Anne Lee

Farmers are being advised that blanket dry cow therapy is no longer appropriate for most herds, and veterinarians are unlikely to prescribe antibiotic drugs for the whole herd at dry off unless there’s a very good reason.

Antibiotic resistance has become an increasing concern overseas with links drawn between antibiotic use in animals in the food chain and an increase in antibiotic-resistant bacteria in both animal and human populations in some countries.

While no such link has been detected yet in New Zealand there are indications of antibiotic-resistant gut bacteria in humans.

That, combined with the growing body of evidence overseas, has prompted veterinarians here to push for more-prudent use of antibiotic drugs.

DairyNZ animal and feed team leader Dr Jane Lacy-Hulbert says treating every quarter of every cow in the herd with antibiotic drugs at dry-off to manage mastitis and intramammary infection will become a thing of the past.

One study involving 36 herds sampled about 80 cows per herd at dry-off, and found that 31% of cows were infected with a pathogen, but only 8% were infected with a major pathogen such as Staphylococcus aureus or Streptococcus uberis.

Instead farmers are being advised to target dry cow antibiotics to those cows that show evidence of bacterial infection.

That doesn’t mean they have to have clinical mastitis or be showing symptoms at dry-off.

Farmers can use somatic cell count (SCC) as an indicator of which cows to treat with antibiotics, while low SCC cows can be protected against mastitis-causing bacterial infections by using teat sealant alone.

Jane says farmers should work with their vets to develop an appropriate plan for their herds.

She says the guidelines for decision-making on treatments is based on several studies carried out over recent years in New Zealand.

One study involving 36 herds sampled about 80 cows per herd at dry-off, and found that 31% of cows were infected with a pathogen, but only 8% were infected with a major pathogen such as Staphylococcus aureus or Streptococcus uberis.

Jane says further studies have found that even a single herd test, providing it is within the last 80 days of milking season, is an effective way to identify cows for dry cow therapy (DCT) treatment with antibiotics.

The research found that the single herd test carried out within 80 days of dryoff was just as accurate as the maximum SCC from three or four tests during lactation or the average of all herd tests for detecting cows which are more likely to have a major pathogen infection.

Further research to determine the threshold SCC farmers should use found that a cut off of 200,000 to 250,000 cells/ml can be used to determine which cows receive DCT.

That’s higher than the previously used 150,000 cells/ml.

The higher the threshold, the fewer cows that will be treated with dry cow antibiotics, and only a few more cows miss being treated for a serious pathogen.

Jane says it becomes a trade-off between reducing antibiotic use, and missing a few cows with a sub-clinical infection.

At a lower threshold, more cows with infections will be treated but there will also be an increase in the number of cows treated unnecessarily that don’t have an infection.

Modelled research data shows that in a 500-cow herd with 7.5% of cows infected with a major pathogen, shifting the cut-point from 150,000c/ml to 200,000c/ml decreases the number of antibiotic tubes from 636 to 492 – a 22% drop. (Table one.)

Compared with a blanket DCT approach, antibiotic use reduced by 63% if the cut point was at 125,000c/ml and 78% for 225,000c/ml.

The number of truly infected cows that miss out on getting DCT lifts from six to eight with a lift in the cut point from 150,000c/ml to 200,000c/ml.

The number of cows that aren’t infected that do get treated with an antibiotic drops from 128 to 94.

If cows below the threshold receive teat sealant only, farmers often express concern over what will happen to those that are infected.

Jane says that for cows with an SCC below 200,000c/ml and no history of clinical mastitis, the bacterial cure rates are relatively high for most pathogens if they receive a teat sealant only.

Three NZ studies showed the incidence of clinical mastitis through the winter or dry period was below 2% of cows when they received teat sealant only.

The cure rate was more than 90% which was similar to outcomes achieved when DCT was used.

If cows get no treatment at all NZ studies have shown that the rate of new infection over the dry period can be high.

In 2015 DairyNZ carried out a study on two Southland farms involving a total of 900 cows. One farm wintered cows on fodder beet and one wintered them in a cubicle barn.

Cows under 250,000c/ml for the whole lactation received either teat sealant or DCT or a combination of them both.

Ten percent of the cows under 250,000c/ml had no treatment at all.

Those that had no treatment had the highest rate of clinical mastitis in the dry period and early lactation at 16%.

There was no significant difference between the teat sealant alone, DCT and teat sealant plus DCT treatments.

Cows that were above 250,000c/ml received either DCT or DCT plus teat sealant.

The cows that receiving the combined treatment had a 6% incidence of mastitis through the dry period and early lactation – significantly lower than those that received DCT alone. They had an 11% incidence.

For those who don’t herd test a rapid mastitis test (RMT), although not ideal can be used to determine which cows receive DCT and which quarters to treat.

The cut point should be set at trace and above.

If a cow is to be treated the decision should be based on the highest reading from all four quarters.

If quarters are treated individually then the decision will be based on each quarter’s result.

Hygiene imperative

Hygiene when administering dry cow therapy or teat sealant is imperative to avoid causing a potentially much more nasty outcome than the mastitis infections we are trying to prevent.

Everybody working with cows to administer the treatments must be trained and fully understand the importance of excellent hygiene.

Teat ends must be cleaned individually, using 70% methylated spirits or medicated wipes, before inserting the tubes. Tubes must be kept clean and dry before use.

Animals should be kept quiet and walked quietly after treatment so the teat sealant remains in place.