In part 1 of this series, we learned historically that the medical officers of the British army who served in India contracted dracunculiasis when they drank water infested with copepods infected with Dracunculus medinensis. This historical account proved that the parasite is transmitted mainly when a person ingests a copepod that was exposed to previously released juveniles from the skin of a definitive host (human). There are 3 conditions that must be met before the life cycle of the parasite can be completed (Mehlorn and Armstrong 2001):
- The skin of an infected person must come in contact with water so that the juveniles will be released and survive.
- Appropriate species of microcrustaceans (e.g. Cyclops vernalis) must be in the water; they will serve as the intermediate hosts of D. medinensis juveniles.
- The infected water must be used for drinking.
There are studies claiming that infection can be acquired by eating a fish paratenic host.
It is surprising that the life cycle of D. medinensis, which is very much dependent on water, is most successfully completed during the dry season. For instance, people in some areas of Africa depend on rivers for their water. During periods of normal river flow, dracunculiasis cases are very few or don’t exist at all, but when the dry season comes, the number of cases dramatically increases. What is the reason behind this phenomenon? During times of drought, rivers are reduced to mere trickles with sporadic deep pools, which are sometimes deepened and enlarged by those who depend on them as a source of water. These pools become warm and semi-stagnant, which favors the population explosion of the intermediate hosts: cyclopoid crustaceans. Because of the hot weather, people frequently visit the standing pools for drinking, bathing, and washing; these activities increase their chances of being infected by the juveniles shed by the crustaceans. Drinking the water is the most effective way of acquiring the parasites. (John 2006; Cox 1993; Fernando and Siew-Yin Long 2001)
In many areas of India, constructing a step well has been an effective method of exposing the groundwater for centuries, but it has also been an effective way of completing the life cycle of D. medinensis parasites. Infected women release juveniles every time they walk on the running water of the step wells.
People living in desert areas usually construct ponds that are filled during the rainy season. These ponds become sources of infection with D. medinensis (John 2006; Fernando and Siew-Yin Long 2001). Even when water wells are available, people still prefer the ponds because they have to pay for getting water in those wells. In addition, the water in the wells is usually saline compared to the pond water.
Given the above examples, it is not surprising that a parasite with an aquatic life cycle should flourish in a desert environment because all animals (especially humans) depend on isolated waterholes for their survival. So does D. medinensis. The fact that the parasite is dependent on isolated waterholes reveals the weakness in its life cycle. Moreover, it is only the helminthic parasite transmitted through drinking water. (Peries and Cairncross1997)
John D, Petri W (contri.), Markell E, and Marietta Voge. 2006. Markell and Voge’s Medical Parasitology. Saunders Elsevier.
Mehlorn H and PM Armstrong. 2001. Encyclopedic Reference of Parasitology. Published by Springer.
Fernando RL and A Siew-Yin Long. 2001. Tropical Infectious Diseases: Epidemiology, Investigation, Diagnosis and Management. Cambridge University Press.
Peries H and S Cairncross. 1997. Global Eradication of guinea worm. Parasitology Today 13:431-437.
Cox F. 1993. Modern Parasitology: A Textbook. Blackwell Publishing.