Epidemiological and molecular investigations of a point-source outbreak of Dracunculus medinensis infecting humans and dogs in Chad: a cross-sectional study.

BACKGROUND: Dracunculiasis (also known as Guinea worm disease), caused by the Dracunculus medinensis nematode, is progressing towards eradication, with a reduction in cases from 3·5 million cases in the mid-1980s to only 54 human cases at the end of 2019. Most cases now occur in Chad. On April 19, 2019, a 19-year-old woman presented with D medinensis in an area within the Salamat region of Chad, where the disease had not been previously reported. We aimed to investigate the connection between this case and others detected locally and elsewhere in Chad using a combination of epidemiological and genetic approaches. METHODS: In this cross-sectional field study, we conducted household case searches and informal group interviews in the Bogam, Liwi, and Tarh villages in Chad. All community members including children were eligible for participation in the outbreak investigation. Adult female D medinensis associated with this outbreak were collected for genetic analysis (18 from humans and two from dogs). Four mitochondrial genes and 22 nuclear microsatellite markers were used to assess relatedness of worms associated with the outbreak in comparison with other worms from elsewhere in Chad. FINDINGS: Between April 12 and Sept 6, 2019, we identified 22 human cases and two canine cases of dracunculiasis associated with 15 households. Six (40%) of the 15 affected households had multiple human or canine cases within the household. Most cases of dracunculiasis in people were from three villages in Salamat (21 [95%] of 22 cases), but one case was detected nearly 400 km away in Sarh city (outside the Salamat region). All people with dracunculiasis reported a history of consuming fish and unfiltered water. Worms associated with this outbreak were genetically similar and shared the same maternal lineage. INTERPRETATION: Molecular epidemiological results suggest a point-source outbreak that originated from a single female D medinensis, rather than newly identified sustained local transmission. The failure of the surveillance system to detect the suspected canine infection in 2018 highlights the challenge of canine D medinensis detection, particularly in areas under passive surveillance. Human movement can also contribute to dracunculiasis spread over long distances. FUNDING: The Carter Center.

Identifying correlates of Guinea worm (Dracunculus medinensis) infection in domestic dog populations.

Few human infectious diseases have been driven as close to eradication as dracunculiasis, caused by the Guinea worm parasite (Dracunculus medinensis). The number of human cases of Guinea worm decreased from an estimated 3.5 million in 1986 to mere hundreds by the 2010s. In Chad, domestic dogs were diagnosed with Guinea worm for the first time in 2012, and the numbers of infected dogs have increased annually. The presence of the parasite in a non-human host now challenges efforts to eradicate D. medinensis, making it critical to understand the factors that correlate with infection in dogs. In this study, we evaluated anthropogenic and environmental factors most predictive of detection of D. medinensis infection in domestic dog populations in Chad. Using boosted regression tree models to identify covariates of importance for predicting D. medinensis infection at the village and spatial hotspot levels, while controlling for surveillance intensity, we found that the presence of infection in a village was predicted by a combination of demographic (e.g. fishing village identity, dog population size), geographic (e.g. local variation in elevation), and climatic (e.g. precipitation and temperature) factors, which differed between northern and southern villages. In contrast, the presence of a village in a spatial infection hotspot, was primarily predicted by geography and climate. Our findings suggest that factors intrinsic to individual villages are highly predictive of the detection of Guinea worm parasite presence, whereas village membership in a spatial infection hotspot is largely determined by location and climate. This study provides new insight into the landscape-scale epidemiology of a debilitating parasite and can be used to more effectively target ongoing research and possibly eradication and control efforts.