Ascaris has probably been infecting humans for thousands of years. It is the most common parasitic
nematode infection and over 1 billion people are estimated to have it. One species, A. suum, typically
infects pigs, while another, A. lumbricoides, affects human populations, typically in sub-tropical and
tropical areas with poor sanitation. A. lumbricoides is the largest intestinal roundworm and is the most
common helminth infection of humans worldwide, an infection known as ascariasis.
Adult A.lumbricoides and A.suum live in the small intestine. A female A. lumbricoides can produce up to
200,000 eggs a day, though the number of eggs produced per female worm is lower when there are large
number of worms present in the gut, a phenomenon called density dependent fecundity. Adult worms can
live 1 to 2 years. A female A.lumbricoides can grow up to 40 cm long and weigh 9 g. An adult male worm
is about half the weight of a female and up to 30 cm long.
Development of the worm is direct with no distinct larva. Ascaris eggs hatch in the small intestine, where
the juvenile penetrates the small intestine and goes into the circulatory system, eventually the juvenile
worm enters the lungs. In the lungs the juvenile worm enters the air passages. It then migrates up the air
passages into the pharynx where it is swallowed, and once in the small intestine the juvenile grows into an
adult worm typically in four molts. This migration process takes about 8-12 weeks. The juveniles undergo
four molts to become adults. Why Ascaris takes such a migration through the body to only end up where it
started is unknown. Such a migration is not unique to Ascaris, as it's close relatives undergo a similar
migration in the bodies of their hosts.
Ascaridae includes the largest nematodes and several members of the family are large enough to be
dissected in invertebrate zoology laboratories. The hemocoel, or body cavity, is filled with fluid under
exceptionally high pressure (higher than that of any other animal) and acts as a hydrostatic skeleton. All
other worm's organ systems are affected by this pressure and function under its influence. The pressure
maintains the body shape and acts as a hydrostatic skeleton against which the body wall muscles act to
Large numbers of worms can accumulate in the intestine which can cause intestinal obstruction, or they
can migrate into the bile duct or pancreatic duct and cause obstruction there. Intestinal, pancreatic, or
appendiceal obstruction may occur from the large and tangled worms. In severe cases, surgical removal
may be necessary.
Tissue damage can occur when the worms migrate. Ascaris pneumonitis may be caused the worms as the
enter the lungs. On chest x-rays it may resemble viral pneumonia. Other symptoms may be fever and/or
unspecific pain in the person's abdomen. However, 85% of infected people show no symptoms.
Typically Ascaris eggs may remain viable in soil for up to 17 months but come researchers claim they
could be viable as long as 20 years. The eggs are found widespread in the environment. Adult worms
feed on digestion products of the host. Infected children with marginal diet may be susceptible to protein,
caloric, or vitamin A deficiency, resulting in retarded growth and increased risk to other infectious
diseases such as malaria.
Infections are diagnosed by finding the typical eggs in the patient's feces; or migrating worms in the
throat, mouth, anus or nose. An Endoscopic retrograde cholangiopancreatography (ERCP) is 90 percent
sensitive in detecting evidence of Ascariasis of the biliary tract. An ultrasound is 50 percent sensitive and
computed tomography (CT) also may help in showing a biliary or intestinal obstruction. ELISA tests are
rapid and sufficiently sensitive to determine infection as well.
Ingestion of eggs and subsequent infection is most common in children due to their habit of playing in
grass and sand and indiscriminately ingesting a variety of potentially contaminated materials. Some of the
heaviest infections with A.lumbricoides are typically found among children of school-age (5 - 14 years)
and especially those who live in moist, warm climates.
The eggs of Ascaris are found in soil and in poorly treated sewage-fertilizer. Consumers of uncooked
vegetables and fruits grown in or near soil fertilized with sewage are at risk for infection. The finding of
large numbers of eggs in domestic municipal sewage implies that the infection rate, especially with A.
lumbricoides, is high in the U.S.