Subsidized veterinary extension services may reduce antimicrobial resistance in aquaculture.

Antibiotic use in aquaculture has become very controversial vis-à-vis driving antimicrobial resistance (AMR) in aquatic bacterial populations. The AMR trends in fish pathogens in Hong Kong over a four-year period suggests that providing small stakeholder farmers with free veterinary advice on fish health issues and treatments, as well as subsidized quality-assured medicines, likely reduced AMR. We observed a dramatic reduction in the proportion of bacteria resistant to oxolinic acid, oxytetracycline, and florfenicol on local aquaculture farms between 2018 and 2021. These decreases coincided with either a change in antibiotic use practices on farms (i.e. with oxytetracycline), or the reduction in the use of specific drugs (i.e. oxolinic acid and florfenicol). We did not observe a similar decline in the resistance pattern to commonly used antibiotics in human medicine in the same fish bacteria. Resistance to these products, which were unlikely to be used by the farmers in our study, was very high. Our finding suggests that both human and veterinary use of antibiotics in Hong Kong may have an influence on the AMR of bacteria in the aquatic environment.

Emamectin Benzoate Treatment of Hybrid Grouper Infected With Sea Lice in Hong Kong.

Sea lice (Copepoda: Caligidae) are ectoparasites which negatively impact marine aquaculture species around the world. There are a limited number of treatments licensed for use against sea lice in tropical and semi-tropical farmed fish species. Emamectin benzoate (EB) was an effective pharmaceutical drug against sea lice infestations in several salmon industries before resistance to the product developed. This drug has not been extensively tested in marine fish within Asia. The objective of this study was to determine whether this drug could be used to treat oral infections with sea lice in hybrid grouper (Mycteroperca tigris × Epinephelus lanceolatus) cultured in saltwater net-pen sites in Hong Kong. We observed an overall reduction in sea lice infections over time, starting on the last day of the treatment up to the end of our study (i.e., 14 days after the last EB treatment). We also observed a large variation in concentrations of EB in fish on the last day of the treatment, which provides an explanation for the variation in response to the treatment. It also suggests that distribution of the medication to fish in saltwater net-pens is difficult, especially when medication is hand-mixed in the feed and possibly unevenly distributed in the daily rations. Overall, this study provides preliminary evidence that EB could be used to treat sea lice found in Hong Kong and potentially in other regions of SE Asia.