Neurocysticercosis is a disease in which tapeworm larvae of the Taenia genus invade the tissues of the brain and central nervous system to form cysts. These cysts, when present in non-neural tissue, are known as cysticerci. They may form in many soft tissues of the body, including skin, muscles, and the heart, but neurocysticercosis is the most dangerous form of infection.
The encysted larvae may exist in a dormant state in the brain for years without provoking an attack from the immune system. It is only when there are a large number of cysts, or when a cyst dies and releases its contents, that the body mounts an immune response against the invading pathogen. Neurocysticercosis was mainly a disease of the developing world, but is becoming more frequently observed in the United States. It is usually detected by magnetic resonance imaging (MRI) during hospital visits for headache or seizure.
Humans come in contact with the tapeworm eggs by ingesting food contaminated with human feces, which contain the eggs. Since the adult form of Taenia solium, the pork tapeworm, can only exist in humans, only human fecal material will contain eggs. Taenia sanginata, the beef tapeworm, rarely causes neurocysticercosis. The protective cover of the eggs is dissolved by stomach acid and the invasive larvae enter the bloodstream. From there, they can travel to a number of tissues.
Cysticercosis can also form in pigs when they ingest food contaminated by tapeworm eggs. These will hatch and form cysts in the pig’s body. If proper care is not taken when processing or cooking pork products for human consumption, the larvae can enter the small intestine through the normal digestive process, hook itself onto the intestinal wall, and develop into an adult tapeworm. In people who carry an adult tapeworm, autoinfection is possible via retroperistalsis, when the contents of the digestive tract move in the opposite direction. This usually occurs during vomiting when the adult tapeworm segments containing eggs are pushed back into the stomach and the eggs hatch via the normal infection route.
Neurocysticercosis can be treated in various ways, according to the amount, condition, and location of cysts present in the nervous system. A single cyst in the eye may require surgical treatment to prevent blindness caused by the pressure of the cyst against the optic nerve; a cyst in the spine may lead to paralysis; cysticerci in the brain can lead to epileptic seizures, depending on their location. A viable larval cyst will elicit only a very weak immune response, but a deteriorating cyst will release its contents into the host environment, which will cause an attack by the immune system.
Whether or not a patient should be treated is a controversial topic. Some researchers believe that using anti-parasitic drugs that kill the larvae will only aggravate the patient’s symptoms, as it is the dead cyst that provokes an immune response. When treatment is prescribed, it usually involves a combination of anti-parasitic and anti-inflammatory drugs. In many cases, patients with a small number of cysts will not be treated with medication. Surgical removal of cysts is accompanied by anti-inflammatory drugs. Patients whose seizures are caused by a dead cyst are usually not treated, but the symptoms should diminish as the larvae are gradually removed by the body with time.
Neurocysticercosis is present in 70% - 90% of patients presenting with acutely symptomatic seizures. Epileptic seizures usually begin when the cyst is still viable or just beginning to degenerate. However, chronic seizures are mostly caused by dead cysts. The first epileptic episode usually occurs when the cyst is beginning to degenerate by releasing its contents into the brain. The location of the cyst in the brain and the patient’s immune response are the factors that set off the first seizure. Neurocysticercosis can be prevented by practicing proper hygiene and cooking or freezing meats thoroughly. Beware of any food from locations where human waste may be used as fertilizer.