Neglected tropical diseases (NTDs) were identified in the early 2000s as a group of mostly communicable diseases, affecting poor, vulnerable, and voiceless populations living in low- and middle-income countries. They are associated with stigma, disability, and discrimination, especially in girls and women,[1] and are neglected by research, meaning that accurate information about their burden in terms of mortality and disability is not available. WHO reports that approximately one billion people are affected by one or more NTDs,[1] but given the neglected nature of these diseases, the exact burden and morbi-mortality are likely underestimated.[2]
It is often difficult to diagnose NTDs early enough for effective interventions, resulting in diagnosis at an advanced stage of disease and with greater disability, if not death, as a consequence. Most NTDs worsen mental health, either due to neurological causes, pain or physical impairment, or stigma and social discrimination – an additional disease burden that could be reduced with earlier diagnosis and care.[3]
WHO defines disabilities as “an umbrella term, covering impairments, activity limitations, and participation restrictions. An impairment is a problem in body function or structure; an activity limitation is a difficulty encountered by an individual in executing a task or action; while a participation restriction is a problem experienced by an individual in involvement in life situations”.[4]
The current WHO list of NTDs includes 20 diseases (see Appendix), which can result in disabilities of various degrees. Some NTDs, such as schistosomiasis, trachoma, onchocerciasis, and lymphatic filariasis, are not diagnosed individually but are treated via mass drug administration (MDA) campaigns, where populations living in an area known to be endemic receive a safe treatment, which reduces transmission levels by treating the human reservoir of the infective agent. This strategy has been shown to be effective, but not sufficient to cover 100% of the population; disease still develops in people who have not been treated.
Some NTDs are more visible, they are skin-associated, disfiguring, and stigmatising like Buruli ulcer, cutaneous leishmaniasis (CL), yaws, mycetoma, post-kala-azar dermal leishmaniasis (PKDL), and the deforming elephantiasis of late stage lymphatic filariasis. In all these diseases, lesions become ulcerated, develop on a large area of the body, affect uncovered body areas (including the face), itch, burn, or hurt, and impair social relations, daily activities, walking, working, and living a normal life. The resulting social stigma is significant. Patients with eumycetoma often have to undergo amputation of a foot, leg, or arm,[5] and since no antifungal is available to treat the infection, they may have to suffer multiple amputations. CL can develop into severe ulcerations that take a long time to heal, and it sometimes invades and causes deterioration of the nasal or pharyngeal mucosa that can lead to difficulties with nutrition.[6] Even though PKDL is mostly non-severe, it scars the face, and women sometimes only seek treatment in order to increase their potential for marriage.[7] Lymphatic filariasis can result in lymphoedema affecting the lower limbs and giant hydrocele. These obvious and terrible deformations mean that people are unable to live normally; they experience pain and severe discomfort, become isolated, and are at risk of severe complications.[8] The social stigma, exclusion, and disability associated with leprosy are well known and emblematic of how seriously NTDs can impact the life of already poor and underprivileged populations.
Other NTDs, such as human African trypanosomiasis (HAT) [also known as sleeping sickness] or visceral leishmaniasis, are life-threatening in the absence of effective treatment. Amongst other more severe symptoms, patients slowly develop fatigue and lose weight; they lose the ability to work, to contribute to the family’s usually modest income and even to travel for healthcare. The development of frightening neuropsychiatric symptoms in people with HAT results in the exclusion of them and their families, as some consider them to be cursed.
Other NTDS, however, develop more silently due to a paucisymptomatic [i.e. presenting few symptoms] post-infection phase and consequent delay in diagnosis, later resulting in chronic and disabling disease. People with onchocerciasis who do not access MDA develop vision impairment that evolves to blindness and sometimes intractable severe pruritus [itch] – endemic villages are full of adults walking with the help of a young child. Approximately 30% of Chagas disease patients will develop an often fatal cardiomyopathy [disease of the heart muscle] for which no treatment exists. Chronic schistosomiasis infection has been associated with liver fibrosis and bladder cancer; [9] genital infection is another potential complication that leads to impaired fertility in men and women. [10]
In children, both chronic schistosomiasis infection and onchocerciasis-associated epilepsy[11] have been shown to impair cognitive development and growth, severely curtailing any hope of a productive future.[12] Another perverse effect of NTD-related disability is that it prevents children from attending school, where they would otherwise receive MDA.
Finally, a highly neglected consequence of most NTDs is their impact on mental health,[13] either directly because of the impact of the disease on already poor people, stigma, community exclusion, and lack of hope for their future, but also indirectly on their families and caregivers who often feel desperate and powerless to help their loved ones.[14]
In most cases, existing case management tools are either ineffective, unsafe, prone to development of resistance, not adapted for field use, expensive, or a combination of the above.
In 2001, DNDi and others published a landmark paper[15] highlighting the imbalance between the burden of NTDs and the research invested in addressing these diseases. This was named the ‘90/10’ gap since only 10% of global health research is devoted to conditions that account for 90% of the global disease burden. In 2013, a follow-up publication[16] showed that progress had been made, but that the gap persisted, with just 4% of new therapeutic products registered in 2000–11 indicated for NTDs.
For most NTDs, proper epidemiological data on gender distribution is limited. However, for some diseases the impact on pregnancy and transmission to the foetus is known, and, for example, WHO is now targeting prevention of mother-to-child transmission of Chagas disease.[17]
Bearing in mind Sustainable Development Goals 3 (good health and well-being) and 5 (gender equality), it is clear that much more needs to be done: [18]
• Firstly, by diagnosing patients earlier and closer to where they live through the development of new tools but also through greater integration within primary health care strategies.
• Secondly, by developing new therapeutic tools, combating the limitations of the existing ones, and, as far as possible, developing treatments that can be administered to pregnant or potentially pregnant women.
In response to some of these challenges, DNDi is currently developing new treatments for HAT, visceral leishmaniasis, CL, PKDL, onchocerciasis, and mycetoma. In the meantime, a more holistic approach should be developed to address social stigma, issues around mental health, and other NTD-related physical disabilities.
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