Among the other major causes of infection of importance in aquaculture are larvae of trematodes. Dactylogyrosis is a common disease caused by species of the genus Dactylogyrus, or the gill fluke, which affects common carp, Chinese carps and other fish. Dactylogyrus is small in size (rarely longer than 1 mm) and occurs on the gill filaments (fig. 9.23). A number of species have been identified, but the ones of epizootic interest for carp appear to be D. vastator and D. extensus. They attach themselves to the gills and, under favourable conditions, such as temperatures below 30°C, develop rapidly. They are hermaphrodites and lay eggs that fall to the bottom of the pond or other rearing facilities. The larvae which hatch out are ciliate and swim around until they attach themselves to the gills, body surface or oral cavity of a host and begin to grow. Irrespective of the place of initial attachment, they all subsequently congregate on the gills and, when mature, start laying eggs, repeating the life cycle. Dactylogyrus vastator seems to prefer warm-water environments and infects mainly young carp, causing heavy mortality depending on the intensity of infection.
Dactylogyrus extensus, on the other hand,prefers temperatures below 17°C. In addition, this species occurs in both young and adult fish and is more pathogenic. The intensity of infection increases with the age of the host. In both cases, older fish are the source of infection. Infected fish become restless and collect near water outlets. The gills are damaged and become covered with mucus, inhibiting normal respiration.
The infections can be controlled by the usual sanitary measures in pond farms. Ammonia baths are recommended for treatment, at concentrations of 2 ml of 25 per cent ammonia solution per litre water for half to one minute.
Bromex-50 and Dipterex have also been used successfully.
The species of Dactylogyrus infecting grass carp have been identified as D. lamellatus and D. ctenopharyngodonis. Dactylogyrus hypophthalmichthys is the common parasite of silvercarp and D. aristichthys and D. nobilis of bighead and Chinese carp hybrids. They do not appear to infect other pond fish. The symptoms of the disease are generally very similar and the treatment recommended is also the same.
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