Neurocysticercosis is caused by the larvae of Taenia solium, a tapeworm found in pork products. The worm is thought to have infected at least 50 million people worldwide.
Neurocysticercosis is most common where pigs are raised and sanitation is poor. In the Western hemisphere, the disease is most common in Mexico and South America, but an increasing number of cases have been seen in the United States as well. It is also common in Eastern countries, such as India.
The Normal Parasitic Life Cycle
Humans often begin to carry T. solium tapeworms after ingesting undercooked pork. The worms exist in the undercooked pig muscle in the form of cysticeri—small sacks of tissue containing the larval head of the organism, which is called the scolex. The scolex consists of a mouth with suckers and hooks, and a base from which thousands of segments arise—one after the other—like links in a chain, to form an adult worm.
After ingestion, the scolex attaches to the human small intestine using its suckers and hooks. There the tapeworm lives, attached to the small intestine, sometimes remaining for years. During this time, the scolex continuously adds sections to its length; the entire worm can reach up to seven meters long. Each section of the worm can produce tens of thousands of eggs to be excreted in human feces. If eaten by pigs, the eggs grow into larval cysts in the muscle, humans eat the meat, and the cycle continues. If eaten by humans, the eggs cause even more serious problems.
How Neurocysticercosis Develops
There is a common misconception that neurocysticercosis is directly caused by eating undercooked pork. Eating cysts does not lead to neurocysticercosis, but rather to an intestinal tapeworm infection as we just discussed. To develop neurocysticerosis, the eggs—not the cysts—must be ingested.
When a human eats Taenia solium eggs that have been shed in the stool of a carrier, the eggs hatch in the small intestine, and worm embryos invade the bowel wall and spread throughout the body, particularly to muscles and the brain. Three to eight weeks later, the next phase of the worm's life cycle begins, and the embryos change into cysticerci: the fluid filled cyst containing the scolex.
The tapeworm cysts are very sophisticated in manipulating and evading the human immune system, and may not cause any kind of inflammatory response for years. Eventually, though, the immune system detects the cysts. The resulting inflammatory response leads to swelling and seizures, and the result of the battle between the cyst and the human immune system is often a calcified granuloma--a kind of circular barrier to wall off the worm from the rest of the body. This granuloma itself can be a source of further seizures.
Symptoms of Neurocysticercosis
Symptoms caused by neurocysticercosis depend on the location of the cyst. If the cyst is in the brain, it can cause seizures or headaches. If the cysts are in the tissues surrounding the brain, the symptoms can indicate elevated intracranial pressure, especially if the cyst blocks the outflow of cerebrospinal fluid from the ventricles, causing hydrocephalus. There is usually no fever associated with this infection. In rare cases, cysts can involve the spinal cord or eye. Numbness, tingling, or weakness can result.
Diagnosis of Neurocysticercosis
Neuroimaging is the best way of diagnosing neurocysticercosis. A CT scan is often sufficient, though in some cases an MRI is also helpful. Most of the cysts found on imaging are small: between 5-20 mm for living cysts and 2 to 4 mm for granulomas. Sometimes, though, the cysts can grow to 20 cm.
Blood tests are also available to check for antibodies to Taenia solium, but a negative blood test should not be taken to mean that no infection is present. A lumbar puncture is not usually necessary for the diagnosis, and may be harmful if intracranial pressure is elevated.
Treatment and Prevention
The first step in managing someone with neurocysticercosis is to manage any seizures or elevated intracranial pressure that have results. Seizures can be handled with anti-epileptic therapy. If the immune system has already caused the cysts to calcify, epileptics may be required for years. Elevated intracranial pressure may need to be managed in any one of a number of ways. If the cysts are so large that they are pressing on important structures, they may need to be surgically removed.
The next step is to start antiparasitic therapy in order to resolve any active cysts, and to reduce the risk of further seizures or hydrocephalus. Medications used to attack the larval Taenia solium cysts include albendazole or praziquantel. Because inflammation and swelling can worsen during this treatment, it is important to give corticosteroids as well in order to avoid potentially disabling inflammation.
Of course, the best option is never to acquire the parasite in the first place. Hand washing prior to preparing food is critical in preventing this infection. Diseased pork has a measly appearance, and should be avoided. Meat must be well cooked or frozen in order to destroy cysticerci. Pickling and salting does not adequately destroy the worm. Being aware of these basic steps can avoid the dangerous consequences of having parasitic tapeworm larvae invade the brain.
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