CHARACTERIZING AND DIAGNOSING SUBCLINICAL MASTITIS IN DAIRY SHEEP

Abstract

Subclinical mastitis, defined as intramammary inflammation in the absence of observable clinical signs, is a major health, welfare, and production challenge in dairy sheep. Yet, its detection and management remain limited by the subtle nature of the disease, lack of methods for accurate early diagnosis, and scarce antimicrobial resistance information. This thesis investigates subclinical mastitis from two complementary perspectives: ewe activity measures and milking-parlor events as potential indicators of disease, and the bacterial etiology and antimicrobial susceptibility profiles of intramammary infections in a commercial dairy sheep farm. In the first research chapter, it was evaluated whether ewe behavior and milking parlor events differed between ewes with and without subclinical mastitis, using the California mastitis test (CMT) to define the ewe’s mastitis status and somatic cell count (SCC) as a quantitative measure of inflammation. Leg-mounted loggers quantified lying time and overall movement outside of the milking parlor, while video observations during milking captured ewe behavioral responses (stepping, weight shifting, and sitting on the rear rail), alongside operational events including human handling and milking-machine detachments. CMT-positive ewes had significantly higher SCC than CMT-negative ewes, consistent with the expectation that elevated CMT scores reflect increased inflammation. Behavioral differences were subtle: although milking-parlor behaviors and overall movement outside of milking did not differ by mastitis status, CMT-positive ewes spent more time lying at night than CMT-negative ewes, suggesting a context-dependent behavioral expression of pain or illness. Parity and seasonal conditions influenced several behaviors such as detachment, weight shifting, and bar sitting, underscoring the importance of environmental and individual factors when interpreting behavioral indicators.

The second research chapter characterized the bacterial species associated with subclinical mastitis and evaluated antimicrobial susceptibility across ten commonly used antibiotics. Non-aureus staphylococci dominated the pathogen profile, particularly Staphylococcus simulans, S. epidermidis, and S. auricularis, while Staphylococcus aureus was absent, likely reflecting effective farm management and culling practices. Antimicrobial susceptibility testing revealed frequent resistance to penicillin-class drugs among non-aureus staphylococci, whereas erythromycin and tetracycline generally remained effective. However, many organism–drug combinations lacked Clinical and Laboratory Standards Institute (CLSI) breakpoints, limiting interpretability and highlighting the need for sheep-specific standards. Taken together, these findings demonstrate that subclinical mastitis in dairy sheep involves subtle behavioral changes, substantial etiologic diversity dominated by non-aureus staphylococci, and notable β-lactam resistance. The results support a management framework that integrates CMT-based screening with targeted bacteriological culture, antimicrobial susceptibility testing, and attention to parity and seasonal factors. This integrated approach can improve early detection, guide responsible antimicrobial use, and enhance both udder health and animal welfare in dairy sheep production systems.