## 2 Materials and Methods  
### Description of Sampled Animal Groups  
The study was conducted in 75 sheep farms located in the region of Epirus, Greece. All farms were situated in a lowland area, sharing pastures with similar vegetation. The size of the flocks was 100-300 animals of a local breed (\"boutsikes\"), and all used the same pastures and followed similar farming practices such as vaccinations, antiparasitic treatments, use of milking machines, administration of concentrated feeds, etc. Samples of milk were collected from healthy sheep with healthy udders (referred exclusively to as the control group-\"marrys\"), from sheep with clinical mastitis before treatment (clinical evaluation points: color and temperature of udders, pain, change in color and viscosity of milk, decreased milk production and California Mastitis Test category of grade 2 and higher), and from the previous animals after treated for mastitis. All animals were in the middle of their milking period and were randomly selected using tables of random numbers based on their ear tag numbers. The animals suffering from mastitis were kept away from the healthy ones under strict hygienic conditions. Diagnosis and treatments were carried out by professional veterinarians. It is important to clarify that identifying the responsible microorganism in clinical practice is usually not feasible, cost-effective, or practical due to the remoteness of the farms and the mountainous terrain. Most veterinarians in Greece determine the treatment regimen empirically, based on clinical signs and the therapeutic history of the flock (efficacy of previous treatments, success rates, etc.). Three groups of animals were formed (Figure 1): (a) control animals with clinically healthy udders--group A--control; (b) animals suffering from mastitis, before treatment (infected udder)--group B; and (c) the animals of group B, at the first day after the end of the withdrawal period of each antibiotic used (treated udder)--group C. The treated ewes were divided into three subgroups, receiving different drastic compounds. The therapeutic protocols used included the following antibiotic substances: (1) penicillin and streptomycin, (2) oxytetracycline, and (3) enrofloxacin (Table 1). On the first day after the end of the withdrawal period when the milk can be given to the dairies for processing, samples were taken from the  
Figure 1: Interpretation of sampled groups—sources of testing milk samples.  
same animals, classified into three subgroups (C1 for penicillin and streptomycin; C2 for oxytetracycline; and C3 for enrofloxacin). It should be noted that most of the milk from these farms is supplied to local dairies, while a smaller portion is reserved by the producers' families for making artisan cheeses for their own consumption. In total, 500 samples of milk were analyzed, originating from the above groups of ewes. Milk samples were collected following a procedure that included cleaning the udder with lukewarm water, discarding the first three streams of milk, and disinfecting the teat ends with an alcohol solution (70% absolute alcohol). All samples were immediately transported to the laboratory within one hour of reception and kept at 4 \({}^{\circ}\)C for a maximum of 24 h before analysis.