Trematodes
While more than 750 million people around the world are at risk for food-borne trematode infections (FBT), an estimated 40 million people are infected with one or more of these parasites (WHO, 1995). The majority of these infections (around 38 million) are fish-borne infections and are mainly occurring in some 20 countries where the parasites are endemic. Although seldom fatal, trematode diseases can cause morbidity and complications leading to death. The cause of infection is the ingestion of viable trematode metacercariae, which can be present in the flesh of, raw, inadequately cooked or minimally processed freshwater fish, molluscans and crabs. Infections are prevalent in several countries and among communities where eating raw, fermented or inadequately cooked fish is a cultural habit.
To control a disease, it is important to know where it is endemic. In trematode disease it is also necessary to have a complete understanding of the biology of the parasite, the life cycle and each stage of the life cycle must be known from the egg via the miracidium to the cercaria to the metacercaria to the adult parasite. All hosts must be determined: the snail (first intermediate), the animal host or vegetation (second intermediate) upon which metacercaria may encyst.
The adult worms are small, flat, slender and measures from a few up to 20 mm in length and 3-5 mm at the widest area. The general life cycle of trematodes, having fish as the second intermediate host is shown in Figure 5.13.
Figure 5.13 Life cycles of trematodes having fish as an intermediate host (redrawn from Strauss 1996).
There are three main groups of fish-borne trematodes infecting man (Table 5.23):
the liver flukes , the lung flukes, the intestinal flukes.
Table 5.23 Trematode parasites transmitted by fish
Parasite
Second inter-mediate host
Geographical area1
Estimated no. of infections (millions)
Liver flukes:
Clonorchis sinensis
> 100 species of freshwater fish, mainly carp
C, RK, J, R, V
7-13
1. Geographical area: SEA: South East Asia, EE: East Europe, R: Russia, U: Ukraine, C: China, RK: Republic of Korea, J: Japan, P: Peru, E: Ecuador, W+CA: West- and Central Africa, EU: Europe, ME: Middle East
While more than 750 million people around the world are at risk for food-borne trematode infections (FBT), an estimated 40 million people are infected with one or more of these parasites (WHO, 1995). The majority of these infections (around 38 million) are fish-borne infections and are mainly occurring in some 20 countries where the parasites are endemic. Although seldom fatal, trematode diseases can cause morbidity and complications leading to death. The cause of infection is the ingestion of viable trematode metacercariae, which can be present in the flesh of, raw, inadequately cooked or minimally processed freshwater fish, molluscans and crabs. Infections are prevalent in several countries and among communities where eating raw, fermented or inadequately cooked fish is a cultural habit.
To control a disease, it is important to know where it is endemic. In trematode disease it is also necessary to have a complete understanding of the biology of the parasite, the life cycle and each stage of the life cycle must be known from the egg via the miracidium to the cercaria to the metacercaria to the adult parasite. All hosts must be determined: the snail (first intermediate), the animal host or vegetation (second intermediate) upon which metacercaria may encyst.
The adult worms are small, flat, slender and measures from a few up to 20 mm in length and 3-5 mm at the widest area. The general life cycle of trematodes, having fish as the second intermediate host is shown in Figure 5.13.
Figure 5.13 Life cycles of trematodes having fish as an intermediate host (redrawn from Strauss 1996).
There are three main groups of fish-borne trematodes infecting man (Table 5.23):
the liver flukes , the lung flukes, the intestinal flukes.
Table 5.23 Trematode parasites transmitted by fish
Parasite
Second inter-mediate host
Geographical area1
Estimated no. of infections (millions)
Liver flukes:
Clonorchis sinensis
> 100 species of freshwater fish, mainly carp
C, RK, J, R, V
7-13
1. Geographical area: SEA: South East Asia, EE: East Europe, R: Russia, U: Ukraine, C: China, RK: Republic of Korea, J: Japan, P: Peru, E: Ecuador, W+CA: West- and Central Africa, EU: Europe, ME: Middle East
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