In order to survive and reproduce in humans, Clonorchis sinensis, the Chinese liver fluke, has to infect a mammal, then a susceptible species of aquatic snail, and finally a fish. The mammal must deposit its feces in fresh water where the snail and fish are present, and the fish must be eaten raw in order to infect a human. How can this set of circumstances, which seem unusual to people in Western culture, happen so often that millions of humans have the worm? Many people in China, Japan, Korea, Taiwan, and Vietnam have C. sinensis worms in the liver (Roberts and Janovy).
How do people get clonorchiasis?
A 2005 paper by Rui Lin and colleagues, published in the Southeast Asian Journal of Tropical Medicine and Public Health, explains why, while some parasitic worm infections are declining in many places, C. sinensis continues to thrive, and even increase. People do everything but lay out a red carpet for this worm: they unintentionally support the life cycle of C. sinensis. The key thing that explains this is the cultural tradition of eating raw fish in the East.
The pleasures of eating raw fish in sushi are relatively new to modern Western culture, but cultures in the East have historically enjoyed fish raw. It is not only a culinary preference, it is often an economic necessity: poor families cannot afford the fuel to cook all their food and thus eat much of it, including fish, uncooked. Lin et al found that, in their sample population, only about a third knew eating raw fish could transmit liver flukes, and about half of these believed the infection is relatively harmless. Almost three quarters of the people surveyed rejected the idea of not eating raw fish as impractical.
Freshwater fish are often raised in ponds in Eastern countries, and those that raise them take advantage of an easy way to dispose of both human and animal waste, and feed the fish at the same time. People deliberately build latrines over or beside the ponds; the waste encourages the growth of algae in the water, and the fish enjoy a healthy diet. These ponds are good habitat for aquatic snails as well. Thus, C. sinensis can rest assured that its eggs will find their way into suitable fresh water, and to the next host.
Feces get into ponds in other ways as well. Lin et al write that “there were… pigpens around 36% (21/56) of [the ponds, and] vegetable plots and other farming land where human nightsoil [sewage] was used as fertilizer around 48% (27/56).” Ponds were only cleaned out about every two years.
Presumably the Chinese liver fluke was once an uncommon parasite of humans; however, when people settled down in permanent communities and started raising both fish and domestic animals, the worm surged. In 2005, in the part of South China studied by Lin et al, 70% of cats, 50% of dogs, and 27% of pigs were infected. None of these animals originated in the Far East, and large populations of them living in human communities there now will clearly have an influence on the epidemiology of a parasite they share with people.
How to prevent clonorchiasis
Lin and co-authors found that 40% of fish tested in their study area carried the parasite. If everyone either disposed of human and animal wastes in a fashion that prevented eggs from entering surface water, or thoroughly cooked fish before eating it or feeding it to animals, the life cycle of C. sinensis would be interrupted, and the prevalence of clonorchiasis would be vastly reduced.
Sources
Lin, Rui, Xueming Li, Chungeng Lan et al. “Investigation on the Epidemiological Factors of Clonorchis sinensis Infection in an Area of South China” Southeast Asian Journal of Tropical Medicine and Public Health 36:5 2005.
Roberts, Larry S. and John Janovy Jr., Foundations of Parasitology 6th ed. McGraw Hill, 2000
Rosemary Drisdelle - Rosemary Drisdelle has been published many times as a nonfiction writer and several times as a poet. Her first book, Parasites: Tales of ...
