Causes, prevention and treatment strategies for tackling mastitis

Mastitis continues to be one of the most prevalent diseases on dairy farms in the UK, representing major welfare issues for the animals affected and, in turn, major financial and emotional strain on the farm business owner (Bradley, 2002; Ryan and O’Grady, 2004).

Veterinarians often find themselves as the source of information and advice pertaining to clinical and subclinical mastitis for farmers; therefore, having a good understanding of aetiology, prevention and treatment is crucial when advising about strategies to improve the mastitis situation on a farm (Agriculture and Horticulture Development Board [AHDB], 2022a).

Aetiology of UK mastitis

While the most common reason for inflammation in the udder is still considered to be bacterial infection or intramammary infection (IMI), since the Five Point Plan was implemented in the UK in the early 1970s, as well as the incidence of mastitis reducing, the species of bacteria predominating in IMIs has shifted. The prevalence of those categorised as environmental, such as Escherichia coli and Streptococcus uberis, has increased, while those categorised as contagious, such as Streptococcus agalactiae and Staphylococcus aureus, have reduced (Bradley et al, 2007; Leigh, 1999; Oliveira et al, 2013; Verbeke et al, 2014).

Reports from two established mastitis bacteriology labs in the UK – Quality Milk Management Services (QMMS) and The Vale Veterinary Laboratory – sit in agreement that S uberis is the most commonly isolated udder pathogen overall on UK dairy farms. S uberis is followed by E coli, S aureus and Streptococcus dysgalactiae in order of overall prevalence at both labs.

The proportions of these isolates and other less commonly identified bacteria can be found in Table 1 (Payne et al, 2013; The Vale Veterinary Laboratory, 2017). S uberis has been reported to have the potential to behave contagiously, whereby 63% of 52 herds studied by Davies et al in 2016 had at least two cows with cases of mastitis caused by the same sequence type of S uberis when assessed by multilocus sequence typing, suggesting the pathogen had spread from one cow to another.

Table 1. A summary of bacteria isolated from samples submitted to Quality Milk Management Services (QMMS) and published by Payne et al (2013), and a summary of bacteria isolated from 2016 samples submitted to The Vale Veterinary Laboratory (data provided in 2017)
	The Vale Veterinary Laboratory		QMMS
Organism	Clinical isolates (as percentage)	Subclinical isolates (as percentage)	Clinical isolates (as percentage)	Subclinical isolates (as percentage)
Streptococcus uberis	32	44	16.95	14.05
Escherichia coli	27	8	19.22	9.31
Staphylococcus aureus	8	14	5.97	9.46
Staphylococcus dysgalactiae	2	4	3.23	2.49
Trueperella pyogenes	1	0	1.03	0.36
Streptococcus agalactiae	0	0	0.08	0.30
Other Enterobacteriaceae	4	2	2.74	2.25
Other major pathogens	0	0	25.42	28.43
Minor pathogens	13	5	11.28	17.91
No growth	12	22	11.59	10.17
Contaminated	1	1	2.51	5.28

This is worth bearing in mind when considering control and treatment strategies where S uberis could be implicated. “Other” major and minor pathogens make up a significant proportion of aetiologic agents as a group, but not as individual species. More than 100 different bacterial species were isolated within the 6,005 samples analysed at QMMS between 2010-13, suggesting a very diverse bacterial aetiology of mastitis in the UK (Payne et al, 2013).

An interesting point to consider, therefore, is perhaps the potential for the most common aetiological agents to shift in future – and, therefore, the need for ongoing monitoring of mastitis-causing bacteria.

To put effective strategies in place to reduce mastitis incidence on-farm – as well as consider bacterial species associated with mastitis and the manner they act in (contagious or environmental) – it is prudent to further categorise mastitis aetiology into a chronological sense, such as dry period origin or lactation period origin.

These can be further subdivided, if necessary, when investigating and developing mastitis plans on farm (AHDB, 2022b). To assess the patterns of infection and whether rates within the various categories are significant enough to warrant intervention, accurate data is required. A number of commercially available programmes and tools are available – such as QuarterPRO, TotalVet and Interherd(+) – which can analyse the data and highlight the areas on farm that warrant further investigation and improvement. Mastitis targets and significance are in Table 2.

Table 2. A selection of mastitis analysis KPIs and their relevance to an investigation
KPI	Target	Significance of KPI
Index cases first 30 days	Lower than 1 in 12 cows	Risk of dry period to mastitis
Index cases after 30 days of lactation (under end of lactation)	Lower than 2 in 12 cows	Risk of lactation period to mastitis
Heifer index cases first 30 days of lactation	Lower than 1 in 12 heifers	Risk of transition/close-up part of dry period to mastitis
Clinical mastitis index case cure rate	Good = greater than 60% Poor = lower than 40%	Good indicates primarily environmental/non cow adapted; poor indicates contagious/cow-adapted organisms
Seasonal variation of mastitis	Case rate greater than 10% higher in a given season than others or case rate greater than 30% with other seasons lower than 30%*	High seasonal variation more suggestive of environmental origin intramammary infections; contagious, likely to have less seasonal variation*
*In seasonally calving herds, where most animals are the same stage of lactation, an apparent seasonal variation may be attributed to stage of lactation rather than the season itself, so these patterns must be carefully analysed. For example, a spring calving herd with high mastitis incidence in autumn (late lactation) may be due to wet/muddy paddocks – a direct effect of season – but also may be due to teat-end hyperkeratosis that has progressively worsened throughout lactation, which is a stage of lactation effect that appears seasonally.

Prevention, investigation and planning

Prevention is better than cure, so strategies employed with regards to mastitis reduction will be primarily aimed at prevention of further cases, rather than management of the animals that succumb to infection. Refining treatment regimens can be associated with improvements in overall incidence, so would warrant a review in most cases (McDougall et al, 2018; 2022).

Prevention strategies are best formulated following on-farm risk assessment of spaces, time periods, groups, actions or objects that may be contributing towards adverse results identified through data analysis of mastitis risk categories – for example, dry period environmental/management, lactation period environmental, lactation period contagious, heifer-specific management and recurrence/treatment success.

Using an evidence-based approach to one’s lines of investigation is likely to yield the answers required to make effective prevention strategies in a much more time (and cost) effective manner, rather than an exhaustive approach to all risk categories (Vandeweerd et al, 2012).

Using this approach will also help to ensure the advice given leads to improvements in performance. This strategy will likely also lead to a more relevant and concise set of instructions or advice for the farm to follow, which will increase the likelihood of compliance (Dunham et al, 2020).

It is worth considering both the animal’s ability to resist infection and the level of infection pressure when formulating plans, with an ultimate goal of either lowering the infection pressure applied to the animal or helping to ensure it can resist infection effectively. Often, however, it is a combination of the two.

Some common examples of lowering infection pressure would include, but are not limited to: diligent bedding management, cow cleanliness, good parlour routine and disinfection protocols, and identification and management of (mastitic) cows at risk of spreading infection (Rodrigues and Ruegg, 2005; Klaas and Zadoks, 2017). Many of these actions are relatively straightforward to implement and can be done rapidly, at relatively modest cost.

Some common examples of helping cows resist infection would include, but are not limited to: ensuring udders, teats and teat-ends are well conformed and in good condition; ensuring cows are not under undue metabolic strain; improving mammary treatment protocols (including judicious usage of internal teat sealants); and inducing an active acquired immune response using vaccination (Rodrigues and Ruegg, 2005; Piepers et al, 2017; Collado et al, 2018; Pantoja et al, 2020).

These strategies often require more in-depth analysis – such as dynamic parlour testing, nutritional investigation or bacteriology over a period of time – to accurately determine where issues lie and, therefore, how the situation can be resolved or improved.

Often, the use of expert third parties at this stage can help with investigation and the formulation of on-farm prevention strategies, as the required equipment and knowledge may not be present in all practices (Rønningen, 2017).

Treatment options and planning

The next area of consideration is the POM-V treatments licensed for treatment of IMIs and mastitis, which include NSAIDs, lactating cow intramammary antibiotics (MCTs), lactating cow parenteral antibiotics licensed for IMIs and dry cow intramammary antibiotics.

From a welfare perspective, all cows with clinical mastitis should receive NSAIDs; this point cannot be understated. Additional clinical outcome benefits are also associated with NSAIDs (McDougall et al, 2009; 2016), which may help with implementing the routine administration of NSAIDs for mastitis on farm, if further evidence is required.

After welfare considerations, treatment strategies need to be formulated alongside farmers, with their business goals and their ambitions for individual animals in mind, as well as the responsibility of all to use antibiotics judiciously. The first step is establishing whether the animal warrants treatment above and beyond NSAIDs. As has been mentioned, mastitis is a costly disease – therefore, does an animal that has had repeated cases of mastitis warrant the costs related with attempted treatment, or should she exit the farm as a cull?

Setting a threshold number of cases to warrant culling with a client is important to ensure animals do not slip through the net and get treated repeatedly without success. The threshold number will be farm dependent, but a commonly adopted number of cases within a lactation would be three (AHDB, 2022c).

If repeated treatment during lactation has failed, but the cow is either at a stage of pregnancy that she cannot be culled or has other reasons for warranting retention on farm, the option exists to dry the affected quarter off early, thereby removing the problem from the current milk pool until the cow is dried off fully. At this point, the best chance of IMI cure will occur (Wilson et al, 1972) – particularly if coupled with dry cow therapy, when compared with treatments during lactation (Newton et al, 2008; Bradley et al, 2010; McDougall et al, 2019). Alternatively, the cow could be fully dried off at an earlier date than planned, to achieve a similar outcome to the aforementioned description.

When antibiotic treatment could be warranted, the first consideration when planning treatment strategies should be whether it needs antibiotic treatment to improve cure rates significantly.

The first stage of this decision should be based on herd history and the likelihood it is an infection that warrants antibiotic treatment. If, for example, we understand the prevalence of S uberis is overwhelmingly high in the herd, selective MCT usage is unlikely to yield a benefit greater than the known benefits of prompt and extended duration/high-frequency MCT treatment to the farm business (Oliver et al, 2004; Deluyker et al, 2005; McDougall et al, 2019).

If the likelihood is that the farm business could benefit from selective MCTs, on-farm culture could be considered within the mastitis treatment plan – assuming the operators meet the criteria required for successful implementation. This method should at least accurately categorise the causative bacteria into either Gram-positive, Gram-negative or no growth. The aim of this approach is to avoid antibiotic treatments in mastitis caused by Gram-negative organisms or where no bacteria is cultured, with little or no effect on cure rates or outcome from the cow, when compared to antibiotic treatment (Lago et al, 2011; Pinzón-Sánchez et al, 2011; McDougall et al, 2018).

If Gram-positive bacteria are cultured, treatment using MCT should be implemented, as treatment of mastitis caused by Gram-positive bacteria is often more successful with antibiotic treatment. Sometimes, further benefit is seen by implementing extended/higher-frequency treatment regimens than might be licensed on certain MCT treatments in the UK (Wilson et al, 1999; McDougall et al, 2022). This approach would see more judicious usage of antibiotics for clinical mastitis, with narrow spectrum MCT being administered on licence for up to five doses at 24-hour intervals (Ubropen, Boehringer Ingelheim) or three doses at 48-hour intervals (Orbenin LA, Zoetis) – depending on product used, and very little or no antibiotic being used for Gram-negative infections.

With some exceptions – such as mastitis, caused by S aureus and severe coliform mastitis – the use of parenteral antibiotics for mastitis will provide no significant benefit over MCTs alone (Barkema et al, 2006; Suojala et al, 2013). When this is considered alongside responsible usage of antimicrobials, cost and the practicalities of being able to promptly define the aforementioned aetiological agents on farm, including the regular usage of parenteral antibiotics in mastitis treatment plans appears to have little reason.

Treatment of subclinical IMIs is a complex and multifactorial subject, and is beyond the scope of this article. Some guidance is available within the referenced materials.

Summary

Mastitis incidence in the UK has reduced, alongside a change in pathogen profile found in IMIs, indicating a shift to more environmental-type mastitis. It is important to be able to monitor udder infection patterns, alongside the aetiological bacteria, to accurately define – and subsequently investigate – mastitis risks on a given farm and allow prevention strategies to be planned.

Even with the best prevention strategies, some IMIs will likely occur, so treatment strategies should be formulated to safeguard animal welfare and provide optimal outcome for the farm. Good opportunities to engage with farm businesses exist for those who are willing and able to engage in mastitis control. As always, good communication and a collaborative approach is key to success.

• This article was amended on 19 October to amend wording in the title of Table 1 and to switch the headings for QMMS and The Vale Veterinary Laboratory, which were previously published above the wrong columns. We apologise for any confusion.