Parasitic diseases are much more widespread than
many people realize. These diseases affect not only impoverished
peoples in remote countries but they are also important health
problems for rich and poor throughout the world, including the
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 prevention
of these infections rather than in their cure.
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 are picked
up on the hands and transferred to the mouth or they enter through
the skin. With the exception of Trichinella spiralis, which
causes trichinosis, mature roundworms eventually enter the intestine
and cause a variety of health problems.
Some of the most common parasitic roundworms in humans
are: pinworms, which cause enterobiasis; ascarides, large
intestinal roundworms that cause ascariasis; hookworms,
which cause ancylostomiasis; whipworms, which cause trichuriasis;
strongyloides stercoralis, which cause strongyloidiasis;
and trichinae, which cause trichinosis. Nematodes
that do not infect human intestines are not discussed in this
Pinworm - Enterobiasis
The pinworm, Enterobius vermicularis, is the
most common roundworm parasite in temperate climates even
in areas where there is a high level of sanitation. In the United
States, it is the most common of all parasitic roundworm infections,
affecting up to 32 percent of the country's children. Because
pinworm infection is spread mainly by children, this infection
is most prevalent in family groups, day care centers, schools,
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 very resistant to drying, they
may persist for a long time after being disseminated in dust and
can cling to the fingers of children playing. 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
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 which, 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 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 the treatment of pinworms. Also, to relieve intense
itching that often accompanies the infection, a physician may
prescribe a soothing ointment or cream.
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 hundreds of 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 laid. Transmission is usually by hand to
mouth, although the use of human feces as fertilizer may also
permit transmission of infective eggs by food which is grown in
the soil and eaten without being thoroughly washed.
When eggs are swallowed and passed 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
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.
However, once mature female worms are present in the intestine,
diagnosis is possible by finding characteristic eggs in the stool.
A few worms in the intestine may cause no symptoms
or may only give rise 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 greater the worm infestation, 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, may, 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.
Ancylostomiasis - 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 optimal. 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, stool specimens are examined
to determine the presence and number of eggs. If the egg output
is large enough more
than 2,000 eggs per gram of stool it
is assumed that the infection may cause anemia and treatment with
drugs should be given.
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. However,
the larvae are unable to complete their migratory cycle. Instead
they move just below the skin producing snake-like markings. This
is referred to as a creeping eruption or cutaneous
Ancylostoma canium, an
illness caused by a particular species of dog hookworm, has recently
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 a type of blood disease called peripheral blood eosinophilia.
Once diagnosed, hookworm can be effectively treated
with drugs such as mebendazole. Drug therapy is frequently supplemented
with dietary iron.
Trichuriasis - Whipworm Disease
This parasitic roundworm infection of the large intestine
often occurs without symptoms and is usually detected by examining
the stool and detecting eggs of the human whipworm Trichuris
trichiura. Heavy infections may result in intermittent stomach
pain, bloody stools, diarrhea, and loss of weight. The name whipworm
comes from the parasite's long, very thin, whip-like shape. Fertilized
eggs develop outside the host, 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. 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 drug most often used to treat
The human body is the principal host 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 larvae in larger numbers
than would develop in the human host.
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
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. A reliable
blood test to detect antibodies to Strongyloides was developed
by scientists at the National Institute of Allergy and Infectious
Thiabendazole (Mintezol) is the recommended treatment,
given twice daily for two or three days. Ivermectin and albendazole
are also effective.
Trichinosis - Trichiniasis
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 through the body and its 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 man 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
belt of the United States. Since many states have adopted laws
requiring the sterilization of all garbage fed to hogs, 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 a
slight stomachache and achy muscles and joints that are frequently
overlooked or ignored. However, invasion by a large number of
parasites 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 for a sufficient time), freezing (16°F for 36 hours), or irradiation. Smoking, pickling, and other
methods of processing or preserving meats do not kill the parasite.
Treatment is nonspecific and is given to relieve
symptoms. 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 also has been shown to 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.
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National Institutes of Health
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