The peculiar epidemiology of dracunculiasis in Chad

Abstract

Dracunculiasis was rediscovered in Chad in 2010 after an apparent absence of 10 years. In April 2012 active village-based surveillance was initiated to determine where, when, and how transmission of the disease was occurring, and to implement interventions to interrupt it. The current epidemiologic pattern of the disease in Chad is unlike that seen previously in Chad or other endemic countries, i.e., no clustering of cases by village or association with a common water source, the average number of worms per person was small, and a large number of dogs were found to be infected. Molecular sequencing suggests these infections were all caused by Dracunculus medinensis. It appears that the infection in dogs is serving as the major driving force sustaining transmission in Chad, that an aberrant life cycle involving a paratenic host common to people and dogs is occurring, and that the cases in humans are sporadic and incidental.

Figure 1.

Photograph of a dog with an emergent worm on left hind leg (Photo by Chad GWEP).

Figure 2.

Map of the Guinea worm-endemic area in Chad, noting villages reporting cases of dracunculiasis in humans 2010–2013* (red dots) and dogs 2012–2013* (blue dots) (* provisional: January–June, 2013).

Figure 3.

Graph showing the month of appearance of Guinea worms in people and dogs for 2012 and 2013* (* provisional: January–June, 2013).

Figure 4.

Photograph of mass fish harvesting in a local lagoon associated with one of the endemic villages in Chad. Not seen in this photograph is an open body of water to the left and to the right that is at least 10 times greater than the area depicted in this image (Photo by Chad GWEP).

Figure 5.

Photograph of mass drying of small fish directly on mats on the ground, Chad (Photo by Chad GWEP).

Figure 6.

Larger fish split, gutted, and drying on elevated mats, Chad (Photo by Chad GWEP).

Figure 7.

Photograph of fish cleaning area with viscera (arrows) on ground and accessible to scavenging dogs. Gray specks on ground are fish scales (Photo by Chad GWEP).

Figure 8.

Pairwise genetic distances between Dracunculus isolates. Data shown are a two-dimensional multidimensional scaling of single-nucleotide polymorphism (SNP) distances between isolates based on genome-wide sequencing data as described in the text. Note that the two axes of the main plot have very different scales: the inset plot shows the same data drawn with two equivalent axes.

Figure 9.

Diagrammatic life cycle for Dracunculus medinensis, showing the typical mode of transmission (1a), which is ingestion of water containing infected copepods. Also shown is potential transmission cycle (1b) that includes ingestion of infected copepods by fish (or other aquatic vertebrates such as tadpoles), which are then eaten under/uncooked by dogs or people leading to infection.