The study findings allowed us to elucidate the determining factors in the occurrence of clinical mastitis at the quarter, cow, and herd-level in smallholders in the province of Ubaté in Colombia. The identified risk factors are essential to establish specific control and prevention measures. The prevalence of CM varied between 0.5–1.1%, while the prevalence per quarter showed variations between 0.1–0.4%, which was lower when compared with studies conducted in Antioquia and the Cundinamarca-Boyacá Highlands, where the prevalence was reported per individual to be between 2.2–4.7% and per quarter between 0.6–1.3% (Calderón et al., 2008; Ramírez et al., 2011; C. M. Trujillo et al., 2011). In this study, the population of interest belonged to small producers with 6.2 ± 5.4 milking animals, with a predominance of manual milking at 92%, and with average milk production of 11 L / day. It is essential to point out that adequate management during the milking routine was marginal at the beginning and improve during the execution of the study; additionally, producers showed little knowledge of control and preventative CM measurements. In contrast, studies in other regions of the Colombia estimated a higher prevalence of CM in medium-sized herds (minimum 29 animals), with a higher proportion of mechanical milking (> 60%), with technical assistance, and some knowledge of bovine mastitis (Calderón et al., 2008; Martínez, 2006; Ramírez et al., 2011). The differences in the results could be consistent with the methodology and epidemiological design implemented in these studies, and the differences in herd management.
There are currently no known studies that report the cumulative incidence of CM in Colombia. International studies have reported a high cumulative incidence of CM, 30–47 new cases per 100 animals at risk per year (Gao et al., 2017; Santman-Berends et al., 2015), attributed to factors such as rainfall, high level of production, and housing (Gianneechini et al., 2002). In intensive systems, the animals are under confined conditions with a higher risk of infection attributable to inadequate management of bedding, the increased animal density, and the lack of hygiene of premises (Gianneechini et al., 2002; Klaas et al., 2017). In Ubaté, intensive-housed management does not exist, and the cows grazed throughout the year. This could explain the low cumulative incidence observed (2–8 new cases / 100 animals at risk per year) in this research; therefore, it suggests a lower risk of CM of cows kept on pasture compared to intensive production systems.
The IRCM observed (2–4 new cases per 100 animals/year at risk) contrasts with a recent study in northern Antioquia, which estimated 13.8 new cases of CM / 100 animals-year at risk (Ramirez et al., 2018). Different studies carried out under confined conditions reported an IRCM range of 23–52 cases 100 cows-year at risk (Hammer et al., 2012; Karimuribo et al., 2006; Kivaria et al., 2007). Interestingly, pasture-based systems represent a low risk for CM compared to free or tie stalls (Morton et al., 2014). In housing management, the hygienic conditions of stalls, bedding, and hallways are challenging, and therefore favor the proliferation of bacteria and pathogenic environmental streptococci associated with clinical mastitis (Klaas et al., 2017).
At the individual level, some CM risk factors could suggest an increased susceptibility among animals. Hyperkeratosis at the tip of the teat caused a higher risk of CM (OR = 2.19, 95% CI: 1.26–3.83 p < 0.05) because it alters the physical barrier of the teal canal protection (Breen et al., 2009). Another study also demonstrated a higher bacterial load on the skin of the teat in the presence of hyperkeratosis (Paduch et al., 2012). Furthermore, the incidence of other lesions such as teat-prolapse and papillomas determines a greater susceptibility of ascendant bacterial infections into the mammary gland (Kivaria et al., 2007; Tolosa et al., 2015). Together, our findings confirm that the disruption of the sphincter anatomy coupled with the disruption of the teat skin may alter the innate defense mechanisms of the mammary gland leading to a high rate of bacterial colonization and infection (Meglia et al., 2001). Moreover, teat skin lesions are associated with the proliferation of coagulase-negative staphylococci, the most prevalent pathogen cultured in the present study (43.0%) (De Visscher et al., 2014). The high bacterial load resulting from skin disruption combined with the lack of a proper routine of disinfection of the teat may favor CM's incidence in this study. The udder hygiene score allows assessing the influence of organic matter, mud, or dirt as a risk factor for clinical mastitis (Kivaria et al., 2007). Animals with more than 50% of the MG covered with dirt had a higher risk of having CM (RR = 4.1, 95% CI (1.6-10-2), p = 0.004). This observation is consistent with the results of Breen et al., 2009, and highlights the importance of controlling situations that promote environmental microorganisms' proliferation (Klaas et al., 2017; Ruegg, 2012).
Interestingly, the isolation frequency of coliforms in this study was low (2.0%) and did not seem to represent a problem compared to infections caused for environmental streptococci (32.0%). In the grazing systems developed in the Province of Ubaté, the milking area is the same where animals stay all day long. So, it is common to deal with unfavorable conditions such as waterlogging and accumulation of mud and manure in places where animals grouped, such as the milking site. For this reason, it is possible to explain the high prevalence of environmental streptococci find in these herds (Gianneechini et al., 2002). It is important to note that the presence of organic matter covering either the udder or the teats may impair the pre-and post-dip disinfectant effect and therefore increase the risk of bacterial survival and infection (Munoz et al., 2008).
Other observed factors that may contribute to the risk of clinical infections commonly associated with poor milking hygiene were the lack of hygiene of the milkers' hands, the paucity of fully dry-off the teats before milking, and the use of dirty workwear or clothes not suitable for milking. Amusingly, factors such as exclusively workwear for milking, cleanness of the workwear, and water source used during milking were associated with the incidence of clinical mastitis in this research. There is no other reported association between clinical mastitis and these factors in the literature, however, they should be considered additional environmental streptococci sources.
Herds using milking machines were at a higher risk of developing CM (RR = 6.8, 95% CI (2.0-20.6), p = 0.002). Contagious microorganisms infect healthy animals through the teat cups or the operators' hands during milking (Klaas et al., 2017; Ruegg, 2012). Likewise, the transmission of infection may increase due to the inappropriate use of milking gloves and disinfectants and milking equipment with little maintenance and poorly calibrated (Keefe, 2012; Ruegg, 2012). The high prevalence of contagious microorganisms (17.0%) observed in conjunction with inadequate handwashing, the non-use of milking gloves, and the use of milking machines with low maintenance validate the importance of these factors in the transmission of the disease. A proper milking routine reduces the transmission of this type of microorganism, preventing the appearance of new disease cases. During this study, it was possible to assess that the average interval of maintenance and calibration of the milking machine was two years, which without a doubt, may increase opportunities and sources of bacterial exposure. On the other hand, an unappropriated use of detergents for washing the milking equipment may facilitate de growth of contagious microorganisms. An observed common practice in the present study was washing the milking system with alkaline and acid detergents and hot water just once a week. The lack of hygiene (De Visscher et al., 2014), coupled with the execution of inadequate practices of maintenance, washing, and disinfection of the equipment (National Mastitis Council, 2015; Ruegg, 2012), have been determining factors in the clinical presentation of mastitis by these pathogens.
This study's estimated CM incidence was relatively low (range 2.0–4.0 new cases per 100 animals-year at risk). Nevertheless, clinical infections were caused primarily by coagulase-negative staphylococci (43%) and environmental streptococci (32%). These findings highlight the importance of establishing prevention and control programs oriented to target identified risk factors such as exclusive workwear for milking, the prevalence of subclinical mastitis, milking method, cleanness of the workwear, source of water used during milking, use of gloves, correct use of disinfectant, pre-milking management, handwashing before milking and foremilk disposal.