health

In humans, there are three major types of worm parasites: flukes (trematodes),
tapeworms (cestodes), and roundworms (nematodes). While worms
complete a part of their life cycle within the gastrointestinal tract, some
may also travel to other parts of the body and invade other organs. Worms
enter the body through different routes and at various life cycle stages. For
example, tapeworms may enter the human body through ingestion of both
the egg and larval forms. There are some types of worm parasites that can
enter the body directly through the skin.
While typically worm infestation is not fatal, it is still a serious health
and economic problem. Parasitic worms create a lot of sickness in the
tropics and subtropics, especially among rural, poor people. They are a
common problem in areas with poor sanitation. Worms may interfere
with the normal growth and development of children and cause chronic
illness in adults. In the United States, cases of worms occur among people
who were infected during travels to endemic* countries.

Flukes (Trematodes) After malaria, the most prevalent tropical disease
in the world is schistosomiasis, which is caused by a fluke of the genus
Schistosoma. The fluke that causes schistosomiasis lives on a type of freshwater
snail. Humans may be infected by contaminated water carrying the
fluke. Schistosomiasis affects large populations in developing countries,
especially among children who become infested while playing in water
containing the snail host. Although it is usually not fatal, schistosomiasis
causes chronic illness that may damage internal organs and impair both
physical and mental development. One form of schistosomiasis involves a
type of fluke that enters the urinary tract and is associated with increased
risk for bladder cancer in adults. Individuals from the United States traveling
to tropical areas may become infested while abroad.

Tapeworms (Cestodes) Human tapeworm infestations usually are
caused by eating meat or fish contaminated with worm larvae, but it may
also be caused by eating soil or water contaminated with human fecal matter
containing the eggs. Meat contaminated with tapeworm larvae has larvae
enclosed in cyst form within the meat. Like other types of worms, tapeworms
frequently cause infestations in areas with poor sanitation, where
livestock animals are exposed to contaminated soil or fish to contaminated
water, and these have parasites in their body tissues. After humans ingest
contaminated tapeworm encysted meat or fish, the larvae travel to the
intestines, where they latch onto the lining of the intestines and gradually
grow into adults. The worms shed their eggs into the feces, from which
they contaminate soil and water, are ingested by animals or fish, and reenter
the cycle. Tapeworms cause the most health problems in areas of Latin
America and Asia. Tapeworm infestation is rare in the United States. Fish
tapeworm infestations in the United States tend to occur among Eskimos
of western Alaska and in individuals from the west coast.

Ascariasis (Nematode) Ascariasis is caused by Ascaris lumbricoides,
an intestinal roundworm. It is one of the most common intestinal parasites
in areas with poor sanitation, affecting people in all parts of the world.
In the United States, ascariasis is extremely common in rural parts of the
Southeast. Exposure to pigs and pig manure increases the risk of infestation.
An estimated 4 million individuals may be infected in the United States.
Approximately 1.4 billion people may be infected worldwide. Indonesia
has an especially high rate of infestation with Ascariasis, with 90 percent
of the people in some regions being affected. The life cycle of this worm
begins when an adult worm lays its eggs in the intestines of an infected
person. The eggs leave the body through the feces and can live in soil for
up to two years. When people eat raw food containing this contaminated
soil, they may ingest the eggs, which hatch in the stomach as larvae. Part
of the life cycle of the larvae is to migrate outside the gastrointestinal tract.
The larvae invade the walls of the gastrointestinal tract, migrate through
the blood to the lungs and then to the throat, where they are swallowed.
Eventually, they pass into the intestines, where adult worms form and begin
the cycle again.

Strongyloidiasis/Threadworm (Nematode) Strongyloidiasis is
caused by a type of roundworm commonly referred to as the threadworm,
Strongyloides stercoralis. Strongyloidiasis is common in the tropics and is
especially prevalent in West Africa, the Caribbean, and Southeast Asia.
Strongyloidiasis is rare in the United States, except in some areas of the
Southeast and Appalachia. It is more common among military veterans
who served in Southeast Asia. Worldwide approximately 35 million cases
occurred in 2008. Although the route of infestation can be fecal-oral, this
infestation most commonly comes from contact with contaminated soil
where the larvae of the parasite can burrow directly through the skin. The
larvae travel to the lungs, are coughed up into the mouth, swallowed, and
enter the intestines. In the intestines the worm matures to adulthood and
begins laying eggs. The eggs can hatch inside the intestines and the worm
can continue to cycle through many generations without leaving the body.
Such an infestation can last for decades. Some larvae may invade the lungs
and other organs. This infestation can be fatal.

Hookworm (Nematode) Hookworms are a type of roundworm
and a common intestinal parasite. The Centers for Disease Control and
Prevention (CDC) estimates that one-fourth of the population worldwide
has hookworm infestations, although improved sanitation has reduced the
number of cases in the United States. Two species can infest humans. The
hookworm eggs hatch into larvae in the soil. Hookworms can directly
penetrate human skin. Humans may be infested by walking barefoot in or
touching contaminated soil, as well as ingesting food with contaminated
soil on it. The hookworm larvae travel to the lungs via the bloodstream.
The larvae then travel to the throat and are swallowed, in a similar fashion
to the ascaris worm. When they reach the small intestine, the larvae latch
onto the intestinal walls and suck blood. There they mature and eventually
lay eggs, which pass out of the body in the feces. Hookworms can live
for one to two years in the human body.

Enterobiasis/Pinworm (Nematode) Enterobiasis, also known as
pinworm infestation, is caused by a staple-size worm known as Enterobius
vermicularis. Pinworm tends to occur in temperate regions, rather than
the tropics. It is the most common worm infestation in the United States
and is found primarily in children. Crowded living conditions are often
associated with pinworm infestation. Outbreaks of pinworm often occur
among individuals who are grouped together in institutions, such as
schools and daycare centers. From there, infested children may spread
the worms to their family members. In the United States, in some small
regions pinworm infestation has affected 12 percent of the population.
School-aged children and adults from 30 to 39 years of age who have
children are most commonly infested.

Trichinosis (Nematode) Trichinosis arises from several varieties of
roundworms of the genus Trichinella. Although once very common, trichinosis
was as of 2009 relatively rare in the United States, with the CDC
reporting an average of just 38 cases per year. Trichinosis is more common
in developing countries. Trichinella larvae live encysted in the tissues of
pigs and wild animals. When people eat their meat raw or undercooked,
the larval cysts travel to the stomach, where acid dissolves the walls of the
cysts and releases the immature, larval worms. These migrate to the small
intestine, mature to adults, and lay eggs. Once the eggs hatch, the worms
travel through the bloodstream to muscles, where they burrow in, forming
new cysts. This ends the cycle in humans.