Diphyllobothrium latum, is probably one of the better-known fish parasites because humans are its final host. This parasite uses 2 intermediate hosts, the 1st a copepod, the 2nd a fish. Plerocercoids in fish muscles can be accidentally eaten by humans when the fish is consumed raw or undercooked.
Many papers have described infections of people with adult broad fish tapeworms (e.g., Dick et al. 1991), but only a few represented cases where humans had become intermediate hosts for this parasite. A good example is so-called“sparganosis”recorded in some Far Eastern countries.
It occurs when humans replace fish as the 2nd intermediate host of broad fish tapeworms by drinking water carrying Diphyllobothrium-laden copepods (e.g., Kim and Lee 2001).
The most important human disease linked to copepods is cholera. The association of Vibrio cholerae, the causative agent of cholera, and its copepod host has been under study for more than 25 yr.
It is now well documented that V. cholerae is autochthonous to aquatic environments and closely associated with copepods. Seasonal cholera outbreaks are associated with algal blooms, which in turn provide enough food to cause rapid growth of planktonic copepods. The V. cholerae associated with copepods also rapidly reproduces.
To cause disease, the number of V. cholerae 01 cells ingested must be high enough to constitute an infective dose, estimated to be 104 to 106 cells (Huq and Colwell 1996). The presence of copepods carrying V. cholerae at aquaculture sites in certain endemic areas can be dangerous.
An open question becomes whether V. cholerae can be transferred to fishes. Other species of Vibrio such as V. anguillarum and V. ordali are specific to fishes, and they are responsible for vibriosis, an important fish disease.
A non-01 V. cholerae was isolated from a fish in Japan (Muroga et al. 1979), and in the early 1990s in the Ukraine, several people contracted cholera apparently from fish caught in a river polluted with municipal sewage.
Conclusions:
Copepods play very important and diverse roles in freshwater aquaculture operations, including some that are beneficial and others that are extremely adverse and may result in complete production losses. Some copepod roles are obvious, for example, copepods that serve as food for small fish or copepods that are fish parasites.
Others, however, are not often considered, such as copepods as micropredators of fish, their role as intermediate hosts of fish parasites, and their role as hosts and vectors of human diseases.
While progress has been made in better understanding the biology of copepods in aquaculture, some areas still require further research, especially including studies of the taxonomy of different copepod species.
Authors:
Wojciech Piasecki, Andrew E. Goodwin, Jorge C. Eiras, Barbara F. Nowak
