Dry cow therapy – does it still deserve a blanket recommendation?

The use of antibiotics in animal husbandry has been subject for debate for many years, and overuse of antibiotics in veterinary medicine is considered one of the main factors responsible for emergence of antibiotic resistance in bacteria. The majority of antibiotics used in dairy herds are related to udder health, of which two-thirds as dry-cow products. Because of the preventive character of dry cow antibiotics and the large potential reduction in antibiotic use, the blanket dry cow therapy (DCT) recommendation, to treat all teats on all cows, needs to be reconsidered.

Summary from presentation and proceedings by Theo Lam, GD Animal Health, from the Netherlands, at NMC Annual meeting in February 2014

Take home message

  • The goal of DCT is to improve udder health by curing intramammary infections (IMI) that exist at the time of drying off, and by preventing new ones to occur.
  • In most countries blanket DCT is recommended, treating every quarter of every cow with antibiotics at drying off, independent of their infectious stage
  • Overuse of antibiotics in veterinary medicine is considered one of the main factors responsible for emergence of antibiotic resistance in bacteria.
  • Studies show that cure rates were no different from blanket DCT when selective DCT was used, and selective DCT successfully eliminated IMI to the same degree as did blanket DCT
  • Selective DCT has shown to be less protective than blanket DCT, but significantly more protective when compared with no treatment
  • There is a potential relation between intramammary antibiotic use and antibiotic resistance in intestinal bacteria through the feeding of waste milk to calves. The same may be true for antibiotic residues of dry cow antibiotics in colostrums.
  • The use of third preference antibiotics (which are also of importance for treating multiresistant bacteria in human patients) can lead to increased resistance, so based on the possible consequences on human and animal health care, antibiotics of third preference should only be used when there are no other alternatives.
  • Second preference antibiotics may only be used if it has been shown that there is a curative need for them, based on the precaution principle, where disease problems due to husbandry should be solved by changing the management rather than by the preventive use of antibiotics.
  • The preventative effect of dry cow therapy in low SCC cows cannot be dropped without adverse effects, but these effects can be counterbalanced by the use of internal teat sealants. Internal teat sealants are a good alternative for antibiotics to prevent low SCC cows from becoming infected during the dry period.

Introduction

Dry cow therapy (DCT) has been used for more than 50 years to improve udder health on dairy farms and has proven to be successful in numerous scientific reports as well as in practice. The goal of DCT is to improve udder health by curing intramammary infections (IMI) that exist at the time of drying off, and by preventing new ones to occur. The incidence of new IMI to occur is highest during the dry period. New IMIs that arise in the dry period can lead to clinical mastitis, either directly after the infection, or later.

Use of antibiotics in animals differs between countries. In most countries blanket DCT is recommended, treating every quarter of every cow with antibiotics at drying off, independent of their infectious stage. Other countries, specifically the Nordic countries have been more restrictive. In those countries the restricted use of dry cow antibiotics generally did not lead to unacceptable high incidence rates of clinical mastitis.

Estimated use of blanket DCT:

USA 72%
Netherlands 76%
Canada 88%

In the Nordic countries the quantity of antibiotics used is much lower:

Norway 1-2%
Denmark 6-7%
Finland 20%
Sweden 25%

The use of antibiotics in Denmark has increased with the introduction of a national udder health program in 2010. Whether the extremely low use of antibiotics with concurrent good udder health in Norway is due to different husbandry system, production level or breed is not fully clear. A fact is that the Nordic countries do perform very well with respect to limiting the use of dry cow antibiotics.

The use of antibiotics for DCT has been stable at least since 1998, and no significant increase in resistance in mastitis pathogens have been seen. Still, there is a link between the use of antibiotics and the development of antibiotic resistance, so the use of antibiotics should be minimized as much as possible. In the Netherlands preventive use of antibiotics is no longer permitted, so it is therefore necessary to look at selection of cows for dry cow therapy, for curative use.

Selective DCT better than no treatment but less effective than blanket DCT

It has been shown in studies (Halasa et al. 2009) that cure rates were no different from blanket DCT when selective DCT was used, and selective DCT successfully eliminated IMI to the same degree as did blanket DCT (Cameron et al. 2014). To evaluate the effect of dry cow treatment, a split-udder study was conducted. In 100 herds, 1,600 cows were selected that had a low SCC at their last milking before dry-off. The study showed that in low SCC cows (<250,000 cells/ml for multiparous cows and <150,000 cells/ml for heifers) the incidence rate of clinical mastitis in the period between drying off and 100 days in lactation was found to be significantly higher in quarters dried off without antibiotics compared to the quarters dried off with antibiotics. A meta-analysis by Halasa et al. (2009a) showed that selective DCT was less protective than blanket DCT, but significantly more protective when compared with no treatment.

The issue now becomes at what level of SCC do we select cows for therapy? Different studies used different approaches:

• Bradley et al. (2010) selected cows in herds with a low bulk SCC (<250,000 cells/ml) that had the last three months individual SCC <200,000 cells/ml and no case of mastitis within that period.

• Huxley et al. (2002) used individual SCC <200,00 cells/ml and no cases of clinical mastitis during the whole previous lactation.

• Cameron et al. (2014) cultured composite milk samples with Petri film, where any pathogen found was considered indicative for the use of dry cow antibiotics. This approach is comparable to the one used in Denmark, where the diagnosis is done by PCR, and any bacteria found (minor or major pathogens) legitimates the use of dry cow therapy.

Antibiotic resistance – the risk involved in feeding calves waste milk

In a Canadian herd level study (Saini et al., 2012) it was found that the use of cephapirin and cloaxillin in DCT was associated with increasing odds of having kanamycin-intermediate or –resistant E.coli isolates at that farm. With the exception of this study by Saini et al. the author is not aware of any other published studies on an increase of antibiotic resistance in mastitis pathogens related to the use of dry cow antibiotics. The number of bacteria in the udder is low, certainly when compared with the intestinal tract. Therefore the risk of development of antibiotic resistance in the udder as a consequence of the use of intramammary antibiotics, is much less likely than in intestinal bacteria after oral or parenteral use of antibiotics.

Intramammary applied antibiotics may invade the body and may have an effect on resistance development in intestinal bacteria, but they are developed to be mainly active in the udder. Bacteria invade the udder through the teat, and intramammary bacteria only seldom lead to bacteraemia. Resistant bacteria from the intestine may invade the udder via the manure in the environment, but this is an indirect route of transmission. The effect of direct transmission of intramammary infections by for instance the milking machine is probably much more important. There is likely only a limited relation between antibiotic resistance patterns of intramammary and intestinal bacteria. However, there is a potential relation between intramammary antibiotic use and antibiotic resistance in intestinal bacteria through the feeding of waste milk to calves. Milk produced during the withdrawal time is unfit for human consumption, and is often fed to calves (Heinrich et al., 2012). Approximately 70% of these samples actually contain antibiotic residues, often in low concentrations, and thus can lead to selective pressure in intestinal bacteria in calves. The same may be true for antibiotic residues of dry cow antibiotics in colostrums. Whether this relation of feeding waste milk to calves or residues in colostrums really exists is unknown.

Types of antibiotics used

The types of antibiotics used, as well as the legislation, differ between countries. In many countries there are ‘formulary groups’ that prepare guidelines for preference of use: antibiotics to be preferably used for major indications per species. First preference are small spectrum and second preference are broad spectrum antibiotics. Third preference are antibiotics that are also of importance for treating multiresistant bacteria in human patients (3rd and 4th generation cephalosporins, some fluoroquinolones and modern long acting macrolides). These third preference antibiotics are only allowed to be used if, at the level of the individual animal, culture results and antibiotic sensitivity data showed there is no alternative. As a consequence, the use of modern cephalosporins in DCT has been virtually banned in the Netherlands. However, as some of these third preference antibiotics are not excreted in the udder, don’t have a withdrawal period for milk and for that reason are safe and attractive to use, they have been used intensively. Evidence exists that the use of third preference antibiotics can lead to increased resistance, so based on the possible consequences on human and animal health care, antibiotics of third preference should only be used when there are no other alternatives.

Dry cow products combining different antibiotics such as penicillin and neomycin, preventing new IMI with gram-positive and gram-negative bacteria, are considered second preference. They may only be used if it has been shown that there is a curative need for them. That can be done on herd level data, but the allowance is only temporary. This is based on the precaution principle, where disease problems due to husbandry should be solved by changing the management rather than by the preventive use of antibiotics. Additionally there should be a good reason not to use small spectrum (first preference) antibiotics, when they are available. Preventive effect is not a good reason in this line of thought.

Internal teat sealants

One of the possibilities to avoid new IMI in the dry period is the use of internal teat sealants. Many papers have been published on the positive effects of internal teat sealants combined with antibiotics, but several recent studies on the effects of internal teat sealants alone have been published. (Huxley et al.,2002; Bradley et al., 2010; Cameron et al., 2014). In the former study the added value of the use of internal teat sealants without antibiotics in dairy cows was shown. The Cameron study showed that internal teat sealants are a good alternative for antibiotics to prevent low SCC cows from becoming infected during the dry period. A key issue with teat sealants is the difficulty to ensure a hygienic application. It has however been proven that if done properly, teat sealants can be applied without problems, even in primiparous, or first-calf, heifers.

The preventative effect of dry cow therapy in low SCC cows cannot be dropped without adverse effects, but these effects can be counterbalanced by the use of internal teat sealants. Thus internal teat sealant seems like a logical alternative to prevent low SCC cows from new IMI in the dry period, although more studies are needed.

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Monica Wadsworth

Monica Wadsworth
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Writer at Milkproduction.com

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