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PARASITE LOADS
I recently acquired (as a review copy) the excellent book
Principles of Medicine in Africa. Browsing through it I
decided to start a short series on parasitic worm infestation
and then saw that the current issue of The Lancet has an
editorial on the topic, so obviously worm infestation is worthy of some attention. However, as The Lancet points out,
worm control ‘rarely makes the headlines’. However, at a
recent meeting in Geneva, the Partners for Parasite Control
who comprise WHO member states, the World Food
Programme, UNICEF, the World Bank, research institutes,
universities and non-governmental organisations, said that
helminth control is vital and neglected. They see it as an
important step in improving public health and reaching
several of the Millennium Development Goals.
Why is this? Looking at the burden of disease caused by
intestinal helminths, it is clear that parasitic worms may be
of the commonest chronic infections in people, and children
in particular. The estimate is that there are more than 3 billion worm infections in the world today. In fact, in many
low-income countries it is more common to be infected than
not. Children in endemic communities are generally infected
shortly after weaning and then constantly infected and reinfected for the rest of their lives. School-age children are
the group most affected and suffer the morbidity associated
with helminth infestation most. Multiparasite infections are
also common. Schistosomiasis or flatworms transmitted by
snails in infected water, and soil-transmitted helminth infections are the commonest forms of worm infestation in the
poorest communities. Although it is school-age children who
bear the greatest burden of disease (an estimated 11 - 12% of
total disease burden), it is those aged 2 - 5 who are most
at risk of morbidity associated with infestation. Anaemia,
vitamin A deficiency, stunted growth, poor intellectual development, impaired cognitive function and damage to the
liver, intestine and urinary tract are all consequences of
chronic worm infestation.
consequences are obviously that the person with a high parasite load is more likely to become ill as a result of his/her
infestation.
Age is another important component in the host-parasite
cycle. T. trichiura and A. lumbricoides typically show their
greatest parasite loads in children between the ages of 5
and 10. After this peak, there is a dramatic decline in intensity to a low level of infestation, which generally persists
through adulthood. The profile for Schistosoma infections is
similar, but with a peak later, between 10 and 14 years.
Hookworms, on the other hand, are far worse in young
adults between the ages of 20 and 25 years.
The effects of these infections on health are often subtle and
insidious in that they constrain normal physical development, particularly as children generally have their highest
parasite loads at a time when they are growing and learning most. Iron deficiency anaemia as a result of hookworm
infestation may prevent children from reaching their genetic
growth potential. Intense whipworm infestation in children
may result in Trichurus dysentery syndrome — the classic
signs are growth retardation and anaemia. Heavy burdens
of roundworm and hookworm are also associated with protein energy malnutrition. At least as important is the growing body of evidence that these infestations can be detrimental to cognition and educational achievement in children. This is again thought to result from iron deficiency
anaemia and undernutrition, both of which are associated
with significantly lower scores on psychological tests.
Evidence from Tanzania and Kenya suggests that simply
treating these children is not sufficient – they need remedial
education intervention to catch up with their uninfected
peers. The study in Kenya also found a relationship
between worm infestation and school attendance, with treated children showing better attendance.
There are around 20 major helminth infections of humans
and all have some public health significance. One of the
most common of all human infections are the geohelminthiases. These comprise the roundworm, Ascaris lumbricoides;
the hookworms, Necator americanus and Ancylostoma duodenale; and the whipworm, Trichurus trichiura. An estimated 25% of the world’s population are infected with one or
more of these worms. In addition, there are probably more
than 200 million people infected with schistosomes.
It is not only heavy worm infestation that has adverse consequences. Studies have shown that infection with as few as
10 roundworms is associated with deficits in growth and
physical fitness in school-age children and that moderate
whipworm infection can cause growth retardation and
anaemia. Further studies have shown that even light hookworm infection can lead to anaemia, in adults and children.
Children, adolescents and pregnant women are particularly
at risk from hookworm infection as the physiological
demands for iron are high during these periods.
The distribution of these helminths among their hosts is interesting. Most hosts have few or no worms, while a small
number carry far larger numbers of parasites. The clinical
Being infected with more than one type of worm can cause
more morbidity as well. There were 47 different protozoan
and helminth species found in association with A. lumbri-
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coides infections in one study, of which 24 were common.
Another study estimated that multiple infections including
Plasmodium falciparum, P. malariae, Schistosoma haematobium and 3 common nematodes occurred in approximately
60% of a Kenyan population. If there are many different
species of human parasite each with high prevalence in the
same area, then the chances of being infected with more
than one increases. It is also apparent that more multispecies infections are seen than would be expected by
chance. It may be that some individuals are actually predisposed to multiple infections. This could be behavioural. T.
trichiura and A. lumbricoides are both transmitted through
the faecal-oral route and factors leading to exposure to one
parasite could lead to exposure to others. There does
appear to be a clear genetic predisposition to susceptibility
to S. mansoni infection.
The sad thing is that worm infestation can be treated in a
cost-effective manner in the community. These drugs are
even safe in pre-school children and in pregnant women.
Once- or twice-yearly treatment can substantially reduce the
burden of disease associated with schistosomiasis and soiltransmitted helminths. A single dose of albendazole or
mebendazole is effective against the common soil-transmitted worms and praziquantel can be given as a single dose
(height-dependent) for schistosomiasis. All these drugs are
cheap, costing around $0.02 - $0.20 per child, depending
on their age. The drugs are heat stable, require no cold
chain and have a shelf-life of up to 4 years. Infection can
also be prevented by provision of safe water supplies,
sanitation and promotion of hygiene such as handwashing,
use of latrines and wearing shoes.
According to the editorial in The Lancet, beyond these simple effects on public health, helminth control could contribute to achieving 7 of the 8 Millennium Development
Goals. Goal 1 is the eradication of extreme poverty and
hunger. Deworming in children improves intellectual functioning, something that is related to income in adulthood.
Goals 2 and 3 are universal primary education and promotion of gender equality — an investment of as little as $3.5
per child on helminth control translates into an extra year of
schooling. Studies have shown that the numbers of girls
enrolling in schools increase where deworming programmes
are effective and their dropout rates decline. By reducing
malnutrition and anaemia, treating worm infections lowers
child mortality and improves maternal health — goals 4
and 5 – and helps to combat HIV and malaria — goal 6.
Goal 8 is the development of global partnerships for development — and linking deworming to other mass treatment
programmes such as those for malaria, measles and trachoma is an obvious example. An approach to parasite
infestation is potentially very simple, but the results of effective interventions could have major implications for health
in poor areas.
Lancet (Editorial) 2004; 364.
Parry E, Godfrey R, Maybe D, Gill G. eds. Principles of Medicine
in Africa. Cambridge: Cambridge University Press, 2004.
Bridget Farham
SINGLE SUTURE
FLU PANDEMIC POSSIBLE
There are those who believe that a new flu pandemic is only a matter of time and it is the spectre of avian flu mixing
with human flu to create a new and highly infectious strain that is causing this belief. Human flu is apparently circulating in areas of Thailand where there have been renewed outbreaks of the H5N1 bird flu in poultry. This strain has
killed at least 4 people recently but so far there are no definite reports of it spreading from human to human. To try to
prevent H5N1 from mixing with human flu strains, people at risk of infection should be vaccinated or given flu drugs.
Unfortunately neither is happening in Thailand because of a shortage of vaccines and the cost of flu drugs. Because it is
the flu season in the northern hemisphere, all the flu vaccine is going to rich countries and there is no surplus for countries such as Thailand. The next southern hemisphere flu season could be interesting.
New Scientist 2004; 25 September.
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