Riskfaktorer för Staphylococcus aureus i mjölk och på has hos mjölkkor
Mastitis is the most common disease of dairy cows. Mastitis can be painful for the dairy cow and causes economical losses for the farmer. These losses are due to decreased milk production, more working hours for the farmer, treatment costs and possible costs due to culling of the cow. The udder inflammation, mastitis, is often caused by bacteria infection. One of the most common bacteria found to cause mastitis in Sweden is Staphylococcus aureus (S. aureus).
The study in this master thesis was part of a larger project about ?Epidemiological studies of mastitis among dairy cows caused by staphylococci? at National Veterinary Institute in Sweden. The aim of the present study was to find risk factors associated with the incidence of S. aureus in milk and on the hock at individual cow level. A total of 177 dairy cows divided on five herds participated in the study. Samples from milk and skin were taken from the cows for analysis for incidence of S. aureus and the hocks on the cows were scored for hock lesions. Moreover, in order to collect information about the management and environment of the cows, an interview using a questionnaire with questions about herd management and routines was performed with each farmer. Because only five herds were included in this study no herd factors could be included in the statistical analysis. The individual cow factors were statistically analysed using logistic regression analysis to investigate individual associations with findings of S. aureus in milk and/or on hock. The statistic analysis included data from the cows milk production, breed, age, udder health, lactations number, sickness and if they had incidence of S. aureus in milk and/or on hocks. Eight different models were built in the statistical analyses of risk factors associated with findings of S. aureus in milk or on hocks. Four models were built for S. aureus incidence in milk and four final models for S. aureus incidence on hocks with different approaches. The first models for each outcome included all cows and all risk factors. The second model for each outcome was as the first with the exception that S. aureus in milk and on hocks were not treated as risk factors in the respective model. The third and forth model for each outcome only included cows with low or high somatic cell counts, respectively.
Of the 177 cows participating in the study 28 % were positive for S. aureus in milk and 49 % of these had two or more udder quarters infected. S. aureus was found on hocks of 24 % of the cows. The hock scoring of 174 cows showed that 52 % of the cows had hock lesions (hair loss or wounds). Of the 83 cows without hock lesions 24 % had S. aureus on their hocks, while 37 % and 59 % of the cows with hair loss or wounds, respectively had S. aureus on their hocks.
Risk factors associated with the presence of S. aureus in milk were presence of S. aureus on hocks and milk yield. Presence of S. aureus on hocks and increased milk yield increased the risk of presence of S. aureus in milk. Risk factors for presence of S. aureus on hocks were presence of S. aureus in milk, presence of hock lesions, high milk urea value, and increased milk yield. Factors associated with lack of S. aureus on hocks were increasing lactation number. Risk factors associated with S. aureus in milk and on hocks were not identical for cows with low or high somatic cell counts. Our study showed thus that individual cow factors are associated with the presence of S. aureus in milk and on hocks, however, herd factors should also be included in future studies. Moreover, different risk factors can influence cows with low or high somatic cell counts differently.