Tapeworms are flatworms that are totally adapted for a life of parasitism. They look as if they are segmented but it is not true segmentation such as that found in the annelids. Instead tapeworms are included in the Phylum Platyhelminthes, because they have no coelom and they share other flatworm characteristics. They do look ribbon or tape-like in the adult forms.
The head of the tapeworm is called a scolex and is characterised by a round rostellum surrounded by hooks and several round suckers. There are no sense organs. Behind the head is a body made up of segments called proglottids, each of which has one or two complete male and female reproductive systems. The life of the adult worm, as with so many endoparasites, is mainly taken up with only two activities: feeding and reproduction. There is no mouth or gut and food is absorbed from the host across the body surface. Once mature, eggs are formed and fertilised in each proglottid.
Gravid proglottids detach from the head and emerge from the host in the feces. Capsules containing larvae are released when the proglottids break down. These are eaten by a herbivorous intermediate host such as a cow as it grazes on the grass infected by the feces. The embryo then develops into a cysticercus stage which has an inverted scolex and hooks for attaching to the host. When the herbivore is eaten by a carnivore, the larvae develop into adults, which doesn’t involve much change as the scolex and major organs are already formed. This is a simplified version of a cestode life cycle. In some species there are several intermediate hosts and larval stages, each of which has a specialised name.
Most tapeworms that infest humans follow this pattern where the larvae live in the intermediate host and the adults are found in the human digestive system. One scary exception is the hydatid worm. Echinococcus granulosis and E. multilocularis live as adult forms in animal hosts while the larvae form cysts in humans. These cysts can lodge in muscle or other tissues, be very painful and life-threatening and require surgery to remove.
There are five kinds of tapeworms that infect humans as adults from larvae found in food animals. The beef tapeworm, Taenia saginata, is transmitted to humans by eating undercooked, infected beef products. Usually only one worm affects the host though it can be long, with a thousand or more proglottids. There may or may not be symptoms such as diarrhea, bloating, cramping and nausea. A close relative, the pork tapeworm, Taenia solium is more dangerous and is also contracted by eating undercooked meat, in this case pork products. The worms mature in the intestine and are usually present in large numbers which can block the intestine, even though the worms themselves are smaller than the beef tapeworms. If one is unlucky enough to be infected by larvae instead, they can migrate and encyst in muscles, brain and eyes, which is a much more serious and life-threatening condition.
The largest tapeworm to infect humans is the fish tapeworm, Diphyllobothrium latum. It is found in many different places, from Russia and Asia to Canada and even Australia. It is picked up by eating undercooked fish and can get to be as big as 4000 proglottids. Pain, nausea and anorexia are some of the symptoms but the worst effect is that it absorbs vitamin B12 leading to pernicious anemia.
The dog tapeworm, Dypilidium canium, is sometimes transmitted to humans through fleas. Kissing an infected dog or letting it lick one’s face can lead to infection, so children are most likely to pick up this parasite. It is also known as the pumpkin seed tapeworm because the first sign is often pumpkin-like proglottids in the feces.
The last tapeworm that can be found in humans is the dwarf tapeworm, Hymenolepsis nana, which uses insects and rodents as intermediate hosts so can be picked up by eating contaminated grains. Symptoms include itchiness, abdominal pain, diarrhea and nausea, weight loss and anemia.
The key to avoiding most tapeworm infections are to cook meat products well and not eat raw meat or fish. And don’t let your dog lick your face!
References: http://pathmicro.med.sc.edu/parasitology/cestodes.htm Meglitsch, P. 1972. Invertebrate Zoology. Oxford University Press.