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A parasite is a living being or organism that exists by depending on another
organism. Parasites that infect humans are much more widespread than many people
realize. These diseases affect not only poverty-stricken peoples in remote areas
of the world, 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. 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 the
roundworm that causes trichinosis, mature roundworms eventually end up or
live in human intestines and cause a variety of health problems.
Some of the most common parasitic roundworms in humans are:
- Enterobius vermicularis, the pinworm that causes
- Ascaris lumbricoides, the large intestinal roundworm that
- Necator and Ancylostoma, two types of hookworms
that cause ancylostomiasis;
- Trichuris trichiura, the whipworm that causes
- Strongyloides stercoralis that causes
- Trichinella spiralis that causes
We discuss roundworms that infect human intestines only in this fact
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A 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 found most often 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 body and requires from three to six weeks to
How do pinworms get into the body?
Pinworms enter the body when eggs are swallowed. The female pinworm
expels thousands of eggs into the environment. Because the eggs are moist
and a bit resistant to drying, they may be able to infect someone for
several days after being distributed in dust. They can cling to the
fingers of children.
Exposure to infective eggs may occur when the person who is infected
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 a person may have no symptoms over a long
period, episodes of infection may return repeatedly.
Folklore is filled with fantastic descriptions of symptoms and abnormal
behavior blamed on pinworm infection. Actually, the symptoms are usually
mild and vague. Movement 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.
How is pinworm infection diagnosed?
A doctor or other health care worker can diagnose pinworm infection by
finding the eggs. The most common way to collect the eggs is a rather
simple one involving swabbing the anal area with the sticky side of a
piece of transparent cellophane tape. The tape is then transferred to a
slide where it can be looked at under a microscope.
How can pinworm infections be prevented?
You can prevent becoming infected or reinfected with pinworms by:
- Bathing frequently;
- Using clean underclothing, night clothes, and bed sheets; and
- Washing your hands routinely, particularly after using the bathroom.
How are pinworm infections treated?
Some doctors believe that no treatment is necessary for pinworm
infections that have no symptoms. This is because children usually outgrow
the infection. Because of the strong probability that small children will
get infected again outside the home, strenuous efforts to eliminate the
eggs from the household are of little help.
If the doctor does prescribe medicine, all members of the household
should take it, regardless of whether they have symptoms. Drugs such as
mebendazole and pyrantel pamoate (Povan) are the most useful in treating
To relieve intense itching that often accompanies pinworm infection, a
doctor may prescribe a soothing ointment or cream.
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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, everyone may be harboring the parasite. Amazingly,
up to a hundred worms can infect one person.
How is ascariasis spread?
Almost more than any other parasitic disease, human carelessness causes
ascaris. Human feces in streets, fields, and yards are a major source of
infective eggs in heavily populated areas. The eggs of ascarids do not
infect humans when first excreted by the worm. The eggs 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 develop and become infective.
When a person swallows the eggs, they pass into the intestine where
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 re-swallowed to be
returned to the small intestine where they grow, mature, and mate. The
worms become mature in about two months.
Can pets transmit these worms to humans?
Other species of ascarids such as Toxocara, which infect dogs
and cats, can, under certain circumstances, be picked up by humans. In
dogs and cats, these ascarids have a migratory cycle similar to A.
lumbricoides. In humans, however, 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. Older dogs, however, have usually become immune to the
What are the symptoms of ascariasis?
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 vomit or in the stool.
How is ascariasis diagnosed?
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.
How is ascariasis treated?
Doctors can treat ascariasis successfully with mebendazole,
albendazole, or pyrantel pamoate.
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One of the most common roundworm infections is hookworm. Like ascarids, people
pick up hookworms 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, usually on a bare foot,
or until they are driven back down by the heat.
Hookworm is widespread 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
How is hookworm disease spread?
People usually get this infection by walking barefoot over contaminated
soil. In penetrating the skin, the larvae may cause an allergic reaction.
It is from the itchy patch at the place where the larvae entered 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.
What are the symptoms of hookworm disease?
Diarrhea, particularly in person who has never been infected, sometimes
starts as the worms mature in the intestines and before eggs appear in the
stool. Other signs and symptoms at this stage include vague abdominal
pain, intestinal cramps, colic, and nausea.
Scientists have learned that people in good health and on a diet
containing adequate iron can tolerate the presence of these worms in small
or moderate numbers without having problems. In chronic infections, if the
number of parasites becomes great enough, a person can develop serious
anemia because of blood loss from the worms attaching themselves to the
intestine and sucking the blood and tissue juices.
If humans come into contact with larvae of the dog hookworm or the cat
hookworm, or larvae of certain other hookworms that do not infect humans,
the larvae may penetrate the skin. But these larvae cannot complete their
migratory cycle in humans. Instead, they move just below the skin
producing snake-like markings and intense itching. 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 certain white blood cells called eosinophils.
How is hookworm disease diagnosed?
A laboratory worker 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.
How is hookworm disease treated?
Once a person has been diagnosed with hookworm disease, a doctor can
prescribe medicines such as mebendazole or albendazole. Frequently, the
doctor will add an iron supplement to this treatment.
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This parasitic roundworm infection of the large intestine often has no symptoms,
but a doctor usually can diagnose it by examining the stool and finding whipworm
eggs. Heavy infections may cause intermittent stomach pain, bloody stools, diarrhea,
and weight loss. The name whipworm comes from the parasite's long, very thin,
whiplike shape. Fertilized eggs develop outside the body, and an embryonated
egg is produced in three weeks in a favorable environment; that is, warm, moist,
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. People can get
infected by accidentally eating whipworm eggs on their hands or in food or
drink. Severe infections in young children can result in serious disease
with bloody diarrhea and a condition called rectal prolapse.
Doctors treat whipworm disease most often with mebendazole and
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The parasitic roundworm called Strongyloides stercoralis
humans. 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 larvae penetrate human skin, enter the blood stream, 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 lining 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.
What are the symptoms of strongyloidiasis?
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.
How is strongyloidiasis diagnosed?
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.
How is strongyloidiasis treated?
Thiabendazole (Mintezol) given twice daily for two or three days is the
one of the treatments doctors recommend. Ivermectin given in one or two
days, or albendazole given in two courses 10 days apart also are
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Trichinosis is an infection by the larvae of a most versatile roundworm, Trichinella
. 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.
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 that 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, fewer people get trichinosis.
Typically, the life cycle of the parasite begins when a person or an
animal eats contaminated meat containing larvae. Digestive juices from the
stomach dissolve the capsule-like cyst and release the parasites. The
larvae then penetrate into the intestine where they mature and mate.
Female worms then pass larvae into the blood stream 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
What are the symptoms of trichinosis?
The average case of trichinosis is not severe and produces no
noticeable discomfort. It can produce 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."
How is trichinosis diagnosed?
Although a doctor may suspect that a patient has trichinosis on the
basis of clinical signs, it is usually diagnosed as the result of: 1) a
blood test that shows an increase in the number of eosinophils, a type of
white blood cell; or 2) microscopic examination of muscle tissue to look
for the larvae.
How is trichinosis treated?
A doctor can prescribe medicine only to relieve symptoms. There is no
treatment for the infection. If the doctor diagnoses infection 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 for any symptoms. In most cases, the chances of
recovery are good.
Thiabendazole may help patients with trichinosis if treatment is begun
very early, during the incubation state. Corticosteroids can relieve the
inflammatory reaction during the larval migration state, and the patient
should take them with thiabenazole. Steroids could, however, prolong the
intestinal phase of the infection.
How is trichinosis prevented?
Researchers and health care providers have known all the basic facts
necessary for preventing trichinosis in humans for years. You can kill the
parasite by cooking (allowing all parts of the meat to reach at least 150
degrees Fahrenheit), freezing (16 degrees Fahrenheit for 36 hours).
Irradiation can also kill T. spiralis. Smoking, pickling, and other
methods of processing or preserving meats do not kill the parasite.
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Researchers at the National Institute of Allergy and Infectious (NIAID) diseases
are conducting basic and clinical research on the prevention, control, and treatment
of a variety of parasitic diseases, including some caused by parasitic roundworms.
NIAID scientists are trying to determine the factors that allow Strongyloides
roundworms to infect humans and cause disease. The findings
from this research may help scientists develop a skin test to diagnose stronglyloidiasis.
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