Bacterial infections (currently Vibrio harveyi) are emerging as mortality episodes related to stress events which are out of abalone farmers’ control. Data for appropriate antibiotic use is unavailable. Inappropriate unregulated use in shellfish including abalone has led to antibiotic resistance and residue problems in several countries. Legal antibiotic access is limited, will worsen with proposed legislative changes, and in the longer term would require at least MUP registration.
MUP would require data on a range of parameters, including tissue levels achieved with various dose rates and how this varies with size, species, temperature and physiological state, duration of residues in tissues, acute and longer term toxicity, and efficacy against the pathogens of concern under the proposed conditions. Environmental safety data requirements would limit the choice of antibiotics to those already available for other aquatic animals, that is oxytetracycline, amoxycillin, potentiated sulphonamides (trimethoprim / sulphadiazine combination), and possibly quinolones such as oxolinic acid. Each of these has major potential limitations in either stability or bioavailability under marine conditions, efficacy against current pathogens, or acceptability for aquaculture use (quinolones). Volumes required for bath treatments, leaching from feeds, and cessation of feeding in infected abalone may all limit practicality.
To address all these parameters for MUP application in not warranted unless sufficient stability, bioavailability, and tissue levels are achieved, hence this preliminary study of the practicality of administration and in vitro and in vivo bioassay of bioactivity. The techniques developed for in vitro assessment of abalone antibacterial defenses and the in vivo infection model will also indicate the role of stress and whether alternate approaches to disease control also warrant investigation. Industry appreciates avoidance is preferable to treatment, and though stress may be unavoidable, understanding its role and being able to measure the effect may assist in developing alternate strategies.
