What is a neglected tropical disease?

When I first sat down to write this post, I thought that I’d start with a quick definition of the term Neglected Tropical Disease (NTD). I thought I knew what an NTD was – I even study a few! – so I was surprised when a definition didn’t immediately pop into my head. And I wasn’t alone. Several people who read our January Parasite Ecology Reading Group Paper also wondered how we decide which human infectious diseases should be considered NTDs.

If you’re like us, you probably thought about Googling it; surely WHO, CDC, etc. have some sort of NTD definition? They sort of do. But while major health organizations all use generally similar verbiage on their websites and in their reports, none of them seem to have a particularly precise definition. It would be hard to use their definitions to decide whether a given human infectious disease was an NTD or not.

What we seem to have instead of a precise definition is the World Health Organization’s list of 18 NTDs. And what an ecologically interesting list it is! There are viruses, bacteria, protozoa, and helminths. There are parasites with vectorborne transmission, fecal-oral transmission, and environmental transmission. Such diversity! In fact, at first glance, the NTDs seem to have little in common. And at second glance, the list seems oddly short. For instance, this article shared by Valentin Greigert asks why hepatitis E, which kills 70,000 pregnant women a year, doesn’t make the list.

The crux of the issue seems to be deciding who is neglecting NTDs. Is it politicians? Is it researchers? Funding agencies? Drug developers? Rich nations? This is a difficult question to answer, because it requires quantifying how much attention different human infectious diseases are receiving.

To figure out which diseases are or are not neglected by research, Furuse (2018) counted the number of publications (i.e., one metric of research effort) for 52 human infectious diseases, to see if NTDs are studied less than non-NTDs. They found that relative to their disease burdens, only a few NTDs are understudied. The only NTDs that were considered understudied relative to their global burdens were lymphatic filariasis, trichuriasis, ascariasis, onchocerciasis, hookworm disease, and trematodiasis. And, as the above article suggested would be the case, some diseases that are not on the accepted list of 18 NTDs had relatively high burdens and relatively few published studies, like paratyphoid fever. (To see the full list, go check out the paper.)

Over on Twitter, there was some interesting discussion about why some diseases had relatively many or few research papers relative to their burdens. In general, it was hard to guess, and Furuse (2018) notes that the reasons are potentially unique to each disease. And thus our conversation kept circling back to whether and how this burden-adjusted research intensity method could be useful in identifying and controlling NTDs. My personal ponderings have been about which types of research papers could be most indicative of neglect vs. attention. For instance, many NTDs already have effective and relatively cheap control methods that are sufficiently deployed in rich nations but not in poor nations, like water sanitation, so we might not need much research on ways to interrupt transmission for those NTDs. Instead, we might need research on where/when those controllable NTDs exist or the best ways to deploy control operations. And thus only some types of research are highly relevant for any given NTD? Anyways, there is a lot to ponder about this neat analysis. You should give it a read and share your thoughts with us!

In closing, I’ll leave you with this description – not a definition – of NTDs. Maybe one day I’ll be able to amend this post with a precise definition. 

Neglected Tropical Diseases…

…are diseases that affect poor populations that lack basic requirements like clean water, sanitation, education opportunities, and access to affordable healthcare. If you don’t study infectious diseases and you aren’t poor, you probably haven’t heard of more than four of the 18 NTDs in the figure below, despite the fact that they affect billions of people.

…trap people in a disease–poverty cycle. No matter how hard they work or how much economic assistance they receive, populations afflicted by NTDs will remain impoverished without disease control efforts because their disease burdens continue to result in lost economic mobility.

…disproportionally affect tropical and subtropical nations because poverty (i.e., people making less than $1 USD per day) is prevalent in “the global south”. But NTDs aren’t restricted to the the tropics. For instance, it is difficult to estimate NTD burdens, but NTDs are thought to affect thousands to millions of people in the United States.

…tend to be chronic diseases that cause substantial human morbidity, rather than mortality, but several of NTDs do cause substantial mortality, especially in children.

…can often be prevented/controlled/treated using existing, effective, and relatively cheap methods, such as education and water sanitation, but not always. When control methods are lacking, NTDs are often neglected by drug research and development efforts, because it isn’t usually profitable to develop drugs for people who won’t be able to pay for them.

…lack public and political visibility and discourse because they affect people with limited economic and political power, they are associated with stigma/shame, and/or they don’t have high, news-worthy mortality rates like HIV/AIDs, tuberculosis, and malaria.

Figure taken from here:

NTD fig

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