Neglected tropical diseases

Neglected tropical diseases, or NTDs, are a diverse group of infections. They primarily affect low-income populations in tropical regions of Africa, Asia, and the Americas. These diseases are categorized as "neglected" because they receive less research and treatment funding than the "big three" infectious diseases: HIV/AIDS, tuberculosis, and malaria. In sub-Saharan Africa, the collective impact of NTDs is comparable to the impact of malaria and tuberculosis. These infections can also make HIV/AIDS and tuberculosis more deadly through co-infection.

NTDs are caused by many different types of pathogens. These include viruses, bacteria, protozoa, and mites called acarines. Many NTDs are also caused by parasitic worms known as helminths. For example, Buruli ulcer is caused by the bacterium Mycobacterium ulcerans. This bacterium produces a toxin called mycolactone, which destroys body tissue.

Other diseases, such as Chagas disease, are caused by protozoa. Because many of these pathogens have long incubation periods, the diseases can remain latent or asymptomatic for a long time. This makes it difficult for doctors to connect a person's symptoms to the actual cause.

There are several distinct categories of pathogens that drive these diseases. The World Health Organization (WHO) recognizes twenty different NTDs. These include protozoan infections like leishmaniasis and Chagas disease. Bacterial infections include leprosy, trachoma, and Buruli ulcer. Helminth infections involve various worms, such as schistosomiasis and lymphatic filariasis.

There are also viral infections like dengue and rabies, as well as fungal infections and ectoparasites like scabies. In 2017, the WHO added snakebite envenomation and deep mycoses to its prioritized list.

Global health organizations have worked to change how these diseases are managed. The WHO maintains a prioritized list of diseases to focus on. In the past, many NTDs were overlooked because they primarily affect the poorest countries and lack prominent cultural figures to champion them. Research has also recently begun to address social stigma. Stigma occurs when people feel shame regarding a disease, which can prevent them from seeking help. In the 1960s, there was only about one citation per year related to social stigma in research. By 2006, that number rose to 458 citations.

Economic factors play a major role in the prevalence of NTDs. Developing economies lose billions of dollars every year due to these diseases. In 2013, NTDs resulted in 142,000 deaths, which is a decrease from 204,000 deaths in 1990. While some treatments are very inexpensive, such as praziquantel which costs $0.20 per child per year, large-scale control requires significant investment. In 2010, it was estimated that controlling these diseases would require $2 billion to $3 billion over five to seven years.

Because treating these diseases is often not seen as profitable, the pharmaceutical industry may view research as highly risky. Between 1975 and 1999, only 16 of the 1,393 new chemical products marketed were related to tropical diseases or tuberculosis. A drug was 13 times more likely to be for cancer or central nervous system disorders than for an NTD. Consequently, many programs rely on donations. The Mectizan Donation Program, for example, has donated more than 1.8 billion tablets of ivermectin.

The Gates Foundation has also funded much of the extra activity needed to counter these diseases.

NTDs are not limited to developing nations; they also exist in developed countries. In the United States, at least 12 million people have neglected parasitic infections. These often affect the poorest members of society. For instance, rates of toxocariasis among African Americans may reach 2.8 million cases. In Europe, these diseases are concentrated in eastern and southern regions where poverty is higher.

Migration can also introduce diseases like Chagas disease to new areas, with as many as 6,000 cases introduced to Europe. Understanding these connections helps scientists address the hidden disease burden in wealthy nations.