Parasite

V

Parasites of Animals

Animals are infected by many parasites including protozoans, worm parasites, and arthropod parasites such as mites, ticks, and fleas. Veterinarians diagnose these parasites in or on pets by checking the animal for visible parasites or by examining blood, tissue, or waste products under a microscope. Common worm parasites of dogs and cats include hookworms, roundworms, and tapeworms.

Hookworm infection occurs when larvae in the soil penetrate the pet’s skin, move into the bloodstream, and eventually travel to the intestine. Adult worms mature in the wall of the intestine and feed on blood from the intestinal lining, sometimes causing serious anemia. Roundworm infections of dogs and cats occur when these pets eat microscopic worm eggs present in the soil. The eggs develop larval stages in the intestine and some of these larvae penetrate the intestinal wall, move into the lungs, are coughed up and reswallowed, and once again enter the small intestine where they mature into 10 to 15 cm (4 to 6 in) worms. Roundworms compete with the pet for food and may cause malnutrition.

While the roundworm enters its host by ingestion and the hookworm enters by active penetration of the skin, the heartworm enters its dog host with the help of a mosquito vector. Microscopic larvae known as microfilariae enter the blood along with mosquito saliva when an infected mosquito bites a dog. The larvae travel via the blood stream to the heart and develop into sexually mature male and female heartworms. They grow 5 to 10 cm (2 to 4 in) in length infesting the heart’s chambers and lodging in the veins that enter the heart.

VI

Parasites of Humans

Humans are subjected to numerous protozoan, worm, and insect-related parasites. Two of the most damaging human parasites are the protozoan Plasmodium that causes malaria and the flatworm Schistosoma that causes schistosomiasis. There are an estimated 400 million to 600 million cases of malaria each year and 200 million cases of schistosomiasis worldwide.

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In malaria, the infective larval stage of the Plasmodium protozoan is transmitted to humans by the bite of a female Anopheles mosquito. The larvae undergo asexual reproduction in the liver producing a cyst that releases new larval stages into the blood stream. Larvae invade red blood cells and reproduce, eventually rupturing the blood cells. Upon rupturing, a toxin is released that causes the chills and fever that are the characteristic symptoms of malaria patients. Drugs such as chloroquine can be used to prevent infection in the blood. Mosquito control by use of repellents and pesticides is also helpful in preventing spread of the parasite.

Humans are infected with Schistosoma when they enter water containing infected snails. The larval stages of this flatworm develop in the tissues of infected snails and eventually release fork-tailed cercariae into the water. The cercariae penetrate human skin, lose their forked tails, enter the blood, and migrate to major veins in the liver, intestine, or urinary bladder. Within about six weeks of infection, the juvenile worms develop into sexually mature adults measuring 1 to 2 cm (0.4 to 0.8 in) in length. The males and females mate and produce microscopic eggs, some of which migrate to the liver and cause a condition known as cirrhosis. Other eggs move into the intestine and are passed out in the feces. When untreated human sewage enters waters containing the snail hosts, the eggs hatch and start a new cycle.

Preventive measures include the use of boots and gloves or special ointments to block penetration of the larvae into the skin. Molluscicides (drugs that kill snails) are used to kill infected snails but they often kill other important fish and invertebrate species. The drug praziquantel has proved effective in killing Schistosoma in humans, although some people experience adverse side effects.

VII

Parasites of Plants

Similar organisms that parasitize animals also infect plants (see Diseases of Plants). Fungi cause the majority of plant diseases. Although they typically feed on dead organic matter, fungi can also feed directly on living tissues. The fungus that causes Dutch elm disease is often carried from tree to tree by beetles. It attacks and eventually kills by blocking water flow through the plant. Protozoans such as phytoflagellates can parasitize milkweed plants. Bacterial plant diseases include fire blights, certain soft rots, and citrus canker. The bacteria that cause these diseases destroy tissue or block the passage of water through the plant. Numerous viruses, such as the tobacco mosaic virus, also attack plants. Certain insects and worms, particularly nematodes, parasitize the roots, stems, and leaves of plants. They secrete chemicals that induce plant cells around the parasite to rapidly divide and produce large growths known as galls. Galls formed by the root knot nematode can cause serious physical damage to the roots of important crops including tomatoes and potatoes.

Some higher plants feed on other plants and cause them harm. One group known as hemiparasites, or water parasites, absorbs water and nutrients from their plant hosts. Witchweed is a hemiparasitic seed plant that damages sugarcane, corn, and other grasslike crops by attaching itself to the host’s roots and absorbing minerals and water, eventually killing the host. Mistletoe, another hemiparasite, parasitizes broadleaf trees including ash, maple, walnut, birch, and some conifers. Mistletoe roots bore into the host’s branches in order to draw out water and nutrients. Birds eat mistletoe berries, which pass through their digestive tracks, are excreted, and sometimes stick to a tree branch where they produce a new mistletoe bush.

True plant parasites lack chlorophyll and cannot photosynthesize. These plants must obtain carbohydrates as well as minerals and water from their plant hosts. True plant parasites include dwarf mistletoe, which primarily parasitizes conifers; dodder, which parasitizes important agricultural crops such as alfalfa, clover, sugar beets, and woody perennials such as olive trees; and broomrape, which causes extensive damage to tomato crops.

VIII

Parasitology

There are many research areas in parasitology practiced by different types of specialists. Microbiologists and virologists primarily work with parasitic bacteria, rickettsiae, and viruses. Plant pathologists work with fungi, nematode parasites of plants, and other plant parasites. Animal parasitologists work with parasitic protozoans, worm groups, and arthropod parasites. Those who specialize in parasitic protozoans are called protozoologists whereas those who study parasitic worms are called helminthologists. Others who examine parasitic insects of humans are called medical entomologists.

Parasitologists who describe new species of parasites are known as systematic parasitologists. Parasite immunologists study ways in which hosts can reject parasites and they also attempt to develop vaccines against parasites. A growing area of parasitology is ecological parasitology including mathematical and computer modeling that predicts how parasites behave in wild populations.

Parasitologists in pharmaceutical industries develop drugs to prevent, control, and eradicate plant and animal parasites. Many parasitologists work to discover the complex life histories of animal and plant parasites whose life cycles remain partially or completely unknown.

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