Ascaris lumbricoides
Common
parasitic roundworms of pigs and humans are in the genus Ascaris. The roundworm that
is commonly found in the human intestine is Ascaris
lumbricoides. The phylogeny of
roundworms is as follows: phylum Nematoda, order Ascaridida, family
Ascarididae.1 Ascaris
lumbricoides infections are cosmopolitan in both tropical and temperate
climates. In the United States
infections are uncommon, with only 4 million people being infected, most of
which are in the rural southeast.
Higher occurrences are in developing countries, where every 1 in 4
people are infected. In Southeast Asia
73% of the population is infected, Africa being 12% and Central/South America
is 8%.3
The reason that it is called a roundworm is
that its body is cylindrical in shape.
A smooth creamy-white cuticle and a mouth with three lips characterizes Ascaris. Differentiation between the male and female is difficult, only
when compared to one another are they distinguishable. The more slender males measure between 10 to
30cm long and have a curved tail with two spicules, but no copulatory bursa.2
The females are very similar, being slightly larger at between 20 to 35cm long,
a vulva approximately a third of the length of the body down from the head, and
have a blunt tail.2
The
life cycle of A. lumbricoides is
direct, with no intermediate host.2 Males and females live in the
lumen of the small intestine feeding on digested food of the human host. There is some evidence that they can bite
the intestinal mucous membrane and feed on blood and tissue fluids.2
It is estimated that the female Ascaris can lay up to 200,000 unembryonated
eggs per day. The eggs are highly
characteristic, with thick shells consisting of a thick transparent inner shell
which is covered in a thick, warty, albuminous coat.2 In order for
the egg to start developing it must be outside of the host and the
environmental temperature must be 30oC with moisture and oxygen
being readily available. After 14 days
the larvae is fully developed, but remains in the egg until swallowed by a
human. The eggs can remain viable for
extensive periods of time, if the environment is suitable. A person contracts A. lumbricoides by ingesting contaminated soil, or unwashed
vegetables. Such infection occurs in
unsanitized areas, mainly where human fecal material is used for fertilizers,
where fecal/oral contact is prolific.
Once
the ova is ingested the larvae hatches out and starts its journey through the
human 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.6 Once inside
the small intestine it takes 2 months to reach full maturity. An adult A.
lumbricoides can live for 6 months or more inside a human small
intestine.
The
disease caused by Ascaris lumbricoides
is called ascariasis. A couple of adult worms in the intestine
won't do much to the human host, except for an occasional abdominal pain. Complications occur when the intensity is
high. Physical blockage of the
intestinal tract will occur, causing severe abdominal pain, vomiting, colic,
and malnutrition.
Most
of the severe cases occur when the larvae are migrating through the body. If large numbers of these larvae are
migrating through the lungs simultaneously this may give rise to a severe
haemorrhagic pneumonia.2 In smaller scale infections migrating
larvae will cause the human host to have complications in breathing, and often
a fever will spike.
In late ascariasis a simple fecal smear or float can be
done to see ova. X-rays are helpful
when diagnosing ascariasis. Early
infections show patchy infiltrates, while late infections show the entire adult
in the small intestine.
Treatment of Ascaris
lumbricoides is mainly targeted to the adult worm. It is unclear if antihelminthic therapy
works against larvae. There are 3 main
drugs used to kill the adult A.
lumbricoides worm. Mebendazole
(Vermox) causes worm death by selectively and irreversibly blocking uptake of
glucose and other nutrients in susceptible adult intestine where helminths
dwell.3 Albendazole (Albenza) decreases ATP production in the worm,
causing energy depletion, immobilization, and finally death.3
Piperazine citrate is recommended for gastrointestinal or biliary obstruction
secondary to ascariasis, causes flaccid paralysis of the helminth by blocking
response of worm muscle to acetylcholine.3 If the physical blockage
by the worms gets so large that it is considered an emergency, surgery will be
performed to remove the entire ball of worms.
Depending on the damage done to the intestinal wall, the damaged part of
the small intestine will be removed.
The
importance of such a parasite to humans has warranted extensive genetic studies
on A. lumbricoides. The entire genetic code has been sequenced
along with the mitochondrial genetic code of A. lumbricoides. When
searching GenBank on the Internet, various numbers of mitochondrial, protein,
and genetic sequences come up.
There
have been various studies done to see if some people are susceptible to A. lumbricoides based on their genetic
information. No racial predilection is
known; however, a genetic predisposition has been described in a study of
families in Nepal.4 Another interesting thing about A. lumbricoides is that people that are
highly allergic to the worm and its eggs have the lowest infestation with the
parasite.4
References
1. Ascarids
1997. http://web.missouri.edu/~vmicrorc/Nematoda/Ascarids/Ascarids.htm Julie Nahm.
Accessed 9 April 2001.
2.
Ascaris lumbricoides
Date unknown.
http://www.ncbi.nlm.nih.gov/htbin-post/Taxonomy/wgetorg?id=6252.
Scott Federhen, Ian Harrison, Carol Hotton, Detlef Leipe, Vladimir
Soussov, Richard Sternberg, Sean Turner.
Accessed 22 April 2001.
3. Ascaris lumbricoides
1998 October
5. http://www.path.cam.ac.uk/~tjs16/Nematodes/Ascaris.html
Dr. Terry Stewart. Accessed 9 April
2001.
4. Ascaris lumbricoides
from Emergency Medicine/Infectious Diseases.
2001 March
2. http://www.emedicine.com/emerg/topic840.htm
Eric L Weiss, MD, DTM&H. Accessed 9 April 2001.
5. How your environment can cause allergies
2001 April
10. http://health.iafrica.com/doconline/allergies/environment.htm iafrica.com. Accessed 9 April 2001.
6. Life cycle of the Ascaris
spp.
http://www.biosci.ohio-state.edu/~parasite/lifecycles/ascaris_lifecycle.html Author unknown. Accessed 9 April 2001.
7. Parasitic Roundworm Diseases NIAID Fact
Sheet.
2001
February. http://www.niaid.nih.gov/factsheets/roundwor.htm
U.S. Department of Health and Human Services.
Accessed 9 April 2001.
8. The Influence of Ascaris Infestation
on the Prevalence of Asthma in Balinese Children
1997 June
11-14. http://www.ipa-france.net/pubs/inches/inch9_1/san.htm
Hendra Santoso. Accessed 9 April 2001.
