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27 August 2019updated 07 Jun 2021 5:12pm

How the world is winning the fight against neglected tropical diseases

By Michael Barrett

August has been a remarkable month in the global fight against poverty-associated disease. A few weeks ago it was announced that two new products were effective at neutralising the Ebola virus and could reduce the incidence of death associated with infection from one person in every two to fewer than one person in ten. A few days later came the news that a new drug, which can kill strains of tuberculosis bacteria that are resistant to other drugs, had also been approved for use. For the first time in years, we are beginning to make significant progress against diseases that afflict the world’s poorest people.

I have spent the last 30 or so years researching ways to combat a disease called human African trypanosomiasis. It is caused by tiny single-celled parasites called trypanosomes, visible only with high-powered microscopes. They are injected into people by blood-sucking tsetse flies, which are confined to certain parts of rural Africa. In the bloodstream, trypanosomes divide, dodging our immune system by repeatedly changing their surface profile. Eventually they invade our brains. Depression, psychosis and changes in sleep-wake patterns that give the disease its common name, sleeping sickness, progressively worsen, until eventually victims die. It is a horrible disease. 

Historically, indigenous Africans avoided tsetse-infested land until late 19th-century European colonisers forcibly moved their subjects into these areas, which superficially appeared ideal for farming. Massive sleeping sickness epidemics ensued. In parts of Uganda, a third of the population died of the disease in the early years of the 20th century. However, once the causative parasite was discovered and the tsetse fly’s role in transmission clarified, efforts began to reverse the problem. By the 1970s the disease had almost been eliminated. Drugs were available and the tsetse were being killed in large numbers. By the 1980s, however, malaria, tuberculosis and the new HIV/Aids epidemic were afflicting hundreds of millions across the continent. Sleeping sickness, with just tens of thousands of cases by then, became neglected. Without control measures it resurged and by the year 2000, around 300,000 people were infected.

Numerous other tropical diseases were resurging too. Maladies with names like schistosomiasis, leishmaniasis, onchocerciasis, elephantiasis and yaws collectively afflicted over a billion people. Yet because individually these diseases were either rare, or else not fatal – just debilitating – they received scant attention. Onchocerciasis, for example, afflicts hundreds of millions of people in the tropics, but seldom kills. It causes profound itching (patients may scratch huge sores over their body), and eventually leads to destruction of the optic nerve and blindness. Its insidious nature, taking years to advance, means that villages exist where entire adult populations are blind, and guided by children, themselves infected but yet to lose their sight. 

A related disease, elephantiasis, is caused by another worm and transmitted by particular mosquito species. Parasites wriggling around beneath the skin cause it to thicken and the limbs swell, resembling those of an elephant. In its most grotesque manifestation, the scrotum of an infected patient can balloon to gigantic proportions. 

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With malaria, TB and Aids devastating already stretched health budgets, developing countries had little left for these other diseases. Western pharmaceutical companies had no incentive to intervene against ailments of impoverished people in distant lands. They were truly neglected. 

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In the early 2000s, frustrated by this, a group of prominent researchers in tropical medicine devised a strategy to change the landscape. David Molyneux and Alan Fenwick in the UK and Peter Hotez in the US classified a subset of these weird-sounding maladies under a single umbrella-term – the neglected tropical diseases (NTDs). I was at a small meeting with Molyneux at the British Medical Association one day in the mid-2000s when he launched into an impassioned tirade about our obligation to intervene. With only a few like-minded people in the room it seemed odd. However, David was honing his rhetoric to educate and inspire policy makers on much larger stages around the world. 

The campaign was a resounding success. Agencies reluctant to fund individual “minor” diseases eagerly joined the fight to support the world’s poorest “bottom billion”. The World Health Organisation (WHO) created a department dedicated to the NTDs. Bill and Melinda Gates, whose foundation is endowed with billions of dollars, became interested. Organisations, including the Drugs for Neglected Diseases Initiative (DNDi) were created to facilitate global efforts at new interventions. Political will and critical cash appeared. The London declaration on NTDs in 2012, sponsored by the Department for International Development, and supported by Gates, WHO and other agencies and pharmaceutical companies set time lines to control or eliminate the NTDs. Drugs companies started providing anti-NTD drugs for free and screening their compound libraries for new drugs. Even the Nobel Prize for medicine in 2015 was awarded to discoverers of new drugs for tropical disease.

And new drugs are now coming along. The first that can be taken orally, instead of by painful injections, has just been approved for use against sleeping sickness. Fifteen years of intensive intervention has already brought the incidence of this disease to a record low, with just 1,500 cases in 2017. Elimination is now a real possibility. 

Elsewhere, Guinea worm, which was infecting millions of people as recently as the 1980s, is down to just a handful of cases across Africa. Guinea worms can reach a meter in length as they crawl through our tissues, and their larvae are transmitted in tiny water fleas that can be drunk in unfiltered water. Former US president Jimmy Carter has led a global campaign to ensure all water in affected areas is filtered before use. Guinea worm is set to become only the second disease of humans, after smallpox in the 1970s, to be eradicated by human intervention. Polio, once widespread globally, is now almost eliminated. Just last week the news that its last stronghold in Africa, Nigeria, hasn’t reported a case for three years means Africa is about to be declared polio-free.

More work is needed though. As we have seen, abandoning control programmes when incidence wanes invariably sees diseases re-emerge. Investment and effort must be sustained if we are to conquer these tractable diseases of poverty. 

Michael Barrett is professor of biochemical parasitology at the University of Glasgow