If therapeutic formalin treatment for gill fluke infestation is to be successful in goldfish, gill lamellae should not have reached the stage of telangiectasis

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.