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What are neglected tropical diseases (NTDs), and what is their global impact?

The term neglected tropical disease, or NTDs, describes a group of infectious disease conditions primarily prevalent in tropical and subtropical areas. Currently more than 1 billion people in 149 countries and territories are estimated to be affected by NTDs, with a resulting economic loss of billions of dollars each year. NTDs are widespread in Africa, South Asia, Southeast Asia, and South and Central America, predominantly among the population in poverty who are forced to live in close contact with vectors and animal reservoirs such as livestock. Currently, the World Health Organization (WHO) defines NTDs as disease conditions that:

Many NTDs are parasitic diseases transmitted to humans through insects or other vectors, such as mosquitoes, flies, kissing bugs, or snails. Some, however, are caused by bacteria or viruses.

Until 2016, WHO classified 17 diseases as NTDs. However, inclusion of mycetoma was endorsed by the World Health Assembly in May 2016, and three additional diseases were further added to the list in 2017, bringing the total to 21.

What makes a disease “neglected”?

NTDs are “neglected” for a number of reasons. First, mortality rates associated with NTDs are not necessarily high as those of HIV/AIDS, malaria, and tuberculosis – the major infectious diseases that receive the greatest resources and attention worldwide. Second, NTDs typically affect individuals who live in poverty for an extended period of time. In other words, those who are residents of developed nations are rarely affected even while travelling to endemic areas. Third, people most affected by NTDs are the poorest of the poor in low- and middle-income countries. Because they cannot afford treatment there is little financial incentive for the private sector to invest in R&D of those treatments. For all these reasons, and because of a perception that the impact of each individual NTD is rather small, these diseases have received little attention from the international community for a long time.

However, the reality is that NTDs make significant health and socioeconomic impacts on endemic populations and countries. NTDs typically cause severe physical deformities and other permanent. For example, trachoma and onchocerciasis cause blindness, and lymphatic filariasis (elephantiasis) causes serious deformation of feet and scrotum. Treponemal infections (such as yaws), leprosy, leishmanisis, and mycetoma involve severe skin morbidity. As a result, affected individuals might not be able to work or get married. They might suffer from discrimination and prejudice in the society and cause significant psychological burden.

Some NTDs also affect physical and cognitive development during critical growth periods of children. This might lead to reduction of the labor force in countries where NTDs are prevalent in the future. While NTDs in general do not immediately kill infected individuals, their socioeconomic impacts are significant.

Please tell us about efforts of the WHO Western Pacific Regional Office and the progress and future prospects of elimination and control of NTDs in that region.

Fourteen NTDs are endemic in 28 countries and territories in the WHO Western Pacific Region. Among those 14 diseases, the greatest advancement has been in elimination of lymphatic filariasis. The Global Programme to Eliminate Lymphatic Filariasis (GPELF) was launched in 2000, preceded by the Pacific Programme to Eliminate Lymphatic Filariasis (PacELF), under the strong leadership of Dr. Kazuyo Ichimori (currently Visiting Professor at Nagasaki University and formerly a scientist in charge of the GPELF at WHO Headquarters).

Dr. Ichimori had devoted her life’s work to the elimination of lymphatic filariasis in Pacific Island countries long before she moved to WHO Headquarters. She launched the PacELF based on her idea that it would be much more efficient and effective to formulate elimination strategies and manage implementation of interventions for the region rather than each individual small island nation managing on its own.

Help us understand what it takes to eliminate lymphatic filariasis.

With some exceptions, the transmission of lymphatic filariasis can be interrupted most effectively by mass drug administration. This strategy has proved effective in the Western Pacific Region where seven countries have validated by WHO as having eliminated lymphatic filariasis as a public health problem over the past two years. Six out of these Seven countries are Pacific Island countries under the PacELF. We believe that most of the countries in the Western Pacific Region are on track for elimination of lymphatic filariasis as a public health problem by the 2020 global target.

Is mass drug administration an important strategy for all NTDs?

While mass drug administration alone can be effective as a primary strategy for elimination of some NTDs, there are many other NTDs for which a combination of multiple strategies is required. For example, schistosomiasis, which is currently widespread in China and the Philippines, infects both humans and other mammals, such as buffalos and cattle. In such case, mass drug administration targeting humans alone is insufficient. Strong partnership and collaboration with WASH and the veterinary health sector is essential.

The global focus for elimination and control of many NTDs so far has indeed been preventive chemotherapy (or mass drug administration). Preventive chemotherapy intervention allows rapid reduction in prevalence of many of the NTDs with inexpensive, safe, broad-spectrum medicines. In many parts of the world, scale-up of preventive chemotherapy intervention still remains the priority. The Western Pacific Region, on the other hand, already has a long history of implementing preventive chemotherapy intervention and is advancing to the next stage with new challenges are faced ahead of other WHO Regions. The WHO Western Pacific Regional Office (WHO-WPRO) has an important role to ensure that countries facing new challenges at varying stages along the road towards elimination and control of NTDs are guided appropriately to move a step forward and also to provide examples and guidance for countries in other Regions.

What led you to work in this field, and what motivates you to continue?

I am not a medical doctor; I am a specialist in the field of environmental and public health. My interest has always been the life cycle of pathogens in the environment. Human pathogens spread through movement between humans, animals, insects, and the environment. We should be able to develop strategies to interrupt the transmission if we fully understand their life cycle and the mechanism of transmission. However, if we cannot do that, infection will continue to spread.

The transmission of parasites is easier to interrupt compared to that of viruses or bacteria because their transmission efficiency is relatively poor for various reasons. If we have a means to interrupt their transmission, we should use it and eliminate the disease. That is our ethical duty.

I had an opportunity to work for WHO while I was a researcher on parasite disease control in Vietnam. I was assigned to the Department of Control of NTDs at WHO Headquarters in 2006 where I met Dr Kazuyo Ichimori, who became my mentor. She taught me everything about the work of disease elimination and eradication and the role of WHO in supporting Member States.

Dr. Ichimori used to say "WHO should be the conductor of an orchestra." Many agencies and organizations are involved in global health, and the number of players keeps increasing. Every player has its own strength and interest as if they have different instruments. WHO should never be the one to play an instrument by itself; instead, WHO should be the one to stay behind and guide each player on when and how best to play their instrument so that all the players join forces and create beautiful music.

In other words, WHO needs to play a leadership role to set out the direction for all the stakeholders and lead them towards a set of common goals by producing evidence-based guidance and identifying the areas for further research. A program where the conductor is fully functioning will be successful. On the other hand, in a program without a functioning conductor, all players began to play as they wish and fail to coordinate and cooperate towards a common goal. Keeping this in mind, I wish to continue working behind the scenes to help as many countries as possible to achieve the NTD roadmap goal for as many diseases as possible.

The affiliations and positions listed in this interview are at the time of publication of the interview in 2017.