A multidisciplinary approach to the control of pathogen-specific mastitis

Mastitis caused by the bacterium Streptococcus agalactiae was virtually extinct among Norwegian dairy cows, but the occurrence increased again in parallel with management changes such as larger herds, increased use of loose housing and introduction of automated milking systems. Recent studies from Norway also show that the bacterium is found in the barn environment. This is contrary to the previous perception that the main reservoir is either in the udder or in humans for some strains. The aim of the project is therefore to increase the understanding of epidemiology as well as the host's genetics and immune response to mastitis caused by Str. agalactiae. The sub-goals are to evaluate a diagnostic test for the bacterium, identify genetic markers related to mastitis susceptibility, estimate losses in milk production caused by Str. agalactiae and to suggest control measures based on the findings. We validated a commercially available PCR test (Mastitis 4) by comparing results for Str. agalactiae from PCR and bacteriology on the same milk sample. Thus, we can calculate the sensitivity (the proportion of affected as positive tests) and specificity (the proportion of non-affected as negative tests) of the tests. The results show that the PCR test is more sensitive than bacteriology, but bacteriology is more specific than PCR. Within immunology and genetics, work is done with macrophage activation in the laboratory and with genetic characterization of animals from the field. Exposure of macrophages to two different strains of Str. agalactiae led to different responses and different microRNA profiles of the cells. It seems that one bacterial strain (ST103), which is most common in cattle, tries to hide from cow's immune system. The other strain (ST1), found in cattle but also associated with pregnant women and infant sepsis, appears to curb the classic activation of the immune system. In addition, macrophages from animals with poor (low) genomic breeding values for milk somatic cell counts produced a lower response in terms of between-cell communication when exposed to the two bacterial strains, compared to macrophages from animals of good (high) breeding values. The analyses have shown that several levels of somatic cell counts should be taken into account for optimal breeding progress. In addition, several chromosome-areas and possible candidate genes associated with chronic subclinical mastitis have been identified. Blood was collected from 10 cows with high and 10 with low genomic breeding values (GEBV) for milk somatic cell counts. The samples have been analyzed for number and type of immune cells (using flow cytometry). Furthermore, analyses are ongoing regarding the expression of selected candidate genes in monocytes from these samples. Results will be available in the spring of 2019. Data from the Norwegian dairy herd recording system show that before the time of diagnosis, cows diagnosed with Str. agalactiae produce more milk than negative cows, but after diagnosis they produce less than the unaffected cows. The effect on milk production is greatest in the period 2-3 months after the diagnosis and persists for more than 3 months. The effect of infection varies between different bacterial strains, based on whether the strain is predominantly cow-specific or not. The estimates from the analyses of the effect of Str. agalactiae on milk production were incorporated into a Danish transmission model modified for Norwegian conditions. The purpose was to estimate the monetary losses associated with Str. agalactiae infections in general, as well as variation in losses by different strain types. Preliminary results for ST1 and ST103 indicate that the rate of new infections was higher for ST1, at prevalences around 0.1. This is because the duration of infection is shorter for the human-associated ST1. Preliminary analyses further show that the financial losses are greater following subclinical mastitis caused by ST1 than ST103, given the same prevalence. The project results show that traits of both host and agent traits will contribute to deciding the duration and consequences of intra-mammary infections. For Str. agalactiae, the human-associated strains ST12 and ST1 gave a reduction in the classic activation of immune response in cows and a shorter duration of the infection, rspectively. It appears that the bovine-associated ST103 has the ability to evade the host immune system, which was observed through microRNA analyses as well as by noting that the somatic cell count in cow milk was lower for ST103 than for the comparison strains. This bovine-associated strain also produced a more pronounced drop in milk production.

Den diagnostiske testen Mastitis4 qPCR benyttes i dag til diagnostikk ved Tine masittlaboratoriet i Molde. Økt kunnskap om testen vil bidra til at veterinærer kan gi bedre råd til produsenter angående anvendelsen. God karakteristikk av fenotypen subklinisk mastitt gir bedre mulighet for genomisk seleksjon, alternative nivåer av celletall kan vurderes hos NRF. Ulike bakteriestammer gir ulik effekt både på kuas immunceller, på melkeytelsen, på spredning av jurinfeksjon i en besetning og på bondens økonomi. Dette har betydning for valg av strategi ved bekjempelse.

Mastitis is the most common and costly disease in dairy cows. It impairs the welfare of the cow and is the main reason for use of antibiotics in dairy farms. Mastitis is a multi-factorial disease, influenced by the infecting microbe, the milking hygiene, and the immune status and genetics of the exposed animal. The Norwegian dairy sector is developing towards fewer and larger herds. Automatic milking in free-stall housing is common and around 30% of the national milk volume is produced in such systems. Th e new production systems have led to new challenges in mastitis prevention, and contagious mastitis caused by pathogens such as Str. agalactiae are emerging. The Norwegian dairy sector is in need of new control strategies to meet these challenges. The o bjective is to increase the understanding of the epidemiology, as well as the host genetics and immune response, regarding pathogens causing intramammary infections in Norwegian dairy herds. The ultimate aim is to develop guidelines for cost-effective con trol of mastitis, focusing on Str. agalactiae, including tools for selective breeding. The control measures will be developed using a multidisciplinary approach focusing on (1) detection of affected animals and herds, (2) determining factors that contribu te to successful eradication at the individual and group level, (3) identifying genetic markers for mastitis resistance, (4) linking genetic background to infection status and immunology and (5) estimating the effect of Str. agalactiae on milk production and quality. The objectives will be reached by combining information from field studies in affected herds, production data recorded in the Norwegian Dairy Herd Recording System, two selection lines of Norwegian Red Cattle and in vitro studies. Important research challenges will be to develop a new diagnostic test for Str. agalactiae, detect SNP markers for mastitis resistance, and study the functional immunology as well as consequences of infection on production.