Occurrence and stability of veterinary antibiotics in Upper Pampanga River Basin located in Nueva Ecija, Philippines

The occurrence of antimicrobial resistance (AMR), resulting from improper use and handling of antibiotics , poses a significant threat to global health, food security, and sustainable development. One major cause of AMR is the misuse of antibiotic substances in the livestock and veterinary industries, where 93,000 tonnes of antibiotics were sold and consumed by the livestock industry in 2017, with an expected increase of 11.5% by 2030. Approximately 80% of livestock receive antibiotic treatment, and up to 75% of these compounds are excreted, eventually entering environmental waters and affecting aquatic biota. In the Philippines, data on antibiotic contamination, particularly in aquatic environments, remain scarce. This study investigates the presence and fate of seven veterinary antibiotics—amoxicillin (AMX), ampicillin (AMP), penicillin G (PenG), tetracycline (TET), oxytetracycline (OTC), sulfamethazine (SMZ), and sulfamethoxazole (SMX)—in the surface waters of the Upper Pampanga River and its tributaries in Nueva Ecija. Water samples from various localities were analyzed using a validated LC-MS/MS-ESI-Positive (MRM) method, following an optimized Solid Phase Extraction (SPE) procedure. The method demonstrated fitness for purpose in determining antibiotics in river water with R2 values of >0.999 at a working range of 0.5-3.0 ng/mL, detection limits ranging from 0.046–0.065 ng/mL, and recovery values between 63.0–113% with precision values of 0.89-4.14% RSD. Five of the seven targeted antibiotics—PenG, TET, OTC, SMZ, SMX—were detected above the established limits, with concentrations ranging from 0.170 to 9.35 ng/mL. OTC, SMZ, and SMX were consistently found across all sampled sites, with OTC having the highest concentration (9.35 ng/mL) detected in Science City of Muñoz, where five antibiotics were present. Moreover, the stability test revealed that the antibiotics follow pseudo-first-order kinetics, with half-lives of 1.58-2.74 days, which is shorter than those previously reported in the literature. This finding may indicate that the substances present in the matrix catalyze the degradation of antibiotics thus affecting their stability. This study is the first to report the occurrence of antibiotics in surface waters of the Upper Pampanga River Basin. It is also the first to report the stability of these antibiotic substances in the said area. Further studies are recommended to investigate the degradation mechanisms of these substances and assess their ecological impact.