Treating clinical mastitis is very costly, but there are efficient measures that can be customized to each farm’s needs. The biggest challenge right now is the careful use of antibiotics. There are tools available, like cell counters that can analyse and help identifying the chronic cases of mastitis, so that the farmer can decide whether to treat or cull. There are producers that achieve superb milk quality, so the knowledge and the tools are there.
Take home messages
1. Basic control plan was developed 40 years ago. It is still essential but can be customised
2. Do not underestimate the value mastitis control, 80% reduction in clinical cases and more than 50% reduction in level of infection
3. No new problems have arisen; residual problems need fresh, additional initiatives
4. On any individual farm, failures are usually in application of control
5. Further advances will come from improved detection and animal genetics
6. Biggest challenge is use of antibiotics
Mastitis, inflammation of the secretory epithelium of the mammary gland, caused mostly by pathogenic bacteria and occasionally other microorganisms, is an abnormality albeit not unusual and found in all mammals. It is the frequency (prevalence) and severity that are important in milk producing animals, as infection and disease influence the volume and quality of milk, and hence its value. Mastitis, certainly the need to treat clinical cases promptly and successfully, is an animal welfare issue.
Developing understanding of mastitis has a long history including identification of the causes; speciation of streptococci from 1870-1940; the application of folk therapies pre penicillin and then a succession of effective medicines: advances in hygiene since the first use of teat disinfection in 1916; and the development of detection tools starting with the California Mastitis Test in 1955 replacing the traditional tasting of milk for saltiness by the milker. For a long time failure to develop a basic mastitis control strategy was probably because too much emphasis was on diagnostic methods, therapy and epidemiology (nothing much seems to have changed!) and not application of knowledge to the multifactorial nature of mastitis.
Frequently, many of the studies carried out were too small, of poor design (issues still happening) and did not take into account that a new infection of the mammary gland is actually a rare event in terms of new infections per unit of time or milking events.
The scientific literature contains many thousands of papers on bovine mastitis, supplemented by many more (but not proportional to species or population) on mastitis in other mammals. Very many of these papers contain useful knowledge. Too many are repetitious of earlier findings and some are truly erroneous (mostly due to chance or poor design of studies). Often papers are published on a problem imagined or created for the opportunity to generate a study. These may be of academic value but the contribution to applied science and the reality of dairy farming may not be obvious. Very few papers report the real advances that inform progress and by which the dairy industry can profit. No detailed reference list is provided but a few key publications are listed. These are essential reading for any serious student of mastitis.
This paper is a commentary based on the author’s views on what is really important to us as dairy farmers. It has prejudices, may be partial in coverage of this topic (mostly because there is neither time nor space to be all encompassing), and is intended to be controversial in parts. The contents are consideration for udder health of:
- What is important for us, as dairy farmers, to achieve; a reflection on the problems and how they vary with farming system and time
- What do farmers do, or need to do, that really matters
- How the problems have evolved and responses to the changes
- What are the (real) gaps and how should they be prioritised?
Mastitis versus intramammary infection
Interchange of the terms ‘mastitis’ and ‘intramammary infection’ (IMI) has long led to confusion as they are not the same, in cause, treatment or implication. Some resolution of this was agreed through the International Dairy Federation in 1967 but carelessness has perpetuated problems. The issue is the presence or absence of a causative infectious agent (or its likely prior existence) in the affected quarter of the bovine udder, the absence or not of an ‘abnormal’ content of blood-derived leukocytes and the ‘normality’ of the secretion.
- If the secretion is visibly abnormal, i.e. an unnatural colour, ‘watery’, contains clotted leukocytes and/or milk proteins or sedimentation, the udder quarter is considered mastitic and if the presence or recent presence of an infectious microorganism is confirmed this is a case of clinical mastitis. In this case the udder tissue will also be abnormal. These cases need some form of interventive treatment. They are a welfare issue.
- If the secretion is visibly normal but the affected quarter contains a leukocyte content above a threshold, currently 100,000 cells/mL, with no visible signs of infection, then the quarter has a latent mastitis. This condition may derive from low milk volume or some trauma resulting in a patho-physiological response (non-specific mastitis), or be the recovery phase from a previous infection. These cases are not recognised by farmers so there is no need for action. On farm no-one will know they happened
- If the secretion is visibly normal but the affected quarter contains a microorganism, usually bacterial, the quarter is subclinically infected. This may be a transitionary phase developing to clinical mastitis (in which case it is pre clinical), or a homeostatic condition where the pathogen and host responses are in equilibrium (sub clinical). Usually the cell count is elevated above normal. These are another form when most of the time they will be unknown events. Only if they persist and other evidence e.g. from a herd test, is known will the problem be identified. If too many cases occur then herd milk quality will be affected. Rarely is there an economic benefit from intervening and at best this is not during the lactation.
Mastitis/intramammary infection is not a simple or single health issue. More than 160 different microorganisms have been reported to cause an udder health problem. Thus ‘mastitis’ can result from any of these infections as well as patho-physiological problems such as trauma.
All infections have different epidemiological circumstances, induce different immune responses and may require quite different methods of control. However, the vast majority of mastitis, especially at the herd level, results from infections by Staphylococcus aureus, three streptococcal species and a small number of Gram negative bacteria. These are the commercially and welfare important forms of mastitis. Other significant pathogens, e.g. pyrogenic organisms cause specialised problems that may be peculiar to the farm system, geographically, or epidemiologically. A final group of ‘minor’ pathogens, including Corynebacterium bovis and several coagulase-negative staphylococcal species, receives a lot of (research) attention. This will be considered later.
The basis of control
For ten years from 1960 a group led by Frank Dodd and Frank Neave at the National Institute for Research in Dairying at Reading in the UK, started to focus attention on hygienic production of milk by applying some old and much new thinking. They first started to think about infection in terms of what is now understood as process control. Back then, some 90% of mastitis was caused by staphylococci and streptococci, primarily infectious diseases with reservoirs of the pathogens in the udder, on the skin and various other body sites. It was assumed then, as it still is, that infection occurred following invasion of bacteria via the streak canal. A new infection was, and is, a rare event consequent to the decreasing likelihood through the series of exposure, invasion, colonization, growth, infection and disease. Prevalence of infection only becomes high because of a high persistence of infection due to a low spontaneous rate of elimination.
A number of steps have to occur before an infection can occur, disease result and any later resolution can occur.
No mastitis control scheme will ever prevent all new infections and give total control. It is possible to devise a control strategy, either
1. Based on careful investigation of the causes of mastitis in the individual herd, relying on laboratory diagnosis or
2. Applying a simple, common scheme to all herds, one that is likely to be effective in the large majority of herds.
The latter approach has been adopted generally because, 50 or so years ago, in virtually every country more than 50% of all cows were infected, on average every cow had more than one clinical case of mastitis each year, and because with such a high prevalence and incidence of mastitis the costs of a diagnostic scheme would be immense. The opposite approach was taken in the Nordic countries that had a heavily subsided state veterinary system already in place.
Practically, the impact of a mastitis control programme can only be measured as a reduction in the prevalence of infection in the herd. No easy, on-farm method exists yet to determine the rate of new intra mammary infection. Clinical mastitis is only one consequence of infection that may or may not occur and also frequently results more than once in the same infected quarter.
At any one time
Prevalence rate of new infection and duration of infection
This is the basis of the control programme, to focus on
- preventing new infections and
- limiting the duration of infections by curing existing infections.
Key trials and outcomes
Hygiene Reducing the rate of new infection involves intervention in the early stages of mastitis which progresses from
Exposure -> Invasion -> Colonisation -> Infection -> Disease
It is easiest to intervene on the farm by concentrating on clean milk production, to prevent the transfer of mastitis-causing bacteria. However, whilst some procedures are effective on individual farms, not all can be applied effectively and economically in a general programme.
A wide range of means of teat preparation has been developed. Various methods have clear effects on the bacterial content of milk but the effects on the rate of new intra mammary infection are not universally convincing, similarly for cluster disinfection between cows.
Post milking teat disinfection aims to kill bacteria on the teat skin, when applied immediately after cluster removal. A huge number of reports vie not to prove effect but to show that a new succession of products can better their predecessors. Intrinsic in the findings from a huge family of studies is that properly applied teat disinfectants significantly reduce the rate of new intramammary infections.
Mastitis control plan component 1. Apply disinfectant to all teats of all cows immediately after removal of the milking equipment, after each milking.
It is imperative that the disinfectant acts to promote healing of teat skin lesions. Secondary infections of any sores on teat skin are predominantly by the staphylococci, streptococci and other Gram positive bacteria that cause mastitis. Prior to the development of teat disinfection, 80% of teats were contaminated with Staphylococcus aureus and 60% of teats had sores. Modern disinfectants contain sophisticated teat-conditioning constituents to counter the adverse effects of all forms of environmental and mechanical stresses to teat skin.
Whilst early trials using good teat preparation and disinfection reduced the rate of new infections by 80%, they only achieved a 14% reduction in the prevalence of infection because they had no effect on the duration of infection.
Elimination of infection (limiting duration) The spontaneous cure rate of mastitis differs according to the pathogen involved. In the early days of development of control systems, the majority of infections were staphylococcal or streptococcal. Both pathogen types are persistent and rarely disappear naturally. The development of penicillins and later antibiotics allowed effective treatment, especially of new cases of clinical mastitis with significant rates of bacteriological cure being achieved. It is economically justifiable, as well as being paramount to animal welfare, to treat with antibiotics all clinical mastitis caused by Gram-positive bacteria.
Mastitis control plan component 2. Treat all cases of clinical mastitis to cure the disease and eliminate the infection, and record the occurrence.
Some twenty years after the first use of penicillin to treat mastitis, a more persistent formulation was developed. This was not useful in lactation, because of the long time that milk had to be discarded to avoid residues, but proved of huge value in two ways when applied immediately after the last milking of the lactation. Persisting infections, occurring in more than 50% of cows, could be treated more successfully with a similar dose of antibiotic with an extended duration resulting from particular formulations. This therapeutic use of intra mammary antibiotic of dry cow treatment led to a major reduction in the duration of infections, curing more than 80% of infections in the majority of trials reported, far greater than the 30-60% cure reported for lactational therapy.
Some 60% of all new infections have been shown to occur in the dry period, either in the first 21 days after cessation of milking or in the few days around parturition. Long-acting dry cow treatment confers a prophylactic effect reducing the rate of new infection, also by 80%, when the product type is matched with the length of the dry period. So dry cow treatment reduces both the duration of infection and the rate of new infection. It is the single most effective procedure in reducing the prevalence of infection.
Mastitis control plan component 3. Use dry cow antibiotic treatment (DCT) on all cows in the herd that will remain in the herd after the next calving.
Antibiotics are available to treat all mastitic infections caused by bacteria. However, even when sensitive to the product used, a proportion fails to cure, largely for pathological reasons. When three or more attempts at treatment in a lactation or dry cow treatment have failed, the only sensible advice is to remove the cow from the production herd, a culling. This may be removal to alternative use such as a beef herd or more usually by killing. Culling obviously reduces the prevalence of infection by limiting the duration of infection. It also has an effect on the rate of new infection by reducing exposure of the rest of the herd, particularly to highly infectious bacteria such as staphylococci. Culling is obviously a limitation to longevity but this must be balanced by the potential for reduction in mastitis and enhancing the rate of genetic gain, including resistance to mastitis.
Mastitis control plan component 4. Cull all persistently infected animals.
A key part of hygienic milk production is to reduce exposure to infectious bacteria. Whilst teat disinfection is highly effective after milking, a number of mechanisms may occur during the milking procedure that lead to contamination from infected teats to uninfected teats within the udder and to uninfected cows milked subsequently. These are very much related to the design and operation of the machine milking process. Changing design of the milking systems is a long-term issue with which much progress has been made over the past 20 years. Modern designs eliminate most of the inherent problems whereby milking increases the risk of infection. Practically, the milking machine has to be configured and operating properly at all times so that the most effective milking vacuum, pulsation characteristics and optimal components are applied. All examinations continue to show that the majority of herds are milked with milking equipment operating sub optimally.
Mastitis control plan component 5. Carry out at least annual inspection and servicing of the milking machine, and regular maintenance and replacement to manufacturers’ specifications.
This is the basic, tried and tested, mastitis control scheme. It is regularly supplemented according to local needs. The National Mastitis Council expanded the basics to produce a 10-point plan for the US. Since then other developments have contributed variations and refinements, such as internal teat sealant augmenting dry cow treatment.
Mastitis and longevity
Good detection and treatment mean that few cows should die from mastitis but this varies with cause, being more likely with coliform cases making it farm system variable. Mastitis (poor milk quality) is a significant reason to cull a proportion of the dairy herd, often second after infertility. Thus mastitis limits longevity; culling is the preferred method for persistently infected cows.
Mastitis costs money from lost production and costs of response e.g. labour and drugs. The costs are rarely understood by farmers. These can all be modelled and calculated but vary with severity of the disease. Prevention also costs and this is more obvious in terms of chemical and equipment purchases, along with labour. For many farmers none of this is real until it affects actual income including milk quality penalties. Certainly the longer-term impacts of failure to recover replacement costs and loss of genetic potential from culling are even less appreciated. Time rather than money is usually the motivation to manage mastitis better.
Pathogens and incidence The mastitis control scheme described above has been hugely successful in reducing the prevalence and incidence of mastitis caused by S. aureus, Streptococcus dysgalactiae and Streptococcus agalactiae to the extent that clinical mastitis has been reduced by up to 80% (from about 150 to 40 cases/100 cows/year). Effectively S. agalactiae has been eradicated from many herds and regions. Its occurrence should be recognised as incompetent mastitis management and poor biosecurity.
The ‘modern’ problem is the ‘rise’ in Streptococcus uberis and coliform clinical mastitis (environmental pathogens). In reality there has been no such thing in well managed herds. The proportion of environmental clinical cases may have risen, say from 10 to 50+%, but the data show that this is a change from 30 of 150 cases to 20 of 40 cases. The relative lack of control is partly related to changes in farm system. The importance of environmental mastitis varies with system. S. uberis predominates in pasture grazed and straw bedded housing systems. Coliform cases are most likely in indoor systems using wood-based, rubber or manure surfaced bedding.
Cell count Forty years ago the milk cell count (white blood cells, termed somatic cell count or SCC) averaged in excess of one million cells/mL. Good mastitis management systems, encouraged by regulatory and financial drivers, have led to national annual average cell counts lower than 200,000 cells/mL in many countries. This would not have happened without the drivers to lower cell count. The happy consequence has been the drop from >50 % to probably fewer than 10% of cows with an infected quarter in most herds in low cell count countries.
Detection The ability to detect something abnormal in milk or the udder varies with the milking system employed. The hand-milker has perfect opportunities for observation whereas the robotic system relies on sensor technology. The ability to detect mastitis is important to avoid sending abnormal milk for sale. This may have implications from challenging human health to the value of milk or legality of selling it. Human observation is not perfect, indeed milker detection of clinical mastitis is probably no better than 70% of cases at any one milking, although regulations appear to ignore this and assume that an artificial system should reach perfection. In effect, they can get close to human accuracy in finding clinical mastitis but often at the cost of too many false positive identifications or identification and intervention in transitory problems promptly and effectively dealt with by the host. Much work is on-going to develop protocols to improve the value of sensors for farmers.
Today’s basic drivers in mastitis control
The problems of yesteryear have not gone away, and never will, but continue to need diligent application of the tried and tested control techniques, although they can always be made more efficient and involve better products. However, a range of new challenges face today’s dairy farmers. These challenges vary widely between farming systems and regulatory jurisdictions. Four examples are described.
1. Resistance to antibiotics
The availability of antibiotics led to highly significant improvements in removal of infection. Two main methods of treatment are possible.
(a) Lactation treatment.
Treating clinical mastitis as rapidly as possible with effective antibiotic products increases the likelihood of a successful cure. Only antibiotic treatment has been proven to aid cure. Various studies of alternative methods have failed to provide evidence of any success. Treating subclinical mastitis, if the farmer can diagnose it, may help eliminate bacteria but has no proven economic benefit.
(b) Dry period treatment.
A more successful form of treatment is the use of long-acting antimicrobial intramammary infusion immediately after the last milking of the lactation. This achieves a bacterial cure rate of up to 80% compared with 20-60% (varying by pathogen) for lactation treatment.
There is no substitute for cure but prevention by an internal teat sealant has offered huge new opportunities for controlling mastitis. This also poses risks if not used properly. Fortunately, the development research has created decision rules to ensure sensible use that minimises the need for antimicrobial use. As dry cow treatment is the main target of the aim to reduce antibiotic use in dairy cows, at least one useful alternative tool is available.
2. Technology changes
Technology will continue to substitute for labour in milking cows. This will require a growing area of sensors and information systems. Smart systems are a long way from distinguishing real events sufficiently accurately. After 20 years, robotic milking systems are an indelible part of dairy farm systems. Now, even more refined robotic technologies are being adopted. However, not all problems have been solved and some old issues have resurfaced because simple gaps have not been plugged. The escalation of S. agalactiae in herds using one version of the system is one example but is easily fixed. Where there is one problem others will exist or arise.
3. Milk quality
Good average cell counts are the norm in many countries, although all still have a tail of poorly conforming suppliers. Increasing market demand will compel those suppliers to comply or force them from the market, eventually. Those who continue to fail in mastitis management have a finite life in dairy farming.
Product specification is increasingly used to dictate what and how much of any substance should be in milk or milk products. This creates a danger in confusing rapidly more sensitive detection limits with thresholds for safety or activity. The recent example of dicyandiamide (DCD) detection at a level 100x lower than the recognised safety level indicates the potential problem.
The problems going forward are moving targets. In one direction is the growing market for ‘natural’ food exemplified in ‘raw’ milk where generations of progress and risk are ignored to the cost of human health and product image. Opposite, is the rapidly stretching drive to ‘improve’ product specification by minimising ‘contaminants’, not on the basis of safety but growingly on the basis of ability to detect. This issue is confused following well reported illegal actions e.g. melamine adulteration; some clear safety issues e.g. alfatoxins from feed supplements; or more ‘accidental’ problems from too much use of iodine sanitisers affecting infant formula.
4. Real udder health issues
From a farmer’s point of view udder health revolves around clinical mastitis and high cell count but more especially on the obvious costs in terms of immediate expenditure, time and milk price. The ‘major’ pathogens are really all that are important. Farmers do not see, and most often do not want to see, invisible costs such as impaired production or very mild/transitory cases. A threshold exists for many farmers on treatment or no treatment. The minor pathogens, and their relatively minimal clinical mastitis impact, are not important to farmers unless they are affect milk value. These minor pathogens may be indicators of how well the mastitis control plan is applied (the C. bovis prevalence is a measure of the quality of teat disinfection) but they are only of major importance to researchers. All those affected, industry to consumer, have to realise the mammary gland, like any other mucosal organ is NOT sterile. It is usually that the means to detect microorganisms having no obvious effect are rarely applied. This is dealt with by proper risk management throughout the food chain.
Vaccines or genetics?
For forty years vaccines to aid in control of mastitis have been promised; some have reached the market with very limited success; most have never left the research bench. The outlook does not look bright. It is necessary to overcome the need to cause a mastitis to prevent a mastitis!
Of greater potential is to understand how some cows of long life never suffer from an intramammary infection – the extent and mechanisms of genetic-based resistance. A growing amount of phenotypic identifications are understood e.g. in teat physiology and immune recognition systems. The phenotypes with defences earlier in the process of infection would appear the most useful in creating a population of cows free of mastitis.
The ultimate tool in mastitis prevention may yet be selecting for physical defence mechanisms.
What’s broken in mastitis control?
The answer is simple – nothing. Good producers achieve superb milk quality, minimal udder health issues and, therefore, have little need to intervene and treat. This is because they prevent mastitis by diligent application of good technologies. They are proactive in management because they are trained and apply good risk assessment. They have incentives to do well and are motivated. Various countries have run milk quality competitions for many years. The longest running may be in the US where, out of 50,000 dairy farms, the finalists regularly include the same farms. They obviously know what to do and how to do it. It is clear that they exploit all the help available to them from the support and supply industry. Their equipment works properly, their products (chemicals etc) are right for the job and the advice available, from veterinarians and others, is pertinent and used properly.
For successful mastitis control, the issue is not necessarily to invent something better but to do better with what is already known.
1. Dodd, F.H. & F.K. Neave (1970) Mastitis control. Biennial Reviews 1970, National Institute for Research in Dairying, Shinfield, UK, p21-60.
2. International Dairy Federation (2010) Mastitis Research into Practice. Proceedings of the 5th IDF Mastitis Conference, VetLearn, New Zealand.
3. National Mastitis Council www.nmconline.org