But How Does the Worm Get in Your Brain?

Robert F. Kennedy Jr.’s disclosure that a doctor apparently found a dead worm in his brain has sparked questions about what brain parasites are, the damage they can cause and how, exactly, they get there.

How do parasites turn up in people? And what can they do to the brain?

Brain parasites encompass far more than worms. There are “legions” of organisms that can affect the brain, said Scott Gardner, a professor of biological sciences at the University of Nebraska-Lincoln who specializes in parasites. In addition to worms, common brain parasites include single-celled organisms such as Toxoplasma gondii and some amoebas.

The damage varies depending on the type of parasite and where it ends up in the brain. “Some of them actively invade the tissues and destroy tissues,” said Dr. Daniel Pastula, chief of neuro-infectious diseases and global neurology at University of Colorado Medicine. Others cause problems because of the inflammatory reaction that they trigger.

Tapeworms

Humans are typically exposed to tapeworms through raw or undercooked food or through food contaminated with feces.

“A lot of these things are transmitted to humans through feces,” said Dr. Edith L. Graham, a neurologist at Northwestern Medicine.

Doctors consulted by The New York Times speculated that Mr. Kennedy described symptoms of an infection with larvae from the pork tapeworm, Taenia solium, one of multiple types of tapeworm that can infect the brain. When a person accidentally swallows pork tapeworm eggs, the eggs hatch in the intestines, and the larvae can travel to other organs, including the brain. There, they form cysts, causing a condition known as neurocysticercosis.

It can take months, or even years, for people to show signs of infection. Symptoms vary based on how many cysts develop and where they are. (Cysts can form in the eyes, muscles and spinal cord.) Generally, though, people with neurocysticercosis experience headaches and seizures, and they sometimes feel confused, struggle to pay attention and have issues with balance. The condition can be fatal.

Amoebas

Another type of parasite that can affect the brain is known as a brain-eating amoeba. Infections of this kind are extremely rare but can lead to a potentially fatal swelling of the brain or spinal cord, said Tajie H. Harris, an associate professor of neuroscience at the University of Virginia. The organisms enter through the nose when people swim in lakes and rivers, and then travel to the brain. People have also been infected through using neti pots or other sinus rinses with untreated or non-sterile water.

Toxoplasma gondii

One of the most common parasitic brain infections is toxoplasmosis. Over 40 million people in the United States may be infected with the parasite that causes that infection, according to the Centers for Disease Control and Prevention. People can be infected by eating undercooked meat or shellfish that is contaminated, by drinking contaminated water or by accidentally swallowing the parasite when they encounter it in cat feces, as when cleaning litter boxes.

The parasite that causes toxoplasmosis can linger in humans for years — potentially, for someone’s entire life — but most people will not develop symptoms. “For the most part, our immune system does an amazing job at just handling and dealing with this parasite, allowing us to live our lives without ever knowing,” Dr. Harris said.

But it can make some people feel like they have the flu. People with compromised immune systems are at risk of serious illness and brain damage if infected, and developing fetuses can be severely affected if pregnant women are exposed. (That’s why doctors sometimes advise pregnant women to avoid coming into contact with cat litter.)

How do you figure out if you have a parasite?

“You wouldn’t know yourself, unless someone is looking,” Dr. Gardner said. Sometimes a patient’s blood can be tested for antibodies that are produced in response to a parasite. In other cases, doctors diagnose infections with an M.R.I. or C.T. scan.

Treatment options vary because infections span such a wide range. Many are treatable with antiparasitic drugs, which patients may need to take for weeks. Doctors may also prescribe steroids to help with inflammation. Some patients will undergo surgery to remove cysts.

In the United States, severe infections from brain parasites are rare, Dr. Pastula said, “but in other parts of the world, they’re more common.”

Fortunately, many of these infections are preventable, Dr. Pastula said. To lower your risk, especially while traveling internationally, he advised washing hands thoroughly before you eat or prepare food, cooking food properly and ensuring that the water you’re drinking is clean.