14 May 2018
Sophie Mahendran looks at how cow udders become infected, and approaches to treatment and prevention.
Sophie Mahendran
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Image: Naj / Adobe Stock
As the seasons change and temperatures rise, farmers again begin to think about turning their young heifers and dry cows out on to pasture. While this has many benefits, the warm weather does increase exposure to risks associated with developing summer mastitis.
This disease is best described as a case of acute clinical mastitis that occurs in non-lactating cattle out at pasture, with the highest risk period in the UK being July, August and September. Estimates of the economic impact of this disease indicate the largest financial costs are associated with lost milk production, with a historic estimated cost of £192 per case.
This figure would equate to approximately £270 today and, when combined with a reported occurrence of 47% of herds having one or more cases of summer mastitis, the importance of understanding the epidemiology – and, therefore, ability – to control (Panel 1) and prevent this disease, is amplified.
Although the main cohort affected is usually pre-calving heifers and dry dairy cows, summer mastitis is also seen in beef cows, occasionally in calves and even in bulls (due to the presence of teats). Non-pregnant heifers appear the least likely cohort to be affected, with mammary tissue development thought to increase their susceptibility to infection, and higher prevalences found in older cows – potentially due to increased ease of bacterial entry into the teat duct and canal.
Clinical signs during the acute phase include pyrexia and anorexia, and the animal may separate itself from the rest of the herd while looking dull and depressed. On examination of the udder, an enlarged quarter will be seen that may appear red, as well as having a swollen and inflamed teat. The discomfort associated with this can lead to an altered gait, so regularly observing cattle for signs of lameness may help farmers with early detection of the disease.
If the quarter is stripped out in the acute phase, a watery secretion with clots will be identified. However, animals are often not spotted until later in the disease course, when a thick, pungent secretion will be identified.
In some cases, the animal will become septicaemic or toxaemic – potentially leading to abortion or death of the affected animal unless adequate treatment is instigated. In severely affected animals, the quarter may become gangrenous and slough away, or an abscess may develop and rupture on the surface of the udder. On recovery from the acute disease, the affected quarter will often lose its function and become a blind (non-milking) quarter due to the presence of thelitis and the development of fibrous scar tissue in the teat canal.
The main bacteria associated with summer mastitis is Trueperella pyogenes (formerly known as Corynebacterium and Actinomyces pyogenes). Although this is the primary pathogen, the presence of anaerobes appears to determine the severity of the disease. These additional pathogens include Streptococcus dysgalactiae, Peptococcus indolicus, Fusobacterium necrophorum, and other Bacteroidaceae and Micrococcus species.
Responses to treatment are often poor – especially as detection rates in the acute stages of the disease are low and quarters are filled with large volumes of purulent material through which antimicrobials poorly penetrate. Administration of intramammary antimicrobials can be difficult due to pasture location of affected animals, as well as having poor efficacy. Parenteral antibiotics are generally required due to the severity of the mastitis (β-lactam antimicrobials), along with NSAIDs and, ideally, frequent stripping of the quarter. This, therefore, usually requires catching and transporting the affected animal back to buildings to enable safer restraint and treatment administrations.
In severe cases, some authors describe amputation of the teat. However, drainage is generally poor due to the highly viscous nature of the secretions, as well as the leaking material, potentially providing an additional source of infection for other animals in the group.
One of the greatest risk factors for summer mastitis is the seasonal activity of the sheep head fly Hydrotaea irritans, which is known to be a mechanical vector for the main causative bacteria T pyogenes. The fly feeds on secretions, so is drawn to teat ends – especially if areas of teat damage are already present. H irritans lay their eggs in sandy soils where they over-winter and emerge around June. Further environmental conditions that appear to increase fly numbers and activity is the presence of standing water or rivers, as well as fields that border wooded areas. Conversely, high wind speeds and heavy rain reduce fly activity.
Another major risk factor for summer mastitis is the occurrence of any trauma or damage to the teat. In multiparous animals, this may have occurred during lactation, with poor milking machine settings (for example, excessive vacuum) or spread of infectious teat conditions, such as papilloma virus, reducing teat end defences.
In addition, the presence of large numbers of spiky plants on pasture (for example, thistles and gorse) can cause scratching and irritation to the udder, which may result in licking and disruption of the keratin seal normally present in the teats of non-lactating cattle.
The final major risk factor for summer mastitis is the presence within a group of an animal that has a predilection for sucking the teats of other animals in the group. T pyogenes bacteria can be found as a commensal in the oral cavity and on the teat skin of healthy animals; therefore transmission of the causative agent can occur during cross-suckling.
Synthetic pyrethroids are based on the naturally occurring pyrethroid produced by flowers such as chrysanthemums.
Pyrethroids are contact insecticides, so when a fly lands on treated cattle, the chemical penetrates the cuticle of the insect. The insecticidal property comes from its ability to prevent the closure of voltage-gated sodium channels in insect neuronal membranes (although they also affect the chloride and calcium channels as well), thereby disrupting electrical signalling and resulting in a paralysed insect. Very little of the chemical is absorbed through the cow‘s skin.
Care must be taken as pyrethroids are highly toxic to cats due to the lack of an enzyme that normally rapidly breaks down the chemical once in the body. The chemical is also highly toxic to aquatic animals.
Types of synthetic pyrethroid include cypermethrin, deltamethrin and permethrin. Resistance to pyrethroids has been reported, so additional use of macrocyclic lactones or combining pyrethroids with piperonyl butoxide may be required.
The control of flies is an important prevention method for summer mastitis, with the added benefit of also helping to reduce the occurrence of infectious keratoconjunctivitis and other effects of fly worry in cows. Pour-on synthetic pyrethroids are commonly used (Panel 2), but will need regular reapplication, at least every four weeks. Pour-on solutions, however, will not provide high concentrations of active chemical around the udder, so it may be necessary to apply the product directly to the ventrum and udder area to deter flies from landing there. Pyrethroid-impregnated ear tags may also be used, but, again, their ability to reduce flies around the udder is limited.
Good pasture management will help reduce the growth of prickly and abrasive plants, as well as improving the quality and nutritional value of the grazing. Avoidance of damp pastures and those bordered by woodland during the high-risk periods is also advisable to help reduce fly exposure.
Maintenance of good teat condition is important to maintain effective teat end defences and reduce the presence of teat cuts and abrasions. Teat condition can be assessed at drying off, and if hyperkeratinisation around the teat end is present, the milking parlour should be serviced and its settings checked. If cross-suckling is observed in heifer cohorts, the application of nose plates can be used.
Internal teat sealant usage in dry cows can help reduce the ingress of bacterial pathogens into the teat canal. These bismuth subnitrate internal teat sealants can also be applied to heifers prior to calving, to help improve internal physical teat barriers. Strict aseptic techniques must be ensured when applying internal teat sealants to prevent introduction of pathogens into the udder.
In addition to teat sealants, the use of long-acting antibiotic dry cow intramammary therapy is known to reduce the incidence of summer mastitis in dry dairy cows. However, the drive to reduce antibiotic usage – and, therefore, reducing the use of blanket dry cow therapy – is an important consideration when deciding how best to tackle summer mastitis prevention.
In severely affected farms, intramammary antibiotics may be required during the highest risk period to prevent the devastating loss of a quarter in a milking cow. This preventive method may be extended to use in heifers, but should only be considered once other preventive methods have been employed.
In farms that experience a high prevalence of disease year after year, even though effective preventive measures have been put in place, consideration must be given to calving cattle during lower risk periods of the year. That is to say, if animals were to calve in the spring and early summer, no dry cows or pregnant heifers would be on the farm during the high-risk time of year, therefore preventing the disease from occurring.
The reduction in value of a cow (especially dairy breeds) due to the loss of a milking quarter from summer mastitis infection can have big financial impacts on the return on investment farmers make when rearing heifers and keeping dry cows. Identifying the most significant risk factors and implementing strategies to mitigate them on individual farms can be challenging, but is worth the investment in both time and labour.
