ANIMAL HEALTH
Drawing the line By Samantha Tennent
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ntibiotic use is under increased scrutiny as any time we use them there is a potential risk that bacteria develop resistance. On dairy farms, around 85% of the antibiotics used are for mastitis control, including antibiotic dry cow therapy (DCT). As we move away from blanket or whole herd treatment and towards more targeted approaches to support reducing antimicrobial resistance (AMR), what methods should we be using to determine which cows should receive what? Identifying offenders In the majority of herds, less than 10% of cows are infected with a major pathogen at drying-off, which means selective DCT is a logical approach for most herds. DCT should be reserved for cows with evidence of infection or if they fall into a high risk category. Evidence includes cows that have been treated for clinical mastitis, have been confirmed by bacterial culture or have a high somatic cell count (SCC). Another group of cows that should be considered, even if they don’t have clear evidence, are older cows producing high volumes. The latest research found that animals over four years old who were producing more than 10 litres of milk at the last herd test were twice as likely to develop clinical mastitis during the dry period. Herd test SCC is currently the most practical tool for selecting which cows need antibiotics. The maximum SCC at any herd test is the best tool to select cows. But, if a farm doesn’t herd test
regularly, trials have shown a herd test within the last 80 days of lactation is sufficient to select cows for DCT. Or if there is no herd testing information available at all, a Rapid Mastitis Test (RMT) performed immediately before drying-off is a viable option. SCC cut-points When using SCC to determine which cows receive DCT, we are aiming to target as many cows as possible that are infected with a major bug. While on average infected cows will have a higher SCC, some infected cows have a relatively low SCC and some uninfected cows have a higher SCC. So we need to select a SCC cut-point that minimises the number of infected cows that are missed, as well as avoid giving antibiotics to uninfected cows. But there is no magic number for where the line should be drawn. Research by DairyNZ and Cognosco has looked at the likelihood of missing truly infected cows when using different SCC cut-points, as well as the likely reduction in DCT usage compared to whole herd use. For example, when using greater than 150,000 as the SCC threshold, approximately 85% of the truly infected cows were identified and only 15% were missed. But the work also demonstrated if the cows below the threshold are treated with Internal Teat Sealant (ITS) they still had good outcomes that were equivalent to cows that had received DCT. So as long as they received something there was a good chance of protection. If we increase the threshold we reduce the total amount of antibiotic
WelFarm general manager Samantha Tennent says selective dry cow therapy is a logical approach for most herds.
required, but we also risk missing a larger proportion of infected cows. The numbers show we could be using 78% less antibiotics for a cut-point of 225,000 cells/mL or 63% at 125,000 cells/mL. And vice versa, when a lower threshold is used, more infected cows are captured, but we also treat a larger number of uninfected cows. The rest of the herd If cows are left unprotected there is a high chance of developing an infection during the dry period. Using ITS provides an effective barrier and reduces the new infection rate by about 70% over the dry period. And the data shows that compared to glands that received no treatment or DCT alone, there is a lower risk of clinical mastitis in the next lactation if they receive ITS. There are no hard and fast rules or exact ways of determining which cows should and shouldn’t receive DCT. Work with your vet to assess your farm when making the plan for drying-off this season and have the confidence to look at options as we move away from a ‘one size fits all’ to a more targeted approach. n
Herd tests and somatic cell counts are a good tool for farmers using a targeted approach to reduce antibiotic use.
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Samantha Tennent is the general manager of WelFarm Ltd
DAIRY FARMER
May 2022
