Guinea worm: what humans and dogs have in common

If, according to my microbiology professor, the Guinea worm parasite has been “virtually” eradicated, then why did the World Health Organization (WHO) push the 2020 Guinea worm eradication date back by 10 years, to 2030? 

It turns out that new cases of Guinea worm have been reported in dogs from Chad, a country in north-central Africa, since the early 2010s. In the past year alone, there have been at least 1,500 new cases in dogs, according to Nature.

More questions are raised rather than solved with this new discovery. Nature reports that while there have been cases of Guinea worm in dogs and baboons, they generally occur at a significantly lower incidence than that of Chad. Now, researchers must understand more about the epidemiology of Guinea worm in animals and its role in infecting humans before they can eradicate the disease.

Photo credit: The Carter Center

Guinea worm, or Dracunculus medinensis, is the largest tissue-affecting parasite in humans; an adult worm can be up to 60-80 centimeters in length! The worm larvae, which are present in cyclops fleas, enter humans through contaminated drinking water to mature and mate.

The disease is debilitating for its victims. While the male dies relatively quickly, the female survives to migrate through the host’s subcutaneous tissues over the course of a year to blister at one’s extremities, releasing hundreds of larvae in a painful process. When wading into bodies of water to relieve the burning sensation, hosts unknowingly facilitate the spread of Guinea worm.

There is no drug or vaccine to alleviate the condition. The WHO, the United States Centers for Disease Control and Prevention (CDC), the Carter Center, and UNICEF have strategized for eradication of the disease through prevention since the 1980s. Methods of eradication, which include filtering drinking water, avoiding bodies of water if infected, slow extraction of emerging worms, and larvicides have proved effective, reducing the number of new infections per year from 3.5 million (1986) to 28 (2018).

Guinea worms are slowly extracted from blisters inch-by-inch, in humans and dogs alike

While conflict zones in certain countries have posed dangers to eradication work and new human cases have emerged, the most urgent issue in eradication is the additional infection and transmission of the disease in the dogs of Chad, according to Nature. Although researchers hypothesized that dogs and people may contract the disease by eating small fish that contain Guinea worm, they have not successfully pinpointed the source of infection.

Completely eradicating a disease is not easy. Smallpox is the only human disease to have been eradicated. And according to National Public Radio (NPR), the deadline for Guinea worm eradication has been pushed back multiple times in the past, unsurprisingly.

Now that we know more about a new host — the dog — that is far more difficult to control, will the Guinea worm ever be eradicated?

Although the Guinea worm has not been explicitly labeled a zoonotic disease, it has all the signs of being one. Zoonotic diseases are spread between animals and people, according to the CDC. Zoonoses are not limited to viruses or fungi — they can include other worms too, such as roundworm, hookworm, and tapeworm. If infected dogs are wading into water sources, the Guinea larvae can be transmitted via indirect contact to humans utilizing that same water source.

If the Guinea worm becomes classified as a zoonotic disease, society and public health officials may have to accept that it is almost impossible to eradicate.

Oftentimes, close proximity of animals and humans lowers the barrier that prevents zoonotic disease. In addition, when “domesticated” animals like dogs and cats roam freely and multiply quickly, acting as reservoirs for the disease, it becomes more difficult to contain and track the disease. This is evident in how the number of dog cases in Chad outnumber the human cases this past year by over tenfold.

Regardless of whether the Guinea worm is a zoonotic disease or not, it is still a threat to public health in certain parts of the world. Introducing diseased dogs into the picture will most likely lead to more stringent prevention methods for dogs and humans alike, and a delayed timeline before society gets closer to complete eradication.

**Edit: In a later zoonotics lecture, my professors clarified that zoonotic diseases do not complete their life cycles (or reproduce) within humans. Guinea worm is therefore not strictly a zoonotic disease. Nevertheless, the presence of a non-human host makes it extremely difficult to control and eradicate the pathogen.

Sources: Nature, World Health Organization, NPR, SPCALA, CDC, Clinical Microbiology Reviews

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