Ascaris, Trichuris, Enterobius,
Ancylostoma, Strongyloides and
Trichinella (Nematoda), and the
diseases that these roundworms
cause in humans
Paul R. Earl
Facultad de Ciencias Biológicas
Universidad Autónoma de Nuevo León
San Nicolás, NL, México
Parasitic nematodes inhabit many thousands
of plant and animal hosts. Some of the most
common parasitic roundworms in humans are:
a) Ascaris lumbricoides, the large intestinal
roundworm that causes ascariasis, b) Trichuris
trichiura, the whipworm that causes trichuriasis,
c) Enterobius vermicularis, the pinworm of
enterobiasis, d) Necator americanus and
Ancylostoma duodenale, 2 types of hookworms
that cause ancylostomiasis, e) Strongyloides
stercoralis responsible for strongyloidiasis and
f) Trichinella spiralis which causes trichinosis
that perhaps ought to be called trichinellosis.
A parasite is an organism that exists by
depending on another organism. Parasites that
infect humans are much more widespread and
prevalent than many people realize. Nematode
diseases are important health problems for rich
and poor throughout the world. As with other
parasitic diseases, some roundworm infections
are more common in warm climates than in
cooler, temperate areas. Many roundworm
parasitic diseases result from human
carelessness and 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 visible to the naked eye to those
several hundredths-of-a-mm 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
trichinellosis found in muscles, mature
roundworms invade the human intestines
and cause a variety of health problems.
Ascaris lumbricoides.
The incidence is over 1500 million
infections annually. Of these cases,
about 210 million are symptomatic.
In some rural settings with poor
sanitation, perhaps half the children
of 2-12 years have ascariasis.
Then many of them will also have
trichuriasis and various of other
chronic illnesses.
40cm
A mass or bolus of Ascaris lumbricoides.
Trichuris trichuriura.
These whipworms cause 800 million infections
per year worldwide. Trichuriasis occurs in the
southern United States and Latin America.
Female whipworm, Trichuris trichiura.
Enterobius vermicularis.
Enterobius egg
in feces.
Enterobius
vermicularis
female.
Enterobius
vermicularis
male.
Necatur americanus and
Ancyclostoma duodenale.
The human hookworms include 2
nematode (roundworm) species,
Ancylostoma duodenale and
Necator americanus.
Adult females measure 10-13 mm
for A. duodenale and 9-11 mm for
N. americanus. adult males:
8 to 11 mm (A. duodenale),
7 to 9 mm (N. americanus).
Ancyclostoma
duodenale.
The second most common human
helminthic infection after ascariasis is
ancylostomiasis. Worldwide distribution,
mostly in areas with moist, warm
climates. Both N. americanus and A.
duodenale are found in Africa, Asia and
the Americas. Necator americanus
predominates in the Americas and
Australia, while only A. duodenale is
found in the Middle East, North Africa
and southern Europe.
Strongyloides stercoralis has both
freeliving and parasitic life cycles. In the
parasitic life cycle, female worms are found in
the superficial tissues of the human small
intestine. There are no parasitic males. The
female worms produce larvas parthenogenically
(without fertilization) and the larvas are passed
in the host's feces. The presence of nematode
larvas in a fecal sample is characteristic of
strongylodiasis. Once passed in the feces,
some of the larvas develop into freeliving larvas,
while others develop into parasitic larvas. The
freeliving larvas will complete their development
in the soil and mature into freeliving males and
females.
Two types of cycles exist in
Strongyloides.
1/ Freeliving cycle: The rhabditiform larvas
passed in the stool can either molt twice and
become infective filariform larvae (direct
development) or molt 4 times and become
freeliving adult males and females that mate
and produce eggs which hatch as
rhabditiform larvas that can either develop
into a new generation of freeliving adults, or
into infective filariform larvas. The filariform
larvas penetrate the human host skin to
initiate the parasitic cycle.
Trichinella spiralis
Muscle can be squeezed between 2 plates
of glass to reveal microscopic larvas.
Trichinosis is probably best known as a parasite
that humans contract from eating raw or
undercooked pork. Through an aggressive
program of meat inspection, the incidence of
trichinosis in pigs in the US has been lowered to
less than 1%, so it is unlikely that those pork
products will contain Trichinella larvas. Most
recent outbreaks of trichinosis in the US have
been traced to pork products from pigs that
have not been inspected and that have been
slaughtered privately. As T. spiralis is not host
specific, hunters should be careful when
preparing meat from their kills, e. g., infections
have been traced to contaminated bear meat.
Endnote.
We have introduced you to soil transmitted
nematodes and to Trichinella spiralis
transmitted in undercooked meat. The much
greater problem is the sanitation of food, water
and waste disposal. The lack of safe water and
drainage is allied to poverty and high birth
rates. No water delivered to the home and dirt
floors are common aspects of poverty. How
many houses have latrines? The migrant can
live anywhere as long as he can get a job.
Shantytown barrios have sprung up over
much of the world and deteriorate their
environments.
Parasitic diseases are part of this, although
generally they cannot be vaccine prevented.
Massive population treatments with one-time
400 mg mebendazole or the like may be the road
to take. These populations are infected yet not
seeking treatment. Worms have never been seen
damaging enough to eradicate. Perhaps the
determination needed to eliminate roundworms
is at the end of the rainbow. Generally, people
with parasitoses are poor, and many are rural. Is
industry the real solution? It is the only solution
! Finally, world economics is such that the pills
like the vaccines MUST be given by generous
donors like Merck.