A parasite is an organism that lives off of or inside of another organism, called a host, during all or part of its life. This is a type of symbiotic relationship in which the parasite needs the host in order to live. The parasite obtains nourishment and/or protection from its host. A parasite may or may not harm the host, but the host never benefits from the parasite. Many organisms, including some plants, animals, spiders, crustaceans, bacteria, and worms, are considered parasites.
Many parasites can enter the human body and cause parasitic infections. Parasites may enter the body through openings in the body, including the skin and mouth. Each type of parasite affects the human body differently. Some feed on humans cells (such as red blood cells), while others live in the intestines and absorb nutrients from food that is consumed by the host. Parasites can cause many life-threatening complications, including anemia, malnutrition, blindness, and organ and tissue damage.
Among the most common types of parasites to affect humans are single-celled organisms called protozoans, worm-like organisms, fungi, and mites. These organisms are most likely to cause diseases such as ascariasis, chiggers, giardiasis, guinea worm disease, histoplasmosis, hookworm infection, leishmaniasis, lymphatic filariasis, malaria, ringworm infection, scabies, tapeworm infection, river blindness, threadworm infection, trichinosis and whipworm infection. Parasites are responsible for the most deaths in tropical and subtropical regions of the world.
Although parasitic infections can cause permanent tissue and organ damage, most patients experience a complete recovery if they are diagnosed and treated quickly. Parasitic infections are treated with medications, called anti-parasitics. These medications, which may be taken by mouth, applied to the skin, or injected into a vein, kill the parasite.

Related Terms

Antihelminthic, antimalarials, anti-parasitic, ascariasis, filariasis, guinea worm disease, histoplasmosis, hookworm, hookworm infection, host, leishmania, loiasis, lymphatic filariasis, malaria, parasite, parasitic fungi, river blindness, roundworms, threadworm infection, trichinella, trichinosis, visceral larva migrans, whipworm infection, worms.

common types and causes of parasitic infections

Ascariasis: Ascariasis is a type of parasitic infection that is caused by a roundworm called Ascaris lumbricoides. Humans become infected after they ingest the parasite's microscopic eggs. The parasite can be transmitted when humans eat produce that is grown in soil that has been mixed with infected human feces. This is most common in developing countries where human feces are used as fertilizer for crops or where there is poor sanitation.
Once the eggs enter the body, they mature into larvae. The larvae then enter the lungs. Eventually, the larvae reach the throat, where they are coughed up and then swallowed. Once swallowed, the larvae enter the intestines, where they develop into adults and feed off of the food that enters the body. Adult worms can grow to be 15 inches long. Adult worms can live up to two years, and female worms can produce more than 200,000 eggs a day. These eggs are released in the patient's feces.
Ascariasis is considered the most common type of roundworm infection in humans. Researchers estimate that about 25% of the world's population is infected with the parasite. In the United States, most infections occur in rural areas that have warm climates, such as the Southern United States.
Chiggers: Chiggers are the parasitic larvae of the harvest mite. While the adults are harmless to humans, the larvae are parasites to many animals, including birds, livestock, reptiles, and humans. Chigger bites only occur during the late summer and early autumn because this is when the larvae are active.
Chiggers are found in many parts of the world, including the Southern United States. Chiggers typically live in brush and grassy areas. Females lay up to 400 eggs in damp areas on the ground.
Once the eggs have developed into larvae (called chiggers), they move to the tips of grasses. When a potential host, such as a rodent or human passes by, the chigger latches on. The chiggers then move to protected areas on the host, such as under socks or under belts.
Chiggers pierce the skin, especially near hair follicles. When they pierce the skin, they secrete digestive enzymes from their saliva and then they suck up and ingest the liquefied host tissues. The rash and intense itching that develops after a chigger bite is an allergic reaction to their salivary secretions. Once the larva is fully fed, it drops off the host.
Giardia infection (giardiasis): Giardiasis occurs when an intestinal parasite called Giardia lamblia infects a human. Giardiasis typically causes abdominal pain, nausea, and diarrhea.
Humans contract giardiasis after consuming food or water that is contaminated with Giardia lamblia. Once consumed, the parasitic eggs (called cysts) hatch in the stomach. The parasite then attaches to the small intestine where it feeds on the food that is consumed by its host.
Giardiasis is common throughout the world, including the United States.
Guinea worm disease: Guinea disease is a painful parasitic infection caused by a roundworm, called the guinea worm (Dracunculus medinensis). Patients become infected with this worm when they drink water that is contaminated with a tiny water flea that is infected with tiny guinea worm larvae. The guinea worm larvae are only found in Africa.
Once the larvae enter the human, they mature into adults in the human gut. Inside the host, these adult worms can grow up to be three feet long. The adult then migrates to an area of the body (almost always the legs) from which it will eventually emerge.
After about one year, the adult worm is ready to release its eggs. When this happens, a small part of the worm will emerge through a painful, round blister in the skin. The tip of the adult worm breaks through the skin to release its eggs into water. The adult worm will continue to emerge and lay eggs whenever the affected limb is submerged in water. This often causes long-term suffering and sometimes crippling effects in the human host.
Histoplasmosis: Histoplasmosis is a parasitic infection of the lungs that may spread to other organs and tissues of the body. Histoplasmosis is caused by a parasitic fungus called Histoplasma capsulatum. This parasitic fungus is most prevalent in Ohio, Missouri, and the Mississippi river valleys.
Humans typically become infected with the parasitic fungus when they inhale the spores of the fungus. The tiny spores become airborne when dirt or other contaminated substances (especially droppings from birds or bats) is disturbed.
Most patients with histoplasmosis do not experience any symptoms of the disease and do not require treatment. This is because healthy individuals have strong immune systems that are able to prevent the parasitic fungi from multiplying and causing an infection. However, patients with weakened immune systems, especially HIV patients and infants, are vulnerable to developing infections that start in the lungs and spread to other areas of the body. When the infection spreads, it is called disseminated histoplasmosis. This condition is fatal if left untreated.
Patients with underlying diseases, such as emphysema, are also vulnerable to developing a long-term infection that primarily affects the lungs. When this happens, the condition is called chronic pulmonary histoplasmosis. Patients with this type of infection require lifelong treatment with antifungals.
In some cases, otherwise healthy individuals may develop histoplasmosis infections. This may happen if the patient is exposed to large amounts of the parasitic fungus. For instance, farmers who are frequently exposed to soil or bird or bat droppings have an increased risk of developing infections.
Hookworm infection: A hookworm infection is caused by one of two different types of roundworms: Ancylostoma duodenale or Necator americanus. If not treated, Hookworm infections can lead to abdominal pain and iron deficiency. Researchers estimate that about 20% of the world's population is infected with hookworm.
Humans become infected with hookworm when they come into contact with contaminated soil or stool. The larvae enter through the skin and travel through the blood to the lungs. Eventually, the larvae reach the throat, where they are coughed up and swallowed. As the larvae enter the digestive tract, they attach themselves to the wall of the small intestine. Here they mature into adult worms and mate. The worms feed on the blood of the host, which may lead to iron deficiency. Adult hookworms may live up to ten years.
Leishmaniasis: Leishmaniasis is a parasitic disease that is caused by protozoa called leishmania. Humans become infected with the parasite after they are bitten by a sandfly that is infected with the leishmania larvae. Sandflies are blood-sucking insects that are commonly found on beaches and marshes. They are especially common in Florida.
There are many different types of leishmaniasis. The most common form is a skin disease called cutaneous leishmaniasis. This infection causes skin sores, which may or may not be painful. The sores have raised edges and a flat center.
Loiasis: Loiasis is a parasitic infection that is caused by a roundworm called the African eye worm (Loa loa). Humans become infected with the parasite after they are bitten by the deer fly chrysops (typically found near the Congo River region, Sudan, and Ethiopia) that is carrying the immature African eye worm. Once the parasite enters the human host, it migrates toward the eyes, where it causes eye congestion and irritation. Sometimes the worms move to the brain, where it causes brain swelling, which is potentially fatal.
Loiasis is most prevalent in tropical areas of Africa.
Lymphatic filariasis: A filarial infection (called filariasis) is a type of infection that is caused by any of the round, thread-like parasitic worms. Lymphatic filariasis, also called elephantiasis, is caused by a worm (either Wuchereria bancrofti or Brugia malayi) that infects the human lymph system. It is the most common type of filarial infection.
The infection is transmitted to humans when a mosquito infected with the larvae bites a person. Once the parasite enters the human, it migrates to the lymph nodes where it develops into an adult. Females release larvae, which circulate in the patient's bloodstream. Adult filarial worms typically live for about seven years, if not treated with anti-parasitic medications.
Although lymphatic filariasis is rarely fatal, it can cause fevers, frequent infections, and serious inflammation of the lymph system if it is not treated.
Malaria: Malaria is an infectious disease of the red blood cells that is caused by protozoan parasites called Plasmodium falciparum, Plasmodium vivax, Plasmodium malariae, and Plasmodium ovale. The protozoa live the first part of their lives inside mosquitoes. When an infected mosquito bites a human, malaria can be transmitted.
This life-threatening disease is most common in tropical and subtropical areas, such as Africa, Asia, the Middle East, South America, and Central America. According to the U.S. Centers for Disease Control and Prevention (CDC), about 350-500 million patients become infected with malaria each year. More than one million of these patients die, most of them young children from sub Saharan Africa.
Malaria can be successfully treated with anti-malarial medications. However, drug resistance is a growing problem in many countries.
Studies on malaria causing parasites have revealed two new distinct forms different from the form already being studied in labs. Understanding the genome of the parasite and all the variations may help researchers discover new methods to combat the disease.
Several genes have been shown to help protect against malaria. Studies on the major histocompatibility complex, class I, B gene, also known on HLA-B, has revealed a link to malaria. HLA-B gene is part of a family of genes known as human leukocyte antigen complex, which helps the body distinguish its own cells from foreign invaders like bacteria and viruses. A mutation on the HLA-B gene known as HLA-B53 has been shown to protect against malaria. Researchers believe that this mutation may help the immune system to respond better to the parasite that causes malaria.
A mutation in the glucose-6-phosphate dehydrogenase (G6PD) gene may also offer protection against malaria. The G6PD gene mutation causes glucose-6-phosphate dehydrogenase deficiency, which is a blood disorder that causes red blood cells to break down prematurely. This blood disorder appears to make it difficult for the parasite to penetrate the blood cells. G6PD deficiency is most common in the world where malaria is also most common.
Another genetic mutation that protects against malaria is the gene mutation that causes sickle cell anemia, a blood disorder that causes red blood cells to be sickle shaped. As a result, the abnormally shaped blood cells may block narrow blood vessels potentially leading to tissue damage. Sickle cell anemia is caused by a mutation on the β globin gene which leads to abnormal hemoglobin. There are two alleles, or variations, of the β globin gene: A and S. Individuals with two normal alleles (AA) have normal hemoglobin and normal RBCs. Individuals who have two mutated alleles (SS) produce abnormal hemoglobin and have sickle cell anemia, but individuals who only carry the allele (AS) produce both abnormal and normal hemoglobin. These individuals are usually healthy and do not develop sickle cell anemia, have been found to be more resistant to malaria than others, and are able to survive in regions with high rates of malaria.
Ringworm: There are many different types of ringworm infections, all of which infect different parts of the body, including the feet, genitals, scalp, and top layer of the skin. Ringworm is named after the round shape that often forms when an infection develops. It does not involve an actual worm in the body.
Athlete's foot (tinea pedis) is a ringworm infection of the foot that causes itching, stinging, and burning. A group of parasitic fungi, called dermatophytes, cause athlete's foot. These fungi prefer warm moist environments. Individuals whose feet are exposed to this type of environment have an increased risk of developing an infection. Patients may be exposed to the fungi when they shower barefoot in a public facility, such as a gym. Once the fungus comes into contact with human skin, it begins to reproduce. As a result, the top layer of the skin produces more skin cells than normal. This causes the skin on the feet to become thick, scaly, and itchy. Most cases of athlete's foot can be treated with over-the-counter (OTC) antifungal medications. More severe infections that do not respond to OTC treatment may require prescription-strength antifungals.
Jock itch, also called tinea cruris, is a ringworm infection that affects the skin of the inner thighs, buttocks, and genitals. Jock itch is caused by a group of fungi called dermatophytes. The fungi that cause jock itch are usually the same organisms that cause athlete's foot and tinea capitis (ringworm of the scalp). These organisms normally live on the skin and do not cause an infection. However, when areas of the body are frequently moist, the fungi may grow uncontrollably. Although anyone can acquire the infection, it is most common in individuals who sweat a lot, such as athletes or those who are overweight. This is because a warm and sweaty environment provides an especially good habitat for fungi. Jock itch is mildly contagious. It may spread through physical contact or after using shared items (e.g. towels) that came into contact with the infected person's skin. Jock itch is not a serious infection. It is usually treated with antifungals that are applied to the skin.
Tinea capitis, also called ringworm of the scalp, is a skin infection of the head. Tinea capitis is caused by a group of parasitic fungi called dermatophytes. Once the disease-causing fungi come into contact with the skin, the organism begins to multiply and an infection develops. Tinea capitis is contagious, and it can spread through skin-to-skin contact. Patients may be exposed to the fungi after touching an infected animal, such as a dog, cat, ferret, rabbit, goat, or pig. Patients may also become infected after touching objects (e.g. towels, clothing, or bed linens) with which an infected person or animal has been in contact.
Tinea corporis is a ringworm infection that affects the top layer of skin on the legs, arms, face, and trunk of the body. It causes a red, itchy rash. Anti-fungal medications are applied to the skin to treat the infection.
River blindness (onchocerciasis): River blindness, also called onchocerciasis, is a parasitic infection of the eyes that is caused by a worm called Onchocerca volvulu. The disease is transmitted to humans by biting black flies (called Buffalo gnats). When these flies bite a human, they allow the parasitic larvae to enter the human's body.
Once inside the human, the larvae begin to mature into adults. Adults then produce millions of tiny worms, called microfilaria, which migrate throughout the body. River blindness often causes severe itching of the skin, and if left untreated, it may lead to blindness.
River blindness is considered an epidemic in more than 25 countries across the central part of Africa. According to the World Health Organizations' export committee on river blindness, about 18 million people are infected with the parasite each year worldwide. Of those infected, an estimated 6.5 million suffer from severe itching or dermatitis, 500,000 suffer serious visual impairment, and 270,000 are blind.
Scabies: Scabies is a contagious skin disease that is caused by microscopic mites that live three to four weeks in a person's skin. The female mite burrows into skin surface to lay her eggs. These eggs cause an inflammatory response in the host that causes itching, redness, and mild swelling.
Scabies is often spread through direct or prolonged skin contact with an infected person or animal. It is easily spread through direct contact with sexual partners or family members. It may also be spread after sharing clothes, towels, bedding, or other linens, with an infected person.
According to the American Academy of Dermatology, about 300 million cases of scabies are reported each year worldwide. It is most common among schoolchildren, individuals living in crowded areas, and people living in poverty.
Tapeworm infection: A tapeworm infection is a parasitic infection that affects the digestive tract.
Humans become infected with tapeworms after they consume food or water that is contaminated with tapeworm larvae. Most tapeworm infections in humans are caused by the pork tapeworm (Taenia solium), the dwarf tapeworm (Hymenolepis nana), the beef tapeworm (Taenia saginata), or the fish tapeworm (diphyllobothrium latum). Tapeworm infections typically occur when a person consumes food, water, or soil that is contaminated with human or animal feces.
Most tapeworm infections cause no symptoms. However, if symptoms do occur, patients typically experience abdominal pain, bloating, nausea, and diarrhea.
Tapeworm infections that are limited to the intestines can be successfully treated with anti-helminthic medications.
Cooking meat thoroughly kills the parasites and prevents an infection from occurring. The meat must reach at least at least 150 degrees Fahrenheit in order to ensure that any tapeworm eggs and larvae have been killed.
Threadworm infection: Threadworm infection, also called strongyloidiasis, is a parasitic infection of the intestines that is caused by a type of roundworm called Strongyloides stercoralis. Unlike the more common roundworm infection ascariasis, a threadworm infection may also spread to the skin.
The infection is transmitted to humans when a person comes into contact with soil that is contaminated with S. stercoralis. This usually occurs when an individual walks barefoot on soil. The larvae enter the human through the skin. Once inside the body, they migrate to the lymph nodes where they are carried into the lungs. Once in the lungs, the larvae migrate to the patient's throat. When the patient coughs, the larvae are swallowed and they enter the digestive tract. Once in the intestine, the larvae mature into egg-producing adults. The eggs are then released in the patient's feces. Adult threadworm may grow to be one to two inches long.
Threadworm infections are not fatal. However, if left untreated, threadworm infections can last for as long as 45 years.
Although this infection can occur in most areas of the world, it is most prevalent in tropical and subtropical climates.
Trichinosis: Trichinosis is a type of parasitic infection that is caused by the roundworm Trichinella. Humans become infected when they eat undercooked meat (usually beef or pork) that is contaminated with Trichinella larvae.
Once inside the human, the larvae mature in the intestine into adult worms over the course of several weeks. The adults then produce larvae that migrate to various body tissues, including muscle.
Some patients may have mild, if any, symptoms. However, if the patient is infected with hundreds of worms, it may lead to permanent tissue damage. This is because the larvae burrow into the patient's muscle and other tissues in the body.
The United States Centers for Disease Control (CDC) recommends cooking meat products until the juices run clear or to an internal temperature of 170 degrees Fahrenheit in order to kill Trichinella larvae.
Whipworm infection: Whipworm infection occurs when a parasitic worm, called Trichocephalus trichiura, infects the large intestine. This infection primarily affects children.
Humans become infected after they consume foods that are contaminated with soil that contains whipworm eggs. Once inside the body, the eggs hatch and attach themselves to the wall of the large intestine.
Whipworms are found around the world, especially in countries with warm and humid weather.