What Are Parasites?

Medically reviewed by Josephine Hessert, DO

Parasites are organisms that infect other living things to survive, live, feed, and reproduce on or inside them. Many types of parasites affect humans. In fact, these organisms can spread through food, water, bug bites, and sexual contact. If a parasite feeds on you, you can develop a parasitic infection. The symptoms of parasitic infections vary a great deal, but most people may experience diarrhea, nausea, vomiting, rashes, and fever.

Since parasites depend on your body for survival, they don't often cause infections that are deadly. However, some parasitic diseases like malaria can be life-threatening. That’s why it’s important to understand what parasites are, the signs of infection, your treatment and management options, and how you can prevent exposure.

What Are Parasites?

A parasite is any organism that lives and feeds on or inside another organism (known as the host). By nature, the way a parasite gets nutrients and reproduces is at the expense of that host organism. A host can be any other living thing, such as a human or another animal. Single-celled organisms, amoebas, worms, ticks, fleas, and lice are among the many types of parasites that infect humans.

Types of Parasites

There are three main types of parasites that infect humans: protozoa, helminths, and ectoparasites. Each type is unique in the ways it spreads, multiplies, and affects your body.

Protozoa

Protozoa are single-celled organisms that infect your body and multiply inside it. These parasites spread to you by either direct contact with the skin or via mosquitos, pets, or ticks. Once they make contact with you, protozoa enter the bloodstream and spread to the lungs, intestines, heart, and pancreas.

Types of protozoa that affect people include:

• Babesia: Babesia infect the bloodstream, causing a rare infection that may cause flu-like symptoms

• Giardia duodenalis: This type causes a giardia infection, which is one of the most common causes of diarrhea

• Cryptosporidium parvum: Often spread through contaminated drinking or swimming water, cryptosporidium parvum, or “crypto” causes diarrhea

• Plasmodium: This class of protozoa causes malaria, which is a severe and deadly disease that’s more prevalent in tropical and subtropical climates

• Toxoplasma gondii: A type of protozoa that causes fever, swollen glands, and muscle aches

Helminths

Helminths are multi-cellular, worm-like organisms that are usually visible to the naked eye in their adult stages. These parasites typically affect the intestines, bloodstream, or tissues beneath the skin. Helminths are only able to multiply inside the body in their younger stages. There are three primary types of helminths that affect people, including:

• Flatworms: Also known as platyhelminths, flatworms are worms that have no segments. Tapeworms and cestodes are subtypes of flatworms that grow in the intestine, while trematodes are a subtype that can affect the liver and lungs.

• Roundworms: Adult forms of roundworms or nematodes are commonly found in the gastrointestinal tract, lymphatic system, blood, or tissues below the skin. In other cases, larval or immature roundworms can also infect tissues in the body.

• Thorny-headed worms: These parasitic worms, also called acanthocephalans, have thorny hooks at one end that latch onto your intestinal walls. These parasites spread through soil or contact with feces. The most common types of thorny-headed worms that affect people are hookworm, ascariasis, and whipworm.

Ectoparasites

Ectoparasites are ticks, fleas, lice, or mites that attach or burrow into your skin. These parasites can stay in your skin for weeks or even months. Not only do they cause health problems, but ectoparasites are also vectors (or, carriers) of more serious viruses and bacteria.

What Do Parasites Look Like?

Since there are many different kinds of parasites, each type can come in its own unique shapes and sizes. The three main types of parasites all have a distinct appearance.

Protozoa

As single-celled organisms, you need a microscope to see protozoa from the human eye. There are four types, which are classified by how they move:

• Sarcodina, or amoeba, which are blob-like and temporarily project their body to move

• Mastigophora, or flagellates, which move using hair-like strands called flagella

• Ciliophora, which have cilia, or hair-like projections that are shorter flagella

• Sporozoa, which do not move and form spores to spread

Helminths

Helminths may be microscopic in their larval stages but are visible when they are fully grown. You can find these parasites in your feces (poop). Tapeworms are ribbon-like and develop segments. But, other flatworms don't usually have segments. Thorny-headed worms have distinctive horns on one end. Roundworms, such as nematodes, appear round and spirally.

Ectoparasites

Ectoparasites are a diverse type of parasite. They include:

• Ticks, which have eight legs and tend to be small and dark-colored

• Head lice, which are six-legged tan or gray-colored insects that are the size of sesame seeds

• Mites, which appear as tiny black dots that cause raised lines on the skin

• Fleas, which look like small dark brown or red insects with a hard exoskeleton

Symptoms of a Parasitic Infection

The specific symptoms of a parasitic infection depend on the exact parasite that infected your body, how severe your infection is, and which part of the body is affected. However, some common symptoms to look out for include:

• Diarrhea

• Abdominal pain

• Gas and bloating

• Nausea

• Unintentional weight loss

• Increased appetite

• Dehydration

• Chills

• Fever

• Muscle aches

• Rash

• Insomnia

How To Know if You Have an Infection

If you're experiencing symptoms of a parasitic infection, it's good practice to see your healthcare provider for proper testing. Your provider will first take your medical history and ask about any recent travels. In addition, they may perform several laboratory tests, including:

• Fecal stool sample: If you have gastrointestinal symptoms like diarrhea or nausea, your provider will need to perform a fecal stool exam, or ova and parasite test. You will have to take a sample of your stool (poop) and then send it to a lab so a technician can screen your sample under a microscope to check for parasites or eggs.

• Endoscopy or colonoscopy: This diagnostic test allows your provider to look inside the intestines for signs of parasites or other disease. With endoscopy, your provider inserts an endoscope (or, a camera that is attached to a specialized tube) through your mouth. A colonoscopy also uses an endoscope, but this exam puts the endoscope through your anus.

• Blood tests: Blood tests can detect certain parasitic infections, which can help your provider understand what's causing your symptoms. One type of blood test, called serology, detects antigens, or chemicals that your body releases as a part of your immune system response. Another common test, known as the blood smear, involves analyzing a sample of your blood under a microscope to detect malaria, babesiosis, and other infections.

• Imaging exams: X-rays, magnetic resonance imaging (MRI), or computerized axial tomography (CAT) scans are all imaging methods that your provider can use to detect parasites that may be present in your organs.

Treatment

What kind of parasite you have determines your exact treatment plan. Generally, parasitic infections don't go away on their own and require some type of antiparasitic medication. Your provider may also prescribe antibiotics or antifungal medications, depending on the symptoms you're experiencing. Most medications are oral pills, but you can apply some treatments as topical creams on your skin.

The most common treatments include:

• Antiprotozoal drugs: Chloroquine (chloroquine phosphate), Humatin (paromomycin sulfate), and Fansidar (sulfadoxine/pyrimethamine)

• Antihelminthic agents: Bilitricide (praziquantel) and Albenza (albendazole)

• Antiscabietic creams: Sklice (ivermectin), Elimite (permethrin 1%), and Kwell (lindane)

• Anti-lice lotions: Ulesfia (benzyl alcohol 5%) and Ovide (malathion 5%)

• Over-the-counter lice treatments: Elimite (permethrin) and Pronto (piperonyl butoxide)

Prevention

There are several strategies you can take to prevent exposure to parasites. These include:

• Washing your hands frequently and thoroughly, especially when handling food

• Taking hygienic measures when cleaning up dog or cat poop and disposing it properly

• Following safe food handling procedures, cooking food thoroughly, and washing vegetables

• Keeping your pet up to date on veterinary care and vaccinations

• Following travel advisories regarding food and drink when traveling abroad

• Taking antimalarial drugs before traveling to regions that have higher rates of malaria

• Wearing insect sprays to prevent tick bites

A Quick Review

Parasites are organisms that live inside or on your body, which survive and multiply at your expense and make you sick. Ranging from single-celled organisms to lice or ticks, parasites can cause a wide variety of infections. Common signs of parasitic infections include diarrhea, nausea, abdominal pain, and fever, among others. Talk to your healthcare provider if you suspect that you have an infection. Your provider can help determine the exact treatment you need to improve your symptoms.

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