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  • Writer's pictureRhiannon Handcock

Mastitis and Antimicrobials

Updated: Apr 11, 2020

I had the opportunity to attend one of the three days of the Dairy Cattle Veterinary Conference in Queenstown in June 2019. There were so many interesting and informative talks so I have attempted to best summarise a handful of the talks over a couple of research summaries.

Part One is all about mastitis and Part Two is about reproduction.

A big theme throughout these talks was the NZVA's proposal to stop using dry cow therapy (DCT) over the whole herd ("blanket" DCT) and from 2020 to only use DCT for the treatment of cows with existing intramammary infections.

Below are 4 papers from the conference.


1. Selection for DCT using the Rapid Mastitis Test

Presenter: Scott McDougall

Approximately 30 – 40% of New Zealand dairy herds do not herd test. This means there is no cow-level information on somatic cell count (SCC) available for these herds. Traditionally, these herds would likely use blanket DCT over the herd at dry-off to treat mastitis. As blanket DCT is no longer advised, another option is required .

This study investigated the possibility of using the Rapid Mastitis test (RMT) to decide which cows would get DCT compared with using herd test SCC information.

Cows from 4 herds were split into 2 groups:

  1. DCT treatment based off herd test SCC – threshold of >200,000 cells/ml – all 4 quarters were treated.

  2. DCT treatment based off RMT – positive test was greater than 0 – only positive quarters were treated.

All cows were infused with internal teat sealants (ITS) at dry-off.

The data were analysed based on the number of quarters that were treated, not the number of cows that were treated.

There were less quarters treated with DCT based off the herd test SCC (26% of quarters), compared with RMT score (64% of quarters). As a result, less antimicrobials were used in the SCC group (2.12 mg/kg LWT) compared with the RMT group (3.47 mg/kg LWT). This equates to approximately 50% more!

The RMT group had a higher cure rate (95% vs 90%) and a lower new infection rate (3.7% vs 5.4%) compared with SCC group.

The authors concluded that selecting cows for DCT based on RMT resulted in a better biological outcome but used much more antimicrobials than selection based on herd test SCC. Hence, for herds that do not participate in herd testing, using RMT to select cows for DCT at dry-off is a possible option.

Interestingly, the authors observed that teat seal alone (i.e. no use of antimicrobials) still had a 85-90% cure rate.


2. Selective and deferred treatment of clinical mastitis

Presenter: Andrew Bates

This next study compared treating mild to moderate cases of mastitis with either:

  1. non-selective treatment - procaine penicillin administered immediately

  2. selective treatment - after 24 hours to allow for detection of the pathogen and it's antibiotic sensitivity using Mastatest (Mastaplex Ltd, NZ)


  • is a rapid bacteria and antibiotic test for on farm

  • tests for the presence of bacteria in a milk sample and which bacteria it is

  • tests which antibiotic might work for the bacteria identified

For the selective treatment group

Quarters from which there was no growth or gram-negative bacteria were not allocated any treatment.

Quarters with growth of gram-positive bacteria were treated according to the sensitivity results of the Mastatest test.

The antibiotic with the highest sensitivity value was used for treatment:

  1. procaine penicillin or

  2. cloxacillin or

  3. lincomycin and neomycin

The cure rate for Staph. aureus was similar for the selective and non-selective groups.

Likewise, the cure rate for Strep. uberis was also similar for the two groups.

The figure below is of the distribution of cure rate for Strep. uberis. In red is the non-selective group and in blue is the selective group. The dashed lines is the predicted median cure rate for each group. As can be seen in the figure, the selective group (blue) appears slightly superior, however statistically they were not different (lots of overlap between the red and blue shading).

Figure 1. Distribution of bacteriological cure rate for quarters infected with Strep. uberis. Red shading indicates non-selective, blue shading selective and dotted vertical lines the predicted median.

At the end of lactation there were also no differences in the SCC at final herd test.

Furthermore, the non-selective treatment used more antibiotics than selective treatment for a similar biological outcome.

I will leave you with their concluding paragraph as I think it is summarised very nicely.

"This study suggests that deferred treatment of mastitis using Mastatest to identify the intramammary pathogen and its antibiotic sensitivity can reduce antimicrobial use for mastitis with no loss in bacterial or clinical cure and with no observed effect on individual cow somatic cell count in the current lactation."


3. Detection of Staphylococcus aureus using herd test milk samples

Presenter: Rosemary Thresher

Staphylococcus aureus is one of the most common pathogens that cause mastitis in cows in NZ. It is a contagious bacteria that can present as clinical mastitis. It often re-occurs and can be difficult to treat. What's more, it can also reside in the udder for lengthy periods as a subclinical infection.

Management of Staph. aureus infection aims to:

  1. reduce disease incidence

  2. prevent spread to uninfected cows

  3. minimise mastitis related costs, and

  4. maintain milk quality standards for milk supplied to dairy processors

At least for the first 2 points, the identification of the infected (clinical AND subclinical) animals is a very important step in an effective management programme, however, detection of Staph. aureus cows can be difficult.

The standard approach to diagnose Staph. aureus cows is to culture milk samples using an aseptic technique from cows presenting with clinical mastitis. To estimate the prevalence of Staph. aureus in a herd, aseptic sampling of a large number of cows needs to be completed which is very time consuming and expensive!

A signal of Staph. aureus infection could be a raised SCC count at repeated herd tests, in addition to an understanding of the prevalence of clinical cases of Staph. aureus on-farm. Herd testing is typically completed 4 times per season on a farm and can be used by the Animal Health Lab at LIC for diagnostic testing.

Although herd test samples are not aseptic samples, LIC has found that they can be used to indicate the presence of Staph. aureus as an aid to estimate the prevalence in the herd and identify cows that may be infected (click here to find out more). Additionally, the use of herd test samples means many cows can be tested in a single day.

Compared with a sample taken using the foremilk and aseptically, using herd test milk had a similar specificity and a slightly lower sensitivity of 70% (see table below).

If sensitivity and specificity confuses you (like it does to me!) click here for a great little explanation.

Table 1. Sensitivity and specificity of detection of Staph. aureus using an aseptic sample of foremilk compared with herd test milk.

There are several explanations for the difference between the two:

  • herd test milk is a sample of all quarters, so infection in a single quarter is diluted

  • foremilk contains a higher bacterial load, so the aseptic sample is expected to have better sensitivity

  • milk volume may be lower in the infected quarter, and

  • sampling days were different, so shedding of bacteria could differ

The results are reported to the veterinarian as one of three categories:

  1. detected

  2. not detected and

  3. suspect infected animals.

The third category are cows that are probably infected with Staph. aureus, but the levels of bacterial DNA present are too low to be certain they are infected.

This category would likely contain cows that had residual milk from the previous cow in her sample, this is not a big problem for most herds unless the herd has high prevalence of Staph. aureus.

Overall, using herd test samples is a handy way of identifying Staph. aureus cows, with a sensitivity of 70% and the additional category of "suspect cows". Added bonus being that the sample is already being taken for milk production analysis, so no extra work!


4. A survey of veterinarian's experience with internal teats sealant administered without concurrent antibiotic dry cow therapy to lactating dairy cows at the end of lactation

Presenter: Scott McDougall

The final presentation on mastitis that I will summarise are the results of a survey on teat sealing.

Infusion of bismuth subnitrate-based internal teat sealants (ITS) on their own has been shown to reduce the risk of new intramammary infection over the dry period compared with no treatment at dry-off.

Given the concerns over antimicrobials (as discussed at the beginning of this page), the national mastitis control programme (Smart SAMM) promotes that cows with a low SCC and no history of clinical mastitis be treated with ITS-alone at dry-off instead of ITS with DCT.

In research studies on the use of ITS alone there have been no cases of mortality after ITS administration, however... there have been reports of morbidity (mastitis infection that was not there before) and in some cases mortality following ITS-alone in the field.

It is not known how many cases occur, hence a survey was undertaken of members of the New Zealand Society of Dairy Cattle Veterinarians in an attempt to define the incidence of morbidity or mortality observed in the 2018 dry period associated with ITS alone and to identify risk factors.

A total of 28 practices responded, which serviced 2,080 herds (approximately 18% of dairy herds in NZ). Among herds using ITS-alone, 13% of herds experienced some morbidities and 7% of herds experienced some mortalities.

31 herds where some morbidities or mortalities occurred provided cow-level data. In these 31 herds; 12,316 of 20,085 cows had been infused with ITS-alone. Of these, 3.7% had morbidity, and 1.5% died.

The average morbidity ITS-alone infused cows within the herd was 6.3% (between 0% and 32.9% of the herd), and mortality was 2.0% (between 0% and 32.1% of the herd).

It is likely that veterinarians servicing herds that had experienced problems after ITS may have been more likely to complete the survey than those that did not experience problems. Therefore, it is likely that the national average morbidity and mortality rates for ITS are lower than these estimates suggest.

One of the risk factors identified was time spent being trained for ITS infusion. Those that received less than 1 hour of training had herd morbidity nearly 3 times higher than those that received more than 1 hour of training.

"This suggests that formal on-farm training of personnel involved with infusion of ITS-alone should be undertaken and documented."

From a farmers perspective, any issues after administering ITS need to be thoroughly followed up with your vet. Also very important to make sure that anyone administering ITS be well-trained and preferably this be documented by your vet.

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Further, you may enjoy one of these summaries...


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