Duct-level evaluation of mammary gland health, somatic cell count and bacterial flora in lactating jennies

Background: Assessing mammary gland health in donkeys is challenging due to the lack of species-specific diagnostic criteria for subclinical mastitis. Although somatic cell count (SCC) is widely used in dairy ruminants, its diagnostic value in jennies remains unclear. Each donkey teat contains two anatomically independent duct systems, potentially influencing microbial distribution. Objectives: To evaluate mammary gland health at the individual duct level in lactating jennies by assessing SCC, bacteriological findings, and total bacterial count (TBC), and to investigate associations with host-related factors. Study Design: Non-randomised cross-sectional observational study. Methods: Milk samples were collected separately from the medial and lateral ducts of both teats (four ducts per animal) in 29 clinically healthy lactating jennies (116 samples). SCC was determined by direct microscopic count; bacteriology was performed with MALDI-TOF identification. TBC was expressed as CFU/mL. SCC and TBC were log10-transformed prior to mixed-effects linear regression, with animal as a random effect. Results: Using an exploratory threshold of ≥200,000 cells/mL, increased SCC was detected in few ducts and was not consistently associated with bacteriological positivity. Median SCC and TBC were low but highly skewed. Bacteriological positivity occurred in 62.1% of jennies, mainly involving environmental and opportunistic microorganisms. Marked microbiological heterogeneity was observed between ducts within the same gland. Lactation stage (4–6 months) was associated with increased SCC (p < 0.05), whereas age, parity, and use were not consistently associated. An apparent association between younger age and higher TBC was not maintained after excluding one outlier. No significant correlation was found between SCC and TBC. Main Limitations: Small sample size, uneven age distribution, cross-sectional design, and absence of differential cell counting. Conclusions: Clinically healthy lactating jennies showed generally low SCC and bacterial loads, but duct-level sampling revealed substantial intra-gland heterogeneity. SCC showed limited concordance with bacteriology, supporting the need for donkeyspecific mastitis criteria.