Effects of Praziquantel on Eggs of the Asian Tapeworm Bothriocephalus acheilognathi

—Praziquantel, an anthelmintic, is commonly used to control the Asian fish tapeworm Bothriocephalus acheilognathi in grass carp Ctenopharyngodon idella and baitfish. We treated 50 individuals from three different cyprinid fish species with praziquantel at 6 mg/L, a dose higher than the minimum recommended for complete Asian tapeworm removal in 24 h but much less than the concentration that would have ill effects on the fish. Praziquantel killed hundreds of adult tapeworms, but many ruptured and released eggs. We observed that the eggs released from Asian tapeworms treated with praziquantel were viable and produced thousands of coracidia over several days. We warn fishery managers that even if fish receive the typically recommended praziquantel treatment regime and all adult tapeworms are killed, viable eggs and coracidia may be present in the holding water or attached to the skin of treated fish, the surfaces of equipment, or treatment personnel. The Asian tapeworm Bothriocephalus acheilognathi is a parasite native to Asia. It is responsible for mortalities of fishes, particularly cyprinids, in its native range. Asian tapeworm has spread rapidly to 102 host species in 14 families and 7 orders of freshwater fishes worldwide (Salgado-Maldonado and Pineda-López 2003) and is found on all continents except Antarctica (Korting 1975; Boomker et al. 1980; Andrews et al. 1981; Heckmann et al. 1987; Font and Tate 1994; Dove et al. 1997). It is believed that Asian tapeworm entered the United States through introductions of grass carp Ctenopharyngodon idella in the 1960s (Stevenson 1965; Hoffman 1976). Asian tapeworm is a generalist whose intermediate hosts are cyclopoid copepods and final hosts are a wide range of fishes (SalgadoMaldonado and Pineda-López 2003) including many endangered species (e.g., Gila and Poeciliopsis spp.). Asian tapeworm was once thought to be limited to warm waters, but the presence of reproducing populations of this parasite in northern U.S. and Canadian fishes (Choudhury et al. 2006) suggests it can live in a wide variety of temperatures. The lifecycle of Asian tapeworm occurs in four stages: egg, coracidia, procercoid, and adult. Adult worms shed eggs that are evacuated in the feces from the final host, usually a cyprinid fish. Eggs settle to the sediment and hatch over 3–5 d. Actual development time is dependent on water temperature; the warmer the water, the faster the development. Free-swimming coracidia hatch from the eggs. Coracidia are round, have three pairs of embryonic hooks, and are covered in cilia used to swim up into the water column. When the coracidia is swallowed by the intermediate host, a cyclopoid copepod, it perforates the intestine and develops into a procercoid. Procercoids are oblong and their posterior is differentiated into a cercomere with three distinct pairs of embryonic hooks. At this stage, the procercoid is considered invasive and is able to infect the final host. It can take 4–11 d to complete this transformation, which is dependent on water temperature (Bauer et al. 1973; Korting 1975). It is thought that the procercoid can actually alter the behavior of a copepod host, causing it to swim erratically near the surface, where it will be more likely to be preyed upon by the final fish host. If a fish digests a copepod, the procercoid attaches to the intestine of the fish with the embryonic hooks. The procercoid then transforms into a tapeworm and within 20–25 d the worm develops into an adult and begins producing eggs. Again, the rate of development is temperature dependent. The entire lifecycle can be completed in 18 d if temperatures are optimal (Bauer et al. 1973). Asian tapeworms have caused up to 90% mortality in grass carp in newly infected ponds (Bauer et al. 1973) and accelerated mortality of bonytail Gila elegans under low food conditions in the laboratory (Hansen et al. 2006). They have reduced growth in roundtail chub G. robusta in the Little Colorado River (Brouder 1999) and bonytail under laboratory conditions (Hansen et al. 2006). This, combined with its * Corresponding author: [email protected] 1 Present address: Arizona Game and Fish Department, 555 North Greasewood Road, Tucson, Arizona 85745, USA. 2 Present address: U.S. Fish and Wildlife Service, New Mexico Fish and Wildlife Conservation Office, 3800 Commons Avenue NE, Albuquerque, New Mexico 87109, USA. Received July 11, 2008; accepted March 25, 2009 Published online September 21, 2009 380 North American Journal of Aquaculture 71:380–383, 2009 Copyright by the American Fisheries Society 2009 DOI: 10.1577/A08-038.1 [Article] D ow nl oa de d by [ U ni ve rs ity o f A ri zo na ] at 0 0: 00 2 1 Se pt em be r 20 13 wide geographic range, suggests it may threaten native fish conservation. Concerns that Asian tapeworm will spread farther (Choudhury et al. 2006) have made it one of the most regulated warmwater-fish parasites in the United States (Mitchell 2004). These concerns have prompted studies into methods of control and treatment for Asian tapeworm, including the use of anthelmintics such as praziquantel (Droncit), developed by Bayer Corporation for treatment of tapeworm infestation in humans and animals (Andrews et al. 1983). Fisheries managers and aquaculturists commonly use praziquantel to treat Asian tapeworm infections in fish (Mitchell 2004; Ward 2007; Mitchell and Darwish 2009). Praziquantel offers an advantage over other control methods because it allows the removal of Asian tapeworm without harming the host fish. When conducting other experiments, we used praziquantel to remove Asian tapeworm from three species of infected cyprinids, including one that was a candidate for listing with the U.S. Endangered Species Act. We observed that adult Asian tapeworms were shed during the treatment, but eggs from the tapeworms subsequently hatched and produced tens of thousands of viable coracidia. Our goal was to report this observation to warn workers who use praziquantel during the transport of fish, and encourage studies that examine the issue in more detail, with the eventual goal of developing treatment protocols that consider both adult tapeworm and immature forms.