Tropical Health Matters

The current table of contents from the American Journal of Tropical Medicine and Hygiene features several important articles on neglected tropical diseases and malaria. Below are abstracts of several with links to the journal.

Agent-Based Simulation for Seasonal Guinea Worm Disease in Chad Dogs

The campaign to eradicate dracunculiasis (Guinea worm [GW] disease) and its causative pathogen Dracunculus medinensis (GW) in Chad is challenged by infections in domestic dogs, which far outnumber the dwindling number of human infections. We present an agent-based simulation that models transmission of GW between a shared water source and a large population of dogs. The simulation incorporates various potential factors driving the infections including external factors and two currently used interventions, namely, tethering and larvicide water treatments.

By defining and estimating infectivity parameters and seasonality factors, we test the simulation model on scenarios where seasonal patterns of dog infections could be driven by the parasite’s life cycle alone or with environmental factors (e.g., temperature and rainfall) that could also affect human or dog behaviors (e.g., fishing versus farming seasons).

We show that the best-fitting model includes external factors in addition to the pathogen’s life cycle. From the simulation, we estimate that the basic reproductive number, R 0, is approximately 2.0; our results also show that an infected dog can transmit the infection to 3.6 other dogs, on average, during the month of peak infectivity (April). The simulation results shed light on the transmission dynamics of GWs to dogs and lay the groundwork for reducing the number of infections and eventually interrupting transmission of GW.

Village Response to Mass Drug Administration for Schistosomiasis in Mwanza Region, Northwestern Tanzania

Are We Missing Socioeconomic, Cultural, and Political Dimensions?

Praziquantel (PZQ)-based mass drug administration (MDA) is the main approach for controlling schistosomiasis in endemic areas. Interventions such as provision and use of clean and safe water, minimizing contacts with infested water, disposal of human waste in latrines, and snail control provide additional key interventions to break the transmission cycle and could complement and perhaps sustain the benefits of MDA. However, all interventions deployed need to be accepted by the targeted communities. A qualitative study was conducted to examine factors that might differentiate villages which did not show a substantial decrease in Schistosoma mansoni prevalence despite repeated, high treatment coverage referred to as “persistent hotspot villages” from villages which showed a substantial decrease in prevalence referred to as “responding (RES) villages.” A convenient sample of adults was drawn from eight villages. Thirty-nine key informants were interviewed and 16 focus groups were held with a total of 123 participants. Data were analyzed manually using a thematic content approach. In both hotspot and RES villages, schistosomiasis was not considered to be a priority health problem because of its chronic nature, lack of knowledge and awareness, and poverty among study communities.

Hotspot villages exhibited poor leadership style, lack of or insufficient social engagement, little or lack of genuine community participation, little motivation, and commitment to schistosomiasis control compared with RES villages where there were commitment and motivation to fight schistosomiasis. We support the view of scholars who advocate for the adoption of a biosocial approach for effective and sustainable PZQ-based MDA for schistosomiasis control.

Assessment of Incubation Period of Cutaneous Leishmaniasis due to Leishmania major in Tunisia

The period between the infective sandfly bites and appearance of cutaneous leishmaniasis (CL) lesions is still hypothetical and little studied. This work aimed at assessing the incubation time of zoonotic CL (ZCL) due to Leishmania major using a standardized methodology. The retrospective analysis used the epidemiological, clinical, and biological information available in the database recording all the CL cases diagnosed at the Parasitology Department of the Pasteur Institute of Tunis during 2015–2019. It allowed for the selection of 92 privileged observations 1) of confirmed CL cases with presentation suggestive of ZCL form 2) living in northern regions free of ZCL 3) with a single infective trip of less than a week to ZCL foci during transmission season and 4) with accurate dates of travel and onset of lesions. Incubation length computed in this population ranged from 1 to 21 weeks, with a median of 5 weeks (interquartile range: 3–8.5 weeks).

Schistosoma mansoni Vector Snails in Antigua and Montserrat

Snail-Related Considerations Pertinent to a Declaration of Elimination of Human Schistosomiasis.

Investigations leading to a WHO-validated declaration of elimination of schistosomiasis transmission are contemplated for several countries, including Caribbean island nations. With assistance from the Pan American Health Organization, we undertook freshwater snail surveys in two such nations, Antigua and Barbuda, and Montserrat in September and October 2017. Historically, the transmission of Schistosoma mansoni supported by the Neotropical vector snail Biomphalaria glabrata occurred in both countries. Transmission on the islands is thought to have been interrupted by the treatment of infected people, improved sanitation, introduction of competitor snails, and on Montserrat with the eruption of the Soufrière volcano which decimated known B. glabrata habitats. Guided by the available literature and local expertise, we found Biomphalaria snails in seven of 15 and one of 14 localities on Antigua and Montserrat, respectively, most of which were identified anatomically and molecularly as Biomphalaria kuhniana.

Two localities on Antigua harbored B. glabrata, but no schistosome infections in snails were found. For snail-related aspects of validation of elimination, there are needs to undertake basic local training in medical malacology, be guided by historical literature and recent human schistosomiasis surveys, improve and validate sampling protocols for aquatic habitats, enlist local expertise to efficiently find potential transmission sites, use both anatomical and molecular identifications of schistosomes or putative vector snail species found, if possible determine the susceptibility of recovered Biomphalaria spp. to S. mansoni, publish survey results, and provide museum vouchers of collected snails and parasites as part of the historical record.

Decreased Mortality of falciparum Malaria in Anemic Prisoners of War?

Modern clinical trials have suggested that anemia protects against malaria mortality. Military records of the Second World War in Asia were examined to see if there was support for this hypothesis. When relatively well-nourished Imperial Japanese Navy sailors captured on Nauru (n = 799) were imprisoned on the Fauro Islands, 26% died from falciparum malaria. Similarly treated but very malnourished colocated Imperial Army soldiers experienced low stable malaria mortality. One-fifth of previously healthy Australian Army soldiers (n = 252) retreating from New Britain died largely because of malaria in April 1942. Malnourished prisoners of war, who were as a group very anemic, both Australian Army soldiers in Thailand and Japanese Army soldiers in Papua New Guinea, had high malaria rates but very low (< 3%) mortality rates. Malaria immunity does not adequately explain this dichotomy, suggesting that severe nutritional deprivation may be protective against malaria mortality possibly because of iron-deficiency anemia.

If it were not difficult enough to guide global health during a pandemic, some world leaders are trying to deflect attention from the real dangers at hand to score on their petty political concerns. In the meantime, we need to focus on what tropical health and disease issues may actually be coming under consideration at the virtual WHA 73.

Agenda item 3 (A73/CONF./1 Rev.1) or “COVID-19 response Draft resolution” directly addresses the concerns of many that other major deadly diseases and essential services should not be further neglected. The large group of resolution proponents urge countries and organizations to,

“Maintain the continued functioning of the health system in all relevant aspects, in accordance with national context and priorities, necessary for an effective public health response to the COVID-19 pandemic and other ongoing epidemics, and the uninterrupted and safe provision of population and individual level services, for, among others, communicable diseases, including by undisrupted vaccination programmes, neglected tropical diseases, noncommunicable diseases, mental health, mother and child health and sexual and reproductive health and promote improved nutrition for women and children, recognizing in this regard the importance of increased domestic financing and development assistance where needed in the context of achieving UHC.”

In Provisional agenda item 23 (A73/32) “Progress reports by the Director-General” we find updates on guinea worm eradication and the burden of snakebite envenoming. The report notes the situation in 2019, which is a far cry from the millions of cases in the 1980d when the dracunculiasis eradication effort was launched. “In 2019, three countries reported a total of 53 human indigenous cases of dracunculiasis (guinea-worm disease), namely, Angola (one case), Chad (48 cases) and South Sudan (four cases), from a total of 28 villages. Cameroon reported one human case, probably imported from Chad.”

It is important to note that, “The global dracunculiasis eradication campaign is based on both community and country-focused interventions,” where community members play an important role in surveillance and notification. This includes at-risk and border areas, as is being done in Cameroon. The challenge of human Dracunculus medinensis infection in dogs continues and points to the importance of One Health in the control and elimination of NTDs. Surveillance is not cheap, and the report stresses that funds are still needed so that international partners can continue to ensure that the last case of guinea worm is detected and contained.

Moving from the smaller serpent to the larger variety, the report recalls the May 2018 World Health Assembly resolution WHA71.5 on addressing the burden of snakebite envenoming. A global strategy, “Snakebite envenoming: a strategy for prevention and control” was launched in  in May 2019. The WHO Secretariat has “fostered international efforts to improve the availability, accessibility and affordability of safe and effective antivenoms for all, through assessments of antivenom manufacturing, training programs and stockpile procedures.

Finally, provisional agenda item 11.8 (A73/8) addresses a “Draft road map for neglected tropical diseases 2021–2030.” This builds on resolution WHA66.12 (2013) on WHO’s earlier road map for accelerating work to overcome the global impact of neglected tropical diseases (2012–2020). The proposed interventions build on important principles including:

1. Tackling neglected tropical diseases through support of the vision of universal health coverage
2. Adopting grassroots approaches that enable access to some of the world’s poorest, hard-to reach communities and people affected by complex emergencies
3. Monitoring progress against neglected tropical diseases as a litmus test of progress towards the achievement of universal health coverage

The report notes that “40 countries, territories and areas have eliminated at least one neglected tropical disease,” most notably dracunculiasis (as mentioned above, lymphatic filariasis and trachoma. Although “substantive progress has been made since 2012, it is evident that not all of the 2020 targets will be met.” Hence, a new draft road map for neglected tropical diseases for 2021–2030 is required. The three pillars supporting the new roadmap are outlined in the attached figure.

It is good to know that the 73^rd World Health Assembly will not be completely overshadowed by COVID-19 and politics. Efforts to sustain and improve NTD control and elimination must not be jeopardized.