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Dry Cow

Dry Period-Associated Mastitis

What Is it?

During lactation the udder is under pressure to continue producing milk after which there is a dry period during which the udder has an opportunity to recover.
 
Structural changes start to occur in the udder within 48 hours in response to an increase in intramammary pressure which arises when milking is abruptly stopped. The udder is particularly susceptible to infections at this time. The udder then becomes quiescent when it acquires some resistance to new intramammary infections1.
 
During the dry period, the secretions in the udder create an environment which traditionally was thought to prevent persistence of gram-negative organisms e.g. E. coli, but did not have much effect on any gram-positive organisms present e.g. Staph. aureus. However, in the last few years it has been shown that gram-negative organisms such as E. coli can also persist through the dry period and may be responsible for clinical mastitis in the subsequent lactation2.
 
In the days leading up to calving and the next lactation, the mammary gland starts producing colostrum for the calf which is due to be born. At this time the gland has increased susceptibility to infection.

Impact

Staphylococcus (Staph.) aureus usually causes chronic sub-clinical infections resulting in very high somatic cell counts in apparently normal milk. This is a contagious mastitis pathogen, so it is not usually transmitted during the dry period. However, a long duration of treatment is often needed and sometimes treatment at drying off may offer the best results3.
 
Escherichia (E.) coli can cause clinical cases of mastitis where the cow can get very ill due to the release of endotoxin from the bacteria. Recent studies have shown E.coli infections picked up during the dry period can persist and cause mastitis well into the next lactation, up to 100 days2.
 
Streptococcus (Strep.) uberis is also a bacterium that can be picked up during the dry period as well as lingering from a previous lactation. In this respect it has some contagious and environmental characteristics, so control is aimed at cure of existing infections and preventing new infections being picked up during the dry period. 
 
Summer mastitis is a type of mastitis that is seen in dry cows usually, as the name suggests during the summer months. It is when the dry udder is infected by one or more gram-positive bacteria such as Trueperella (Arcanobacterium) pyogenes, causing a low grade mastitis with the cow often being clinically well but with a very enlarged and painful quarter. Flies are implicated in transmission of the infectious agents and infection often results in the quarter being unproductive in following lactations.
 
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Control of mastitis in the dry cow

Although the dry period is a crucial time for self-curing of existing infections, use of antibiotics and/or sealants may also be required to eliminate existing infections and reduce new mastitis infection rates during the dry period. 
 
Dry cow therapy normally involves the infusion of long acting antibiotics and/or internal sealant into all four quarters of the udder at “drying off”. Increasingly and particularly in some other EU countries, it is normal practice to use antibiotic infusions only in the high cell count cows and indeed only in the high cell count quarters of these cows, using teat sealers for whole herd application.   
 
When using an antibiotic dry cow therapy, it is important to check that the therapy you are using provides suitable cover against both gram-positive bacteria such as Strep. uberis and gram-negative cover such as E. coli. Some chronic mastitis cases caused by gram-positive bacteria are more easily cured during the dry period3. The ideal dry cow tube would have good activity against the persistent gram-positive bacteria, but also act against the gram-negative and environmental bacteria that pose a risk as new infections.
 
For summer mastitis, additional use of a fly repellent is crucial in reducing the risk of this very costly mastitis. Long acting pour-ons or impregnated ear tags should be considered and pastures with sandy soils, tree cover and water should be avoided. At times it may be necessary to house animals.  Any animals with visible teat lesions should be treated and covered with fly protection and it may be prudent to isolate these animals from the rest of the herd.  To reduce the risk of skin lesions, areas of rough grazing (thistles and long grasses) should be avoided. 

Responsible use of antibiotics

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References:

 
1. Hurley (1989) Mammary gland function during involution. J. Dairy Sci. 72:1637
2. Bradley and Green (2000) Proceedings of the British Mastitis Conf. (2000) p28-36 
3. Andrew Biggs. Mastitis in Cattle. The Crowood Press Ltd, 2009. ISBN 978 1 84797 071 8

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Date of preparation: Jul 2013. AHD 7729. Use Medicines Responsibly (www.noah.co.uk/responsible)