Brain Parasites

Parasites (PAIR-uh-sites) are organisms that live inside a host organism and derive their nutrients from that host. Human brain parasites are either single-celled animals called protozoa * or worms. Most parasites initially infect other parts of the body and only rarely reach the brain.

What Are Brain Parasites?

Parasitic brain diseases can be caused directly by the parasite or by the body's reaction to the parasite. Brain parasites frequently cause encephalitis (en-seh-fuh-LYE-tis) or inflammation of the brain; meningitis (meh-nin-JY-tis) or inflammation of the meninges (the membranes that surround the brain and spinal cord); or both (meningoencephalitis).

Naegleri fowleri

N. fowleri is known as the “brain-eating amoeba.” This microscopic protozoan lives in warm freshwater lakes and rivers, in hot springs, and in the soil. Very rarely, N. fowleri occurs in inadequately chlorinated swimming pools or heated contaminated tap water. If the amoeba enters the nose, it can travel to the brain and destroy brain tissue, causing primary amebic meningoencephalitis (PAM). Swallowing N. fowleri does not cause infection. PAM is usually fatal in a very short time.

Loa loa

L. loa is a thread like roundworm or nematode * that causes loaiasis. L. loa is transmitted to humans by two species of biting flies in the genus Chrysops. Loaiasis is also called subcutaneous filariasis, because the worms (filaria) generally live under the skin in the subcutaneous tissues. It is also called African eye worm, because the worms are sometimes seen moving across the eye. Rarely, severe L. loa infections reach the brain and cause encephalitis. Encephalitis also can be caused by large numbers of worms dying simultaneously during treatment. Although L. loa encephalitis is rare, it is an important public health problem, in part, because it interferes with the use of drugs for treating other filarial diseases.


Neurocysticercosis is a form of cysticercosis that affects the brain and spinal cord. Cysticercosis is caused by ingesting eggs of the pork tapeworm Taenia solium that are found in infected feces * or in undercooked or improperly prepared contaminated foods. T. solium forms larval cysts * in various body tissues. Cysts in the brain and spinal cord cause neurocysticercosis.

Loa loa Life Cycle

The L. loa worm has a complicated life cycle that is dependent on both fly and human hosts. When infected deerflies and mango flies of the genus Chrysops bite humans, they introduce third-stage larval L. loa into the open wound. Over about five months, the larvae develop into adults in the tissues beneath the skin. Each day, the adult females produce thousands of microfilariae that enter the human host's lymph vessels, blood, lungs, and spinal fluid. Adults can live for up to 17 years in the human body, producing microfilariae. When flies take a blood meal from an infected human, they ingest the microfilariae, which then undergo development into third-stage larvae that can infect other humans.

Other Brain Parasites

How Common Are Brain Parasites?

Although parasitic diseases are very common, brain parasites are relatively rare because the blood-brain barrier * usually protects the brain from foreign invaders. Some parasites only invade the brains of people who have weakened or compromised immune systems, such as from HIV/AIDS * , organ transplants, diabetes, or drugs such as steroids or chemotherapy.

Symptoms of meningitis can include:

N. fowleri

Early symptoms of PAM can be similar to those of bacterial meningitis, beginning one to seven days after infection. Later symptoms include loss of balance, confusion, hallucinations, and seizures. PAM progresses rapidly, usually resulting in death within about five days.

L. loa

Most people infected with L. lia have no symptoms. If symptoms do develop, they usually do not appear until many months after infection. Symptoms are more common in visitors to areas where L. lia is endemic. The most common symptoms are eye worm and recurrent itchy swellings called Calabar swellings. Encephalitis may develop before or after treatment. Spontaneous L. lia encephalitis can cause a variety of symptoms that last for one to three months and are usually fatal.


This is the most serious form of cysticercosis, although it sometimes does not cause any symptoms. Symptoms may develop months or even years after infection, when the cysts begin to die and the surrounding tissue swells, causing increased pressure in the brain. Pressure from the cysts themselves can also cause symptoms. Specific symptoms depend on how many cysts form in the brain and the parts of the brain that are affected. They can be similar to symptoms of a brain tumor and may include:

A. cantonensis

This parasite may cause only mild, short-lasting symptoms or none at all. However, sometimes angiostrongyliasis causes meningitis with symptoms that include tingling or painful skin sensations.


Symptoms of baylisascariasis depend on how many eggs have been ingested and where the larvae migrate from the intestines. Symptoms generally develop about one week after infection, but ingestion of just a few eggs may not cause any symptoms. Infection of the brain and spinal cord can cause:


Trypanosomiasis symptoms begin within weeks to months after infection. East African trypanosomiasis progresses much more rapidly than the West African type. Symptoms can appear suddenly and, in the first stage of infection, may include:

In the second stage of trypanosomiasis, the parasite crosses the bloodbrain barrier and infects the brain and spinal cord. Symptoms can include progressive confusion, personality changes, sleep disturbances, balance and walking problems, partial paralysis, hearing and speech loss, blindness, and coma. Untreated trypanosomiasis can cause brain damage and death within months or years.

Chagas disease begins with an acute phase lasting for a few weeks or months with vague, mild symptoms or no symptoms at all. Rarely, meningoencephalitis develops during the acute stage, especially in young children and people with weakened immune systems. In the chronic phase of infection, symptoms may not appear for decades—or ever—although some people develop complications.


Symptoms of schistosomiasis are caused by the body's reaction to the worm eggs. Most people have no symptoms during the early phase of infection. However, eggs in the brain or spinal cord can cause inflammation, paralysis, and/or seizures.

How Do Doctors Diagnose and Treat Brain Parasites?


Diagnosis of a brain parasite is based on the symptoms and a physical exam, as well as circumstances: whether the patient has lived in or traveled to areas where brain parasites are endemic, been exposed to insects or animals, eaten certain types of food, or engaged in activities such as swimming in potentially contaminated water. The neurologic exam assesses brain and nerve function, including hearing and speech, vision, coordination and balance, mental status, and mood and behavior changes. Electroencephalography * (EEG) can indicate abnormal electrical activity in the brain and help diagnose seizures. If there are signs and symptoms of infection elsewhere in the body, tissue may be removed (biopsied) to identify the parasite under a microscope. Cultures may be grown from body samples to test for bacterial or viral causes of encephalitis or meningitis. Blood tests may be able to detect antibodies against a parasite or reveal high levels of cells called eosinophils. However, a positive antibody test indicates only that the patient once had the infection—not whether parasites are still living in the body. Computed tomography * (CT) scans or magnetic resonance imaging * (MRI) of the brain may show inflammation, swelling, bleeding, or other signs of encephalitis, meningitis, or the parasite itself. A spinal tap (lumbar puncture) removes cerebrospinal fluid * (CSF) from around the lower spinal cord to test for signs of inflammation and parasitic infection.

Other diagnostic techniques depend on the specific parasite:


Encephalitis is treated by supportive nursing, nutrition, and rehydration with fluids, possibly in an intensive care unit. Other treatments depend on the particular parasite:


Brain parasites can lead to brain damage, blindness, seizures, and death. Some parasites can cause complications elsewhere in the body, such as heart or kidney damage. Chagas disease can cause gastrointestinal and heart problems as well as brain abnormalities.

Can Brain Parasites Be Prevented?

Prevention of N. fowleri infection involves reducing the risk of contaminated water going up the nose. This includes avoiding bathing and swimming in contaminated water, as well as keeping human waste away from water sources. Only boiled, filtered, sterile, or distilled water or water disinfected with chlorine bleach should be used for flushing, rinsing, or irrigating the sinuses.

Where L. loa is endemic, the risk of infection can be reduced by avoiding areas such as muddy, shaded riverbanks and wood fires that attract the deerflies and mango flies that spread the parasite. Weekly doses of diethylcarbamazine reduce the risk of loaiasis. Other measures for reducing the risk of insect-borne parasites include:

Neurocysticercosis and other parasitic diseases that are spread through feces can be prevented by:

A. cantonensis infection can be be prevented by not eating raw or undercooked snails, slugs, frogs, shrimp, or prawns. Snails and slugs should be handled with gloves, and hands should be washed after contact. Fresh produce should be thoroughly washed, and uncooked vegetables should not be consumed when traveling to areas where the parasite is endemic.

Toxoplasmosis is an opportunistic infection in people with suppressed immune systems, such as those with HIV/AIDS. Therefore, prevention includes treating HIV, avoiding undercooked meat and fish, and wearing gloves and a face mask when cleaning cat boxes and washing thoroughly afterward. Drugs are available to help prevent toxoplasmosis in susceptible people.

See also Chagas Disease • Encephalitis • Malaria • Meningitis • Parasitic Diseases: Overview • Toxoplasmosis • Trypanosomiasis



Centers for Disease Control and Prevention. “Naegleria fowleri—Primary Amebic Meningoencephalitis (PAM)—Amebic Encephalitis.” (accessed March 25, 2016).

Centers for Disease Control and Prevention. “Parasites—Cysticercosis.” (accessed March 25, 2016).

Centers for Disease Control and Prevention. “Parasites—Loaiasis.” (accessed March 25, 2016).

MedlinePlus. “Cysticercosis.” U.S. National Library of Medicine, National Institutes of Health. (accessed March 25, 2016).


Centers for Disease Control and Prevention. 1600 Clifton Rd., Atlanta, GA 30329-4027. Toll-free: 800-232-4636 (800-CDC-INFO). Website: (accessed March 25, 2016).

National Institute of Allergy and Infectious Diseases. Office of Communications and Government Relations, 5601 Fishers Ln., MSC 9806, Bethesda, MD 20892-9806. Telephone: 301-496-5717. Toll-free: 866-284-4107. Fax: 866-284-4107. Website: (accessed March 25, 2016).

* protozoa (pro-tuh-ZOH-uh) are single-celled microorganisms (tiny organisms), some of which are human parasites.

* nematode (NEE-muh-tode) is a parasitic roundworm that infects humans.

* feces (FEE-seez) are the wastes excreted from the gastrointestinal tract through the anus.

* cysts (SISTS) are shell-like enclosures that contain tiny organisms in a resting stage.

* larvae (LAR-vee) are the immature forms of an insect or worm that hatch from eggs.

* coma (KO-ma) is an unconscious state in which a person cannot be awakened and cannot move, see, speak, or hear.

* seizures (SEE-zhurs) are sudden bursts of disorganized electrical activity that interrupt the normal functioning of the brain, often leading to uncontrolled movements in the body and sometimes a temporary change in consciousness.

* blood-brain barrier is a biological shield in blood vessels that helps prevent parasites or other potentially harmful organisms or materials in the blood from entering the brain and spinal cord.

* HIV/AIDS is infection with the human immunodeficiency (ih-myoo-no-dih-FIH-shen-see) virus that causes acquired immunodeficiency syndrome that severely weakens the immune system.

* endemic (en-DEH-mik) describes a disease or condition that is present in a population or geographic area at all times.

* dementia (dih-MEN-shuh) is a loss of mental abilities, including memory, understanding, and judgment.

* electroencephalography (EEG) is the monitoring of electrical activity in the brain.

* computed tomography (toeMAH-gruh-fee) (CT) or computerized axial tomography (CAT) uses x-rays and computers to visualize structures within the body.

* magnetic resonance imaging (MRI) uses magnetic waves to scan the body and produce detailed pictures of internal structures.

* cerebrospinal fluid (seh-ree-broSPY-nuhl) (CSF) is the fluid that surrounds the brain and spinal cord.

* antibody is a protein produced by the body's immune system to help fight a specific infection caused by a foreign organism such as a parasite.

Disclaimer:   This information is not a tool for self-diagnosis or a substitute for professional care.

(MLA 8th Edition)