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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
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
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.
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.
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.
The name Ascaris lumbricoides
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.
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.
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
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.
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.
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
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.
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.
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.
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, 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. 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
mainly infects 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.
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) 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 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.
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
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
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."
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.
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.
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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This infection is caused
by a small, white intestinal worm called Enterobius vermicularis. Pinworms
are about the length of a staple and live in the rectum of humans. While
an infected person sleeps,
female pinworms leave the intestines through the anus and deposit eggs
on the surrounding skin.
Itching around the anus,
disturbed sleep, and irritability are common symptoms. If the infection is
heavy, symptoms may also include loss of appetite, restlessness, and
difficulty sleeping. Symptoms are caused by the female pinworm laying her
eggs. Most symptoms of pinworm infection are mild; many infected people
have no symptoms.
Pinworm is the most
common worm infection in the United States. School-age children, followed
by preschoolers, have the highest rates of infection. In some groups
nearly 50% of children are infected. Infection often occurs in more than
one family member. Adults are less likely to have pinworm infection,
except mothers of infected children. Child care centers, and other
institutional settings often have cases of pinworm infection.
Pinworm eggs are
infective within a few hours after being deposited on the skin. They can
survive up to 2 weeks on clothing, bedding, or other objects. You or your
children can become infected after accidentally ingesting (swallowing)
infective pinworm eggs from contaminated surfaces or
If pinworms are
suspected, transparent adhesive tape (often called the "scotch tape test")
or a pinworm paddle (supplied by your health care provider) are applied to
the anal region. The eggs become glued to the sticky tape or paddle and
are identified by examination under a microscope. Because bathing or
having a bowel movement may remove eggs, the test should be done as soon
as you wake up in the morning. You may have to provide several samples to
your health care provider for examination. Since scratching of the anal
area is common, samples taken from under the fingernails may also contain
eggs. Eggs are rarely found during lab examinations of stool or urine. At
night, the adult worms can sometimes be seen directly in bedclothes or
around the anal area.
With either prescription
or over-the-counter drugs. You should consult your health care provider
before treating a suspected case of pinworm. Treatment involves a two-dose
course. The second dose should be given 2 weeks after the
The infected person
should be treated with the same two-dose treatment. Close family contacts
should also be treated. If the infection occurs again, you should search
for the source of the infection. Playmates, schoolmates, close contacts
outside the house, and household members should be considered. Each
infected person should receive the usual two-dose treatment. In some cases
it may be necessary to treat with more than two doses. One option is four
to six treatments spaced 2 weeks apart.
- Bathe when you wake up
to help reduce the egg contamination.
- Change and wash your
underwear each day. Frequent changing of night clothes are
- Change underwear,
night clothes, and sheets after each treatment. Because the eggs are
sensitive to sunlight, open blinds or curtains in bedrooms during the
- Personal hygiene
should include washing hands after going to the toilet, before eating
and after changing diapers.
- Trim fingernails
- Discourage nail-biting
and scratching bare anal areas. These practices help reduce the risk of
continuous self reinfection.
Cleaning and vacuuming
the entire house or washing sheets every day are probably not necessary or
effective. Screening for pinworm infection in schools or institutions is
rarely recommended. Children may return to day care after the first
treatment dose, after bathing, and after trimming and scrubbing
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Amebiasis is a disease
caused by a one-celled parasite called Entamoeba histolytica.
Although anyone can have
this disease, it is most common in people who live in developing countries
that have poor sanitary conditions. In the United States, amebiasis is
most often found
in immigrants from developing countries. It also is
found in people who have traveled to developing countries and in people
who live in institutions that have poor sanitary conditions. Men who have
sex with men can become infected and can get sick from the infection, but
they often do not have symptoms.
- By putting anything
into your mouth that has touched the stool of a person who is infected
with E. histolytica.
- By swallowing
something, such as water or food, that is contaminated with E.
- By touching and
bringing to your mouth cysts (eggs) picked up from surfaces that are
contaminated with E. histolytica.
On average, about one in
10 people who are infected with
E. histolytica becomes sick from
the infection. The symptoms often are quite mild and can include loose
stools, stomach pain, and stomach cramping. Amebic dysentery is a severe
form of amebiasis associated with stomach pain, bloody stools, and fever.
Rarely, E. histolytica invades the liver and forms an abscess. Even
less commonly, it spreads to other parts of the body, such as the lungs or
Usually 1 to 4 weeks
later but sometimes more quickly or more slowly.
See your health care
Your health care provider
will ask you to submit stool samples. Because E. histolytica is not
always found in every stool sample, you may be asked to submit several
stool samples from several different days.
Diagnosis of amebiasis
can be very difficult. One problem is that other parasites and cells can
look very similar to E. histolytica when seen under a microscope.
Therefore, sometimes people
are told that they are infected with E.
histolytica even though they are not. Entamoeba histolytica and
another amoeba, Entamoeba dispar, which is about 10 times more
common, look the same when seen under a microscope. Unlike infection
E. histolytica, which sometimes makes people sick,
infection with E. dispar never makes people sick and therefore does
to be treated.
If you have been told that you are
infected with E. histolytica but you are feeling fine, you might be
infected with E. dispar instead. Unfortunately, most laboratories
do not yet have the tests that can tell whether a person is infected
withE. histolytica or with E. dispar. Until these
tests become more widely available, it usually is best to assume that the
parasite is E. histolytica.
A blood test is also available.
However, the test is recommended only when your health care provider
thinks that your infection has invaded the wall of the intestine (gut) or
some other organ of your body, such as the liver. One problem is that the
blood test may still be positive if you had amebiasis in the past, even if
you are no longer infected now.
Several antibiotics are
available to treat amebiasis. Treatment must be prescribed by a physician.
You will be treated with only one antibiotic if your E. histolytica
infection has not made you sick. You probably will be treated with
two antibiotics (first one and then the other) if your infection
has made you sick.
I am going to travel to a
country that has poor sanitary conditions. What should I eat and drink
there so I will NOT become infected with
E. histolytica or other
- Drink only bottled or
boiled (for 1 minute) water or carbonated (bubbly) drinks in cans or
bottles. Do not drink fountain drinks or any drinks with ice
cubes. Another way to make water safe is by filtering it through an
"absolute 1 micron or less" filter and dissolving iodine tablets
in the filtered water. "Absolute 1 micron" filters can be found in
camping/outdoor supply stores.
- Do not eat
fresh fruit or vegetables that you did not peel yourself.
- Do not eat or
drink milk, cheese, or dairy products that may not have been
- Do not eat or
drink anything sold by street vendors.
Yes. However, the risk of
spreading infection is low if the infected person is treated with
antibiotics and practices good personal hygiene. This includes thorough
hand washing with soap
and water after using the toilet, after changing
diapers, and before handling food.
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The cestodes (tapeworms) Taenia saginata
and T. solium
(pork tapeworm). Taenia solium
Humans are the
only definitive hosts for Taenia saginata and Taenia
- Eggs or gravid proglottids are passed with feces;
- the eggs can survive for days to months in the
environment. Cattle (T. saginata) and pigs (T. solium)
become infected by ingesting vegetation contaminated with eggs or gravid
the animal's intestine, the oncospheres hatch,
the intestinal wall, and migrate to the striated muscles, where they
develop into cysticerci. A cysticercus can survive for several years
in the animal. Humans become infected by ingesting raw or
undercooked infected meat.
- In the human intestine, the cysticercus develops over 2 months into
an adult tapeworm, which can survive for years. The adult tapeworms
attach to the small intestine by their scolex
reside in the small intestine.
- Length of adult worms is usually 5 m or less for T.
saginata (however it may reach up to 25 m) and 2 to 7 m for T.
solium. The adults produce proglottids which mature, become
gravid, detach from the tapeworm, and migrate to the anus or are passed
in the stool (approximately 6 per day). T. saginata adults
usually have 1,000 to 2,000 proglottids, while T. solium adults
have an average of 1,000 proglottids. The eggs contained in the
gravid proglottids are released after the proglottids are passed with
feces. T. saginata may produce up to 100,000 and T.
solium may produce 50,000 eggs per proglottid respectively.
Both species are worldwide in distribution. Taenia
is more prevalent in poorer communities where humans live in
close contact with pigs and eat undercooked pork, and in very rare in
taeniasis produces only mild abdominal
symptoms. The most striking feature consists of the passage (active
and passive) of proglottids. Occasionally, appendicitis or
cholangitis can result from migrating proglottids. Taenia
taeniasis is less frequently symptomatic than Taenia
taeniasis. The main symptom is often the passage
(passive) of proglottids. The most important feature of
taeniasis is the risk of development of
Microscopic identification of eggs and proglottids in feces is
diagnostic for taeniasis, but is not possible during the first 3 months
following infection, prior to development of adult tapeworms.
Repeated examination and concentration techniques will increase the
likelihood of detecting light infections. Nevertheless, speciation
is impossible if solely based on microscopic examination
of eggs, because all Taenia
species produce eggs that are
morphologically identical. Eggs of Taenia
sp. are also
indistinguishable from those produced by cestodes of the genus
(tapeworms of dogs and other canid hosts). Microscopic
identification of gravid proglottids (or, more rarely, examination of the
scolex) allows species determination.
detection may prove useful especially in the early invasive stages,
when the eggs and proglottids are not yet apparent in the stools.
- Morphologic comparison with other intestinal
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