Ascariasis is caused by the intestinal roundworm Ascaris
lumbricoides. The World Health Organization has estimated
that more than 800 million people are infected with A lumbricoides, with
the largest number of infections occurring in Asia.1,2 In
areas of high prevalence, infection rates as high as 95% have
been noted. Tropical areas with warm, wet climates favor year-round
transmission and are more likely to have high prevalence rates.
In the United States, the highest infection rates are among immigrants
from developing countries (20–60% infected in
some surveys). Young children are infected most frequently, with
peak prevalence in children between the ages of 3 and 8 years living
in the tropics. Intensity of infection or worm burden typically
decreases significantly after the age of 15 years. Infections tend
to cluster in families. Individuals may be asymptomatic and shed
eggs for years, thus enhancing transmission.
Ascaris is the largest intestinal roundworm that
commonly infects humans: females measure 20 to 40 cm long, and males
measure 15 to 30 cm long (eFig. 324.1).3 The
female lays approximately 200,000 eggs daily; eggs are broadly ovoid
and 45 to 75 μm by 35 to 50 μm.
Fertilized eggs have a 3-layer coat with a bile-stained, mamillated outer
eggs are broader and longer (ie, approximately 90 μm
by 45 μm) and usually lack the mamillated outer
coat (Fig. 324-1).
Eggs are passed in the host’s feces and become infective
in the environment only after the first-stage larva molts within
the egg (embryonation). The eggs are resistant
to drying, low temperatures, and many chemicals. Children often
infect themselves and others by playing in the same areas where
they eliminate their wastes. Where human wastes are used as fertilizer
(eg, Asia), Ascaris infection is especially frequent.
When embryonated eggs are ingested and stimulated by enzymes
in the duodenum, the larvae emerge, traverse the intestinal mucosa, and
enter the mesenteric lymphatics and venules (Fig.
324-2). They then enter the portal circulation and reach the
pulmonary vascular bed, perforate the alveolar wall, ascend the respiratory
tree to the epiglottis, and are swallowed. The vast majority of
ascarids finally settle in the jejunum, where mature worms mate
and females begin laying eggs in 2 to 2.5 months. After a life span
of 10 months to 2 years, they are passed in the stool. A video showing
a live worm observed during colonoscopy is available at: http://www.youtube.com/watch?v=HOaZCkA8Zvk (accessed
December 4, 2009).
Ascaris lifecycle (see text). ...