Scientists estimate that up to 50% of all animal species are parasitic symbionts. Some phyla such as the Platyhelminthes, Nematoda, and Arthropoda contain large a number of parasitic species. Hosts and parasites have coevolved together and under natural conditions many have become mutually tolerant. Host organisms can live independently, but, in most cases, the parasite's association with its host is obligatory. Animal parasites affect the health of humans and domesticated animals throughout the world. In most warm climates parasitic infections from flukes, nematodes, and arthropods greatly diminish the quality of life for people.
Helminths are widely distributed parasites of vertebrates. Infections caused by helminths such as schistosoma, hook worms, and filarial nematodes are a major cause of sickness of humans inhabiting the tropics. Helminths have complex lifecycles. They live for a long period within host animals, and they often possess a remarkable ability to evade the host's defense mechanisms. The prevalence of helminthic infections in some areas is high; however, only a few hosts develop disease. Helminths do not multiply in humans, and therefore the severity of the disease depends on the extent of the original infection. However, some helminthes may accumulate after repeated infection of a host.
Flukes (including the liver fluke, lung fluke, and human blood fluke) are obligate endoparasites of vertebrates as adults. After mating, the female fluke produces eggs into the host environment which are then are passed out of the host with feces or urine. There is a series of larval stages that multiply asexually in snails, serving as the first intermediate hosts. A larval stage (cercaria) with a characteristic tail emerges from the snail and either penetrates a vertebrate host immediately, encysts on to vegetation, or is eaten by a second intermediate host such as a crab or a fish, which may then be ingested by a vertebrate. Fasciola hepatica, the sheep liver fluke, commonly inhabits the bile duct, liver, or gallbladder of cattle, horses, pigs, and other farm animals. The Chinese liver fluke, Clonorchis sinensis, is an important parasite of humans and other fish-eating mammals in Far Eastern countries. Fish farming in east Asia is a major source of fluke infections in people. In other areas, dogs and cats serve as reservoir hosts of C. sinensis. Paragonimus westermani, the lung fluke, infects humans, cats, dogs, and rats. Occurences of this fluke are extremely prevalent in the people of China, the Philippines, Thailand, and other Asian countries.
Next to malaria, schistosomiasis is the most important parasitic disease in the world, affecting more than 200 million people in more than 75 countries. Schistosomes are blood flukes, and they reside in the mesenteric blood vessels of humans. Adult flukes are elongated and wormlike, and the female fluke is permanently held in the ventral groove of the male fluke. The presence of blood fluke eggs in various host tissues triggers an immune response, causing the affected person to show symptoms of disease that include enlargement of the liver and spleen, bladder calcification, and kidney disorders. Three important blood flukes that infect humans are Schistosoma mansoni, S. japonicum, and S. haematobium. Schsito-soma mansoni has been known to cause intestinal bilharziasis among people in South America, Central America, and the Middle East. Urinary schistosomiasis is caused by S. haema-tobium and is thought to occur in about 40 million people in Africa and the Middle East.
Tapeworms represent the ultimate example of biological adaptation in order to live in another organism. All tapeworms are obligate symbionts of vertebrates and arthropods. Sexually mature tapeworms live in the intestines of vertebrates; in their larval stages they develop in the visceral organs of an alternate host, which may be a vertebrate or an arthropod. Serious diseases are caused by the progressive larval stages that take place in the muscles and nervous tissue of the vertebrate host. Some scientists view the adult tapeworms in the alimentary canal as endocommensals living in a nutrient-rich environment. Tapeworms lack a digestive system and absorb all their nutrients through their tegument, which is remarkably similar to that of flukes. Diphyllobothrium latum, the fish tapeworm, is a common inhabitant of the alimentary canal of fish-eating mammals, birds, and fishes. In temperate climates, people who eat raw fish often carry D. latum. The fish tapeworm is well known for its ability to absorb vitamin B12, thereby causing the host to be deficient in a vitamin that is essential for the development of red blood cells. Humans become infected with pork tapeworm (Taenia solium) when they eat undercooked meat. Humans acquire the beef tapeworm, Taenia saginata, by eating raw or undercooked beef. Hy-menolepis diminuta, the rat tapeworm, has been a favorite experimental subject to investigate the nutrition, biochemistry, immunology, and developmental biology of tapeworms.
Roundworms are second only to insects as the most abundant animals on earth. Most nematodes are free living. They occur in freshwater, marine, and soil habitats, feeding on microorganisms and decaying organic matter. Many nematodes are adapted for a parasitic lifestyle in plants, fungi, and ani-
mals. Scientists estimate that every kind of animal is inhabited by at least one parasitic nematode. Many nematodes live in the alimentary canals of their hosts, while others parasitize organs such as the eyes, liver, and lungs, often causing destruction to the host tissue. Ascaris lumbricoides is one of the largest intestinal nematodes present in humans and is prevalent throughout warmer climates. The two most important hookworms are Ancylostoma duodenale—the oriental hookworm of China, Japan, Asia, North Africa, the Caribbean Islands, and South America, and Necator americanus—which is primarily in South and Central America but also present in Africa and Asia. An estimated one billion people who live in the warmer climates of the world are infected with these nematodes, but most are asymptomatic. Trichinella spiralis, one of the most common parasites of vertebrates, has been studied extensively by physicians, experimental biologists, and ecologists.
Filarial nematodes are obligate parasites with complex life-cycles involving humans, other vertebrates, and arthropod vectors. Wuchereria bancrofti and Brugia malayi occur in the lymphatic system and cause the elephantiasis disease. The filarial worm, Onchocerca volvulus, causes skin tumors and blindness and is prevalent among the people of Africa and the Middle East. A typical filarial lifecycle begins when humans acquire the parasite from the bite of an infected bloodsucking insect. Once in the bloodstream or lymphatic system the nematode larvae become sexually mature. The mature female gives birth to larval stage microfilariae, which infest the biting insects that continue the lifecycle. Symptoms of filarial diseases are the result of host immune response and the physical blockage of ducts.
Insects are the dominant form of life on earth, and nema-todes have successfully evolved symbioses with many of them. Nematodes that are symbionts of insects have intricate life-cycles that are synchronized with those of their hosts. Het-erorhabditis and Steinernema are nematodes that parasitize insects and transmit bacteria that kill the hosts. The possibility of developing a biological control for mosquitoes has heightened interest in the mermithid nematode, Romanomer-mis, which kills mosquito larvae.
Most nematodes that attack plants are obligate parasites and include root-knot nematodes (Meloidogyne spp.) and cyst nematodes (Globodera and Heterodera spp.) that cause destructive infections in crop plants. There are more than 2,000 species of plant-parasitic nematodes, but few species are pests. Cell proliferation, giant cell formation, suppression of cell division, and cell wall breakdown are some of the host responses to nematode parasitism.
Bursaphaloenchus exylophilus is a nematode that lives in weakened or dead pine trees. A beetle that may carry up to 15,000 juvenile nematodes to a new location spreads the ne-matode, which feed on wood tissue and are suspected of killing pine trees. The relationship between the plant-parasitic ne-matode and the bark beetle is thought to be an example of mutualistic symbiosis.
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