Abstract

The Red Snakehead (Channa micropeltes) is a freshwater fish native to Thailand. These piscivorous fish are commonly imported into the United States for sale in the ornamental fish industry. Necropsies of several debilitated individuals revealed the presence of large cestodes (3-6 cm in length) in the intestines. These cestodes appear to belong in the genus Senga. While these fish probably support this parasite burden without difficulty in the wild, it was felt to be an added stress on fish already stressed by recent transcontinental shipping.

Praziquantel (Droncit® Mobay Corn.) is commonly used in veterinary medicine to kill cestodes in mammals and digenetic and monogenetic trematodes in fish. Oral doses are reported for praziquantel to treat cestode infections and doses used in bath form exist for treatment of trematodes in fish.

In the initial study, a bath treatment using 10.0 ppm praziquantel was employed since dosing fish orally or by injection at the wholesale level of the tropical fish industry would be both costly and impractical. The initial results were very encouraging since fish could be observed shedding intact cestodes from the cloaca within a period of 3 hours post treatment. Subsequent experiments showed that concentrations of praziquantel as low as 0.5 ppm will remove cestodes from the intestinal tract of Channa micropeltes.

In an attempt to objectively quantify the results of these experiments, a single blind study was performed using one group of untreated fish and another treated for 3 hours with 1.0 ppm praziquantel. Gross and histopathological findings confirmed the presence of adult cestodes in the intestines and liver of the untreated fish and revealed the treated fish to be entirely void of cestodes.

Specific dosages, water quality parameters and other experimental conditions and protocols will be discussed in detail as well as the implications of treating other teleost fishes cestode infections with praziquantel.

Review

In a large tropical fish wholesale facility many thousands of fish come and go in a relatively short period of time. Most fish in good health spend less than 5 days in a typical facility. In the case of the Red Snakehead (Channa micropeltes), a species imported directly from Thailand, periodic bouts of high morbidity and mortality were observed within 24 hours of arrival in the wholesale tanks. Many affected fish appearance cachexic weak and anorexic. These traits are all abnormal for the characteristically aggressive and voracious snakehead. Gross necropsy revealed the presence of large mature castnodes in the intestines. In cases the parasitic load was severe.

Praziquantel is a cestocide which exerts its effect on the neuromuscular system of the Parasite. Cestodes which are exposed to this compound become paralyzed and the adhesive suckers lose their function. This drug is routinely used to treat cestodiasis, in domestic mammals. Recommended routes of administration include both oral and parenteral injection. These routes are difficult or impractical with large numbers of tropical fish. Heckman (1984, 1987) experimented with sculpin using praziquantel as a tank treatment for helminth parasites in combination with other parasiticides including ivermectin and mebendazole. Heckman had success using praziquantel as a bath treatment at a concentration 2.0 ppm. He found this to be safe and when used in combination with ivermectin a variety of helminth parasites (none of which were intestinal cestodes) were readily killed. Heckman (1984) described experiments using injectable praziquantel and reported that it was adequate to kill all the life stages of the digenetic trematode, Diplostomum spathaceum. Moser et al., (1986) used praziquantel to kill larvae-of the fluke, D. spathaceum. Moser et. al., (1986) also treated electric rays (Torpedo californica) with oral and injectable praziquantel to control the cestode, Acanthobothrium californica. Their controlled study found praziquantel to be quite effective using both routes of administration when appropriate doses were utilized. Schmahl and Mehlhorn (1-085) found praziquantel to he effective in killing monogenetic gill flukes belonging to the genus Dactylogyrus which parasitized the gills and skin of the carp, Cyprinus carpii.

Due to the high cost of praziquantel, our initial bath treatment studies were limited but encouraging. Doses as high as 10.0 ppm caused the Snakehead fish to expel intact cestodes from the cloaca within 30 minutes of dosing. Subsequent clinical trials revealed that concentrations of praziquantel as low as 0.5 ppm in a bath treatment produced similar results. In order to perform a blind, controlled study, 2 groups of Snakeheads (7 fish per group) were isolated. One group was treated with 1.0 ppm praziquantel and the second group remained untreated. The fish were shipped by air to a co-investigator in separate bags marked with the letters "A" and "B". Clinically, those fish in the treated group arrived in much better condition than those in the untreated group. Several fish from the untreated group died in transit and the rest appeared unthrifty. All of the fish were necropsied and upon examination every treated fish was found to be void of cestodes while all those in the untreated group were found to have cestodes in the intestines and liver.

When used as a bath treatment, concentrations of praziquantel up to 10.0 ppm appear safe for Channa micropeltes. Recent unpublished work by Heckman and Zhatkanbaeva (1989) suggests that praziquantel is a safe and effective treatment for controlling cestode infestations in the largemouth bass (Micropterus salmoides). These findings combined with our study suggest that praziquantel is a safe and effective way to rid fish of mature cestode parasites. Future work will focus on other species of tropical ornamental fish and on determining the safest and most cost effective treatment regimen using praziquantel.

Acknowledgements

We are grateful to David O'Beirne of O'Beirne Tropical Fish for providing the fish and access to the aquatic systems.

References

1.  Heckman, R. 1984. The efficacy of praziquantel and other pharmaceuticals against the eye fluke of fish, Diplostomum spathaceum. American Fisheries Society Newsletter, Fish Health Section, 12(4), p 7.

2.  Heckman, R.. 1987. The efficacy of praziquantel (Droncit) and ivermectin in combination as a helminthicide for fish parasites. American Fisheries Society Newsletter, Fish Health Section, 15(3), p 7.

3.  Heckman, R. and Zhatkanbaeva. 1989 Personal Communication.

4.  Moser, M. Sakanari, J. and R. Heckman. 1986. The effects of praziquantel on various larval and parasites from freshwater and marine snails and fish. J. Parasit., 72(1). pp. 175-176.

5.  Schmahl, G. and H. Mehlorn. 1985. Praziquantel effective against Monogenea. Z. Parasitenkd, 71: 727-737.