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Setback for campaign to eradicate Guinea worm disease
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Abstract

Arabella Gray discusses new research investigating the role of domestic dogs in sustaining Guinea worm disease in Africa.

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This is a compelling example of the need for a One Health approach to achieve successful disease eradication

Domestic dogs have emerged as a potential animal reservoir of Guinea worm disease (GWD), prompting concern that the animals are a likely source of GWD in people, according to research published by a team at the University of Exeter.

GWD, or dracunculiasis, was set to become the second human disease in history to be eradicated, following a remarkably successful eradication campaign – reported cases have fallen from 3.5 million across 21 countries in 1986, to just 28 in 2018. The majority of these recent cases were found in Chad, but the disease is also still endemic in Ethiopia, South Sudan, Mali and Angola. This impressive progress in reducing the number of cases in people is due to basic public health measures, surveillance, clean water pumps and improved education.

However, the recent detection of clusters of Guinea worm infection in domestic dogs, published in PLoS Neglected Tropical Diseases, could be a major setback in its eradication as little is known about the epidemiology of Guinea worm in its non-human hosts.

GWD is a debilitating neglected tropical disease caused by the nematode Dracunculus medinensis, and its fidelity to human hosts made it the first parasitic disease to be targeted for eradication. The transmission pathways in people are well documented, and usually include drinking water that is contaminated with copepods (‘water fleas’), infected with worm larvae. The larvae penetrate the wall of the intestine and migrate, mature and reproduce before female worms expel more larvae from a blister in the skin that forms where the worm exits its human host.

Although direct transmission from dogs to people is not possible, there are concerns that dogs may shed Guinea worm larvae in shared environments. Indeed, the infection transmission pathways for dogs are currently unknown, but there are two main possibilities: dogs may follow the classical pathway, as in people, and drink water containing infected copepods, or they may consume paratenic hosts, such as fish or amphibians, that have themselves consumed infected copepods.

The rationale behind this second, novel putative pathway is the high levels of seasonal fishing in Chad, and the likelihood of dogs consuming uncooked entrails and scraps and therefore being exposed to Guinea worm larvae. Consequently, to understand the epidemiology and control of Guinea worm in dogs, the research focused on the ecology and diet of free-ranging dogs, their access to water and their consumption of aquatic foods.

The team from the University of Exeter – in collaboration with the World Health Organization and The Carter Center – used GPS trackers fitted to collars to determine the location and ranging of dogs at various study sites along the Chari River in Chad. They also performed stable isotope analysis of samples of the dogs’ whiskers and from putative food items to understand the variation and composition of the dogs’ diets.

Results from the studies in Chad (in 2016 and in 2018/19) showed that the majority of dogs visited only a small number of ponds close to their households. Additionally, the households that provided clean water to their dogs displayed a significantly reduced risk of Guinea worm infection, indicating that the classical transmission pathway operates in dogs.

By determining the growth rate of the dogs’ whiskers, the team was able to associate sections of the whiskers progressively with a dog’s diet going back in time. Not only did this stable isotope analysis indicate that fish and wildlife made up a noteworthy proportion of the dogs’ diets, it also highlighted the monthly variation in the diet for each individual dog.

Subsequently, the team was able to narrow down dogs’ main fish consumption to a period between March and May – this is during dry season, and correlates with a major increase in fishing and human activity.

By focusing disease management on these seasonal patterns linked to human behaviour, control measures can be much more targeted, and therefore effective. Robbie McDonald, one of the ecologists on the team, described the study as a ‘compelling example of the need for a One Health approach to achieve successful disease eradication’.

• This paper was discussed at the World One Health congress, held virtually on 30 October–3 November. ●

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