Efficacy of Bath Treatments of Formalin and Copper Sulfate on Cultured White Bass, Morone chrysops, Concurrently Infected by Onchocleidus mimus and Ichthyophthirius multifiliis

Abstract

Concurrent ectoparasite infections on fishes are commonly encountered in the wild and in aquaculture (Rohde 1984; Hoffman 1999), for example, infections of both Ichthyophthirius multifiliis Fouquet, 1876 (Hymenostomatida: Ichthyopththiriidae; Colorni 2008) and gill‐infecting monogenoids (Platyhelminthes: Monogenoidea; Whittington and Chisholm 2008). The decision to aggressively treat infections of either of these parasites can be prudent in aquaculture systems. Both taxa have direct life cycles and short generation times, which may result in high infection intensities that can debilitate or kill the fish host (Noga 2010). Various chemical therapies have been evaluated to prevent, reduce, or eliminate such infections. Typical treatments for killing monogenoids include: freshwater or saltwater baths, formalin, copper sulfate (CuSO4), hydrogen peroxide, mebendazole, trichlorphon, and praziquantel (Whittington and Chisholm 2008). Treatments for infections of I. multifiliis (Ich) include: temporary exposure to high temperature, formalin, malachite green, CuSO4, potassium permanganate, and sodium chloride (Colorni 2008). Despite the common occurrence of these parasites on fish, little species‐specific information exists for the vast majority of fish–parasite combinations across the diversity of culture systems and captive settings. Formalin is approved by the US Food and Drug Administration (US FDA) for use as a treatment for external monogenetic trematodes in fish at up to 250 mg/L for up to 1 h (US FWS/AFS 2008). The ambiguity of this recommendation suggests a wide therapeutic range with no interpretation of repeated applications. Likewise, little information exists on the safety or efficacy of repeated formalin treatments on fish, and CuSO4 has an indeterminate approval status but is widely accepted as having high efficacy. In October 2009, white bass, Morone chrysops (Rafinesque, 1820; Perciformes: Moronidae), in a flow‐through culture system were diagnosed with concurrent infections of the gill‐infecting monogenoid Onchocleidus mimus (Mueller, 1936; Monogenoidea: Ancyrocephalidae) and Ich. O. mimus has a direct life cycle with no intermediate host required and as the intensity of infection increases can become problematic to a population. This study reports an opportunistic evaluation of the efficacy of repeated applications of formalin and CuSO4 to reduce, control, or eradicate these infections in cultured white bass.