New research from the University of Georgia provides insight about how the parasite responsible for Guinea worm disease in humans may be maintained by dogs, frogs, and small crustaceans known as copepods. The findings, recently published in the International Journal for Parasitology, suggest that reducing copepod populations might be the most effective control method for this transmission route.
Guinea worm has long taken a devastating toll on human health in parts of Africa. People typically come in contact with the parasite by drinking water contaminated with infected copepods. When ingested, it causes Guinea worm disease, which can lead to death or disability along with follow-up health problems.
Efforts to eliminate human contact with the parasite by providing access to clean drinking water have been largely successful, and cases of Guinea worm disease have declined significantly in recent years. However, complete eradication of the disease has been complicated by the presence of infected domesticated dogs.
“We’ve seen an increasing number of cases in domestic dogs and this is a worry because the parasites hiding out in dogs might compromise some of the control efforts that are focused on reducing human exposure,” said Richard J. Hall of the Odum School of Ecology and College of Veterinary Medicine, the study’s senior author.
Like humans, dogs can become infected by drinking contaminated water. But dogs may also acquire the worm by eating frogs that, as tadpoles, consumed infected copepods. Infected dogs can then shed worms into water bodies used by people, which can increase human exposure risk.
The researchers, led by John Vinson, a postdoctoral associate in the Odum School and the Center for the Ecology of Infectious Diseases, developed a mathematical model to explore the role of frogs in Guinea worm transmission. The model calculated the theoretical transmission potential of Guinea worm among copepods, frogs and dogs under a variety of scenarios. In one, frogs play no role; in the second, frogs consume infected copepods but do not transmit the parasite; and in the third, frogs both consume infected copepods and transmit the parasite to dogs.
They then applied their findings to determine how the model’s results could inform control strategies.
Surprisingly, the model showed that frogs are often helpful in preventing Guinea worm transmission to humans because they remove infected copepods from the water body, lowering the direct consumption of infected copepods by dogs.
“A lot of the times the presence of frogs would reduce transmission potential because they were vacuuming up the copepods and weren’t leading to transmission or infection in the dogs,” Vinson said.
The researchers also found that having more dogs present did not necessarily mean there was a higher outbreak risk. The model showed that high numbers of dogs could mean that worm-carrying copepods are typically eaten before the worms mature enough to be infectious to dogs, thus lowering transmission potential.
Vinson said this finding is important because it indicates that prevention measures geared toward reducing or controlling the dog population may not be the answer, and in some circumstances the transmission potential increases as the number of dogs decreases.
The research also highlights the importance of taking into account the ecological context of host-parasite interactions when trying to devise control strategies. The model revealed that, in the case of Guinea worm, eradication methods directed towards dog or frog populations could have unintended consequences.
“The best way to reduce transmission would probably be focusing on trying to control the copepod populations,” Vinson said. “Because doing that would reduce the pathway of direct consumption of the copepods by dogs, and consumption through the tadpoles and frogs. But we should be cautious that these interventions do not harm other aquatic organisms in the community.”
Besides Vinson and Hall, study coauthors were Vanessa Ezenwa (now at Yale University) and Andrew Park of the Odum School of Ecology and College of Veterinary Medicine and Christopher Cleveland and Michael Yabsley of the College of Veterinary Medicine and Warnell School of Forestry and Natural Resources. This research was funded by donations made to the Carter Center.