An oxytetracycline bath protocol to eliminate the agent of withering syndrome, Candidatus Xenohaliotis californiensis, in captive abalone populations

Abalone (Haliotis spp.) populations are in decline due to overfishing and/or disease in all regions that historically supported important fisheries worldwide. Two species have been federally listed as Endangered in the USA. Recovery plans for these and other highly depleted species typically include outplanting of progeny from captive-reared broodstock, for which maintaining good health is essential to promote gonad development and maturation. In California, USA, the principal disease of concern for abalone is withering syndrome, caused by a gastrointestinal intracellular Rickettsiales-like prokaryote named Candidatus Xenohaliotis californiensis (Ca.Xc). We report the application of an antibiotic bath exposure method to eliminate the pathogen in captive red abalone (H. rufescens) populations. A series of eight, 24-h baths consisting of 500 ppm oxytetracycline in seawater were delivered over a period of 21 days. Testing of feces by PCR and histology indicated complete elimination of Ca.Xc by day 105 following the first bath treatment. Mean oxytetracycline levels in the digestive gland reached a peak of 670 ppm at the end of the treatment period and remained above 100 ppm for 125 days. Survival rate, body condition index, growth and a measure of foot muscle health were similar between treated and control groups, despite tetracyclines being potentially toxic or immunosuppressive in some animal systems. This method should prove useful for creating Ca.Xc specific-pathogen-free captive abalone populations for land-based production and to prevent pathogen transfer when abalone are introduced to growing regions where the pathogen is not present.