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Neglected Tropical Diseases
The term ‘neglected tropical diseases’ (or NTDs) covers
15 diseases that have often been overlooked in the battle against poverty.
The most prevalent are:
- Soil-transmitted helminthiasis (STH)
- Schistosomiasis
- Lymphatic Filariasis (LF)
- Onchocerciasis
- Trachoma
The first four of these are caused by parasitic worms inside one's body,
while Trachoma is bacterial.
The Diseases
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Soil-transmitted Helminthiasis (STH)
STH results from infection by roundworm, whipworm, or hookworm.
They are the most common infections in the world and affect the
most deprived communities: roundworm infects over 1 billion people,
whipworm infects 795 million, and hookworm infects 740 million.
Infection is caused by accidentally ingesting eggs from contaminated
soil or by the larvae in the soil actively penetrating the skin.
Soil-transmitted helminths produce a range of symptoms including
diarrhoea, abdominal pain, general malaise, and weakness. They
can affect working and learning capacities and impair physical
growth. Hookworms cause chronic intestinal blood loss that results
in anaemia.1
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Schistosomiasis
Schistosomiasis is a parasitic disease caused by flatworms.
Larval forms of the parasites, which are released by freshwater
snails, penetrate the skin of people in the water. Symptoms include
progressive damage to the bladder, ureters, and kidneys, as well
as progressive enlargement of the liver and spleen, intestinal
damage, and hypertension.
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Lymphatic Filariasis (LF)
LF (which is commonly known as elephantiasis) is caused
by thread-like worms known as filariae which are spread from human
to human by mosquito bites. Once inside the skin, they migrate
to the lymphatic system where they cause significant damage and
produce millions of offspring. LF is most well known for causing
a disfiguring swelling of the limbs or genitals, that often leads
to disability and social isolation.3
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Onchocerciasis
Onchocerciasis, or river blindness, is caused
by a certain type of filarial worm. It is transmitted from human
to human through the bites of infected blackflies. The larvae
form nodules under the skin, where they mature to adult worms.
After mating, each female adult worm can release up to 1,000 offspring
per day. These move through the body, and when they die they cause
a variety of conditions, including blindness, lesions, and intense
itching.4
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Trachoma
Trachoma is a bacterial infection. It spreads from person
to person, especially where there are shortages of water,
numerous flies, and crowded living conditions. Infection often begins
during infancy or childhood and can become chronic. If left untreated,
the infection eventually causes the eyelid to turn inwards, which
in turn causes the eyelashes to rub on the eyeball, resulting in
intense pain and scarring of the front of the eye. This ultimately
leads to irreversible blindness, typically at between 30 and 40
years of age. 5
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Cost-Effectiveness
These conditions can each be treated/prevented by taking the appropriate
medication every 6–24 months. Recently people have started using
a combination of five drugs, known as the rapid impact package,
which allows all of the above diseases to be treated at once.
Charities which focus on neglected tropical diseases often advertise
their cost-effectiveness in terms of the amount of money it takes to distribute
one drug treatment:
The DCP2 and WHO-CHOICE
cost-effectiveness estimates are as follows:
In other words, according to the DCP2 reports, $100 would prevent a total
of 30 Disability Adjusted Life Years
due to soil-transmitted helminthic infections. This is a health gain that
is roughly on a par with saving a life. $1,000 would thus produce health
benefits on a par with saving 10 people’s lives.
The reason these figures differ is that DCP2 and WHO measure cost-effectiveness
in DALYs, whereas the charities’ estimates are simply for the cost
of distribution of the drug. Not everyone who takes the drug suffers from
the conditions listed, and ridding someone of these diseases for one year
does not amount to one DALY averted (unless they would have died immediately
from that disease were it not for the treatment). At the moment we are
uncertain exactly how effective the combined-treatment Rapid Impact Package
is; we are working on the conservative assumption that it is at least
as cost-effective as treatment for STHs alone.
Other Information
Why is this treatment so cost-effective?
There are many reasons why these treatments are so effective, including:
- The drugs need only be given once a year, and treatment through the
school years can offer life-long protection.
- Often, the drugs are donated by pharmaceutical companies. They are
also exceptionally cheap to buy at market rates.
- The drugs keep for 4 years and do not require refridgeration, which makes distribution much easier.
- The high proportion of people infected, the very low price of the
drugs, and the fact that the drugs are safe to take even if one is not
infected, combine to mean that everyone in an affected area can be given
the drug. Thus there is no need for a potentially costly screening process.
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There is considerable geographical overlap and coinfection of
these diseases, so it is often possible to treat many of them with one
package of four drugs.
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Because these diseases have been neglected in the past, the easiest
and most cost-effective interventions have still not been performed.
Having said this, there are some other considerations that must be taken
into account:
- First, in general these drug treatments provide smaller benefits
to many people, rather than very large benefits to fewer people. This
means that small changes in the disability-weighting assigned to the
impairments caused by NTDs, or changes in estimates about how much damage
these NTDs do, can have large effects on cost-effectiveness estimates.
However, it seems that, if anything, the disability weightings given
are underestimates, possibly by a factor of 20.
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- Second, one might think that many small benefits don’t add
up to a large benefit: one might think that 10 DALYs are worth more
if they are all averted for one person (e.g. by curing them of blindness)
rather than if they are spread across 100 people (e.g. by treating them
for minor parasitic infections). However, there are two things that
must be borne in mind. Firstly, the health experts and ethicists who
developed the DALY system recommend that it be used to aggregate benefits
in precisely this way. Secondly, one must be careful not to underestimate
the suffering that NTDs cause, which can include severe disability,
social ostracization, and death.
Why are NTDs so little-known?
There are several reasons why these conditions have received relatively
little attention. For example:
- Unlike HIV/AIDS and tuberculosis, they do not affect rich countries,
and therefore do not receive as much publicity within those countries.
This means that it is more difficult for people in rich countries to
know about them, and to empathize with those who suffer from them. It
also means that there is little research done on them, so the extent
of the damage that they cause was not appreciated until very recently.
- NTDs have lower mortality rates than HIV/AIDS, tuberculosis and malaria.
It is easier to convey the importance of diseases that are more likely
to cause death.
Are there side-effects?
All health interventions have side-effects, both good and bad. In the
case of drug treatments for neglected tropical diseases, it seems that
the positive side effects may be even larger than the direct benefits.
Positive side-effects:
- Prevention of NTDs is the cheapest known way to improve school attendance.
It is estimated that deworming programs improve school attendance by
one day for every $0.02 spent; or $3.27 for every aggregate year of
additional school attendance. They thus have a claim to being the most
effective educational intervention as well as a claim to being the most
effective health intervention.7
- They decrease the infectiousness of the disease in an area. Thus
even people in that area who were not treated are less likely to contract
the disease.
- A mass administration of drugs for lymphatic filariasis for five
years may almost completely eliminate the existence of that parasite
in the area treated.8
- Treatment often decreases the chance of the recipients contracting
other diseases, such as malaria, and HIV.9 Treatment also decreases the likelihood of death of sufferers of malaria,
tuberculosis and HIV/AIDS.10
- The rapid impact package simultaneously treats other, more minor
parasitic infections, such as scabies.11
- There are significant economic benefits, as these neglected tropical
diseases can severely impair the ability to work. The Copenhagen Consensus
2008 estimated that each deworming treatment cost $0.49 but generated
an additional $30 of wages.12
- NTDs tend to affect the very poorest people in the world. As treatment
provides economic benefits, it helps to decrease economic inequality.
- Since NTDs have low mortality rates, treatment benefits the people
without having much effect on population growth.
Negative side-effects:
- It is possible that the parasites may become resistant to drugs over
time.13 However, it is not clear whether this is a major issue14 and the WHO are sufficiently aware of the problem that they are taking steps to maintain drug efficacy.15
Conclusion
Charities which focus on STH, schistosomiasis, and LF are at the very
top end of cost-effectiveness — about 100 times more effective than
typical developing-world health interventions — and they produce
important positive side-effects. For these reasons, we have no hesitation
in recommending that donors give to charities which work in this area.
In particular, we recommend Schistomiasis
Control Initiative (SCI). For more information about them, see our
list of recommended charities.
If you are looking for more information on neglected tropical diseases,
a good place to start is this
article.
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