Subclinical mastitis in Lacaune sheep: Causative agents, impacts on milk production, milk quality, oxidative profiles and treatment efficacy of ceftiofur

Mastitis is a major disease affecting dairy sheep. It is caused by microorganisms that generate inflammation of the mammary gland in response to tissue invasion. This syndrome affects the welfare of ewes, as well as the production and quality of the milk, thereby reducing its productive efficiency. Because mastitis causes inflammation process, it also increases the production of free radicals that cause lesions via lipoperoxidation, causing damage to proteins, cells and tissues. One way to minimize the impact of the disease is antimicrobial treatment. Nevertheless, the continuous use of antimicrobials contributes to microbial resistance, in addition to producing residues in the milk and derivatives if not given during the grace period. Therefore, the objective of this study was to evaluate the consequences of subclinical mastitis on ewe health, milk production, milk composition and quality. We also evaluated the susceptibility of the bacteria in vitro using disk diffusion antibiograms. Finally, we performed two-way testing of efficacy of treatment in Lacaune ewes using the same agents. In the first stage of the study, 30 lactating ewes (+/- 90 days) were used, 10 of which were negative on the CMT (California Mastitis Test) used as control group (CG) and 20 sheep with subclinical mastitis diagnosed by CMT (MG). Samples were collected and several analyses were performed on the milk and blood. We found that ewes in the MG had higher lipid peroxidation in serum and milk, as well as lower production, with reduction of the total dry extract in milk. There were 15 isolates of Staphylococcus hyicus, four isolates of each S. epidermidis and S. intermedius, and two isolates of Corynebacterium spp. The primary hematological result was leukocytosis in ewes with mastitis. Based on the antibiogram, we chose ceftiofur for in vivo tests. In this stage, we divided the sheep with subclinical mastitis into two subgroups of 10 ewes each, to receive drug by two routes: intramuscular (IM) and intramammary (IMM). In the IMM group, of the 10 CMT-positive ewes at the beginning of the experiment, seven were already negative by the racket test 120 h after the last application (70% efficacy). In the IM group, of the 10 positive ewes, only four were negative after 120 h of the final application, a low efficacy treatment (40%). We evaluated antimicrobial residues in the milk of treated animals. We found this material within 5 days after treatment in the two forms used; despite the fact that the product's stated withholding period is 3 days. We conclude that ewes with mastitis produce less milk of lower quality. We also conclude that, although ceftiofur is 100% effective in vitro, when used in ewes with mastitis, the efficacy did not exceed 70%, and was more efficient when administered via the intramammary route.