Epidemiology & Risk Factors

Collecting water from a stagnant pool. Photo credit: Emily Staub, 2004, The Carter Center.

In 1986, there were about 3.5 million GWD cases per year in parts of Africa, Asia, and the Middle East. Today, GWD affects poor communities in remote parts of Africa that do not have safe water to drink. Thanks to the Guinea Worm Eradication Program, there were only 22 cases reported worldwide in 2015. Chad reported 41% of the GWD cases in 2015, replacing South Sudan as the country with the most cases. The remainder of the 2015 GWD cases were from Ethiopia, Mali and South Sudan [1].

People become infected with Guinea worm by drinking stagnant water containing copepods (tiny "water fleas") that carry Guinea worm larvae (immature forms of the worm). These larvae are eaten by the copepods that live in these stagnant water sources. The larvae need about 2 weeks to mature inside the copepods before they can infect humans. Unsafe stagnant water includes ponds, pools in drying riverbeds, and shallow uncovered wells. Anyone who drinks from contaminated water sources can become infected. GWD is not normally caught from drinking flowing water (rivers and streams)^{[2, 3, 4]}.

GWD transmission has a seasonal pattern. In dry regions, people generally get infected during the rainy season, when stagnant surface water is available. In wet regions, people generally get infected during the dry season, when surface water is drying up and becoming stagnant^{[3, 4]}.

The risk for disease varies by sex, age, profession, and ethnicity. These differences reflect how and where people get their drinking water in different areas and countries. In general, about the same number of men and women get infected. GWD occurs in all age groups but it is more common among young adults 15–45 years old. This may be because of the type of work done by people this age. Farmers, herders, and those fetching drinking water for the household can become infected more often. It is thought they are more likely to drink contaminated stagnant water while away from home. In certain areas, GWD affects some ethnic groups more than others^{[2, 3, 4]}.

The greatest risk for GWD is having GWD the year before. People do not become immune to infection. Many people in affected villages suffer from GWD year after year. This is probably because the same water sources are repeatedly contaminated and conditions that support the spread of disease have not changed. It might also be related to some biological factor of the person that increases susceptibility. Not everyone drinking from the same contaminated water supply will become infected. A few people seem to keep getting infected while others drinking the same water do not^{[2, 4, 5]}.

Ten years after reporting its last GWD case, Chad reported 10 confirmed GWD cases in 2010. It has subsequently reported 9–14 cases each year from 2011 through 2015^[1]. In a highly unusual pattern for GWD, the vast majority of cases have occurred alone in separate villages, unconnected to other cases by person, place, or time. Few cases have been linked to each other or have shared common water sources^[6]. Guinea worm infections in dogs in Chad were confirmed in 2012^[7]. Since then, the number of dogs infected with Guinea worms has increased annually and far surpassed the number of human cases: 21 infections reported in 2012, 54 in 2013, 113 in 2014, and 483 in 2015^{[1, 8]}. GW infections in both humans and dogs tend to cluster around the Chari River. The worms infecting dogs are genetically indistinguishable from the worms infecting humans in Chad^[7]. Other countries have reported dogs infected with Guinea worms as well. Ethiopia reported 13 infections in dogs in 2015 in addition to three human cases. Mali and South Sudan each reported one infected dog and five human cases in 2015. Further, sporadic GWD infections have been reported historically in dogs and other animals in other countries and in laboratory animals^[1]. However, the current pattern and volume of GWD transmission in dogs in Chad has never been documented before in other countries or within Chad prior to 2012. The working theory for the unusual circumstances in Chad is that dogs and the occasional human are getting infected by eating an aquatic animal that is itself infected with Guinea worm (called a paratenic host)^[7]. Studies are underway to evaluate a drug to prevent GWD infection in dogs and to evaluate which aquatic animals can be experimentally infected with Guinea worm in order to try to identify a potential paratenic host (for example, a particular type of fish or frog) ^[9]. Because Chad has a large domestic and commercial fishing industry along the Chari River and because of the clustering of human cases and dog infections along the Chari river, villagers are being educated to cook their fish well, to bury fish entrails left over from fish processing, and to not allow their dogs to eat fish entrails. A reward is being offered to report dogs with suspected GWD infections and dog owners are encouraged to tether infected dogs until the worms emerge to prevent the dogs from entering and contaminating water^[9].