Another unfavorable impact of copepods on aquaculture facilities is that they may serve as intermediate hosts of important fish parasites such as tapeworms or nematodes.
The presence of parasites may lead to fish mortalities or adversely affect the market value of the fish or fish products when parasites are present in fish muscle. Some important human parasites utilize copepods and fishes as intermediate hosts.
A number of parasitic diseases can affect humans, when an infected copepod is accidentally ingested with water. One of the better known and the most spectacular parasites are representatives of the genus Ligula.
They commonly occur in Europe, Asia, and North America in temperate zones. The final hosts are piscivorous birds. The first intermediate hosts are planktonic copepods of the genera Cyclops, Eucyclops, Megacyclops, Acanthocyclops, and Eudiaptomus.
The 2nd intermediate hosts for these parasites are fishes, mainly of the family Cyprinidae. This tapeworm occurs in the peritoneal cavity of fish in the form of larvae known as plerocercoids. Plerocercoids are usually between 10 cm and 1 m long (Dubinina 1966). Infection leads to cachexia, impairment of fish growth, arrested reproduction, and death (Bryli´ nski 1972).
Bream infect themselves in their 1st years of life when they feed on plankton. The infection is fatal. All large, market-size bream are those fortunate individuals which avoided infection in their early life. Once they grow up and change the planktonic copepods in their diet to benthic macrofauna, their chances of becoming infected with Ligula spp. decrease. Large plerocercoids can occur in the muscles of fishes (Je ewski and Karbowiak 2002).
In the US, ligulosis is a sporadic problem in fathead minnows, but given the small size of the host and the large size of the parasite, outbreaks can be quite severe. Another interesting tapeworm transmitted by planktonic copepods is Triaenophorus nodulosus (or T. crassus). Its scolex is armed with 4 characteristic anchors. The final hosts are predacious fishes such as pike.
The 1st intermediate hosts are copepods representing Cyclops, Eucyclops, Mesocyclops, Paracyclops, Acanthocyclops, Orthocyclops, Diaptomus, and Eudiaptomus. In the intestine of the final host, the adult tapeworms are no longer than 30 cm and are not pathogenic.
However, plerocercoids occurring in fishes serving as the 2nd intermediate host can be pathogenic, settling in the liver where they become encapsulated. The presence of such plerocercoids in fish fry can lead to abdominal distension, other serious lesions, and death.
Mortalities of fry associated with Triaenophorus spp. infections have been recorded at many European aquaculture facilities and lakes. In Königsee (Bauern, Germany), this parasite was a factor that strongly limited the size of the local population of Arctic charr (Shäperclaus 1992).
The Asian tapeworm, Bothriocephalus acheilognathi, occurs in grass carp (Ctenopharyngodon idellus) and carp, not only in Asia but also in Europe and North America. This tapeworm has only 1 intermediate host, which are various species of free-living cyclopoids. Bothriocephalus acheilognathi is less than 20 cm long, and in Europe it can be pathogenic, occurring in 1-2-moold fish in high numbers (e.g., 20-40 parasites per fish). Intensities in 2-yr-old fish varied from 1 to 362 parasites per fish.
Mortalities have been reported, even in older fish, especially after overwintering (Klenov and Vasil ,kov 1972). The Asian tapeworm was introduced into the US more than 20 yr ago. It is a major problem in grass carp and golden shiners.
Infections severe enough to cause loss of fish are rare, but there are many regulations regarding interstate transport that prohibit the movement of infected fish. The State of Utah requires that all grass carp coming into the state to be treated with the antihelmintic, praziquantel, before transport.
Other important cestodes include representatives of Proteocephalus. These species have 4 characteristic circular suckers on their scolices, and they only use cyclopoids as intermediate hosts. Proteocephalus exiguus infects coregonids, and densities of this species and its congeners may be high and can lead to local destruction of the intestinal mucosa. In the US, a closely related tapeworm, Corralobothrium sp., is commonly seen in cultured channel catfish but is of no commercial importance.
The broad fish tapeworm, Diphyllobothrium sp., is potentially pathogenic to humans and can be very abundant in fishes in some areas in North America. In the area of along the border of Maine and the Province of New Brunswick, Canada (e.g., Spednic Lake) in the mid-1990s, the viscera and muscle of fishes were so heavily infected that the fish were useless for human consumption and were thus unmarketable (Piasecki unpubl. data).
The most interesting parasite in eel management in Europe has been the nematode Anguilicola crassus. This species was accidentally introduced to Western Europe in the 1970s along with imported Japanese eels, and by the end of 20th century, it managed to spread with eels to most of Europe. This nematode ,s only intermediate hosts are planktonic copepods. Small fish serve as paratenic hosts, and the adult nematode inhabits the swimbladder of eels.
In the estuary of the Odra River (Szczecin Lagoon) in Poland, almost 70% of eels were infected (Garbacik- Weso owska et al. 1994), and intensities can be very high. In recent years, the entire swimbladder is sometimes found to be filled with these uglylooking nematodes, and infections may affect the marketability of the eels.
Parasitic copepods are potentially capable of transmitting viruses and bacteria that cause important fish diseases. Nylund et al. (1991 1993) discussed the possibility of transmission by sea lice of the furunculosis bacterium, Aeromonas salmonicida, and the virus that causes infectious salmon anemia (ISA). Sea lice are considered marine copepods, but freshwater fish-lice (Caligus lacustris) can also become abundant in aquaculture cages (Rokicki 1987) and may be disease vectors.
Salmincola is reported to transmit IHN virus between fishes (Mulcahy et al. 1990), but the actual transmission mechanism is unclear because Salmincola adult females are immobile.
Authors:
Wojciech Piasecki, Andrew E. Goodwin, Jorge C. Eiras, Barbara F. Nowak
