Public health problems associated with consumption of unpasteurized cow’s milk and raw-milk products have been well documented (De Buyser et al., 2001; Harrington et al., 2002). There is no evidence that the risk from unpasteurized ewe’s milk is any lower (Allerberger et al., 2001; McIntyre et al., 2002). Pathogenic microorganisms can gain access to milk either by faecal contamination or by direct excretion from the udder into the milk.

The importance of subclinical mastitis as a limiting factor in sheep's milk production is well-known. In addition to decreased milk yield, decreased viability of lambs, subclinical mastitis also greatly reduces the hygienic quality of milk as well as its technological properties.

Current knowledge of mastitis in small ruminants has been reviewed by some authors (Bergonier et al., 1999; Bergonier and Berthelot, 2003; Lafi et al., 1998; Contreras et al., 2007). The causative organisms of mastitis are categorized as major or minor pathogens (Harmon, 1994). The most common major pathogens include Staphylococcus aureus, Streptococcus agalactiae, coliforms and enterococci, while other pathogens such as Streptococcus spp., Pseudomonas aeruginosa, Mannheimia hemolytica, Corynebacteria, Coagulase negative staphylococci and fungi, are considered to be minor pathogens which can produce intramammary infection in small ruminants, but occurrence rates are lower (Contreras et al., 2007).

Differences in climatic conditions, production patterns, breeding management and breeding practices influence the different epidemiology and clinical manifestation of sheep milk. The aim of this work is to identify the spectrum, frequency and antimicrobial resistance of bacterial pathogens occurring in sheep dairy herds in Slovakia.

Of a total of 310 samples of sheep's milk coming from three farms, at least one potential pathogen was isolated from 102 samples (32.9%). The results are shown in the Table 1. The most represented pathogens were coagulase negative staphylococci CoNS, namely Staphylococcus chromogenes, Staphylococcus epidermidis, Staphylococcus xylosus (75.6%), followed by Streptococcus agalactiae (10.7%), Staphylococcus aureus (6.9%), Streptococcus dysgalactiae (4.6%), Escherichia coli (1.5%), Enterococcus faecium (1.5%) and others (Streptococcus uberis, Streptococcus parauberis, Candida sp., Klebsiella sp.). The cause of the sheep's mastitis may be a number of microorganisms, according to the literature, they are mainly representatives of the genus Staphylococcus (Bergonier and Berthelot, 2003; Zigo et al., 2011). The results of this work also confirm this bacterial genus as dominant in the farming conditions included in our observation, in both consecutive seasons.

Several authors state that CoNS are most frequent pathogens responsible for subclinical mastitis of dairy sheep (Fthenakis, 1994; Burriel, 1997; Lafi et al., 1998; Pengov, 2001; Ariznabarreta, Gonzalo and San Primitivo, 2002; Gonzalo et al., 2002) and Staphylococcus aureus is more frequent in meat sheep (Jones, 1991; Watson et al., 1990; Hariharan et al., 2004; Mork et al., 2007). The farms included in this work are focused on the production of sheep's milk and CoNS represent a significant prevalence of all isolated pathogens. Staphylococcus aureus represented less than 7% of isolates.

Streptococci are probably the second most common cause of ovine mastitis (Bergonier et al., 1999). The most commonly isolated species are Streptococcus agalactiae, Streptococcus uberis and Streptococcus dysgalactiae (Las Heras et al., 2002). In this work streptococci were mainly represented by Streptococcus agalactiae, but only in the first of the monitored seasons. In the second year of follow-up, the occurrence of streptococci in the samples taken decreased significantly. This difference could have been caused by certain breeding measures in the context of milk management and overall hygiene in the farm. The presence of other pathogens in the isolates obtained in this work was negligible.

Antibiotic resistance pattern for staphylococci isolated from subclinical mastitis refers mainly to cattle, and little is known about dairy sheep.

A total of 99 isolates of CoNS were tested for antimicrobial resistance. Of these, 63.6% were resistant to at least one antibiotic. A total of 24.2% of the tested isolates were resistant to 3 antimicrobials simultaneously. The highest resistance was observed to lincomycin (57.6%) and neomycin (36.4%), the lowest to sulfamethoxazolum+trimethoprim (0%) and enrofloxacin (3.0%). The results are shown in Table 2.

The high multidrug resistance rates observed in CoNS are in accordance with previous reports (Kumar, Yadav and Singh, 2009; Sawant, Gillespie and Oliver, 2009; Vasiľ et al., 2018) and support the hypothesis that CoNS might play an important role as a source of genes resistant to S. aureus (Taponen and Pyörälä, 2009). Considering such an assumption, studies about host specificity in Staphylococcus aureus and other pathogenic agents could provide useful information about the epidemiological importance of CoNS as reservoirs of genes resistant to pathogenic strains for humans. However, because CoNS are the major mastitis causing agents in small ruminants, the high frequency of resistant genes in such species reported here is worth noting.

The drugs showing the lowest resistance rates were enrofloxacin and sulphonamides, which are in accordance with other studies on mastitis in several ruminant species (Kumar et al., 2009). Virdis et al. (2010) reported high sensitivity to quinolones, but not to aminoglycoside in subclinical mastitis-causing staphylococci.

From the point of view of epizootology, CoNS are frequently found to be a pathogenic especially in the case of subclinical mastitis. They do not represent a major specific pathogen but can potentially cause infections that tend to have a mild clinical manifestation but cause losses in the milk production, in quantity and quality of milk.

Based on the results of this work, it is possible to assume a similar spectrum of pathogens and their antimicrobial resistance described in the literature also within the Slovak sheep farms focused on milk production. Coagulase negative staphylococci, as the most common pathogen, poses a risk in the form of a subclinical course of inflammatory changes in the mammary gland that often escape the attention of breeders, but can have a significant impact on the quality of milk production.