Individual risk factors for intramammary infections affecting dairy herds in Western Alps

Mountain dairy farming plays a vital role in preserving landscapes and biodiversity, socioeconomically supporting local communities, but is threatened by intramammary infections (IMIs), the main cause of mastitis and economic losses. While IMI risk factors are well documented in lowland farms, little is known for mountain herds. This study aimed to identify the risk factors for IMIs caused by contagious pathogens in transhumant dairy cows. Eight herds from the Lanzo Valleys, western Alps (Italy), were studied: 6 transhumant and 2 sedentary. Milk was sampled from up to 30 dairy cows per herd thrice between April and August 2024: T0 in spring before transhumance; T1 within five days after transhumance, and T2 approximately one month later. Composite milk samples (n=488) were analyzed for composition (fat, protein, lactose) and somatic cell count. Quarter milk samples (n=1,956) underwent bacteriological analysis for detecting major contagious pathogens (Staphylococcus aureus, Streptococcus agalactiae, Corynebacterium bovis). Data on cow breed, parity, days in milk, body condition score (BCS), litter hygiene, and cleanliness of cows were also collected. We tested the same cows repeatedly and used a mixed-effects logistic regression model to assess risk factors for IMI. Transhumant cows had nearly threefold the odds of IMI at T1 compared to sedentary cows (odds ratio: 2.74, 95% CI: 1.05–6.16), likely due to stressors associated with transport, walking and environmental changes. Additional findings included a 1.2% daily increase in IMI odds likely due to advancement in hot season and lactation stage (95% CI: 1.0–2.5%), a 32.6% decrease in IMI odds per BCS point increase (95% CI: 5.3–52.1%), and 27.2% higher odds per lactation rank increase (95% CI: 11.8–44.6%). A 0.1% rise in milk fat increased IMI odds by 2.5% (95% CI: 0.3–5.2%), probably due to the advancement of lactation stage. This study emphasizes the need for targeted strategies for transhumant dairy cows to protect udder health and sustain this traditional practice.