FHC wet lab facility

Columnaris causes severe erosion of skin and gill.

The California – Nevada Fish Health Center (CA-NV FHC) assisted the Service’s Aquatic Animal Drug Approval Partnership in several drug registration trials for the use of Florfenicol (INAD 10-697) to control columnaris in juvenile steelhead trout. Schering-Plough Animal Health produces the florfenicol antibiotic Aquaflor ™  that is currently labeled to treat a catfish bacterial disease and “coldwater disease” in salmonid fishes.  This research was aimed at expanding the use of Aquaflor for another serious fish disease. Public and private aquaculture in the United States has struggled for many years because of a severe shortage of FDA approved drugs and therapeutants for use in aquatic species.  A number of tanks were purchased by the sponsor, Schering-Plough Animal Health, for this study.

Columnaris is a bacterial disease caused by Flavobacterium columnare and is a common health problem for hatchery and wild fish when water temperatures climb above 65°F.   The disease has affected both Steelhead trout (Oncorhynchus mykiss) and Chinook salmon (O. tshawytscha) juveniles reared at Coleman National Fish Hatchery during the spring and summer. This bacteria literally eats away the skin, muscle, and gill tissue of affected fish (see photo).

In May and June 2007, two separate studies were conducted at the CA-
NV FHC ‘s wet laboratory using juvenile Steelhead trout from Coleman National Fish Hatchery.  The wet lab facility has an effluent disinfection system and can control a number of water quality parameters such as pH, oxygen, and temperature.  A major challenge in conducting therapeutic drug trials is having a test population that is representative of hatchery fish undergoing a disease epizootic.  We created such a scenario by artificially infecting a small number of fish with the bacteria (donors) and subsequently placing them with a larger group of trout that were stressed by crowded conditions. 

At the water temperatures used for the trials (17 – 20°C), columnaris epizootic can occur in 24 – 48 hours.  This pattern occurred in the first trial and control was not obtained during the 10 day treatment period.  Fish were fed at a relatively low feed rate (2% body weight / day) in this trial and cannibalism of diseased fish was a likely factor in continued transmission.  In the second trial, the feed rate was increased to 3.5% bodyweight / day while the dosage (10 mg / kg fish / day) remained the same.  While relative disease control was obtained in 3 of 4 treated replicates, the cumulative mortality was similar for both treated and untreated groups. The occurrence of drug resistant bacteria was monitored during the trials and was not detected in the fish.  No tissue changes to the liver, kidney, and intestinal tract was observed in histological sections.  Future studies with Aquaflor and columnaris will need to focus on drug concentration delivered to the majority of fish in the first 3- 5 days of a 10 day treatment.