Taking the guesswork out of mastitis

As veterinarians and farmers, it is our responsibility to ensure that antibiotics are both prescribed and administered appropriately so that their efficacy against microorganisms is well-preserved in years to come.

Bacterial resistance against antibiotics is a real and growing concern in both human and animal medicine.

Over the years, there have been an array of antibiotics used to treat bovine mastitis in an attempt to cure them. Decisions on which antibiotic to use have historically been based on timing since calving, age of cow, number of quarters affected and severity, etc. often without diagnostic evidence of the actual cause and determination of the antibiotic’s sensitivity against a certain bacterial isolate. In many situations, cows are treated with multiple antibiotic types due to the inefficacy of preceding treatments.

Nowadays, there are many ways that we can determine both the causal organism and how sensitive a particular bacterial isolate will be against a certain antibiotic. One method is called Mastatest. This is a rapid on-farm diagnostic technology which removes the guesswork out of mastitis treatment and determines both the bacterial cause and what antibiotic is likely to give you the best chance of a cure for an individual cow. Results are sent to you and your vet via email 24 hours after processing the milk sample (which takes just a few minutes).

Currently, we have approximately 30 dairy farms in the Manawatu using this technology to help them to identify the cause and determine which antibiotic is likely to be most appropriate against the bacterial isolate. This has allowed farmers to not only establish a bacterial profile for mastitis on their farm but has also increased their mastitis cure rates significantly because they are using the most appropriate antibiotic right from the start. Because of this, many farms have reported fewer recurrent cases and ultimately there has been a considerable reduction in antibiotic use on these farms (and increased profit!).

Out of 3183 samples on 30 farms analysed through Mastatest between November 2018 and May 2022, the most common bacterial isolate was the environmental bacteria Streptococcus uberis (34 %). See findings HERE, as predicted, Strep. uberis detection was most prevalent over the calving period (blue line on Table 1) but also had the highest incidence of all bacteria throughout lactation in all years.

This does not imply that we should only select treatments that are likely to be effective against Strep. uberis but does suggest that perhaps our treatment decisions based on timing since calving, age of cow, etc. are no longer appropriate and that we should be opting for treatments based on knowledge of the cause of individual mastitis cases. This will significantly vary between farms depending on the bacterial profile on each farm.

Mastatest uses minimum inhibitory concentrations (MICs) to determine sensitivity of bacteria to certain antibiotics. Put simply, MICs predict which antibiotics are likely to offer a higher chance of cure for an individual case.

HERE are some treatment indicators for Strep. uberis and Staph. aureus of our commonly used antibiotics used on dairy farms in the Manawatu, Benzylpenicillin (e.g. Intracillin 1000 MC), Cloxacillin (e.g. Orbenin LA), Lincomycin/Neomycin (e.g. Albiotic) and Tylosin (e.g. Tyloguard). Green indicates a higher chance of cure (lower MICs), blue, a moderate chance, and yellow, a lower chance of cure (higher MICs).

Benzylpenicillin MICs indicate a higher chance of cure in 87% of the Strep. uberis isolates compared to 6%, 37% and 12% of SU isolates for Cloxacillin, Lincomycin/Neomycin, and Tylosin respectively. For Staph. aureus, 58 % isolates were likely to have higher chance of cure when treated with benzylpenicillin compared to Cloxacillin (3 %), Lincocin/Neomycin (6 %) and Tylosin (0.5%). This does not suggest that the only antibiotic that will be effective is benzylpenicillin but does provide a strong indication that benzylpenicillin is the most appropriate choice of therapy Strep. uberis and Staph. aureus.

Other bacteria, including Strep. agalactiae/dysgalactiae, CNS, and other gram-positive bacteria show similar sensitivity profiles. However, with coliforms (the “poo bugs” e,g, E. coli, Klebsiella species) and other gram negative bacteria e.g. Serratia, pseudomonas species, results indicate that none of the antibiotics are likely to be effective.

In conclusion, technology like Mastatest will help us to better understand the cause of mastitis and perhaps the potential source of infection on individual farms. It also creates a platform to change our approach towards treating mastitis so that more appropriate first-line therapies are being used on a “case by case” basis. This is likely to lead to a reduction in antibiotic use over time and have a positive effect on maintaining their efficacy on farms in the future.

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