Mastitis: staphylococcus aureus

ISSN 2398-2993

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Synonym(s): Staph, S.aureus, intramammary infection, IMI, contagious, cow-adapted, staphylococcal mastitis

Introduction

Cause: infection of the mammary tissue with Staphylococcus aureus bacteria (S. aureus).
Signs: clinically, inflammation of the affected mammary gland, altered visual appearance of the milk and reduced milk production may be observed. Sub-clinical presentations are common, with increased somatic cell count in the absence of visible signs.
Diagnosis: bacterial culture of milk from quarters of the udder where clinical signs have been detected. Other diagnostic tools are available, for example milk polymerase chain reaction (PCR) tests, as discussed in more detail.
Treatment: intramammary and systemic antibiotics have been combined with various local and systemic anti-inflammatory preparations in many varied protocols.
Prognosis: a clinical cure may be readily affected, but a bacterial cure is less frequently achieved, leading to persistent infection, with recurrence of the clinical and sub-clinical presentations being common.

Geographic incidence

S. aureus is found worldwide Staphylococcus spp.
As a rule, prevalence of S. aureus mastitis in a region is correlated to predominant milking practices in the locality.
- Areas in which measures to reduce contagious transmission of pathogens are widely adopted will generally have lower prevalence of intramammary infection with Staphylococcus aureus.

Age predisposition

In common with mastitis of other etiologies, there is a positive association between age and incidence of S. aureus intramammary infection.
- Multiparous cows are significantly more likely to acquire clinical or subclinical intramammary infection with S. aureus than primiparous animals.
- This age predisposition is multifactorial, with differences in immune function, previous/concurrent disease status and teat anatomy all implicated in the predisposition of multiparous animals to S. aureus infection.
- Invasion of the mammary tissue by S. aureus is countered by a marked influx of polymorphonuclear neutrophils, leading to phagocytosis and intracellular killing of the bacteria. However, it has been demonstrated that multiparous animals have reduced phagocytic and bactericidal neutrophil function in comparison to their primiparous counterparts and this is thought to be a key risk factor for S. aureus intramammary infection.

Breed/Species predisposition

Resistance to intramammary infection, including that due to S. aureus, is influenced by genotype. As such, different genetic lines will have differing predispositions to S. aureus infection.
It is possible to evaluate the predicted transmitting ability (PTA) of bulls for mastitis resistance and low somatic cell counts in their female progeny. Heritability for these traits is relatively low, with mastitis resistance having a heritability score of four per cent. However, with PTAs currently ranging from negative five to positive five, it is estimated that a bull may reduce the probability of mastitis in a daughter by up to five per cent Introduction to genetics and genomics.
However, despite this variation in genotypic susceptibility to mastitis in general, there is currently little to suggest a direct effect of cow ‘breed’ on predisposition for S. aureus intramammary infection. Breed affects numerous traits which can influence the odds of intramammary infection, such as milk yield, udder and teat conformation and susceptibility to other diseases. These traits may all influence a breed’s likelihood of acquiring a S. aureus infection, but the breed would not be classed as predisposed to mastitis per se, as the same traits could also exert an effect in individuals of other breeds.

Public health considerations

S. aureus species are pathogenic to humans, infecting various organ systems with severity ranging from mild to fatal.
Of particular significance to human health is the emergence of methicillin-resistant S. aureus (MRSA), with significant increases in resistant isolates since the end of the twentieth century.
The first isolations of MRSA in animals were from bovine mastitis samples, perhaps unsurprisingly as milk is one of the most commonly tested animal food products.
Humans may come in to contact with bovine S. aureus in two ways: firstly, those working with cows, particularly the milking team, may become contaminated with S. aureus directly from the skin of the cow or from contaminated milk; and secondly, members of the wider population may encounter viable S. aureus in unpasteurized milk.
Direct transmission of MRSA between cow and human has long been suspected, with strain typing giving definitive confirmation in 2007, although direction of transmission in this case was not established.
It is common for household pets to be infected with MRSA strains identical to human MRSA clones, with human to animal direction of transmission being postulated in these cases. However, regardless of the common direction of travel, the risk of transmission of S. aureus (including MRSA) from dairy cows to humans through direct contact is present and significant.
Indirect transmission of S. aureus from cow to human in milk and dairy products is a significant possibility if pasteurization of the raw milk does not occur. Risks of ingestion are mainly associated with the production of enterotoxins by some strains of Staphylococcus aureus. Bovine S. aureus isolates appear to be highly variable in their production of enterotoxins, with different studies reporting enterotoxigenicity ranging from zero to fifty-six per cent.
It is generally concluded that risk of transfer of S. aureus (including MRSA) from cows to the general population is low, unless raw milk is consumed.
Transmission by direct contact should be considered a possibility for those working with cattle.
It is in the interest of public health to reduce the prevalence of intramammary S. aureus infection within the dairy herd.

Cost considerations

Intramammary infection due to S. aureus accrues significant cost for the dairy herd.
Cost of any mastitis case may be considered in three areas:
- Firstly, the cost of any measures employed to prevent S. aureus infection.
- Secondly, the cost of treating cows identified as infected.
- Thirdly, the losses in potential revenue caused by reduced production.
In addition, S. aureus mastitis has the potential cost of further infection, through its contagious transmission.
Preventative measures are often not included in mastitis costings, being thought of as an investment in animal health, rather than a cost of disease. However, in the absence of S. aureus intramammary infection there would be no need for such an investment, rendering these preventative measures a true cost of the disease.
Direct costs of clinical mastitis due to S. aureus are relatively easy to calculate, being an addition of the cost of therapeutic treatment (both the medicines and the staff time required for administration) and the cost of discarding the milk for the treatment period plus the relevant withdrawal period for medicine residues.
Lost income due to S. aureus clinical cases is harder to quantify. The reduction in yield following clinical mastitis varies depending on the individual cow, the severity of the case and the stage of lactation. However, a significant and prolonged production loss is common to most scenarios.
Further to losses from clinical cases, sub-clinical infection plays a significant role in the cost of S. aureus. Chronically infected animals, with elevated milk somatic cell counts but no visual signs, invariably have lower milk yields than they would have in the same circumstances without S. aureus infection. Different methods of calculating the relationship between somatic cell count and milk yield all agree that increased cell counts are associated with significant milk loss. Meta-analysis by Hortet and Seegers concluded an approximate reduction in daily yield by 0.4kg for primiparous and 0.6kg for multiparous animals for every twofold increase above 50,000 somatic cells/ml.
Elevated somatic cell counts on a herd level may also lead to financial losses due to milk not meeting the criteria for sale set by the purchaser. Some milk contracts have payment penalties or bonuses for being above or below a cell count threshold respectively.
To control somatic cell count at a herd level many producers will discard milk from sub-clinically infected cows, increasing the losses due to unrealized income.
Additionally, the culling of chronically infected animals adds replacement costs to the overall financial impact of S. aureus intramammary infection. The impact of this will vary with replacement prices, as well as with milk and feed costs for herds that are replacing cows with heifers of a lower production level.
Many studies have attempted to derive a mean cost for a S. aureus intramammary infection, but the inputs to this calculation are not only highly variable between herds, but even within herds at different points in time, giving a huge variation around the mean thus rendering any ‘average cost’ to be of little practical relevance to the individual herd.
Estimates of cost at herd level must take in to account current milk prices, feed prices, replacement costs, cull cow prices and production parameters for the herd in question within the time period of interest.
Cost of future cases through the contagious nature of this pathogen can also be ascribed to a case.
- This factor that has been largely overlooked in the past but is highly significant in contributing to total herd mastitis cost.
- Transmission rate is a key reason for S. aureus being one of the financially more important mastitis pathogens.

Pathogenesis

Etiology

S. aureus Staphylococcus spp is one of many etiological agents for clinical and sub-clinical mastitis.
The S. aureus bacterium is often described by the result of various bacteriological tests, being referred to as a Gram-positive and coagulase-positive coccus.
Numerous strains of S. aureus have been isolated, from a wide number of animal species. Specificity to the original host is low, with strains readily being transmitted across species, including between humans and other mammals.
Different strains have a moderate degree of genetic variation, resulting in a range of virulence factors.
Virulence factors include:
- The ability to hydrolyse casein and ferment lactose, making milk an ideal growth medium.
- Bacterial wall surface adaptations that enhance the ability to adhere to epithelial cells and colonize the teat canal.
- Antiphagocytic properties which allow evasion of the neutrophil response in the udder.
- An ability to survive inside macrophages and mammary cells.
The origin of the S. aureus pathogen is the skin surface of cows within the herd. Not all strains of S. aureus isolated from the skin are associated with intramammary infection, but the skin sites of the udder, teats and hocks are particular reservoirs of mastitis-associated strains.
Survival times of S. aureus in the environment tend to be low, although there is a positive relationship between the number of bacteria present and the duration to negative culture result. Some researchers have demonstrated survival times ranging from one to fifty-six days for S. aureus on surfaces such as cotton and plastic. This indicates a significant role of fomites in the transmission of S. aureus between cows Pathogen transmission: overview.

Predisposing factors

S. aureus intramammary infection requires two key events to occur:
- The entry of the bacteria in to the teat.
- Evasion of the host’s immune response.
These events can be facilitated by several key predisposing factors.
- Compromised integrity of the anatomy of the teat sphincter is associated with increased intramammary infection, presumably as a result of an increased likelihood of bacterial invasion of the teat.
  - Teat sphincter damage may be due to teat injury, excessive compressive load from the milking machines liner on the teat during the massage phase of pulsation Machine induced changes in teat condition, or infectious diseases such as ‘black spot’ Teat: conditions caused by infectious agents .
- In addition to exerting an effect on teat end integrity, the milking machine may also be directly responsible for transferring S. aureus bacteria across the teat sphincter.
- Under certain conditions, detachment of the milking unit can generate a vacuum gradient that reverses the direction of flow through the teat. This has the potential to cause injection of milk, air and pathogens up in to the teat from the teat end. The significance of these events in generating intramammary infections is not yet fully established .
- The introduction of S. aureus bacteria to teat sinus and mammary tissue does not automatically result in an established infection. The mammary immune defences, particularly the recruitment of polymorphonuclear neutrophils, can successfully eliminate the bacterial population resulting in self cure. However, it has been demonstrated that if only low numbers of neutrophils are recruited, or if the function of the neutrophils is impaired, S. aureus infection is more likely to occur. As such any physiological or pathological process that impairs immune function may predispose intramammary infection with Staphylococcus aureus.

Pathophysiology

For S. aureus to establish an intramammary infection, firstly it must be present at the teat orifice.
- This teat end contamination does not have to be extensive, as an intramammary infection may be established from as little as ten colony forming units of S. aureus.
S. aureus bacteria may inhabit the teat end naturally or may be transferred there from other sources.
- Hock skin often contacts the teat when the cow is recumbent, and this may result in transfer of bacteria between these sites.
- Udder skin may harbor S. aureus with gravity aiding the transfer from udder to teat, particularly if the udder skin is wet.
- Other common methods of teat end contamination include the hands of milking staff passing from cow to cow, or the liners of the milking machine being used on consecutive animals without disinfection .
Once present at the teat orifice the S. aureus bacteria must traverse the teat sphincter to enter the teat canal.
- These bacteria are not motile and so require environmental factors to aid this transport.
- This process is incompletely understood, but there is a growing body of evidence to implicate certain conditions of machine milking as having a role in transfer of pathogens across the teat sphincter.
On entering the teat canal S. aureus bacteria adhere to the epithelial cells, colonising the teat canal. It is thought that the closer this adherence is to the teat sinus the greater the risk of intramammary infection.
From the teat canal, S. aureus bacteria enter the gland cistern, either by progressive colonisation or, as some researchers have proposed, by altered intramammary pressure dynamics when the animal walks, lies down or is milked. Within the glandular tissue, the bacteria adhere to mammary epithelial cells, thought to facilitate the establishment of infection, and also adhere to fat globules, thought to allow dissemination throughout the quarter of the udder.
S. aureus bacteria are able to invade mammary epithelial cells, leading to erosion and ulceration of the lactiferous sinus and ducts as well as lesions of the alveolar secretory cells.
Occlusion of the alveolar ducts allows localized populations of S. aureus to evade both intramammary antimicrobial treatments and host leukocytes, surviving to propagate the infection on release from the occluded alveolus.
These focal regions of trapped bacteria may progress to form microscopic abscesses or granulomas.
S. aureus also causes tissue damage remote to the location of the bacteria, by secretion of toxins and enzymes in to the mammary gland.

Timecourse

A period of 3 to 6 days is commonly seen between invasion of the teat and establishment of an infective population within the mammary tissue.
- After this period clinical mastitis may be observed, but in the case of subclinical infection the lag time between initial infection and detection may be much longer.
Left untreated, or with poor response to treatment, a S. aureus intramammary infection will often become chronic, persisting for numerous months -sometimes for the remainder of the animal’s lactation Mastitis: approach to the cow with chronic mastitis.

Epidemiology

S. aureus is described as a ‘contagious’ mastitis pathogen, with infection following transmission between cows, invariably through fomites associated with the milking process such as the liner of the milking machine; teat-preparation materials that are shared between cows such as dip-cups or teat brushes; and the hands of the milking team Pathogen transmission: overview.
Transmission within the herd can be described by the infection ‘reproduction number’ -that is the number of new cases that each S. aureus intramammary infection will generate during its duration. This number can be affected by factors such as the general level of herd immunity, or the teat end condition within the herd. Poor herd level immunity (or a high proportion of rough teat ends) will increase the reproduction number and lead to a higher number of new infections within the population.
This contagious ‘reproduction’ of infection gives a relatively consistent epidemiological picture between S. aureus positive herds.
Incidence of clinical mastitis is usually moderate, although poor cure rates lead to a moderate to high prevalence.
Prevalence of intramammary infection is dictated by new infection rate, duration of persistence and elimination rate. New infection rate is itself affected by prevalence, with a positive association between herd prevalence and odds of new infection.
Data for both clinical and subclinical infection will show no evidence of seasonality and no strong association with stage of lactation. Case distribution is that of ‘lactational’ origin rather than ‘dry period’ origin. It may appear as if there is a high incidence of early lactation cases, but many of these are uncured infections from the previous lactation.
A key indicator of a significant S. aureus presence in the population is the cure rate over the dry period. Of the cows that start their dry period with a cell count above 200,000/ml, more than eighty per cent would be expected to enter the next lactation with a cell count less than 200,000/ml. If this benchmark is not achieved a persistent pathogen such as S. aureus may be suspected, particularly if there is no evidence of a high incidence of intramammary infections of dry period origin.
The notable feature of S. aureus infection at a herd level is an ongoing increase in the proportion of the milking herd with chronically high somatic cell counts Somatic cell count:
- This will be highlighted by individual cow milk recording data, but may or may not be reflected in the bulk milk cell count, depending on the herd’s policy of discarding milk from animals with high cell count.
- Cows can be classed as ‘chronically’ infected if they have two consecutive months with somatic cell counts over 200,000/ml.
- For primiparous animals a threshold of 150,000 cells/ml may be appropriate.
  - This is partly due to a naturally lower level of cells in the milk, but also due to the importance of maximising test sensitivity in primiparous animals to allow reduction of culling and yield loss in the first lactation.
- These parameters are chosen to reflect the likelihood of infection of an individual quarter when using a composite cell count of all four quarters.
- Higher or lower thresholds can be used to improve this assumption’s specificity and sensitivity respectively.

Mastitis: staphylococcus aureus