A sweeping new systematic review and meta-analysis has produced the most detailed global picture yet of Staphylococcus aureus in bovine intramammary infections, the udder infections broadly known as mastitis that rank among the most economically damaging diseases of dairy cattle worldwide. Pulling together 143 peer-reviewed studies published between 2010 and 2025, spanning 40 countries and regions across every inhabited continent, the research team estimated that S. aureus accounts for roughly a quarter of these infections globally, while revealing alarmingly high and geographically uneven levels of antimicrobial resistance in the isolates recovered from infected glands.

The scale of the underlying data is considerable. Of the 143 studies ultimately included, 126 reported prevalence data covering a combined 145,457 milk samples, of which 14,527 tested positive for the bacterium. Pooling these results with a random-effects model, the researchers calculated a global prevalence of 24.78 percent, with a 95 percent confidence interval running from 21.27 to 28.47 percent. That figure, however, conceals enormous variability between individual studies, from a low of just 0.70 percent in Poland to estimates reaching 100 percent in a single Indian study, a spread captured by the extraordinarily high statistical heterogeneity of the pooled dataset.

Geography mattered enormously. At the continental level, Africa recorded the highest pooled prevalence at 29.21 percent, followed by North America at 28.15 percent and Asia at 25.88 percent, while Europe lagged at 14.19 percent and Oceania at just 5.27 percent. Country-level estimates painted an even starker picture: Cameroon topped the table at 67 percent, with Sri Lanka at 64.32 percent, Indonesia at 56.98 percent, Mexico at 48 percent, Japan at 47.44 percent and Uruguay at 47.07 percent. At the other extreme, Poland reported 0.70 percent, Australia 5.49 percent and New Zealand 5.32 percent. The authors attribute these disparities to differences in herd management, diagnostic methods, biosecurity and mastitis control programs rather than to biology alone.

Time also left its mark on the data. Prevalence estimates fluctuated substantially across publication years, bottoming out at 8.53 percent in 2014 and peaking at 41.39 percent in 2018. Notably, estimates from studies published between 2018 and 2025 were generally higher than those from 2010 to 2017, a pattern that runs counter to earlier reports of declining trends. The authors suggest this may partly reflect better surveillance and wider adoption of sensitive diagnostic tools that detect infections previously missed, as well as the intensification of dairy production, with larger herds, greater animal density and more frequent milking all favoring the persistence and spread of contagious udder pathogens.

The resistance findings are perhaps the most consequential part of the analysis. Across 66 studies reporting phenotypic susceptibility data, resistance to the older beta-lactam antibiotics was pervasive: penicillin resistance was estimated at 65.19 percent, ampicillin at 62.80 percent and amoxicillin at 52.07 percent. Tetracycline-class drugs fared little better, with doxycycline resistance at 70.73 percent and oxytetracycline at 62.42 percent. Moderate but worrying levels appeared for oxacillin at 40.23 percent, a marker of methicillin resistance, and vancomycin at 44.62 percent, a drug often reserved as a last resort in human medicine. By contrast, amikacin at 18.31 percent, chloramphenicol at 26.82 percent, gentamicin at 27.51 percent and ciprofloxacin at 30.81 percent retained comparatively more activity, likely because they are used less frequently in dairy practice.

The continental heatmap of resistance added further nuance. Beta-lactam resistance was highest in Europe and South America across all antimicrobial classes, while Europe also showed elevated resistance to glycopeptides and macrolides. Asia demonstrated significant resistance to tetracyclines, sulfonamides and macrolides, and Africa showed intermediate to high resistance to aminoglycosides and sulfonamides. At the country level, beta-lactam resistance was consistently high in India, China, Egypt, Brazil and Turkey, while tetracycline and macrolide resistance was moderate to high in Bangladesh, India, Iran and Indonesia. North America generally showed the lowest resistance for several classes, including aminoglycosides, fluoroquinolones and macrolides.

Beneath the phenotypic data lies a molecular explanation. Among the 32 studies reporting resistance genes, the beta-lactamase gene blaZ and the mecA gene were highly prevalent. blaZ encodes an enzyme that chemically cleaves the beta-lactam ring shared by penicillins, rendering them inert, while mecA produces an altered penicillin-binding protein, PBP2a, with greatly reduced affinity for methicillin and related drugs, the defining mechanism of MRSA. Genes conferring macrolide, lincosamide and streptogramin resistance, including ermC, ermB, msrA and mphC, were also common, as were tetracycline resistance genes tetK, tetL and tetM, which work through active efflux and ribosomal protection respectively. Aminoglycoside-modifying enzyme genes and additional determinants of trimethoprim and vancomycin resistance rounded out a portrait of accumulating multidrug resistance.

The authors emphasize that these genes rarely stay put. Resistance determinants in S. aureus are frequently carried on mobile genetic elements such as plasmids, transposons and staphylococcal cassette chromosomes, which shuttle DNA between cells through conjugation, transformation and transduction. This horizontal gene transfer allows resistance genes to jump across species and even genus boundaries under antibiotic selection pressure, promoting co-selection of multidrug-resistant strains. The biology of the pathogen compounds the problem: biofilm formation on udder tissue impedes antimicrobial penetration and is closely linked to treatment failure, which helps explain why S. aureus mastitis is so often chronic and stubbornly unresponsive to therapy.

The study is candid about its limitations. The analysis was not prospectively registered, studies with incomplete reporting were excluded, and not every country within each continent was represented. More importantly, the funnel plot showed significant asymmetry, and Egger’s regression test confirmed small-study effects, indicating that studies reporting low or null prevalence may be underrepresented in the published literature. When the researchers applied a trim-and-fill adjustment, imputing 59 potentially missing studies, the pooled prevalence dropped dramatically from 24.78 percent to 7.44 percent, a sobering reminder that publication bias can inflate global estimates. A leave-one-out sensitivity analysis, however, showed that no single study disproportionately drove the headline result.

The broader stakes extend well beyond the dairy shed. Mastitis caused by S. aureus costs the global dairy sector billions of dollars annually through reduced milk yield, treatment expenses and culling, while contaminated milk can carry heat-stable enterotoxins responsible for one of the world’s leading causes of acute food poisoning. Livestock-associated MRSA adds a zoonotic dimension, with occupational exposure and foodborne transmission raising One Health concerns, and the authors note that multidrug-resistant clones isolated from bovine mastitis, including human-associated lineages such as ST239 and ST8, hint at bidirectional exchange between people and animals. With projections suggesting antimicrobial resistance could claim as many as ten million lives annually by 2050 if unchecked, the authors argue that stricter regulation of antibiotic use across the food chain, routine surveillance of resistance patterns, prudent antimicrobial stewardship, hygienic milking practices, selective dry cow therapy and sustained farmer and veterinarian education are no longer optional extras but essential defenses for animal health, food safety and human medicine alike.

Subject of Research: Global prevalence and antimicrobial resistance of Staphylococcus aureus in bovine intramammary infections

Article Title: Staphylococcus aureus in bovine intramammary infections: A global systematic review and meta-analysis of prevalence and antimicrobial resistance patterns

Article References: Ahmed, M. J., Rahman, M. A., Chalise, R., Bhandari, P., Bhuiyan, M. I. H., Hossain, M. I., Kaderi, I. J., Rahi, M. A. I., & Hossain, D. (2026). Staphylococcus aureus in bovine intramammary infections: A global systematic review and meta-analysis of prevalence and antimicrobial resistance patterns. New Microbes and New Infections, 74, Article 101860. https://doi.org/10.1016/j.nmni.2026.101860

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Keywords: Staphylococcus aureus, bovine mastitis, intramammary infection, antimicrobial resistance, MRSA, meta-analysis, dairy cattle, One Health, beta-lactam resistance, mecA gene, food safety, systematic review

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Tags: Antimicrobial Resistanceantimicrobial resistance in dairy pathogensbeta-lactam resistancebovine intramammary infections epidemiologybovine mastitisdairy cattledairy cow udder infection prevalencedairy farm disease managementdairy industry economic impactfood safetygeographic variation in bacterial infectionsglobal antimicrobial resistance in dairy cattleglobal dairy cattle healthintramammary infectionmecA genemeta-analysismeta-analysis of mastitis studiesMRSAOne Healthregional differences in antibiotic resistanceStaphylococcus aureusStaphylococcus aureus bovine mastitissystematic reviewsystematic review of bovine mastitis