Development of an oral administration of oxytetracycline to control losses due to withering syndrome in cultured red abalone Haliotis rufescens

Abstract

Withering syndrome (WS) is a lethal bacterial disease of wild and cultured abalone, Haliotis spp., along the west coast of North America. Therapeutic treatment of WS was assessed using oxytetracycline (OTC) by injection and oral administration. Survival of OTC-injected red and black abalone was significantly higher than that of sham-treated animals (p<0.001 and p=0.036, respectively). Microscopic examination revealed that these animals died from WS and that tissue damage was beyond the ability of the animals to repair. Feeding rates of treated abalone were also significantly higher than for sham-treated control animals (p<0.001 for black abalone and p<0.0001 for red abalone). Oral administration (oxytetracycline medicated feed for 14 consecutive days) also effectively controlled WS in both pilot and production scale applications. The treatment caused significant and persistent long-term reductions in the intensity of bacterial infection as well as in the degree of morphological changes in the digestive gland (p<0.001 and p<0.05, respectively). On a pilot scale, using a 14-day treatment with a 4.2% OTC wet feed, mortality dropped from 36% in the controls to 5% in the treated populations (p<0.001). On a commercial scale, a dry floating feed containing 2.6% OTC was found to be just as effective. Withering syndrome caused heavy (41%) mortality in control production tanks, but treated tanks experienced only 9% mortality (p<0.001). Treated tanks produced 56% more market sized abalone than control tanks (p<0.001), and larger abalones were harvested from treated than control tanks. This resulted in 70% more biomass of market abalone in treated tanks relative to control tanks (p<0.01). Concentrations of oxytetracycline in individual abalone foot muscle were variable and peaked at over 40 ppm. Residues declined to <2 ppm, the federal tolerance level, between 15 and 22 days post-treatment.