The Open Door Web Site
Part XXII: The Interdependence of Living Things : Special Relationships between Animals and Plants Index
Robert Hooke's drawing of a flea as seen under the microscope.
Drawing of a louse as seen from above
Drawing of a louse as seen from the front
The life cycle of a tapeworm
The life cycle of the liver fluke
The life cycle of the malaria parasite
THE INTERDEPENDENCE OF LIVING THINGS
Parasitic relationships: Human parasites
The flea is a successful parasite. The body of the flea is long and thin. It is able to jump from one host to another. When the flea jumps it reaches a height of 200 times its body length at an acceleration of 200 times the normal gravity.
Fleas are not found on monkeys, apes or horses. They are, however, common on rats, mice and squirrels. Between 1349 and 1352, a disease called the bubonic plague killed one third of the population of Europe. It quickly became known as the Black Death. This disease, which is caused by a bacterium, was started by infected rat fleas biting humans.
Lice (sing. louse) are common human parasites, particularly in dirty and overcrowded places. The louse is an insect. Its body grows to 11mm in length and is flattened. It has hooks on its legs which help it to hold onto its host. Lice spread to a new host by body contact with an infected person. This is particularly easy for head lice if an infected person's hair is in contact with someone else's hair.
Lice feed by biting the skin and sucking up the released blood. They lay their eggs, called nits, on hairs. These eggs are difficult to remove because the lice produce a glue-like substance to keep the eggs attached. The eggs hatch into miniature adults which quickly grow as they feed from their host.
Human head lice are quite common but they can be removed by washing hair with a special shampoo. The nits are more resistant and may need to be combed out with an especially fine comb. Before the introduction of insecticides, lice were much more widespread than they are today. They were responsible for transmitting diseases such as typhoid fever.
Pelicans preening, Bristol Zoo, UK
Lice are found on birds and all mammals except the duck-billed platypus, anteaters, armadillos, bats and whales. Lice are especially numerous on sick or injured animals. One sick fox was infested with 14000 lice and a sea bird with a damaged beak was home to 7000 lice! Healthy animals manage to keep the numbers down by grooming, seen in mammals, and preening, seen in birds. Most birds have at least 4 species of lice on different parts of their bodies.
Drawing of a tapeworm
The tapeworm is a human parasite. It belongs to the flatworm phylum and can grow up to 15 metres in length. The adult lives in the human gut. Around its small head are found hooks and suckers which it uses to attach itself to the wall of the small intestine. Its body produces chemicals which protect it from the digestive juices of its host. The tapeworm has no need of a digestive system of its own. It is able to absorb its host's dissolved food directly into its body.
The tapeworm's body is divided into hundreds of flat segments. Each of these segments contains both male and female reproductive structures. The segments at the end of the body mature first and fall off the tapeworm. They leave the host's body in the faeces. Each of these segments contains a potential 40000 fertilized eggs!
It is at this point that the life history of the tapeworm becomes a little complicated. If the segments which have moved out of the host are eaten by an animal, such as a pig, the eggs hatch into tiny larvae in the gut of the animal. These larvae then burrow their way through the tissues of the pig until they reach the muscle. Once in the muscle tissue the larvae become still and form a protective coat around themselves. Each larva is now in the form of a cyst.
The cysts are able to remain dormant in the muscle of a pig for some time. If the pig is slaughtered and eaten uncooked or undercooked, the cysts break open in the human gut to release small tapeworms. Each tapeworm is capable of attaching itself to the wall of the human gut but only one will eventually survive to grow up to 15 metres in length. The life cycle will then start all over again.
Since the pig is one of the animals in which the larvae can develop into cysts, pig meat, which is called pork, must always be cooked well before it is eaten. Other animals in which the larvae can develop are dogs, camels and monkeys. Another species of tapeworm is found as a parasite in cattle.
The complicated life cycle of the liver fluke
Many parasites have very complicated life cycles involving a main host, which provides a home for the adult, and a temporary host, which provides a home for the larvae. Sometimes an adult parasite is hermaphrodite. This means that it contains both male and female sex cells. For a parasite such as the tapeworm, where only one individual is living inside the host, it would be impossible to meet a mate! Therefore it has to make both types of sex cells.
Malaria is Italian for "bad air". Years ago people believed that malaria could be transmitted by breathing in the air around an infected person. In fact, we now know that the malaria parasite is a one-celled protozoan which is passed from one person to another by a species of mosquito. To be exact, it is the female mosquito only which is responsible for transferring the parasite. The female mosquito feeds by drinking blood and, if she is carrying the malaria parasite, she introduces it into the bloodstream of her victim.
Drawing of a female mosquito
The parasite cells live and multiply in the bloodstream. People who have malaria suffer from frequent attacks of fever which can eventually cause death if they are not treated. The best way to prevent the spread of malaria is to eradicate the mosquitoes. Since their larvae live in water, one solution is to drain stagnant swamps and pools. Another solution is to cover the mosquito breeding grounds with a thin layer of oil to prevent the larvae from breathing.
Rabies is caused by a virus. All viruses are parasites; they can only reproduce themselves in the living cells of their hosts. When the rabies virus enters the body of a mammal it travels to the nervous system, where it reproduces, and to the salivary glands. Rabies is a dreadful disease which kills the infected animal if it is not treated quickly. The anti-rabies vaccine is only effective if it is given within two weeks of the start of the infection.
In Europe, rabies is mostly carried by dogs and foxes. In South America, rabid vampire bats pass the disease to cattle and this has serious economic consequences for the farmers.
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