Neglected Tropical Diseases (NTDs) such as Schistosomiasis and Onchocerciasis are normally caused by worms or bacteria and occur in tropical climates. These diseases can be treated very cheaply using a 'rapid impact package' comprising five 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. According to our research, two of the most cost-effective charities in this area are the Schistosomiasis Control Initiative and Deworm the World.
The term 'neglected tropical diseases' covers 15 diseases that have often been overlooked in the battle against poverty. We believe that fighting these diseases may be among the most cost-effective ways of helping people in the developing world, making this a particularly important area for further 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 can result in anaemia. 
Schistosomiasis (also known 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 exposed to affected 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. 
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. 
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. 
Trachoma is a bacterial infection. It spreads from person to person through eye, nose and throat secretions, especially in areas where there are water shortages, 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. 
The first four of the above diseases are caused by parasitic worms, while Trachoma is bacterial. These conditions can each be treated/prevented by taking the appropriate medication every 6-24 months. People have recently started using a combination of five drugs, known as the 'rapid impact package', which allows all of these diseases to be treated at once.
There are many reasons why the rapid impact packages are so exceptionally cost-effective:
- The drugs need only be given once a year and treatment throughout one's 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 co-infection of these diseases, so it is often possible to treat many of them with one package of five 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, as announced in this blog post: a decimal point was misplaced in the DCP2 and the figure should have been $341.00 per DALY. GiveWell's own estimate of the cost per 'life-equivalent' saved through combination deworming is $5000. If we consider saving one life to be equal to 30 DALYs, this suggests a cost of around $170 per DALY.
However, we've also discovered several methodological issues which pull in the opposite direction, suggesting that these studies may underestimate the cost-effectiveness of NTD treatment. Based on our research, we estimate the cost to be between $30-80 per DALY, which is in 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 that treating neglected tropical diseases might nevertheless be very cost effective. See our blog post on this topic for a more comprehensive overview.
- Because the burden of NTDs tends to result from lower quality of life and economic hardship rather than deaths, the DALYs associated with curing these diseases depend greatly on their associated disability weight. However, some argue that the Global Burden of Disease methodology unfairly underestimates the suffering caused by these diseases, and should be updated accordingly. 
- It is possible that there are substantial education benefits to treating NTDs, as these conditions inhibit children's ability to go to school. The Cochrane report raised suspicions about the significance of these effects,  but other experts in the field disagree.  The low cost of these interventions means that even small benefits, difficult to statistically distinguish from zero, could be extremely cost-effective, although much more research is needed on this matter.
- SCI and Deworm the World both have good records of handing over their programmes to local governments, which continue funding them after the charity has left. This potentially allows donors to leverage their donations towards redirecting a large amount of the country's taxpayer funding that otherwise wouldn't have been spent on deworming. We estimate this could increase the cost-effectiveness of NTD donations by around 5 times. Again, more research is needed, but if these predictions are correct, investing in NTD treatment would remain one of the most cost-effective ways to improve the wellbeing of others.
Negative side effects:
- It is possible that the parasites may become resistant to drugs over time. 9 However, it is not clear whether this is a major issue 10 and the WHO are taking steps to maintain drug efficacy. 
WHY ARE NTDS SO LITTLE-KNOWN?
There are several reasons why these conditions have received relatively little attention.
- Unlike HIV/AIDS and tuberculosis, these diseases do not affect rich countries, and therefore do not receive as much publicity within those countries. This means that it is less likely that people in rich countries know about them, and can empathize with those who suffer from them. This also means that little research has been done on these diseases, so the extent of the damage they cause has not been 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 result in 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 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 these charities, see our list of recommended charities.
- WHO summary of intestinal worms
- WHO summary of schistosomiasis
- WHO summary of lymphatic filariasis
- WHO summary of onchocerciasis
- WHO summary of trachoma
- For example, Charles King's paper Asymmetries of Poverty: Why Global Burden of Disease Valuations Underestimate the Burden of Neglected Tropical Diseases
- See www.onlinelibrary.wiley.com/doi/10.1002/14651858.CD000371.pub4/full
- See for example http://www.poverty-action.org/blog/cochrane%E2%80%99s-incomplete-and-misleading-summary-evidence-deworming
- 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.
- Personal communication with Professor Alan Fenwick of Imperial College.
- Lawrence D, 2008. 'What's in a name? Drug resistance in helminth parasites' , The Lancet Infectious Diseases , 8:536-536.