Study finds multidrug-resistant Aeromonas hydrophila in subclinical bovine mastitis in Cairo dairy farms
Researchers report Aeromonas hydrophila in 20.8% of subclinical mastitis cases from Cairo dairy farms, with high rates of multidrug and extensive drug resistance and multiple virulence genes.
A study of quarter milk samples from 200 dairy cows on commercial farms in Cairo, Egypt found subclinical mastitis in 42.5% (340 of 800) of examined samples. Among those subclinical mastitis cases, Aeromonas hydrophila was identified in 20.8% (71 of 340).
All recovered A. hydrophila isolates carried the aerA virulence gene. Other virulence determinants were present at lower frequencies: alt in 53.3% of isolates, ast in 45.1%, ser in 29.6%, act in 26.8%, and hylA in 22.5%.
Antimicrobial susceptibility testing showed universal resistance to amoxicillin (100%) and high resistance rates to tetracycline (90.1%), amoxicillin/clavulanic acid (85.9%), sulfamethoxazole/trimethoprim (84.5%), ceftriaxone (83.1%), erythromycin (76.1%), and gentamycin (53.5%). Norfloxacin retained significant activity against the isolates.
A substantial proportion of isolates were multidrug-resistant. The study reports 28.2% of A. hydrophila strains as extensively drug-resistant (XDR) across eight antimicrobial classes; these XDR strains carried blaTEM, blaCTX−M−1, blaOXA−1, tetA, aadA1, and sul1. Separately, 15.5% of isolates were MDR to six classes and carried blaTEM, blaCTX−M−1, blaOXA−1, tetA, and sul1, while 14.1% were MDR to five classes with the same resistance gene profile minus aadA1.
The authors state this is the first documentation of MDR and XDR A. hydrophila recovered from subclinical bovine mastitis in Egypt and highlight a potential public health risk given the combination of multiple resistance genes and virulence determinants in milk-origin strains.
The study notes frequent associations between virulence genes (aerA, alt, ast, ser) and resistance genes (blaTEM, tetA, blaOXA−1, sul1, blaCTX−M−1, aadA1) among the MDR/XDR isolates. Datasets from the study are available from the corresponding author on reasonable request. The manuscript is presented in unedited form and may contain errors prior to final publication.