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Neglected Tropical Diseases
Neglected Tropical Diseases (NTDs) such as Schistosomiasis (also knows as bilharzia, bilharziosis or snail fever) and Onchocerciasis are normally caused by worms or bacteria in tropical climates. They can be treated very cheaply using a “rapid impact package” of five important drugs that need only be taken once a year and do not require refrigeration. Such NTD interventions typically have a range of additional positive side-effects. As two of the most cost-effective charities that we have found, we recommend Schistosomiasis Control Initiative and Deworm the World.
The term ‘neglected tropical diseases’ (or NTDs) covers 15 diseases that have often been overlooked in the battle against poverty. We think they might be among the most cost-effective ways of helping people, making them a particularly important area for research. The most prevalent of the NTDs are:
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
Schistosomiasis (also knows as bilharzia, bilharziosis or snail fever) 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.2
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
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
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
The first four of these are caused by parasitic worms inside one's body, while Trachoma is bacterial.
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.
There are many reasons that help these treatments be unusually cost-effective.
- The drugs need only be given once a year, and treatment through school years can offer life-long protection.
- The drugs are cheap to buy at market rates, and are often donated by pharmaceutical companies.
- The drugs keep for 4 years and do not require refrigeration, which makes distribution 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, mean that everyone in an affected area can be given the drug. Thus there is no need for a potentially costly screening process.
- 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.
- Because these diseases have been neglected in the past, the easiest and most cost-effective interventions have still not been performed.
Previously, we thought that this “combination deworming” approach was extremely cost-effective, with DCP2 estimating its cost-effectiveness to be as good as $3.41 per DALY. However, GiveWell subsequently discovered major problems with DCP2's methodology. Correcting for these errors requires an adjustment in cost per DALY of roughly 100x. GiveWell’s own estimate of the cost per “life-equivalent” saved through combination deworming is between $500 and $5000, which corresponds to roughly 5 to 50 DALYs averted per $1000. Translating 'save a life' as 30 years of DALYs, suggests around $170 per DALY.
However, we've also discovered several methodological issues which pull in the opposite direction, suggesting this may underestimate the cost-effectiveness of NTD treatment. Our estimate is between $30-80 per DALY, which is on the same range as our estimate for insecticide-treated bednets, though considerable work remains to be done.
Reasons to be optimistic about NTD treatment.
There are several reasons to suspect the Givewell estimate is an underestimate.
- Because the burden of NTDs tends to be from lower quality of life and economic efficiency rather than deaths, the DALYs associated with curing them depends greatly on the associated disability weight. However, some argue that the Global Burden of Disease methodology, which hasn't been revised for decades, unfairly under-estimates how bad it is to live with one of these diseases.6
- It's possible that there are substantial education benefits to treating NTDs, as the conditions inhibit children's ability to go to school and invest in human capital in other ways. The Cochrane report raised suspicions about the significance of these effects,7 but other experts in the field disagree.8The low cost of these interventions means that even small benefits, difficult to statistically distinguish from zero, could be extremely cost-effective.
- Finally, a controversial benefit is that of leveraging government funding. SCI and Deworm the World both have good records of handing over their programs to local governments, which continue funding them after a charity has moved on. This potentially allows donors to leverage their donations by redirecting a large amount of taxpayer funding which would otherwise probably be spent on less desirable causes. We estimate this could increase the cost-effectiveness of NTD donations by around 5 times. By not considering this multiplier, the GiveWell estimate effectively sets it to 1, which is unrealistically low. Since this number makes such a big difference, it needs to be taken into account and we need more information on a realistic range for it. However, these benefits are difficult to model - the issue of who is leveraging whom is particularly problematic.
For example, Charles King's paper Asymmetries of Poverty: Why Global Burden of Disease Valuations Underestimate the Burden of Neglected Tropical Diseases(Close footnote)
|9.||Albonico M, Engels D, Savioli L, 2004. ‘Monitoring drug efficacy and early detection of drug resistance in human soil-transmitted nematodes: A pressing public health agenda for helminth control’, Int J Parasitol 34:1205–1210. (Close footnote)|
|11.||Lawrence D, 2008. ‘What's in a name? Drug resistance in helminth parasites’, The Lancet Infectious Diseases, 8:536–536. (Close footnote)|
Why are NTDs so little-known?
There are several reasons why these conditions have received relatively little attention.
- 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.
Unfortunately, it has turned out that combination deworming is not as cost-effective as DCP2 estimated it to be. However, the more reliable estimates from GiveWell still show it to be fairly cost-effective, and there is some reason to think that these estimates may be on the lower than the true value. We therefore continue to believe that de-worming charities are worth recommending. In particular, we recommend Schistosomiasis Control Initiative (SCI) and Deworm the World. For more information about them, see our list of recommended charities.
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