ConditionInfections
Parasitic Diseases
Parasitic diseases are infections caused by organisms that live on or inside the body and feed off it. There are three main types: protozoa (single-celled organisms such as Giardia, Toxoplasma and the malaria parasite), helminths (worms such as pinworms, roundworms and tapeworms), and ectoparasites (lice, scabies mites and fleas on the skin). They spread through contaminated food and water, soil, insect bites and close contact. Most are treatable with specific antiparasitic medicines once diagnosed.
Also called: Parasitic infection, parasitosis, parasitic infestation, worm infection, protozoan infection
At a glance
- What they are
- Infections or infestations by protozoa, worms or external parasites.
- How they spread
- Contaminated water and food, undercooked meat, soil, insect bites, person-to-person contact.
- Common symptoms
- Diarrhea, abdominal pain, bloating, itching, rash, fever, weight loss, anemia, or no symptoms.
- Diagnosis
- Stool tests, blood tests and smears, skin scrapings, imaging.
- Treatment
- Specific antiparasitic medicines; hygiene measures to prevent reinfection.
When to get help
Call 911 or go to the ER
- Fever after travel to a malaria area, especially with confusion, drowsiness, breathing difficulty, jaundice or dark urine
- Seizures or severe headache in someone with a history of travel or pork tapeworm exposure
See a doctor within a day or two
- Fever within 3 months of returning from a tropical country
- Severe or bloody diarrhea, or diarrhea with signs of dehydration
- Diarrhea in pregnancy or in someone with a weakened immune system
- Muscle pain, swollen eyelids and fever after eating undercooked pork or game
Book a routine appointment
- Diarrhea, bloating or greasy stools lasting more than 2 weeks
- Unexplained weight loss, anemia or itching after travel
- Itching around the anus at night, especially in a child
Usually safe to manage at home
- Head lice in a child, treated with an over-the-counter product and wet combing
Types of parasitic diseases
Protozoa (single-celled parasites)
| Parasite | Disease | Spread | Main symptoms |
|---|---|---|---|
| Giardia | Giardiasis | Contaminated water, person to person | Greasy diarrhea, gas, bloating, weight loss |
| Cryptosporidium | Cryptosporidiosis | Pools, contaminated water | Watery diarrhea; severe in weak immunity |
| Entamoeba histolytica | Amebiasis | Contaminated food and water | Bloody diarrhea, liver abscess |
| Toxoplasma | Toxoplasmosis | Undercooked meat, cat feces | Often none; risky in pregnancy and weak immunity |
| Plasmodium | Malaria | Mosquito bites | Cyclical fever, chills, anemia; can be fatal |
| Trypanosoma cruzi | Chagas disease | Triatomine bug feces | Heart and gut problems years later |
| Leishmania | Leishmaniasis | Sand fly bites | Skin sores, or fever and enlarged spleen |
| Trichomonas | Trichomoniasis | Sexual contact | Vaginal discharge, itching |
Helminths (worms)
| Worm | Disease | Spread | Main symptoms |
|---|---|---|---|
| Pinworm | Pinworms | Swallowing eggs from hands and surfaces | Itchy bottom at night, especially in children |
| Roundworms (Ascaris, hookworm, whipworm) | Soil-transmitted helminthiasis | Contaminated soil, walking barefoot | Abdominal pain, anemia, poor growth |
| Strongyloides | Strongyloidiasis | Skin contact with soil | Rash, diarrhea; dangerous with steroids |
| Pork tapeworm | Taeniasis and cysticercosis | Undercooked pork; eggs from carriers | Often none; cysts in the brain cause seizures |
| Echinococcus | Hydatid disease | Dog feces | Cysts in liver or lungs |
| Trichinella | Trichinosis | Undercooked pork or game | Diarrhea, then muscle pain, swollen eyelids |
| Schistosoma | Schistosomiasis | Freshwater contact in Africa, Asia, South America | Rash, fever, blood in urine or stool |
| Toxocara | Toxocariasis | Dog and cat feces | Usually none; eye or organ involvement |
Ectoparasites
- Head lice, body lice and pubic lice: itching, visible nits
- Scabies: intensely itchy rash, worse at night, burrows between fingers
- Fleas and bed bugs: itchy bites
- Tungiasis and myiasis: in some tropical areas
How parasites spread
- drinking untreated water or ice in areas with poor sanitation
- eating raw or undercooked meat, fish or unwashed produce
- poor handwashing, especially in childcare settings
- walking barefoot on contaminated soil
- swimming or wading in contaminated fresh water
- insect bites (mosquitoes, sand flies, triatomine bugs)
- close personal or sexual contact
- contact with pets and their feces
Symptoms
Many parasitic infections cause no symptoms. When present, they depend on the parasite and where it lives:
- gut: diarrhea, abdominal pain, bloating, nausea, greasy stools, weight loss
- blood: fever, chills, anemia, fatigue
- skin: itching, rash, sores, visible lice or nits
- muscles and organs: muscle pain, liver cysts, cough, seizures
Diagnosis
- stool tests: ova and parasites microscopy, antigen tests and multiplex PCR panels; several samples may be needed
- blood tests: blood count (eosinophils are often raised with worms), malaria smears and rapid tests, antibody tests
- tape test for pinworms: sticky tape pressed around the anus in the morning
- skin scrapings for scabies
- imaging: ultrasound, CT or MRI for cysts in the liver, lungs or brain
Tell your doctor about travel, diet, water sources, pets and work exposures.
Treatment
Specific antiparasitic medicines depend on the organism, for example metronidazole or tinidazole for Giardia, albendazole or mebendazole for many worms, praziquantel for tapeworms and schistosomes, ivermectin for strongyloides and scabies, permethrin for scabies and lice, and artemisinin-based combinations for malaria. Household members may need treatment at the same time for pinworms, scabies and lice. Rehydration, nutrition and treatment of anemia support recovery.
Prevention
- wash hands with soap after the toilet, changing diapers and before eating
- drink safe water: boiled, filtered or bottled in high-risk areas
- cook meat thoroughly and wash fruit and vegetables
- wear shoes outdoors in endemic areas
- use insect repellent and bed nets; take malaria prophylaxis when recommended for travel
- deworm pets and clean up pet feces
- avoid swimming in untreated fresh water in schistosomiasis areas
Frequently asked questions
What are the main types of parasitic diseases?
The three main types are protozoan infections (such as giardiasis, toxoplasmosis and malaria), helminth or worm infections (such as pinworms, roundworms and tapeworms), and ectoparasite infestations (such as lice and scabies).
What are the symptoms of a parasitic infection?
Common symptoms include diarrhea, abdominal pain, bloating, nausea, weight loss, fatigue, itching, rashes and fever. Many infections cause no symptoms at all. Symptoms depend on the parasite and the part of the body affected.
How do you test for parasites?
Doctors usually test stool samples for parasites, eggs, antigens or DNA. Blood tests can detect malaria and antibodies to some parasites. The tape test detects pinworms, and skin scrapings detect scabies.
Can parasitic infections be cured?
Yes, most are curable with the right antiparasitic medicine. Some, such as malaria, need urgent treatment, and others may need repeated courses or treatment of household contacts to prevent reinfection.
Sources
This guide was written from current patient and clinical guidance, including:
- CDC: Parasites (DPDx and parasite topic pages)
- World Health Organization: Neglected tropical diseases; Soil-transmitted helminth infections
- MedlinePlus: Parasitic diseases
- Merck Manual Consumer Version: Overview of parasitic infections
- IDSA: Guidelines on infectious diarrhea (2017)