severe the symptoms. Symptoms range from none at all to severe abdominal pain, vomiting and insomnia. The worms may be visually identifiable in an infected person’s vomit or stool. Ascariasis is a common worm infection, although it is not frequently seen in the United States.


Any of several tickborne diseases due to infection with protozoa of the genus Babesia. More than 100 species have been reported, but only a few infect humans and they generally do not produce symptoms. Infections are transmitted from animals to humans.


A parasitic disease caused by infection with species of the roundworm Capillaria.

Cercarial dermatitis

Also known as swimmer’s itch, this patchy, red skin rash causes itching on the parts of the body that have been in the water. It is caused by a larval parasite known as a cercaria. Typically, it is not serious and clears without treatment.


A diarrheal disease caused by the parasite Cryptosporidium parvum. It can live in the intestines of humans and animals before being passed in the stool. It is a significant cause of waterborne illness throughout the world, including the United States, where it is sometimes found in drinking water and recreational water (e.g., swimming pools, waterparks). Symptoms may include diarrhea, nausea, cramping and a slight fever. The health risk associated with this infection is greatest in people with compromised immune systems, such as HIV and AIDSAIDS. In these patients, cryptosporidiosis can have long-lasting and potentially fatal consequences. All cases of cryptosporidiosis in the United States are required to be reported to the CDC.


Infection with the Cyclospora cayetanensis parasite that occurs when a person ingests food or water contaminated by the parasite. Symptoms may include diarrhea, nausea, cramping, vomiting, muscle aches, a low fever and fatigue. The infection was previously confined to developing areas but cases now occur worldwide.


Also known as Guinea worm disease, this painful, debilitating infestation with the parasite Dracunculus medinensis is contracted by drinking stagnant water contaminated with Guinea worm larvae. The larvae matures inside a person's abdomen until the worm emerges through a blister. International efforts are under way to eradicate Guinea worm disease in the African nations where it still exists.


Infection caused by the trematodes Fasciola hepatica (the sheep liver fluke) and Fasciola gigantica. Human infections are sometimes found where sheep and cattle are raised, generally in sporadic outbreaks in Africa and South America.

Giardiasis (also known as beaver fever)

A parasitic infection from consuming contaminated water or food. It may cause diarrhea, nausea, cramping and bloating. Giardiasis is one of the most common waterborne illnesses in the United States and is responsible for most cases of childhood diarrhea in developing nations. It is a highly contagious parasitic infection and may be spread through drinking or swimming in contaminated water, consuming contaminated food and person-to-person contact.

Hookworm (also known as ancylostomiasis)

A common roundworm infection in which parasites generally enter the body through the skin. It is commonly spread through unsanitary conditions (e.g., walking barefoot on contaminated soil). An itchy patch typically develops where the parasite entered the body. Symptoms depend on the number of worms in the body, and may range from minor abdominal pain and diarrhea to serious anemia.


Infection with the dwarf tapeworm, it is the most commonly diagnosed tapeworm infection throughout the world, and the leading source of tapeworm infections in the United States.


Infection of the epithelial cells of the small intestine with the coccidian parasite Isospora belli. It causes diarrhea and has been the source of some diarrheal outbreaks in daycare centers and nursing homes. In Africa, isosporiasis is commonly the first infection experienced by people infected with HIV.


A parasitic disease spread by the bite of infected sandflies. There are several different forms, including cutaneous (causes skin sores) and visceral (affects some internal organs). It occurs in tropical and subtropical areas where sandflies are common.


Illness caused by tiny, wingless, parasitic insects that feed on a person’s blood. The major symptom associated with lice is extreme itchiness. Forms of lice include head lice, body lice and pubic lice. Lice are highly contagious and are spread through personal contact or sharing personal items. For example, outbreaks of head lice among school children are commonplace.


A potentially fatal disease caused by a parasite that is marked by high fevers, shaking, chills and flu-like illness. Five types of malaria parasites infect humans - Plasmodium falciparum, P. vivax, P. ovale, P. malariae and P. knowlesi. All cases of malaria in the United States are required to be reported to the U.S. Centers for Disease Control and Prevention.


An intestinal infection that causes diarrhea and wasting in people with HIV. Two species of the single-celled parasite microsporidia cause the illness -- Enterocytozoon and Septata.


Infection of the biliary tract with trematodes of the genus Opisthorchis. Transmission occurs through ingesting raw or inadequately cooked fish infected with the parasites. It generally occurs in parts of Europe and Asia.


Infestation with a worm of the genus Paragonimus. It typically invades the lung, producing chronic bronchitis with cough and reddish or brownish sputum. Most cases are native to areas of Asia, Africa and Latin America.

Pinworm infection (also known as enterobiasis)

A parasitic infection by a small, white roundworm called Enterobius vermicularis that lives in the human rectum. Females exit the anus and deposit eggs on the surrounding skin. Itching in the area is common. According to the National Institutes of Health, pinworm infection is the most common of all roundworm infections in the United States. It is mainly spread by children.


Infection caused by Pneumocystis jiroveci (formerly known as Pneumocystis carinii), a fungus that lives in the lungs. Healthy people usually do not experience symptoms, but it can cause pneumonia in people who have compromised immune systems. It is the most common serious infection in people with advanced HIV disease in the United States.


A contagious and itchy skin disease caused by a species of mite. The mites burrow under the skin and deposit their eggs, forming a burrow that resembles a pencil mark. Scabies is spread by direct contact with infected individuals. Scabies is found worldwide.


An infection caused by one of the five species of parasite known as Schistosoma. Transmission occurs through contact with contaminated fresh water in Africa, Asia, South America and the Caribbean.


Infection caused by the roundworm Strongyloides stercoralis. It is fairly common in warm, moist regions and is transmitted through exposure to contaminated soil. In many cases, it does not cause symptoms. Moderate infections may include symptoms of burning pain in the abdomen, nausea, vomiting, as well as alternating diarrhea and constipation. Severe infections may cause anemia, weight loss and chronic diarrhea.


Infection with pork or beef tapeworms. It is typically transmitted by consuming undercooked pork or beef contaminated with tapeworm eggs or larvae. Humans are the definitive hosts of these tapeworms, and the organisms’ eggs are passed in human feces. Symptoms may include abdominal pain, weight loss, digestive problems and itching near the anus.


Infection with the parasite Toxoplasma gondii. Cats are the definitive host of the parasite. Transmission methods include ingestion of contaminated soil, careless handling of cat litter or ingestion of undercooked meat. It may also be transmitted from an infected mother to her fetus. Most people with toxoplasmosis exhibit no symptoms. Minor symptoms may resemble the flu. Severe symptoms include encephalitis (inflammation of the brain), eye diseases and respiratory failure. Infants born with an infection may develop problems later in life, such as blindness, hearing loss, as well as mental and learning difficulties.

Risk factors and causes of parasitic diseases

Many parasitic diseases rarely or never occur in developed countries, which significantly lowers the risk of contracting these illnesses in nations such as the United States. However, people who travel to developing parts of the world increase their risk of exposure to certain parasitic diseases. Developing areas, especially developing tropical areas, are much more likely to have insects or other creatures that can transmit parasites, in addition to contaminated water supplies that harbor them.

In the United States, situations in which people come into physical contact with many other people (e.g., school, daycare facilities) can increase the risk of contracting a parasitic infection. Poor hygiene habits (e.g., not washing the hands after contact with potentially contaminated matter, such as feces or food) can also increase the risk of a parasitic infection.

People with weakened immune systems also are at increased risk of health problems brought on by exposure to certain parasites. This includes HIV and AIDS patients, organ-transplant recipients, and patients undergoing chemotherapy.

People who drink from untreated water supplies or poorly maintained sources of private water (e.g., private wells that are not properly disinfected after flooding) are at greater risk of ingesting water that contains parasitic agents. Hikers and campers who drink from untreated water in lakes and streams also are more vulnerable.

Signs and symptoms of parasitic diseases

In many cases, people who are infected with a parasitic disease do not experience any symptoms. Others may notice eggs, larvae or portions of adult parasites in their stool. Some people may experience mild symptoms that resemble influenza. Such symptoms include:

  • Abdominal pain and cramps
  • Diarrhea
  • Fatigue
  • Fever
  • Itchy skin
  • Irritation of the perianal area (area around the anus)
  • Loss of appetite
  • Muscle aches
  • Nausea
  • Weakness
  • Vomiting

More severe cases of parasite infections can lead to severe and possibly even life-threatening symptoms, including:

  • Breathing problems
  • Coordination difficulties
  • Damage to the eyes or brain
  • Heart problems
  • Inflammation of the brain (encephalitis) and meninges (meningoencephalitis)

Diagnosis methods for parasitic diseases

In diagnosing a parasitic disease, a physician will perform a complete physical examination and compile a thorough medical history. Some parasitic infections may produce symptoms or complications years after the initial infection, which makes the medical history especially important. There are also several types of laboratory tests that physicians use to diagnose parasitic disease.

The type of test performed usually depends on factors such as a patient’s symptoms, history of disease and pattern of recent travel. In many cases, more than one type of test may be ordered. It is also common for tests to be performed several times before a parasite is detected. Some commonly used diagnostic tests include:

Fecal exam

Also known as an ova and parasite test (O&P), it is used to identify the presence of infectious parasites or parasite eggs in a patient’s stool. Patients may collect multiple samples over a period of days. These samples are specially stored until they can be delivered to a healthcare provider or laboratory, where the samples are examined. Use of certain medications may make it difficult to detect parasites in a stool sample. Such medications include antacids, antibiotics and laxatives.


Used when a fecal exam fails to reveal the source of a patient’s diarrheal illness. During the procedure, a tube is inserted into the patient’s mouth or rectum and through the intestines to allow a physician (e.g., a gastroenterologist) to examine the area. If a parasitic infection is suspected, endoscopy will be used to identify the presence of parasites within the intestines.

Blood tests

Some parasites can also be found through this method, but only if the test is designed to identify a specific type of parasitic infection. There is no blood test that can identify all parasite infections. Serology tests look for certain types of antibodies produced by the body’s immune system in response to infection by a particular parasite. Serology tests can also find antigens associated with a specific type of parasite. Another test known as a blood smear uses microscopic examination to reveal the presence of parasites found in the blood, including those that cause malaria, filariasis or babesiosis.

Various imaging tests also may be used to looks for parasitic disease that causes swelling of internal organs or scarring. Such procedures include x-rays, magnetic resonance imaging (MRI) and computed axial tomography (CAT scan).

In some cases, a biopsy of skin tissue may be used to look for the presence of a parasite.

Treatment options for parasitic diseases

Some types of parasitic diseases resolve on their own and do not require treatment. Other infections require medications, such as antiparasitic agents. These are drugs used to treat infestations of parasites in the body. These medications work by either paralyzing parasites or by killing them outright.

Antiparasitic medications are usually well-tolerated by patients and cause few side effects. Adverse reactions that do occur tend to be minor. Most people can use these drugs safely and effectively regardless of any other medical conditions they might have. Antiparasitic agents are available in the form of pills, creams, lotions and shampoos. Some of these drugs are available in over-the-counter formulations, whereas others can only be obtained with a physician’s prescription. Antiparasitic agents known as antimalarials have been developed to treat malaria. Patients may need to take these medications for several months.

The U.S. Food and Drug Administration (FDA) has classified most antiparasitic agents as belonging in either category B (which means that either animal studies have shown no risk to an animal fetus, or human studies have shown no risk to a human fetus) or category C (which means animal studies have shown a risk to animal fetuses, but has not been adequately studied in humans). Some antiparasitic agents are passed into human breast milk, though it is unknown whether others appear in breast milk. Patients are urged to discuss the potential risks and benefits of using antiparasitic agents during pregnancy and breastfeeding.

In some cases, parasites are becoming resistant to medications typically used to treat them. For example, new drug-resistant strains of the parasite responsible for the most deadly form of malaria, Plasmodium falciparum, have been identified.

Prevention methods for parasitic diseases

There are no vaccines available to prevent parasitic diseases. While people in industrialized nations are at a low risk for contracting most parasitic diseases, anyone who travels to developing nations is urged to take precautions to prevent infection with parasites. However, parasitic diseases such as malaria can be treated with prophylactic (preventative) medications that can sharply reduce the risk of contracting the illness.

Avoiding risky foods and drinks in developing countries can substantially reduce the risk of contracting parasitic diseases. People are urged to drink bottled water only while in these nations. If bottled water is not available, bringing water to a rolling boil for one minute before drinking is advised. This is also important when hiking or camping. Drinks with ice and popsicles should be avoided.

People also are urged to avoid raw vegetables and fruits that cannot be peeled. All foods should be thoroughly cooked so that they are hot and steaming, and foods and beverages from street vendors should be avoided. In addition to food precautions in developing areas, travelers should avoid swimming in fresh water that may harbor parasites.

People who travel to countries where mosquitoes carry serious disease are urged to use mosquito-repellents that have an active ingredient registered with the Environmental Protection Agency (EPA), such as N,N-diethyl-meta-toluamide (DEET). Wearing long sleeves and pants also helps deprive mosquitoes of the exposed skin on which they like to feed. Keeping well-maintained screens on windows and doors can help prevent mosquitoes from getting indoors.

People can also reduce mosquito breeding grounds by eliminating standing water by dumping it from buckets, barrels, flower pots and other containers. Water should be changed in bird baths and pet dishes at least weekly, and holes should be drilled in tire swings to allow water to drain out. Keeping children’s wading pools empty of water and resting on their sides also can eliminate potential breeding areas. Clogged gutters should be cleaned to prevent water from pooling.

Even when not traveling, thorough cooking of all meat products is advised. Meat must be heated to an internal temperature of at least 160 degrees Fahrenheit (71 degrees Celsius) to kill germs. No pink should remain in the meat, and juices should run clear. Poultry should be cooked to a minimum internal temperature of 165 degrees Fahrenheit (73.9 degrees Celsius), according to the Food Safety and Inspection Service.

Practicing good hygiene (frequent hand-washing) can help prevent infection by many different types of germs, including parasites. Hand washing is particularly important after contact with fecal matter as well as before and after preparing food. Food preparation sites, utensils and cutting boards should be cleaned thoroughly before and after use.

If possible, contact with potentially contaminated material should be avoided. If avoidance is not possible, wearing gloves and a mask (that covers the nose and mouth to avoid inhaling contaminated dust particles) during contact with suspect matter may help reduce the risk of contracting a parasitic infection.

For many parasitic infections, people infected once may be reinfected again in the future. Patients who have contracted a parasitic disease should not assume they have immunity to future illness. Patients should consult their physician about their likelihood of contracting the disease again the future.

Questions for your doctor on parasitic diseases

Patients may wish to ask their doctor the following questions related to parasitic diseases:

  • What are the signs of parasitic disease?
  • How will you diagnose my parasitic disease?
  • What type of parasite do you suspect caused my infection?
  • How serious is my parasitic disease?
  • What are my treatment options?
  • What are the risks associated with these treatments?
  • How long will I have to take my medication?
  • How will I know if treatment has been effective?
  • What tips do you suggest for preventing parasitic disease?
  • What steps can I take to protect myself while traveling?

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