Lernaeosis, or anchor worm disease, caused by species of the genus Lernaea, has been observed in a number of fresh-water fish, including common and Chinese carps. They are very sensitive to salinity and cannot survive even low concentrations of salt. The two species of epizootic importance appear to be L. cyprinacea (fig. 9.21) and L. ctenopharyngodonis.The mature females of the species have a long, unsegmented body and the head has branched processes with which the parasite can penetrate the body of the host (fig. 9.22). There are no intermediate hosts for the species and the free-living larvae parasitize the skin or gills of fish. L. cyprinacea will parasitize any species of fish,but L. ctenopharyngodonis appears to prefer the grass carp.
The metamorphosis of L. cyprinacea is rather complex, with three nauplii and five copepodid stages. Each female develops two egg sacs with 300–700 eggs. The optimal temperature is reported to be 23–30°C and the embryonic development takes three days. Hatching takes place on the fourth day. The development of nauplius stages lasts four to five days, followed by the copepodid stages in the next 9–10 days. The copepodid podid larva must find a host and attach itself for further development into the free-swimming cyclopid stage. Mating takes place during this stage, after which the males die and the females penetrate the skin of fish, settling in the muscle and remaining attached to the host. Depending on the temperature, between two and eleven generations have been observed in one year.
The parasite enters the fish-rearing facilities through water supplies. When it penetrates the skin of the host, reaching the muscles, deep ulcers, abscesses or fistulas are formed at the point of attachment. The margins of the ulcer are bright red or greyish in colour. Secondary infections may set in at this time. The inflammation at the point of penetration results in the formation of parasitic fibrous nodules. The parasite may also penetrate the liver, causing focal traumatic hepatitis.
It is difficult to eradicate adult parasites as they are very hardy and resistant to most chemicals. Potassium permanganate baths at a concentration of 2 ppm for 1–2 hours may kill most of the parasites, but it has been reported that within a short time young parasites begin to develop again. The free-living larval stages can more easily be controlled. Bromex, at a concentration of 0.12–0.15 ppm active ingredient in pond water, can kill these stages. The application has to be repeated three times, at intervals of about seven days in summer and 12–14 days at lower temperatures.
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