Indicators and Bacterial Diversity of Subclinical Mastitis in Iran's Industrial Cattle Farms

Mastitis is an important and prevalent infectious disease in dairy herds that imposes high economic losses annually to the animal industry. Numerous infectious agents, including bacteria, fungi and viruses, can cause mastitis, with Staphylococcus spp. and Streptococcus spp. as prevalent causative agents. Mastitis is typically classified as clinical or subclinical, which impacts lactose and somatic cell count (SCC) volumes in milk.

In this cross-sectional study, 108 Holstein Friesian cows from three industrial farms in Iran were sampled; after exclusion of five samples for culture contamination, a total of 427 milk samples were analysed. Samples with SCC > 200,000 cells/mL were considered positive for subclinical mastitis.

Overall, 238/427 samples (55.7%) exceeded this SCC cut-off. Staphylococcus aureus and Klebsiella spp. were the most frequent isolates among culture-positive samples. Antibiotic susceptibility testing was performed on 11 representative isolates (selected to cover the main species and herds). A significant negative association between milk lactose and log-SCC was observed (p < 0.05).

These results indicate lactose reduction as a useful indirect indicator of subclinical mastitis and document the bacterial profile of industrial Iranian farms.