A clinical investigation to study the relationship between somatic cell count and bacterial counts in quarter milk samples of dairy cows

Abstract

Mastitis is one of the most common and costly problems in the dairy industry, and is caused by intramammary infections (IMI). IMI can be diagnosed by measuring somatic cell counts (SCC) as values increase during mastitis. Upon the emergence of automatic milking systems (AMS) SCC can be monitored daily. Results of this monitoring showed daily fluctuating SCC patterns in some cows that were associated with persistent infections. These fluctuating patterns have not been evaluated before. hypothesized is that fluctuating bacterial concentrations in the milk over time may explain the fluctuating SCC response. Therefore, the aim of this study was to evaluate whether the SCC is associated with bacterial concentrations in the milk. In addition, differences in mean log10 SCC of the identified bacteria species were also investigated. During the study, 9 cows in different stages of lactation were sampled for 19 days, and the SCC of each individual quarter was determined daily. Bacterial colonies, if present, were counted and identified in each sample. The means of the log10 SCCs of all identified bacteria species were compared to the culture-negative samples by a t.test. All pathogens except from Bacillus spp. and some Streptococcus spp. showed an increased mean log10 SCC in comparison to the culture-negative samples. The mean log10 SCC of major mastitis pathogens were compared the mean log10 SCC of minor mastitis species, showing a higher mean log10 SCC in major pathogens. The quarters were subdivided into three groups: quarters with a fluctuating SCC pattern, stable high values and stable low values. The mean log10 SCC of the quarters with fluctuating patterns and stable high values were significantly higher than the mean log10 SCC of the low SCC quarters. Random effect regression models showed no significant association of the SCC with bacterial count (BC) in any of the three groups. The results suggest that there was no relationship between the SCC and the BC in the cows . However, previous studies showed that this correlation is possible, but is affected by multiple factors such as breed, stage of lactation and the species of bacteria causing the infection. The timeframe in which the reaction of the SCC follows the course of the BC could also differ from the analysis done in this study. The timeframe used might have been too short to discover a true fluctuating pattern. Also the different reaction of the SCC to different bacteria cannot be excluded. More research is needed to determine whether the increase and decrease in the SCC can be explained by a similar increase and decrease in bacterial concentration in the milk and which factors affects the association of the SCC and the BC in cows.