The fish tapeworm is a broad, long worm, often growing to lengths of 3-7 feet at maturity and capable of attaining 30 feet. It is the longest tapeworm invading humans with as many as 4,000 segments (proglottids). The main body of the worm is virtually filled with male and female reproductive organs allowing it to produce an incredible number of eggs, often more than 1,000,000 a day. The adult attaches to the wall of the intestine with the aid of two sucking grooves located in its head (scolex).
This tapeworm is sometimes called a broad fish tapeworm, because the reproductive segments are usually broader than they are long. The adult is ivory or grayish-yellow in color and can live in humans for 20 years. Humans are the final host of this worm, but first it must pass through a tiny freshwater crustacean, and then to a fish. The larva that infects people, a 'plerocercoid', is frequently found in the intestines of freshwater and marine fish. It is sometimes found in the flesh of freshwater fish or in fish that are migrating from salt waters to fresh water for breeding.
You can be infected by eating raw, lightly cooked, under-processed freshwater or certain migratory species of salmon, perch, pike, pickerel, and turbot. The popularity of eating raw fish dishes, such as Japanese sushi and sashimi, helps to spread this disease. Cooks who sample their fish dishes before they are properly cooked put themselves at risk of being infected. Fish tapeworms are found wherever humans, bears, and other fish-eating mammals defecate in the same lakes and streams from which this fish are obtained.
Most infected people do not produce any symptoms. During the acute stage of infection, which has its onset about 10 days after eating raw or insufficiently cooked fish, the symptoms may be similar to other tapeworm infections. This includes symptoms such as diarrhea, abdominal discomfort and pain, flatulence, vomiting, nausea, and weakness. Chronic infestations may produce some of the same symptoms or only vague discomforts including fullness in the upper abdomen, water retention, loss of weight, and malnutrition. Some people are constantly hungry because the tapeworms are eating most of the food. There are times when the worm gets so large that it will cause a colon blockage. In some people a severe anemia may develop, because of this tapeworm's ability to consume most of its host's vitamin B12. Folate may be reduced as well. With the anemia that results, neurological symptoms can manifest including numbness, loss of vibration sense, and even some eye symptoms.
Evidence of Diphyllobothrium spp. has been found in 4,000-10,000 year old human remains on the western coast of South America. There is no clear point in time when Diphyllobothrium latum and related species were 'discovered' in humans, but it is clear that diphyllobothriasis has been endemic in human populations for a very long time.
Life Cycle Diagram (Courtesy of the DPD)
Hymenolepis is generally found in the feces of rats which is consumed by its secondary hosts: beetles. The dwarf tapeworms mature into a life form referred to as a "cysticercoid" in the insect; in H. nana, the insect is always a beetle. Humans and other animals become infected when they intentionally or unintentionally eat material contaminated by insects. In an infected person, it is possible for the worm's entire life-cycle to be completed in the bowel, so infection can persist for years if left untreated. Hymenolepis nana infections are much more common than Hymenolepis diminuta infections in humans because, in addition to being spread by insects, they can be spread directly from person to person by eggs in feces. When this happens, H. nana oncosphere larvae encyst in the intestinal wall and develop into cysticercoids and then adults. Hymenolepis nana infections can grow worse over time because, unlike in most tapeworms, dwarf tapeworm eggs can hatch and develop without ever leaving the definitive host.
Dwarf tapeworm infections occur worldwide, but is most prevalent in the southern United States. Children are frequently infected, and infections in the same family are a common occurrence.
This is a short worm, only growing 1.5 inches long at maturity with about 200 segments (proglottids). The dwarf tapeworm does not need an intermediate host, but only one mammal to host its entire life cycle. The head is small with a single ring of small hooks and four cup-shaped suckers.
The dwarf tapeworm can infect humans when the eggs are ingested from contaminated food or water, or when infected food handlers pass this organism to others. The eggs also develop in grain beetles and many other insects, who then infect the grains that they eat. When humans eat these grains, they are also eating the organism. Rats, mice, hamsters, and dogs can also be infected by ingesting this organism and passing it along to humans. Most human infections result from human-to-human contact through the fecal-oral route. It is possible to be self-infected with this organism when the eggs pass out in the stool, depending upon a person's hygiene habits.
Mild infections are usually without symptoms, but if enough worms are present, symptoms can include diarrhea, itching, abdominal pain, headaches, and other vague digestive complaints. If the infection is severe, the symptoms may present as body weakness, weight and appetite loss, insomnia, abdominal pain with or without diarrhea, vomiting, dizziness, allergies, nervous disturbances, and anemia. The infection may still be without symptoms even when the person has a heavy infestation.
Life Cycle Diagram (Courtesy of the DPD)
Dr. Ross Andersen, N.D.
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