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Tropical Worms: They Infect A Third Of The Planet, So Why Aren’t We Doing More?

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Mark Booth

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1818 Tropical Worms: They Infect A Third Of The Planet, So Why Aren’t We Doing More?
A blood fluke from bilharzia. Pan Xunbin

Several years ago the international health community wanted to raise global awareness about several dozen afflictions that had previously been neglected in favour of HIV/AIDS and malaria. Scientists coined the term neglected tropical diseases to cover everything from sleeping sickness to dengue fever to leprosy, but by far the most prevalent are four parasitic worms: roundworm, hookworm, whipworm and schistosoma (bilharzia).

These parasites infect several billion people in poor countries across the world – often in combination (the numbers are somewhat imprecise because these diseases were neglected for so long). They have received substantial new funding and research in recent years, but while it has certainly not all been wasted, there are numerous obstacles in our way. Until we address these, our prospects for bringing these afflictions under control are far slimmer than they ought to be.


All four parasitic worms are closely linked to poverty, poor sanitation and hygiene. Roundworm, hookworm and whipworm, collectively known as soil-transmitted helminths, live in the intestines of infected individuals; whereas schistosoma worms live in the blood vessels near the gut or bladder. Eggs of all these infections are passed out with human faeces or urine. The schistosoma worms are spread through contact with water containing specific snail species, whereas soil-transmitted helminths spread through contaminated soil.

Signs and symptoms depend on the species and number of worms, among other factors. Symptoms include anaemia, stunting, intestinal obstruction, cancer, diarrhoea, liver fibrosis, haematemesis (vomiting blood) and splenic rupture. Death is relatively rare, given the large number of infections, but estimates still run into tens of thousands per species each year.

Affected people live mainly in low-middle income countries in the tropics and sub tropics. The staggering number of sufferers is partly explained by the fact that each pair of adult worms in each species produces tens of thousands of eggs per day.

Incidence Of Roundworm, Hookworm And Whipworm


Dark brown = >100m, brown = 20m-99.9m, orange = 5m-19.9m, yellow = 1m-4.9m, pale yellow = <1m WHO

The Rescue Effort

The neglected tropical disease label has undoubtedly increased investment into controlling and researching these parasites. Funding comes from the likes of the US and UK governments and philanthropists including the Bill and Melinda Gates Foundation, into non-profit groups such as the Schistosomiasis Control Initiative.

Funding is devoted to distributing deworming medicines to at-risk populations on the assumption that doing this at regular intervals will save lives and prevent illness. It goes further thanks to cut-price drugs and donations from pharma companies. But it is a one-size-fits-all approach, known in the parlance as mass drug administration, which shows no signs of an exit strategy.


Interventions are driven by estimates of the benefits of deworming on various outcomes including anaemia, school performance and higher economic productivity. Funding for research is not very substantial, however. In 2013, for example, R&D funding totalled: schistosoma $94m (£60m), hookworm $7m, whipworm $0.9m, roundworm $0.6m. The figures for other years are here.

A hookworm under the microscope Sebastian Kaulitzki

The war metaphor is never far from the vernacular. And all seemed to be going well until recently, when there was an episode that looked distinctly like friendly fire. In summary, a clinical trial in Kenya that had reported strong results from deworming on school attendance in 2004 was re-analysed. The apparent benefits were discovered to depend on the analytical method, throwing doubts on the original paper (this was well covered in social media under the hashtag #wormwars).

This is unlikely to be a one-off. There are a few things about these parasites that everyone needs to bear in mind. Infections by the different worm species are distributed differently across time, space and populations. The symptoms can be affected by physiological and immune status, age, previous exposure, genetic background, intensity of infection and so forth. Co-infections with multiple species can also affect symptoms, and this unfortunately is the norm. Changes to the environment can influence infection too. Last but not least, human behaviour is the central factor behind transmission.


It is true that the investment to date has led to millions of treatments. But mix all the variables together and it is not difficult to come to the conclusion that one size is unlikely to fit all when it comes to intervention. To stretch my previous metaphor further, they create a fog of war that makes “friendly-fire” inaccuracies likely to happen again.

Fine-Tuning Suggestions

How to clear this fog? In 2013 the amount of funding for R&D in malaria was $549m – more than five times higher than the combined R&D spend for the four worm infections. Malaria analysts argue that R&D against malaria requires more investment, while researchers studying neglected tropical diseases are celebrating the fact that spend has been increasing in recent years – the UK’s Department for International Development, for example, increased its expenditure from £50m to £245m from 2011-2015.

Yet R&D funding into these diseases needs to significantly improve – and not just on new medicines. One argument goes that these ailments will only disappear through economic development and that until then, we should stick to mass drug administration. My argument is that we should never stop giving people access to effective and safe medicine, but we must accept that such programmes may not be sustainable and research ways of mitigating and adapting as necessary.


Schoolchildren in Kenya, where parasite success data proved wrong Juliya Shangarey

This means empowering a much greater range of people to take part in research. To do this, I think we need to create a new army of practitioners. Civic scientists placed in affected communities could collect information in great detail. Until recently this would have been a laborious activity requiring field laboratories, forms, data-entry clerks and the like. But now we have smartphones.

Give a civic scientist a smartphone and some training and away they go. Combined with advances in high-performance computing, satellite imaging and so on, we could substantially improve our view of the landscape in which neglected tropical diseases thrive. Hopefully this will help us develop more nuanced and effective interventions than one-size-fits-all without depending on economic development to get us there more slowly. There is so much more that we could and should be doing now.

The Conversation

Mark Booth is Senior Lecturer in Epidemiology at Durham University


This article was originally published on The Conversation. Read the original article.


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