Disease-causing bacteria are called pathogens. The most common mastitis pathogens are found in the udder tissues, spreading from cow-to-cow (contagious pathogens) or in the herd's surroundings (environmental pathogens), such as bedding materials, manure and soil. This distinction may be important when assessing the challenges present in a herd and the measures which may be taken to reduce or treat mastitis.
Contagious pathogens that cause mastitis tend to live on the cow's udder and teat skin and transfer from affected cow (or quarter) to unaffected cow (or quarter) during milking. They adhere easily to the skin, colonising the teat end and then 'grow' into the teat canal, where infection occurs; because of this, post-milking teat disinfection and dry cow therapy play an important role in controlling contagious mastitis. Farms with a high level of contagious mastitis often have high Somatic Cell Counts (SCCs) with relatively normal Bactoscan results.
Present in the housing and bedding - can transfer during milking or between milkings, when the cow is loafing, eating or lying down. The pathogen can enter the teat canal by force during milking, for example, when liner slippage occurs. These environmental pathogens do not generally possess the same ability as contagious pathogens to adhere to and colonise the teat; dry cow therapy has little value in their control as these kinds of infections do not carry from one lactation to the next. High levels of environmental pathogens in a herd may cause normal SCCs but higher than average Bactoscan results.
Mastitis pathogens can infect cows both during the dry period and when cows are lactating, and it is important to identify and recognise the source of these infections, as approaches to control, prevention and treatment of the pathogen's effects can differ according to whether the infection occurs when the cow is dry, or in lactation.
The main mastitis-causing pathogens are Escherichia coli (E. coli), Streptococcus uberis and Staphylococcus aureus, and a wide variety of other organisms have been identified as potential mastitis pathogens. These organisms are termed major pathogens and are generally regarded as those commonly associated with clinical mastitis in dairy cattle.
It is not always possible to identify the causative pathogen of the case of clinical mastitis from the symptoms presented without laboratory testing of milk. Other bacteria that may be present in the udder and often have an overall beneficial effect on protection from infection caused by major pathogens, due to the production of natural anti-bacterial substances or competition with other bacteria, are termed minor pathogens. Due to their complex interaction with the udder they can be implicated in instances of increased SCCs and thus the incidence of sub-clinical mastitis but they do not usually cause clinical forms of the disease.
Mastitis causing pathogens
In the UK the main bacterial species that cause mastitis in dairy cattle are Streptococcus uberis, Staphylococcus aureus, and Escherichia coli (E. coli), although more than 200 organisms have been identified as mastitis-causing pathogens.
Streptococcus uberis is by and large a pathogen which causes environmental mastitis. It is found in much of the cow's environment and is particularly prevalent in straw bedding and straw yards. In contrast, sand-based bedding systems have been found to be an unsupportive environment for the bacterium.
Mastitis caused by the bacterium is very acute, with a sudden onset, but is normally particularly responsive to a range of antibiotics used for the treatment of mastitis. Many S. uberis infections can occur during the dry period, and only become apparent as clinical symptoms during the lactation, stressing the importance of good dry cow management.
Although generally designated as an environmental pathogen, some strains of the bacterium have been shown to possess characteristics of the contagious forms of the disease, where they may be responsible for chronic mastitic cases in particular, and the bacterium can be found in significant numbers on the cow herself while not being present in the faeces, (unlike E. Coli).
Staphylococcus aureus is notoriously difficult to treat with antibiotics, as it has an ability to become trapped in fibrous tissues and within cell structures; areas that antibiotics cannot reach with great effectiveness. Its particular characteristic in chronic cases is the formation of fibrous lumps on the udder where the infection has effectively become untreatable and will result in recurrent bouts of mastitis.
The bacterium is a contagious mastitis pathogen, and has a good ability to adhere to teat and udder skin where it can colonise into the teat canal. Hence, post-milking teat disinfection is an important means of controlling the bacterium and dry cow therapy has an important role in controlling levels of infection and the spread of the disease.
Because these two methods have become so crucial in controlling the bacterium, and are both widely-used, the proportion of mastitis caused by S. aureus has dropped significantly in the past few decades. In some extreme cases, the bacterium can cause a very unpleasant acute gangrenous mastitis.
E. coli is an environmental pathogen that is present in significant numbers in the general farm environment due to being present in large numbers in faeces. It can become responsible for some very acute, painful and potentially fatal forms of mastitis and is the biggest environmental cause of mastitis on dairy farms. Cows with acute infections often secret a thin, watery discharge instead of healthy milk.
In recent decades due to the changes in practices on dairy farms involving a number of factors including more intensively-house cattle with higher average yields, the proportion of mastitis cases caused by E. coli has increased significantly. Poorly-designed housing and bad bedding management can exacerbate the situation where poor ventilation increases humidity levels. The bacterium infects the teat canal when infected milk is forced into the teat canal during milking if liner slippage occurs or the milking plant is unable to momentarily cope with large amounts of milk, the clawpiece fills with milk and the teat end is bathed in potentially-infected milk residues.
The bacterium produces very strong toxins, which can quickly cause severe symptoms, in some cases fatal, particularly as some cows, for reasons not yet fully understood, fail to mount an effective immune response to the disease.
Vaccines are available, but their use is heavily compromised by the fact that several strains of the bacterium may be present. Infection can occur during the dry period with symptoms not apparent until the lactation begins: an increase in stress levels or suppression to the immune system can be a trigger.
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