Is there a worm in your brain? It's actually a global problem

Magnetic resonance imaging (MRI) of a 25-year-old patient's brain shows cysts (purple) caused by infection with the pork tapeworm. © Zephyr/Getty Images

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The habits of parasites can be truly disgusting - they invade the host's body, live inside it, and feed on the host's tissues. Some of these parasites are particularly bizarre and terrifying. Not only do they eat away at the host's body, but they hijack the host's brain, causing the host to behave in a way that better suits the parasite's needs.

Cordyceps is a compound of the fruiting body of the Cordyceps sinensis and the sclerotium (corpse of the larvae) of the ergot family of the Hypocreales order. It parasitizes the larvae of the bat moth in the soil of the alpine meadows, turning the larvae into zombies. Under suitable conditions, in the summer, a long rod-shaped fruiting body grows from the head of the zombie insect, forming a complex of the fruiting body of the Cordyceps sinensis and the zombie insect's sclerotium (larvae corpse). © Parasite of the Day

There are hundreds of known parasites that can change their host's mind. Fortunately, most of them only parasitize insects. For example, Cordyceps sinensis is a fungus that, when it develops to a certain stage inside its insect host, "commands" the host to climb to a higher place, such as a tall blade of grass. And when the host climbs up high, it clings to it and dies. Then, the fungus's fruiting bodies slowly emerge from the insect's corpse, and finally the fungus's spores fall off. The fungus commands the insect to climb high in order to spread its offspring farther. Each fungus only attacks one or two specific host species, and then Cordyceps sinensis grows in various shapes, many of which are beautiful, terrifyingly beautiful.

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Illustration of a cysticercus: the one on the left is from a pig, showing the scolices turned inward. The one on the right is from the human intestine, showing the scolices turned outward. The scolices will eventually develop into the head of the adult tapeworm. © wikipedia

Several parasites affect the human central nervous system and can cause cysts in brain tissue. While such infections are relatively rare in the United States, they are devastating in many parts of the world.

The World Health Organization estimates that between 2.56 million and 8.3 million people worldwide suffer from neurocysticercosis, a brain infection caused by the pork tapeworm Taenia solium.[2] “It’s a really big problem in Latin America, sub-Saharan Africa, India, and other parts of Asia. It’s also a leading cause of LKS seizures,” says Clinton White, a parasitologist and professor of infectious diseases at the University of Texas Medical Branch at Galveston. “It’s kind of funny that people are focusing on neurocysticercosis when it’s a major disease.”

Scientific American spoke with Shafir and White about how the parasite infects the brain, what symptoms it causes, and how the infection can be diagnosed and treated.

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What are parasites and which ones can infect our brains?

Shafir: Normally, we don't have adult worms in our brains. What do occur in the brain are early developmental stages of the parasite, such as eggs or larvae. So the parasitic infections that can affect the brain are those that are in early developmental stages and most of them enter the brain accidentally.

I study a number of parasitic infections that affect the central nervous system. The most common of these is the pork tapeworm. It has a very complex life cycle, and humans become infected by eating undercooked infected pork .

Adult bloodworms in Guangdong. © Microboids

I also study Baylisascaris procyonis and Angiostrongylus cantonensis, larvae that travel through the body to try to find tissues they like and may accidentally end up in the brain. They can cause some very significant neuropathology.

Tell us about the life cycle of the pork tapeworm and how it infects humans.

Shaffir: Exactly, the pig is the natural host. So, the pig can get infected with tapeworms. In the pig, the tapeworms penetrate the intestinal wall and enter the muscle tissue, which is what people usually eat. So humans can get infected with the intestinal form of the tapeworm if they eat infected pork that is undercooked or raw. They excrete eggs in human feces, and if those eggs are eaten by pigs, the pigs get infected, and the cycle continues.

The pork tapeworm can grow more than 20 feet long in the human intestine, using four suckers on its head segment to attach to the intestinal wall and absorb nutrients. © Centers for Disease Control and Prevention

However, if an infected person accidentally comes into contact with feces (contaminants) themselves or another person (because many people do not have good hand hygiene habits) and swallows the eggs, they will hatch, penetrate the intestinal wall, and enter the muscle tissue. They can then reach any organ in the body, most commonly the subcutaneous tissue, as well as the brain and eyes.

So if you eat raw pork, you can get tapeworms - but it's an intestinal problem. If you come into contact with the feces of someone who has intestinal tapeworms, you can get neurological symptoms.

[Editor's note: There are two forms of human infection. One is the "primary host," called Taeniasis, which is caused by eating undercooked pork containing tapeworm cysts , resulting in the development of adult worms in the intestines, also known as intestinal parasites. This form usually has no symptoms; the infected person does not know they have tapeworms. Infection is easily treated with anthelmintic medications.

The other is a "secondary host," called cysticercosis, which results from ingesting tapeworm eggs (not cysts) through consumption of food or drinking water contaminated with feces from an infected adult worm. The eggs form cysts primarily in muscle.

What are the specific symptoms?

WHITE: The disease caused by pork tapeworm infection in human tissue is called cysticercosis, and when it infects the brain, the disease is called neurocysticercosis. In most parts of the body, the larvae don't cause too many problems until they eventually die.

The larvae that end up in the brain can live for several years and usually don't cause many problems. These cysts are small, round, bubble-like structures, about a centimeter in diameter, that are clear, fluid-filled vesicles.

From left to right: Tapeworm eggs at 400x magnification; a scan of a brain from a patient with neurocysticercosis, where the small black dots are tapeworm larval cysts; and a tapeworm cyst from human brain tissue. © Centers for Disease Control and Prevention

Sometimes, the cysts get large enough that if they get into the fluid surrounding the brain, called cerebrospinal fluid (CSF), they can block its flow. This can cause hydrocephalus, which is usually fatal unless the patient has emergency surgery. But the cysts don't usually cause many problems - problems usually arise from the inflammatory response caused when the larvae start to die - especially seizures.

Shaffir: Once the eggs of the pork tapeworm reach muscle tissue -- whether it's pig or human -- they form an encyst, which is a spherical membrane wall that surrounds the larvae and protects them. This process can also happen in the brain. Depending on where in the brain the cysts are located, they can interfere with pretty important brain functions.

© Medscape Reference

In fact, the Centers for Disease Control and Prevention has some pretty striking images of what neurocysticercosis looks like, with these cysts, or balls of eggs, in the brain that make it look like Swiss cheese (pictured above).

Does this cause memory loss?

White: No, it doesn't usually lead directly to memory loss. If you do careful psychological testing, you might find some mild (memory) problems, but that's not a common symptom. However, if a person is having frequent seizures, then they may eventually lose their memory. Those are more typical symptoms of mercury poisoning, by the way. (Editor's note: Kennedy said he was experiencing mercury poisoning at the same time he learned about his parasitic infection, according to The New York Times. Mercury exposure has been linked to damage to the central nervous system and can lead to memory loss and Alzheimer's disease.)

According to The New York Times, Kennedy claimed that the worm that infected him "ate" part of his brain. Can tapeworms "eat" brain tissue?

Shaffir: Discussions about brain-eating are best left to zombie movies, not scientific discussions. Parasitic infections that affect the brain do not eat brain tissue. However, that does not mean they cannot damage brain tissue. Such hyperbole clearly lacks basic scientific literacy and is highly problematic.

How is neurocysticercosis diagnosed and treated?

White: The primary diagnosis is an imaging test of the brain, such as a computed tomography (CT) scan or magnetic resonance imaging. However, the results of these scans can be confused with other findings, so confirmatory tests are needed to look for antibodies to the parasite. The CDC has developed a test for the pathogen—many commercial tests are not that accurate. Recently, scientists have developed PCR tests and antigen tests that can be very helpful in severe cases.

Treatment starts with relieving symptoms. If the person has seizures, they should be given anti-seizure medications. If they have hydrocephalus, neurosurgery may be needed. Anti-inflammatory and anti-parasitic medications often speed up the death of the parasites and also reduce seizures. Sometimes multiple treatments are needed to kill the parasites, but they will die. Even without treatment, they cannot live forever. In some cases, calcified lesions (caused by the larvae) have been there for a long time, and these lesions may cause damage to the hippocampus in the brain. This may be related to seizures that will not subside until surgery is done.

How common are these infections?

Shafir: Generally speaking, cysticercosis (and neurocysticercosis) is more common in low-income countries and regions, including Latin America. We do a lot of work in communities there. Since part of the tapeworm life cycle requires individuals to eat raw or undercooked pork, and this infected pork is allowed to be bought and sold, we don’t generally see tapeworm transmission in the United States because we have strict inspections by the Department of Agriculture. This means that people who are infected either come from communities where pork infection is common or have traveled to those countries. In the United States, there are about 1,000 hospitalizations each year, and these cases occur in states with the most international travel, such as New York, California, Texas, and Illinois.

By Lauren J. Young

Translated by tamiya2

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