Intramammary infections (IMI) caused by bacterial pathogens lead to the inflammation of the mammary gland (mastitis) and cause significant economic losses in dairy cow herds worldwide. Microorganisms causing new IMI can be either transmitted from infected quarters (contagious transmission) or from extra-mammary reservoirs (so called “environmental transmission”). The causative species are often assigned to have either one or the other transmission behaviour and this assignment is used to select prevention and control measures. In recent years however, the strict assignment of entire species to one mode of transmission has been challenged and increasing evidence shows that some pathogens might spread mainly contagiously in some herds whereas environmental reservoirs play a bigger role in others. While the main mode of transmission determines the type of potentially successful preventive measures, in the first place, the necessity of establishing certain measures needs to be evaluated. Whether the prevention and control of IMI with a specific pathogen is economically meaningful and the treatment of an IMI with antimicrobial substances can be justified is, among other factors, determined by the duration of infections. However, the duration of IMI episodes have for most mastitis pathogens to date only been reported for a limited number of herds and often also treated IMI were included when average durations of infection episodes were estimated.
The aim of the present PhD project was therefore to explore the transmission pathways of selected pathogens regularly associated with IMI in dairy cows and to describe the durations of untreated IMIs with these pathogens.
One commercial dairy cow herd was visited ten times in 14-day intervals. At every visit, milk samples (in total n=8056) from all lactating udder quarters and samples from the housing and milking environment (in total n=251) were collected. All samples were subject to microbiological examination and the somatic cell count of milk samples was measured. The species of cultured isolates was determined by MALDI-TOF MS. All isolated Staphylococcus (Staph.) aureus, Staph. epidermidis, Staph. haemolyticus, Streptococcus (Strep.) uberis and Strep. dysgalactiae were subsequently typed by RAPD-PCR. The number of infection episodes caused by each strain of these species was determined and infection durations were estimated using survival analysis. All infections cured due to antimicrobial treatment were excluded from the estimation of infection durations. In addition, strain types isolated from milk samples and milking- and housing-related samples were compared.
Among Staph. aureus positive milk samples, one strain dominated and caused 83% of all observed infection episodes. This strain was also cultivated from milking liners and milker gloves. One additional strain was isolated from both, milk samples and a milking liner. Similarly, only two strains of Strep. dysgalactiae caused 69% of all infections with this pathogen. However, this species was not isolated from housing- or milking-related samples. Furthermore, about half of the observed Strep. uberis infections were caused by three strain types only. This pathogen was isolated from a milking liner and a bedding material sample. However, the isolated strains did not match with those cultivated from milk. The estimated median infection durations were 80 days for Staph. aureus and more than 84 and 70 days for Strep. dysgalactiae and Strep. uberis, respectively. The results from the present study indicate that Staph. aureus and Strep. dysgalactiae were mostly transmitted contagiously in the study herd. Contradictory to its usual categorization as an environmental pathogen, also Strep. uberis has likely spread to a considerable extent between quarters. In addition, the median infection durations of all three pathogens were longer than 70 days. This low rate of spontaneous cure implies that the prevention of IMI with these pathogens is an important pillar of mastitis management. Based on the results of this study and the existing literature new Staph. aureus IMI will likely be most effectively prevented by impeding the transmission between quarters in most herds. Also Strep. dysgalactiae and Strep. uberis can both be transmitted contagiously. The best suitable measures for each farm will however depend on the main mode of transmission in the respective herd which can be determined using strain typing.
Both, Staph. epidermidis and Staph. haemolyticus, were regularly isolated from milk samples. However, almost no matching strain types were found in milk samples from different udder quarters. Staph. epidermidis IMI were often associated with a clear inflammatory response of the affected quarter, while the isolation of Staph. haemolyticus was mostly not associated with a clearly increased somatic cell count. Although both species were cultivated from housing- and milking-related niches and especially Staph. haemolyticus was often isolated from milking liners and milker gloves, no isolated strain type matched with those found in milk. The median estimated durations of infections with Staph. epidermidis and Staph. haemolyticus were 28 and 22 days, respectively. Subclinical IMI with Staph. haemolyticus should probably not be treated with antimicrobial substances (e.g. at drying-off) as the spontaneous cure rate was high in all quarters, its isolation was not associated with a clear inflammatory reaction of the affected quarter and there was no indication of contagious spread. Also, Staph. epidermidis infections were manly short and not transmitted contagiously. However, positive milk samples often had clearly elevated somatic cell counts. This indicates that IMI with this pathogen should probably not be treated, but that the introduction of some preventive measures against Staph. epidermidis IMI might be advisable in some herds. Overall, proper diagnostics of IMI with non-aureus staphylococci (such as Staph. epidermidis and Staph. haemolyticus) should include species identification as their importance for udder health and the necessity of prevention and control of IMI with these might differ within a herd.
Also, the presence of Corynebacterium (C.) spp. in housing- and milking-related samples was investigated. At least two different Corynebacterium spp. were isolated from all sampled locations and the three most frequently isolated species were C. stationis, C. xerosis and C. amycolatum. Especially milking-related samples were positive for many different Corynebacterium spp. and transmission of this group of bacteria from teat to teat during milking is therefore likely. The importance of this finding for the establishment of new IMI with Corynebacterium spp. can however not be evaluated in the present study. The predominant species isolated from milk samples was C. bovis followed by C. amycolatum. Remarkably, C. bovis was not once isolated from housing- or milking-related samples. The investigated locations seem therefore to be of minor importance as reservoirs for this species. However, further studies using more selective methods should confirm this finding.
The present study adds upon the available literature on transmission pathways of several Staphylococcus and Streptococcus species. Furthermore, it reports durations of infections corrected for the present strain type and excluding infections that cured due to antimicrobial treatment. Studies considering these two aspects have been reported only rarely for several of the investigated pathogens. Finally, it is the first study to report potential reservoirs of Corynebacterium spp. in the housing and milking environment of dairy cows. Overall, the findings of this study can help to further improve the management of bovine mastitis in the future.