Parasites

Parasitic diseases are much more widespread than many people realize. These diseases affect not only impoverished peoples in remote countries but they also can be important health problems for rich and poor throughout the world, including the United States.

As with other parasitic diseases, roundworm infections are more common in warm climates than in cooler, temperate areas of the world. Many roundworm parasitic diseases result from human carelessness and a lack of appropriate personal hygiene and sanitation measures. Thus, the best solution to the problem rests in preventing these infections rather than in curing them.

Roundworms, or nematodes, are a group of invertebrates (animals having no backbone) with long, round bodies. They range in size from those plainly visible to the naked eye to those several hundredths of an inch long and visible only under a microscope. Most roundworms or their eggs are found in the soil and can be picked up on the hands and transferred to the mouth or can enter through the skin. With the exception of Trichinella spiralis, which causes trichinosis, mature roundworms eventually end up or reside in the intestine and cause a variety of health problems.

Some of the most common parasitic roundworms in humans are: Enterobius vermicularis, the pinworm that causes enterobiasis; Ascaris lumbridoides, the large intestinal roundworm that causes ascariasis; Necator and Ancylostoma, two types of hookworms that cause ancylostomiasis; Trichuris trichiura, the whipworm that causes trichuriasis; Strongyloides stercoralis that causes strongyloidiasis; and Trichonella spirae that causes trichinosis. Nematodes that do not infect human intestines are not discussed in this fact sheet.

Pinworm

The pinworm is the most common roundworm parasite in temperate climates – even in areas with high levels of sanitation. In the United States, it is the most common of all parasitic roundworm infections, affecting up to one-third of the country's children. Because pinworm infection is spread mainly by children, it is most prevalent in family groups, day-care centers, schools, and camps.

Pinworms are small, threadlike roundworms found primarily in the colon and rectum. The life cycle of the pinworm - egg, larva, and mature worm - takes place inside the human host and requires from three to six weeks to complete. Pinworms enter the body when eggs are swallowed. The female pinworm expels thousands of eggs into the environment. Because the eggs are moist and moderately resistant to drying, they may remain infectious for several days after being disseminated in dust and can cling to the fingers of children. Exposure to infective eggs may occur when the person harboring the infection scratches the contaminated area (the area around the anus where the female worm deposits her eggs) and then transfers the eggs to the fingertips and from there to the mouth. The eggs may be scattered into the air from bed linen and clothing, and can cling to doorknobs, furniture, tubs and faucets, and even food. Although an individual may have no symptoms over a long period, there may be repeated episodes of infection.

Folklore is filled with fantastic descriptions of symptoms and abnormal behavior attributed to pinworm infection. Actually, the symptoms are usually mild and vague. Migration of egg-laden female worms from the anus will often produce itching of the anus or vagina that, in some cases, may become very intense and even interfere with sleep.

Diagnosis of pinworm infection is made by detecting characteristic eggs. The most common procedure for collecting the eggs is a rather simple one involving swabbing the anal area with the sticky side of a piece of transparent Scotch tape, and transferring the tape to a slide for examination.

Some physicians believe that no treatment is necessary for pinworm infections that are asymptomatic, since children usually outgrow the infection as they grow older. The strong probability of small children becoming reinfected outside the home makes the strenuous efforts to eliminate the eggs from the household of little help. Frequent bathing; clean underclothing, night clothes, and bed sheets; and routine hand washing, particularly after using the bathroom, will help prevent pinworm infection or reinfection.

When medicine is given, all members of the household should take it, regardless of whether they show symptoms of infection. Drugs such as mebendazole and pyrantel pamoate (Povan) have proven most useful in treating pinworm infection. Also, to relieve intense itching that often accompanies the infection, a physician may prescribe a soothing ointment or cream.

Ascariasis

The name Ascaris lumbricoides reflects the resemblance of this intestinal roundworm to the common earthworm known as Lumbricus. Ranging in length from six to 13 inches, the female worm may grow to be as thick as a pencil. Ascaris infections are common throughout the world in both temperate and tropical areas. In areas of poor sanitation, an entire population may be harboring the parasite. The worm burden can reach staggering levels with up to a hundred worms infecting a given individual.

Almost more than any other parasitic disease, ascaris infection is a result of human carelessness. Human feces in streets, fields, and yards provides a major source of infective eggs in heavily populated areas. The eggs of ascarids are not infective for humans when first excreted. They are very resistant to extremes of temperature and humidity. They usually are transmitted by hand to mouth, although the use of human feces as fertilizer may also permit transmission of infective eggs by food that is grown in the soil and eaten without being thoroughly washed. The eggs require several weeks to embryonate and become infective.

When eggs are swallowed and pass into the intestine, they hatch into larvae. The larvae then begin their journey through the body. Once through the intestinal wall, they reach the lungs by means of the blood or lymphatic system. In the lungs, they pass through the air sacs, are carried up the bronchial tree, and are reswallowed to be returned to the small intestine where they grow, mature, and mate. The worms reach maturity in about two months.

A large number of larvae invading the lungs at one time may cause pneumonia. This stage of the disease precedes the intestinal phase by weeks, and the symptoms are difficult to diagnose. Once mature female worms are present in the intestine, however, a doctor can diagnose the infection by finding characteristic eggs in the stool.

A few worms in the intestine may cause no symptoms or may give rise only to vague or intermittent abdominal pain. Heavy infection may cause partial or complete blockage of the intestine resulting in severe abdominal pain, vomiting, restlessness, and disturbed sleep. The heavier or greater the worm infection, the more severe the symptoms are likely to be. Occasionally, the first sign of infection may be the presence of a worm in the vomitus or in the stool.

Other species of ascarids such as Toxocara, which infect dogs and cats, can under certain circumstances be picked up by humans. In their natural hosts, these ascarids have a migratory cycle similar to A. lumbricoides; however, in humans they fail to reach the intestine. Instead they remain active in other body tissue for some time. This state of larval migration is known as visceral larva migrans. Young puppies and kittens contribute most to contamination of soil by eggs that must incubate for some time in the soil. Almost all dogs are infected at birth; however, older dogs have usually become immune.

Ascariasis can be successfully treated with mebendazole or pyrantel pamoate.

Hookworm Disease

One of the most common roundworm infections is hookworm. Like ascarids, hookworms are picked up as a result of unsanitary conditions. Hookworm eggs are passed in human feces onto the ground where they develop into infective larvae. When the soil is cool, the worms crawl to the nearest moist area and extend their bodies into the air. They remain there - waving their bodies to and fro - until they come into contact with the skin of a suitable host or until they are driven back down by the heat.

Hookworm is endemic in those tropical and subtropical countries in which people defecate on the ground and soil moisture is most favorable. Necator americanus is the prevailing species in the southeastern United States.

The infection is usually contracted by persons walking barefoot over contaminated soil. In penetrating the skin, the larvae may cause an allergic reaction. It is from the itchy patch at the site of entry that the early infection gets its nickname "ground itch." Once larvae have broken through the skin, they enter the bloodstream and are carried to the lungs. (Unlike ascarids, however, hookworms do not usually cause pneumonia.) The larvae migrate from the lungs up the windpipe to be swallowed and carried back down to the intestine.

Maturation of the worms in the intestine before eggs appear in the stool is sometimes marked by the onset of diarrhea, particularly in a previously uninfected person. Other signs and symptoms at this stage include vague abdominal pain, intestinal cramps, colic, and nausea.

Scientists have learned that persons in good health and on a diet containing adequate iron can tolerate the presence of these worms in small or moderate numbers with no ill effects. In chronic infections, if the number of parasites become great enough, serious anemia can occur as a result of blood loss from the worms attaching themselves to the intestine and sucking the blood and tissue juices of the host.

To diagnose hookworm, the doctor will examine stool specimens to look for and count the number of eggs. If the egg output is large enough – more than 2,000 eggs per gram of stool – the doctor will assume that the infection may cause anemia and start treating the patient.

If humans come into contact with larvae of the dog hookworm or the cat hookworm, or of certain other hookworms that do not infect humans, the larvae may penetrate the skin. But the larvae are unable to complete their migratory cycle in humans. Instead, the larvae move just below the skin producing snake-like markings. This is referred to as a creeping eruption or cutaneous larva migrans.

Ancylostoma canium, an illness caused by a particular species of dog hookworm, has been described in Australia. This worm may almost complete its development in the lower small intestine, but produces a severe inflammatory reaction in the bowel, causing abdominal pain, diarrhea, and an increase in peripheral blood eosinophils.

Once diagnosed with hookworm, the patient can be treated effectively with drugs such as mebendazole. Frequently, the doctor will supplement the drug therapy with dietary iron.

Whipworm Disease

This parasitic roundworm infection of the large intestine often occurs without symptoms, but a doctor usually can diagnose it by examining the stool and detecting whipworm eggs. Heavy infections may cause intermittent stomach pain, bloody stools, diarrhea, and loss of weight. The name whipworm comes from the parasite's long, very thin, whiplike shape. Fertilized eggs develop outside the host, and an embryonated egg is produced in three weeks in a favorable environment; that is, warm, moist, shaded soil.

Although the incidence of whipworm infection is high, its intensity is usually light. In the United States, the infection occurs principally in warm, moist climates, most frequently among children. Infection results from eating eggs via hands, food, or drink. Severe infections in young children can result in serious disease with bloody diarrhea and a condition called rectal prolapse.

Mebendazole is the medicine most often used to treat whipworm disease.

Strongyloidiasis

Humans are the principal hosts of the parasitic roundworm called Strongyloides stercoralis. This parasite has different types of life cycles. One is direct, similar to that of the hookworm. After a short feeding period and development in the soil, the infective larvae penetrate human skin, enter the circulation, and pass through the right side of the heart to the lungs. From the lungs, the adolescent parasites go up the windpipe into the mouth, are swallowed, and reach the upper part of the small intestine where they develop into mature worms. Under certain conditions, parasites may undergo an indirect life cycle in which free-living mature male and female worms develop in the soil and produce a new generation of large numbers of larvae.

At times, the larvae may develop rapidly into the infective state in the intestine where they penetrate the intestinal mucosa instead of passing out of the body in the feces, as occurs normally. This modification of the life cycle, called internal autoinfection, explains persistent strongyloidiasis, as long as 40 years in patients who have moved to areas where the disease is not generally found. Autoinfection may produce heavy infections and severe disease, especially in patients with reduced immunity such as those receiving corticosteroids or other immunosuppressive drug treatment.

Many Strongyloides infections are mild and go unnoticed. Moderate infections may cause a burning pain in the abdomen. Nausea and vomiting may be present, and diarrhea and constipation alternate. Severe infections result in anemia, weight loss, and chronic diarrhea. Laboratory diagnosis includes the examination of feces and duodenal contents for larvae. Scientists at the National Institute of Allergy and Infectious Diseases have developed a reliable blood test to detect antibodies to Strongyloides.

Thiabendazole (Mintezol) is the one of the recommended treatments, given twice daily for two or three days. Ivermectin given in one or two days, or albendazole given in two courses 10 days apart also are effective.

Trichinosis

Trichinosis is an infection by the larvae of a most versatile roundworm, Trichinella spiralis. This parasite can infect virtually every meat-eating mammal. Unlike the other parasitic roundworm diseases that have been discussed, trichinosis is not an intestinal infection in the usual sense. It is the migration of T. spiralis larvae through the body and their encystment (becoming enclosed in a capsule) in a muscle that creates serious problems. The parasite is especially common in rats and in swine who feed on uncooked garbage. The disease occurs in humans when they eat undercooked infected pork.

Although trichinosis is sometimes found in cities, it is much more common in rural areas, particularly in the hog-raising areas of the United States. Because many states have adopted laws requiring that all garbage fed to hogs be sterilized, the incidence of infection has been reduced.

Typically, the life cycle of the parasite begins when an individual or an animal eats contaminated meat containing larvae. Digestive juices from the stomach dissolve the capsule-like cyst and release the parasites. The trichinae larvae then penetrate into the intestine where they mature and mate. Female worms then pass larvae into the circulatory system where they make their way through the capillaries (tiny blood vessels) into the muscle fibers. Once in the muscle fibers, they encyst again and begin a sometimes long life.

The average case of trichinosis is not severe and produces no noticeable discomfort or symptoms that are frequently overlooked or ignored – a slight stomachache and achy muscles and joints. Invasion by a large number of parasites, however, produces symptoms that mimic food poisoning followed by severe "muscular rheumatism."

Although trichinosis may be suspected on the basis of clinical signs, it is usually diagnosed as the result of: 1) a blood test that detects an increase in the number of eosinophils, a type of white blood cell; or 2) microscopic examination of muscle tissue to detect the larvae.

All the basic facts necessary for prevention of trichinosis in humans have been known for years. The trichinae can be killed by cooking (allowing all parts of the meat to reach at least 150° F), freezing (16° F for 36 hours), or irradiation. Smoking, pickling, and other methods of processing or preserving meats do not kill the parasite.

A doctor can prescribe medicine only to relieve symptoms. There is no treatment for the infection. If infection is diagnosed while the patient is still having digestive symptoms, standard antiparasite drugs can be used to dislodge some of the worms. Once encystment of the parasite has begun, treatment is symptomatic. In most cases, the chances of recovery are good.

Thiabenazole may help patients with trichinosis if treatment is begun very early, during the incubation state. Corticosteroid therapy can relieve the inflammatory reaction during the larval migration state, and it should be given together with thiabenazole. Steroids could, however, prolong the intestinal phase of the infection.

National Institute of Allergy and Infectious Diseases (NIAID), National Institutes of Health (NIH) Bethesda, MD 20892, October 1998.