Antibiotic Use in Shrimp Culture in the Mekong Delta, Vietnam: Pharmacokinetics, Pharmacodynamics and Impact on Shrimp and Consumer Health

Whiteleg shrimp (Penaeus vannamei) farming has developed rapidly in Vietnam during the last decades, especially in the Mekong Delta, where shrimp production has significantly contributed to the aquaculture sector. However, intensive shrimp aquaculture is often associated with the use of different chemicals and antibiotics for disease control and farm management. The improper use of antibiotics, including high doses and non-compliance with antibiotic withdrawal times, may lead to antibiotic residues in shrimp products and an increase in antimicrobial resistance, raising concerns about food safety and public health. In this context, the present thesis aimed to investigate antibiotic use in shrimp aquaculture and to determine the pharmacokinetics (PK) and depletion of selected antibiotics in whiteleg shrimp.
The first part of this research examined the use of chemicals and antibiotics in super-intensive whiteleg shrimp farming systems in the Mekong Delta, Vietnam. Based on sample-size calculation, a survey involving 210 randomly selected shrimp farming households across seven Mekong Delta provinces was conducted to investigate farming practices, chemical use, and disease occurrence. The results showed that a wide range of chemicals were used during both pond preparation and grow-out stages, mainly for disinfection purposes. Probiotic products were also mostly applied by farmers to improve water quality and feed digestibility. The acute hepatopancreatic necrosis disease and white feces disease were the most frequent diseases reported by farmers. Farmers used several antibiotics, mainly cefotaxime (CFT), doxycycline (DOX), oxytetracycline and florfenicol (FF) to control bacterial infections. It is worth noting that a small number of farmers still reported the use of antibiotics such as enrofloxacin and ciprofloxacin, which are prohibited for use in aquaculture production in Vietnam. These findings suggest that antibiotic use in shrimp farming remains an important issue and that better guidance and management practices are needed to ensure responsible use and food safety.
Based on the survey findings, several commonly used antibiotics in shrimp farming were selected for subsequent PK and withdrawal time investigations. For the PK studies, shrimp were fed antibiotic-medicated feed as a single oral dose, and tissue samples were collected at different time points up to 24 h after administration. For residue depletion studies, shrimp were fed medicated feed once or twice daily for three consecutive days before withdrawal time determination. Doxycycline was investigated at a dose of 20 mg/kg body weight. The DOX concentrations were measured in hemolymph, hepatopancreas and muscle tissues. The results showed that DOX was rapidly absorbed and distributed in shrimp tissues, with the highest concentrations observed in the hepatopancreas, where the maximum concentration (Cmax) reached 2.37 µg/g at the time to reach maximum concentration (Tmax) of 0.23 h. Based on the obtained exposure in shrimp tissues, DOX showed limited potential to achieve effective antibacterial activity against Vibrio parahaemolyticus (V. parahaemolyticus) at the investigated dose. The withdrawal time in shrimp muscle ranged from 8 to 10 days at the maximum residue limit (MRL) of 50 µg/kg, depending on the dosing regimen. Histological examination indicated that DOX administration caused temporary changes in hepatopancreatic tissue during treatment, but the hepatopancreas structure recovered within seven days after stopping medication.
Florfenicol was also investigated under similar experimental conditions at a dose of 10 mg/kg body weight. As observed for DOX, FF showed rapid absorption and higher concentrations in the hepatopancreas. Residues in shrimp muscle declined quickly and fell below the quantification limit within 24 hours after stopping medication. Based on the MRL of 100 µg/kg, the withdrawal time was estimated to be approximately two days at a water temperature of 26.5°C. However, the concentrations measured in plasma and hepatopancreas of whiteleg shrimp were lower than those required to effectively inhibit V. parahaemolyticus, suggesting that the investigated dose may not be sufficient for treatment. In plasma, Cmax reached 60.56 µg/L at 1.77 h (Tmax), while in the hepatopancreas, a higher Cmax of 386.92 µg/kg was observed at 0.19 h. Histological observations indicated that no negative effects on the shrimp hepatopancreas occurred after cessation of medication.
The PK and residue depletion of CFT were also studied in whiteleg shrimp following oral administration at a dose of 25 mg/kg body weight. Similar to the other antibiotics (DOX and FF), CFT was rapidly distributed in shrimp tissues, with higher concentrations observed in the hepatopancreas (Cmax of 19.45 ± 2.10 mg/kg at Tmax of 1.00 ± 0.20 h). Based on the minimum inhibitory concentration of V. parahaemolyticus, the obtained exposure suggested that CFT administered once daily could potentially be effective for bacterial disease control in shrimp. The withdrawal time was estimated to range from approximately 2.0 hours to 77.7 hours depending on the applicable MRL and the dosing regimen when shrimp were fed medicated feed for three consecutive days. Histological analysis indicated that CFT administration did not cause detectable damage to shrimp hepatopancreas.
Finally, the occurrence of antibiotic residues in commercial whiteleg shrimp products and the potential risk for consumers were evaluated. Thirty shrimp samples were collected from markets and supermarkets in Can Tho City and analyzed for antibiotic residues. Five antibiotics were detected by liquid chromatography-tandem mass spectrometry, including DOX, oxytetracycline, enrofloxacin, ciprofloxacin and sulfamethoxazole. Among them, DOX showed the highest concentration in shrimp samples. Based on shrimp consumption data collected from local consumers, the estimated daily intake values under high-consumption conditions corresponding to the 95th percentile consumption ranged from 0.002 to 0.241 µg/kg body weight per day. These exposure values were much lower than the acceptable daily intake values established for these antibiotics, indicating negligible health risks for consumers. However, the detection of antibiotic residues in some shrimp samples emphasizes the importance of continued monitoring and responsible antibiotic use in shrimp production.
In general, this thesis provides additional information on antibiotic use practices in shrimp culture in the Mekong Delta, as well as on the PK, residue depletion and histological effects of selected antibiotics in whiteleg shrimp. In addition, the occurrence and dietary exposure assessment of antibiotic residues in shrimp products were evaluated. These findings may be useful for improving antibiotic management and food safety in shrimp production.