Cestodes

Cestodes are tapeworms: class Cestoidea; phylum Platyhelminthes; subclass Cestoda. They are specialized flatworms, looking very much like a narrow piece of adhesive tape. Tapeworms are the largest, and among the oldest, of the intestinal parasites that have plagued humans and other animals since time began. Found all over the world, tapeworms exist in many different forms, but they have no close relatives living outside of animal hosts. Tapeworms do not have a mouth like the fluke, nor do they have a head or a digestive tract with digestive enzymes. The ends differ, but neither has any organs or sensors that could be associated with what is commonly thought of being a "head." However, through a segment called a scolex, they are able to absorb predigested food. The scolex attaches to the intestinal wall by hooks or suckers. The body contains hundreds of segments (proglottids), and each is a sexually complete unit that can reproduce, if necessary. Some tapeworms have reached lengths of more than ten meters (thirty feet) with a lifespan, inside a host, of thirty years or more. Cestodaria is the unsegmented subclass of tapeworm affecting various fish and some reptiles.

Tapeworms are dependent on two hosts for their development, one human and the other animal. Larvae are found in animal hosts, while the adult worm is found in humans. However, there are two species where this development is reversed. Echinococcus granulosis and E. multilocularis differ from other tapeworms in that it is the adult worm that infects an animal host, while the larvae form produces slow growing cysts in humans. This condition is known as echinococcosis or Hydatid disease, which takes surgical intervention to remove the cysts.

Human infection comes as a result of eating insufficiently cooked meat (especially beef, pork, and fish), where the larvae are buried in the tissues of the animal involved. The fleas of both dogs and cats can also transmit tapeworm larvae. Rice-shaped particles that resemble pumpkin seeds in the stool can be a sign of a dog tapeworm, which can easily be mistaken for pinworms. It is important to deflea household animals frequently.

Some of the conditions and symptoms that tapeworms can cause include the following: mineral imbalances, abnormal thyroid function, intestinal gas, blood sugar imbalances, bloating, jaundice, fluid buildup, dizziness, fuzzy thinking, hunger pains, poor digestion, allergies, sensitivity to touch, weight changes, and symptoms of pernicious anemia. Treatment can take months before the entire worm is expelled. It is suggested that long periods of fasting not be undertaken since tapeworms cannot be starved, but will only leave the person feeling weak and nauseated. It is better to eat foods that tapeworms do not like, as onions and garlic. This weakens the worm so that it loses its grip. Then it is easily dislodged and can be expelled.

Tapeworms known as Spirometra and a roundworm called Gnathostoma can develop in humans after eating raw snake. Spirometra causes a bizarre eye disease called Sparganosis. In the Far East, sometimes poultices of raw frogs or snake muscle are put directly on the eye to cure various ailments. Tapeworm larvae then migrate into the tissue around the eye. This is not solely a condition possible only in underdeveloped countries. For example, raw meat is often used in the western world for bruises and black eyes. The other unusual tapeworm is the gnathstomes worm, which attaches to the stomach wall of animals and humans. It eventually passes the eggs of a tiny one-eyed bug called a "cyclops". Fish, frogs, or snakes later eat these infected bugs, which ultimately infect that animal. As it moves up the food chain, humans become infected.

There are five types of tapeworms that can infect humans.

  1. Taenia saginata is the beef tapeworm that is transmitted to humans after the ingestion of undercooked or raw beef. The worm can grow to a strand of 1,000 to 2,000 segments. Each segment is known as a proglottid, and contains both male and female reproductive organs, capable of bearing fertilized eggs. Tapeworms thrive on the diet of the host, and is dependant on their carbohydrates, but also utilizing the tissues to obtain proteins. Usually, there is only one worm that will infect the system. Despite its length, it usually does not produce any marked symptoms. Therefore, it can be quite surprising when it passes out of the body. If there are symptoms, they will include diarrhea, abdominal cramping, nervousness, nausea, and loss of appetite.

  2. Taenia solium is the pork tapeworm, and the one capable of causing the greatest harm to the human host. It is similar to the beef tapeworm, but shorter, having fewer than 1,000 proglottids. When a tapeworm egg is ingested, the shell around the egg is dissolved in the stomach, and a living embryo called an "oncosphere" is released. After about sixty to seventy days, these oncospheres become mature bladder worms called cysticerci or "cysts" that attach to the intestinal, using a head composed of four suckers and eight hooks. When its wastes are absorbed by the host, it produces toxic effects, as well as intestinal obstruction as the worm swells. This worm can remain in a human host for twenty-five to thirty years, reaching lengths of two and one-half to three meters (eight to ten feet). Humans become infected after eating undercooked pork or smoked ham or sausage where cysts are imbedded in the tissue. Unlike the beef tapeworm, pork tapeworm infestation is usually caused by multiple worms rather than just one. Infection with the adult stage of the pork tapeworm is called taeniasis, which is not a serious health threat. However, the eggs can still be carried under the fingernails of those preparing foods or be on their skin or clothes. Eggs will appear in the stool eight to twelve weeks after eating infected pork, but eggs from a carrier can take several days to ten years to develop in another person. Infectiousness remains as long as the worm is in the intestines. Twenty to thirty thousand eggs every day can be shed into the feces of a carrier. On the other hand, the larval stage causes a much more serious condition, known as cysticercosis, producing seizures and brain deterioration that is often misdiagnosed as epilepsy. Over time, the tapeworm can riddle the brain with its grape-sized bladders, causing progressive brain deterioration to the point of death. The larvae can also develop and spread through the CNS into the muscles, heart, and eyes. Amazingly, this parasitic worm does not alert the immune system. It secretes a substance that suppresses the inflammatory response to its presence, and also controls the amount of fluid passing across its membranes from the brain. Not until the death of the worm, does the body begin to respond to the "foreigner" in its midst, when these once-protective substances are no longer produced by the worm.

  3. Diphyllobothrium latum is the fish tapeworm, and is the largest parasite found in humans, with its length reaching 4,000 proglottids. It is commonly found in Scandinavia, Russia, Japan, Australia, the Great Lakes of Canada, and Alaska. It is usually picked up after eating raw or lightly cooked freshwater fish or such species of migratory fish as Alaskan salmon, perch, pike, pickerel, and American turbot. In the intestine, a fish tapeworm can consume 80 to 100% of the host's vitamin B12. It is this deficiency (pernicious anemia) that is the most debilitating effect. After it has been eliminated from the body, it can take up to a year for B12 levels to return to normal. Digestive disturbances that include pain and fullness in the upper abdomen, nausea, and anorexia are common symptoms.

  4. Dipylidium canium is the dog tapeworm. It is transmitted to humans by infected dog fleas. Children are the most frequently affected. By kissing a dog or having it lick the face, an infected dog flea can easily be swallowed. Called the "pumpkin seed" tapeworm, the first hint of infection may be finding seed-like particles in the stool or undergarments. These particles are actually the egg-bearing segments of the tapeworm. After the flea is swallowed, the larvae is liberated, reaching maturity in about twenty days. Symptoms are vague, but include restlessness and persistent diarrhea.

  5. Hymenolepsis nana is the dwarf tapeworm. Although it is the most prevalent in the southern US, it does occur worldwide, requiring an intermediate host. It is a short worm, growing only about one and one-half inches, complete with about 200 segments. The head is small with a ring of hooks and four sucker cups. The tapeworm infects humans only when the eggs are ingested. Eggs can be transmitted by infected food handlers, grain beetles and other insects that infest grains, as well as rodent contamination of foods. Mild infestations are usually without symptoms, but if enough are present, symptoms of diarrhea, itching, abdominal pain, headaches, and other vague digestive complaints occur, especially in children. In severe cases, symptoms will include the following: general body weakness, weight and appetite loss, insomnia, abdominal pain, with or without diarrhea, vomiting, dizziness, allergies, nervous disturbances, and anemia. The eggs in fecal samples are easily identified, displaying two membranes enclosing an embryo with six hooklets. White blood cells may also be elevated, especially the eosinophils. A similar tapeworm called Hymenolepis diminuta can infect humans after infected mice and rats contaminate grains, flours, and baked goods with their feces, leaving meal worms and flour beetles.