Abstract
Background:
Bovine subclinical mastitis is a significant disease that significantly impacts the health of dairy cows and subsequently affects milk production. Among the various causes of subclinical mastitis (SCM), bacterial infections, particularly Staphylococcus aureus(S. aureus), play a crucial role.

Aim:
The aim of this investigation was to extract and identify S. aureus strains from cows with subclinical mastitis, assess their antimicrobial susceptibility virulence genes, and investigate the potential antibacterial activity of Moringa oleifera ethanolic extract and selenium nanoparticles against both somatic cell count (SCC) and S. aureus count in subclinical mastitic cows.

Methods:
We collected 110 milk samples from dairy cows with subclinical mastitis in the Kafrelsheikh governorate before treatment with intramammary antibiotics. Samples were identified using pH detection, somatic cell count (SCC) at ≥200,000 cells/ml, and California Mastitis Test (CMT).

Results:
The effect of Moringa oleifera ethanolic extract, selenium nanoparticles, and ciprofloxacin antibiotic in reducing somatic cell count (SCC) and S. aureus counts was significant. Selenium nanoparticles showed the highest reduction in the S. aureus count, comparable to that of ciprofloxacin, while Moringa oleifera extract also demonstrated considerable efficacy. Additionally, SCC was significantly decreased in all treatment groups, with the most substantial reduction observed in the ciprofloxacin-treated group. These findings highlight the potential of Moringa oleifera extract and selenium nanoparticles as effective alternatives or adjuncts for managing subclinical mastitis.

Conclusion:
This study highlights the prevalence of S. aureus in subclinical bovine mastitis and underscores the potential of Moringa oleifera ethanolic extract and selenium nanoparticles as effective therapeutic options. Selenium nanoparticles exhibited significant antibacterial activity against South aureus and reduced somatic cell counts, comparable to ciprofloxacin. Moringa oleifera extract also demonstrated promising efficacy, suggesting its potential as a natural adjunct or alternative to conventional antibiotics for managing subclinical mastitis.

Key words: mecA gene, MRSA, Moringa oleifera, South aureus, Selenium nanoparticles, Subclinical mastitis