CYSTICERCOSIS

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Cysticercosis is a parasitic tissue infection caused by larval cysts of the tapeworm Taenia solium. These larval cysts infect brain, muscle, or other tissue, and are a major cause of adult onset seizures in most low-income countries. A person gets cysticercosis by swallowing eggs found in the feces of a person who has an intestinal tapeworm.  People living in the same household with someone who has a tapeworm have a much higher risk of getting cysticercosis than people who don’t. People do not get cysticercosis by eating undercooked pork.  Eating undercooked pork can result in intestinal tapeworm if the pork contains larval cysts.  Pigs become infected by eating tapeworm eggs in the feces of a human infected with a tapeworm.

Both the tapeworm infection, also known as taeniasis, and cysticercosis occur globally. The highest rates of infection are found in areas of Latin America, Asia, and Africa that have poor sanitation and free-ranging pigs that have access to human feces. Although uncommon, cysticercosis can occur in people who have never traveled outside of the United States. For example, a person infected with a tapeworm who does not wash his or her hands might accidentally contaminate food with tapeworm eggs while preparing it for others.

Epidemiology & Risk Factors for Cysticercosis

Cysticercosis is an infection caused by the larvae of the tapeworm, Taenia solium. A person with an adult tapeworm, which lives in the person’s gut, sheds eggs in the stool.  The infection with the adult tapeworm is called taeniasis.  A pig then eats the eggs in the stool.  The eggs develop into larvae inside the pig and form cysts (called cysticerci) in the pig’s muscles or other tissues. The infection with the cysts is called cysticercosis. Humans who eat undercooked or raw infected pork swallow the cysts in the meat. The larvae then come out of their cysts in the human gut and develop into adult tapeworms, completing the cycle.

People get cysticercosis when they swallow eggs that are excreted in the stool of people with the adult tapeworm. This may happen when people

  • Drink water or eat food contaminated with tapeworm eggs
  • Put contaminated fingers in their mouth

Cysticercosis is not spread by eating undercooked meat. However, people get infected with tapeworms (taeniasis) by eating undercooked infected pork. People who have tapeworm infections can infect themselves with the eggs and develop cysticercosis (this is called autoinfection). They can also infect other people if they have poor hygiene and contaminate food or water that other people swallow. People who live with someone who has a tapeworm infection in their intestines have a much higher risk of getting cysticercosis than other people.

Human cysticercosis is found worldwide, especially in areas where pig cysticercosis is common. Both taeniasis and cysticercosis are most often found in rural areas of developing countries with poor sanitation, where pigs roam freely and eat human feces. Taeniasis and cysticercosis are rare among persons who live in countries where pigs are not raised and in countries where pigs do not have contact with human feces. Although uncommon, cysticercosis can occur in people who have never traveled outside of the United States if they are exposed to tapeworm eggs.

Biology of Cysticercosis

Causal Agent of Cysticercosis

Cysticercosis is the disease associated with the development of the larval form (cysticercus) of the pork tapeworm, Taenia solium, within an intermediate host. Swine are the usual intermediate host for T. solium but humans, the usual definitive host, can serve as accidental intermediate hosts following ingestion of infectious eggs. Note that cysticercosis is only acquired from the fecal-oral route (ingestion of eggs)not via the ingestion of cysticerci in undercooked pork, which is associated with intestinal taeniasis.

Figure 1. Life cycle of Cysticercosis. Cysticercosis is an infection of both humans and pigs with the larval stages of the parasitic cestode, Taenia solium. This infection is caused by ingestion of eggs shed in the feces of a human tapeworm carrier (1). These eggs are immediately infectious and do not require a developmental period outside the host. Pigs and humans become infected by ingesting eggs or gravid proglottids (2,7). Humans are usually exposed to eggs by ingestion of food/water contaminated with feces containing these eggs or proglottids or by person-to-person spread. Tapeworm carriers can also infect themselves through fecal-oral transmission (e.g. caused by poor hand hygiene). Once eggs or proglottids are ingested, oncospheres hatch in the intestine (3,8), invade the intestinal wall, enter the bloodstream, and migrate to multiple tissues and organs where they mature into cysticerci over 60–70 days (4,9). Some cysticerci will migrate to the central nervous system, causing serious sequellae (neurocysticercosis). This differs from taeniasis, which is an intestinal infection with the adult tapeworm. Humans acquire intestinal infections with T. solium after eating undercooked pork containing cysticerci (5). Cysts evaginate and attach to the small intestine by their scolices. Adult tapeworms develop to maturity and may reside in the small intestine for years (6).

Hosts of T. solium

Humans are normal definitive host for T. solium; cysticercosis results from humans acting as accidental intermediate hosts for the parasite (this role is normally fulfilled by swine).

Geographic Distribution of Cysticercosis

Taenia solium is found nearly worldwide. Because pigs are intermediate hosts of the parasite, completion of the life cycle occurs in regions where humans live in close contact with pigs and eat undercooked pork. Poor sanitation leading to environmental fecal contamination is a major factor in transmission. Cysticercosis mainly affects low- and middle-income countries in Africa, Asia (e.g., India, China, and Nepal) and Latin America (e.g., Guatemala, Nicaragua, El Salvador).

It is important to note that human cysticercosis is acquired by ingesting T. solium eggs shed in the feces of a human T. solium tapeworm carrier (e.g. on contaminated food items), and thus can still occur in populations that neither eat pork nor share environments with pigs, as long as the human carrier is present.

Clinical Presentation of Cysticercosis

The symptoms of cysticercosis vary depending upon the location and number of cysticerci. Cysticerci may develop in skeletal and heart muscle, skin, subcutaneous tissues, the lungs, liver, and other tissues, including the oral mucosa. In most locations, cysticerci cause few symptoms and spontaneously degenerate.

Cysticerci can migrate to the central nervous system and cause neurocysticercosis (NCC), which is associated with serious neurological and epileptic manifestations (Figure 2). Death can occur suddenly. NCC is typically divided into parenchymal and extraparenchymal disease. Parenchymal NCC occurs when cysticerci develop within the brain tissue. Extraparynchymal NCC occurs when cysticerci develop in other parts of the nervous system, such as the subarachnoid space, meninges, ventricles, spine, or eyes. A rare variant called “racemose cysticercosis” involves the development of cysts that are unusually large, multilobular, and clustered; they may lack a scolex. This form usually occurs in extraparenchymal sites (e.g., subarachnoid space, meninges). Mixed parenchymal and extraparenchymal disease can occur.

Figure 2. Taenia egg at a high magnification of 400x. Left: When consumed by humans, Taenia solium eggs can lead to cysticercosis, including a serious condition known as neurocysticercosis. Center: A radiographic image of the brain of a patient who has neurocysticercosis; the small dark spots within the brain are larval cysts of T. solium. Right: A cross-section through a T. solium cyst from a human brain tissue specimen, stained with hematoxylin and eosin (H&E).

How the disease (Cysticercosis) progresses in the human body

Cysts, called cysticerci, can develop in the muscles, the eyes, the brain, and/or the spinal cord.  Symptoms caused by the cysts depend on the location, size, number, and stage of the cysts.

Cysts in the brain or spinal cord:

  • Cause the most serious form of the disease, called neurocysticercosis
  • May cause no symptoms
  • May cause seizures and/or headaches (these are more common)
  • May also cause confusion, difficulty with balance, brain swelling, and excess fluid around the brain (these are less common)
  • May cause stroke or death

Cysts in the muscles:

  • Generally do not cause symptoms
  • May cause lumps under the skin, which can sometimes become tender

Diagnosis of Cysticercosis

The diagnosis of neurocysticercosis usually requires magnetic resonance imaging (MRI) or Computed Tomography (CT) brain scans. Blood tests may be useful to help diagnose an infection, but they may not always be positive in light infections.

Treatment of Cysticercosis

Some people with cysticercosis do not need to be treated. There are medications available to treat cysticercosis for those who do need treatment. Sometimes surgery may be needed. It is better for people with symptoms of Cysticercosis to seek medical intervention by visiting a hospital.

Prevention and control of Cysticercosis

To prevent cysticercosis, the following precautions should be taken:

  • Wash your hands with soap and warm water after using the toilet, changing diapers, and before handling food
  • Teach children the importance of washing hands to prevent infection
  • Wash and peel all raw vegetables and fruits before eating
  • Use good food and water safety practices while traveling in developing countries such as:
    • Drink only bottled or boiled (1 minute) water or carbonated (bubbly) drinks in cans or bottles
    • Filter unsafe water through an “absolute 1 micron or less” filter AND dissolve iodine tablets in the filtered water; “absolute 1 micron” filters can be found in camping and outdoor supply stores.

Source:

https://www.cdc.gov/parasites/cysticercosis/

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