Abstract:
Dactylogyrus sp. (a monogenean gill fluke) is one of the most common disease causing agent
found in gills of goldfish, Carassius auratus reared in mud ponds. Several aquaria owners have
reported that when the goldfish, infested with gill flukes were treated with the recommended therapeutic
dosage of formalin (200 mgl-1 for 15 to 30 minutes), mortality of fish increased drastically;
therefore, the present study was carried out to find out the possible reason for poor results of the
treatment. Three slightly different stages of severity in breathing difficulty of gill fluke infested
goldfish (5.5±1.5 cm SL) were identified using the behaviour exhibited by the fish. These three
stages were confirmed with wet mounts under the microscope by the presence of gills covered
with thick mucus (stage I) and the gills with different degree of hyperplasia (stage II) and gills under
telangiectasis (dilation of groups of small blood vessels in the secondary lamellae; stage III). The
fish in each different stage of breathing difficulty/gross appearance of gills were treated separately
(3 replicates from each stage) with the recommended dosage of formalin; apparently healthy fish
reared in cement tank were used as control and were subjected to the same treatment. During the
treatment period each fish that reached the terminal stage of life was euthansiated and wet mounts
of gills were observed under the microscope.
Ninety eight percent of goldfish that reached the terminal stage of fish during the treatment had gill
lamellae under telangiectasis before the treatment. Fish that had gills with thick mucus and hyperplasia
of gill lamellae before the treatment could tolerate the treatment. It seems that the therapeutic
dosage of formalin recommended for killing monogenean gill flukes is also toxic for fish if the gill
lamellae have reached the stage of telangiectasis due to the presence of the parasite. Therefore it
is essential to identify gill fluke infestations at an early stage, if the therapeutic treatment with
formalin is to be successful in protecting goldfish while controlling the flukes.