Summer Mastitis - The warmer months


Summer mastitis is a term for the type of mastitis which tends to occur during the warmer months in dry cows and heifers at pasture. It differs from other types of mastitis in several ways, and is classed neither as contagious nor environmental in its origin. It is often called 'August bag'.

Its spread is linked to flying insects, specifically the sheep headfly (Hydrotaea irritans). Summer mastitis is caused by a number of bacterial varieties, most notably Streptococcus dysgalactae and Arcanobacterium pyogenes. The exact means of infection is not entirely understood, as experimental infection via the sheep headfly has proved inconclusive. The actual means of infection is likely to be more complex, with a mixture of routes, and other anaerobic bacteria such as Peptococcus indolicus are implicated in this.

In addition to potential external routes of infection via teat skin or teat canal, particularly if damaged, there is the possibility of infection originating internally, with bacteria spreading from other parts of the body, particularly via the blood. Once a quarter is affected then spread can then occur via infected material between quarters or even between animals.


Initial symptoms are a swollen, painful teat or quarter, and can be easily identified by careful observation, particularly when flies become attracted to the teat's unpleasant-smelling yellow secretions, which as the disease progresses will become worse. If left untreated, the bacterial toxins will damage the udder tissues irreversibly; in many cases the quarter affected will be lost entirely or so badly affected by the infection that it will be effectively lost. Further signs as the illness becomes systemic are swelling of the hind legs, obvious lethargy and separation from the herd, abortion and even death.


Treatment is most often via regular and repeated stripping of the affected quarter, to remove as much affected material as possible, followed by intra-mammary antibiotics and an antibiotic injection to counter the systemic effects of bacterial toxins. Heifers and cows with summer mastitis are best isolated to prevent the spread of the illness.

Summer mastitis can be avoided by various measures:

  • Having effective dry cow therapy, including the use of long-term intra-mammary antibiotics, teat end sealants and good hygiene measures at drying-off. In some circumstances, intra-mammary antibiotics may require re-administration during the dry period, although care should be taken with milk withdrawal periods.
  • Implementing measures to control and minimise exposure to flies. Flying insects should be controlled from early on in the fly season by the use of pour-on anti-parasitic treatments, the use of fly ear tags, and the application of teat fly repellents to teats, such as traditional Stockholm tar and brown salves.
  • Maintaining good teat condition pre-drying off, having good dry cow nutrition and observing/checking cattle on a regular basis.
  • Avoiding areas where teats may be damaged or areas where flies are a particular problem, such as near rivers and woods.

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