Scientists are striving to develop a vaccine against hookworm that is rampant in the Third World
A HAU-INCH long parasite that thrives on
blood extracted ftom the gut of the host,
the hookworm is playing havoc with the
health of about 900 million people
worldwide, predominantly in the Third
World. According to the World Health
Organization, the parasite affects 132
million people in Africa, 104 million in
Central and South America and about
685 million people in Asia. Now, some
recent discoveries have suggested excellent possibilities of developing a vaccine to conquer the worm (Scientific
American, Vol 272, No 6).
The life cycle of a hookworm begins
with the initial development of larvae in
the ground. Infective larvae generally
sneak into the body through the skin,
enter the blood circulatory system and
then break out of it and attach themselves to the small intestine. Tliere they
mature into adults and mate to produce
fertilised eggs that are transported back
to the ground through excrement. The
larvae can also be ingested along with contaminated food.
In the intestines, the parasite
extracts a fraction of a teaspoon of
blood from the circulatory system
everyday with its sharp teeth.
The infection causes anaernia
and protein malnutrition in the
host. The worst hit are children,
in which the parasite can cause
irreversible retardation of physical and intellectual growth.
Among infants, the worm can even prove fatal.
The hookworm menace has
been aggravated by the unavailability of medicines in places
where the disease is common.
Also, there is a tendency of the
dormant larvae of the parasite to
cause reinfection after the treatment is complete. Moreover, the
larvae can also enter the mammary gland of pregnant women,
and get passed on to newborns
through breast milk. Scientists at
the Institute of Parasitic Diseases in Shanghai recently identified many infants with severe hookworm infection.
For this reason, researchers at the
Yale University School of, Medicine
have started investigations on the molecular aspects of hookworm infections.
Two genera of hookworms, Ancylostoma and Necator, are being explored
to identify hookworm proteins that
can serve as preventive vaccines.
Unexpectedly, many of these proteins
also seem to have therapeutic value
for cardiovascular and immunological disorders.
Researchers have set out to identify
molecules that enable hookworms to
burrow through the skin, mature and
survive in the human body. As part of
the efforts to identify and isolate
molecules that could be enlisted as vaccines, biochemical events that occur
after a hookworm fastens to the intestinal wall are being examined. Some
enzymes secreted by the hookworm
that cause tissue breakdown and
vascular rupture have been identified.
These include proteases which degrade
host proteins, as well as hyaluronidase
that breaks down other structural components of the intestine.
The human host's defence to the
hookworm invasion involves coagulating blood at the site of vascular damage.
An immunologic defence is also
launched by activating leukocytes and
despatching antibody molecules especially Immunoglobin E, which act in
concert with leukocytes.
Investigators have established that
hookworms secrete a protein that
inhibits blood clotting, thus thwarting
the host's vascular defence. This small,
highly potent protein has now been isolated from A caninum, the hookworm
infecting dogs. The molecule is called
ACAP (A caninum anticoagulant peptide).
Larvap of hookworm are also shown
to produce interesting chemicals. For
instanc@, A caninum larvae secrete a
proteas@ that helps the organism to bore
through, the skin. Another protein -
Asp (An 4Mostoma secreted polypeptide)
- is shown to help in its maturation.
The discovery that hookworms act
as chemical factories suggests exciting
ideas lor, vaccine development. Taking
assistance of genetic engineering
techniques, researchers are all set
to test hookworm products like
ASP, AcAP and other anticlotting
factors, as potential vaccines.
Though the best approach to
control the infection is to
improve sanitation conditions in
vulnerable places, and also to
popularise protective clothing to
reduce exposure, these measures
are unlikely to be successful in
developing countries at present.
So the most logical alternative is
the admin 'istration of vaccines to
boost host defences. But funding
for fighting hookworm infection, which mainly afflicts the
Third World, is scarce and this
factor could slow down development of a hookworm vaccine.
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