Molecular and mathematical epidemiology of Staphylococcus aureus and Streptococcus uberis mastitis in dairy herds

Mastitis is the most common and costly production disease affecting dairy cows. Staphylococcus aureus and Streptococcus uberis are two major mastitis-causing pathogens. Staphylococcus aureus is traditionally classified as contagious pathogen, while Streptococcus uberis is classified as environmental pathogen. Infected udders form the main reservoir of contagious pathogens. Exposure to contagious pathogens is largely limited to the milking process. Environmental pathogens are present in the environment of the cow. Cows may be exposed at any time in life. We used a combination of molecular and mathematical methods to study whether S. aureus and S. uberis are indeed contagious and environmental pathogens, respectively. Pulsed-Field Gel Electrophoresis and binary typing were used for strain typing of S. aureus and had higher typeability and discriminatory power than phage typing. Comparison of clinical and epidemiological observations with typing results revealed significant associations between distinct genotypes and severity of disease, suggesting strain-specific virulence. Analysis of human and bovine S. aureus isolates showed that strains clustered predominantly in host specific clones. Circumstantial evidence suggested an environmental origin of one S. aureus strain. Teat skin did not appear to be an important source of intramammary S. aureus. Random-amplified polymorphic DNA typing was used to characterize S. uberis strains. Within two herds, a variety of strains were found in addition to a predominant strain, suggesting co-existence of strains with environmental and contagious epidemiologies. Infections with dominant strains lasted longer than infections with unique strains. One dominant strain caused an outbreak of mastitis that was limited to lactating cows. The strain was also isolated from the milking machine. The combined evidence strongly supports contagious transmission during milking. SIR-models were used to describe the spread of mastitis and simulate the impact of control measures. Prevalence of infection was a significant predictor for the number of new infections with S. uberis in one herd, as would be expected under the assumption of contagious transmission. Presence of other pathogens did not affect the susceptibility of udder quarters to new infection, but quarters that had recovered from infection were more susceptible than quarters that had not experienced mastitis. The same was true for S. aureus, as shown in a study of cow and quarter-level risk factors for new infections and by means of an SIR-model. Treatment of subclinical mastitis and prevention of influx of infected animals contribute to elimination of S. aureus mastitis from a herd under the assumption that S. aureus is spread solely through contagious transmission. However, even when R, the reproduction ratio, was lower than the threshold value of 1, indicating that contagious transmission was curbed, new infections continued to occur. Among other things, this may be the result of infections that originate from environmental sources. In summary, stain typing studies and mathematical models suggest that both S. aureus and S. uberis are genetically varied species that include strains with contagious transmission as well as strains with environmental epidemiology. This implies that both options have to be considered when studying mastitis epidemiology or choosing control measures for dairy herd managemen