Role of Haematobia irritans as a potential vector of subclinical mastitis pathogens in dairy herds under tropical conditions.

Subclinical mastitis is a major challenge in dairy production, associated with economic losses and public health concerns. This study investigated the potential role of Haematobia irritans (horn fly) as a mechanical vector of pathogens causing subclinical mastitis in dairy cows. The research was conducted on two dairy farms in Alegre, Espírito Santo, Brazil, with assessments of herd hygiene and milking practices. Cows were screened using the California Mastitis Test (CMT), and milk and teat swabs were collected from CMT-positive animals. Horn flies were captured during milking sessions. All samples were cultured on MacConkey agar, Brain Heart Infusion agar, and Sabouraud Dextrose agar. Bacterial isolates were identified by colony morphology, Gram staining, and biochemical tests. Staphylococcus aureus was the most frequently isolated pathogen from both milk and teat swabs, and was also commonly found in Haematobia irritans, along with other Gram-positive and Gram-negative bacilli. Principal Component Analysis (PCA) showed a strong association between fly microbiota and Staphylococcus aureus prevalence on Farm-1, while Streptococcus spp. was more common on Farm-2, likely due to hygiene differences. These results suggest Haematobia irritans may contribute to the spread of mastitis pathogens and emphasize the need for ectoparasite control and microbiological monitoring in tropical dairy systems.