A Survey of Mastitis Pathogens in the South Eastern Australian Dairy Industry

 Neil Charman1, Rodney Dyson2 , Andrew Hodge1, Natalie Robertson1 and Sarah Chaplin2

 1 Pfizer Animal Health Australia, Parkville, Vic; 2 Dairy Focus, Tongala, Vic

(This is an edited version of the presentation given at the Countdown Mastitis Symposium in Melbourne in July 2012)


Knowing which bacteria are causing mastitis and high cell counts, gives an understanding of the source of the infection, how it is spreading, and helps to suggest treatment options.

To assist our dairy industry, Pfizer Animal Health asked Dairy Focus to co-ordinate a survey of mastitis pathogens affecting the South Eastern Australian dairy industry.

Thirteen veterinary practices enrolled 65 farms for the purpose of conducting this survey, and samples have been collected from 2986 cases of clinical mastitis and 1038 cases of subclinical mastitis between February 2011 and March 2012.

All farms enrolled in the survey had a herd size of over 250 cows, herd tested on a regular basis, and maintained an electronic herd recording system.

From the start of each calendar month, enrolled farms collected a milk sample from the first ten cases of clinical mastitis for that month.

In addition, on a single occasion, many of the farms collected a further 20-30 samples from a selected group of cows that displayed a high somatic cell count in their milk in mid-to-late lactation.


Clinical Mastitis Submissions

On the surveyed farms, Strep uberis is clearly the most common mastitis pathogen – in fact, more than half (54%) of the samples that returned a positive result were Strep uberis!

Staph aureus, E. coli & Strep dysgalactiae were the next most common bacteria, but at a much lower level than Strep uberis.

And, overall, about 90% of mastitis was caused by these four bacteria – we can probably think of these as “the big four major pathogens” in terms of bacteria causing mastitis in our industry.

Whilst a number of other bacteria were isolated, they were at relatively low levels.

A significant finding was that 39.3% of samples were either contaminated (16.1%) or produced no growth (23.2%).

Consequently, the lesson for industry in general is that about 40% of milk samples from clinical cases of mastitis fail to produce a positive result - this knowledge will assist advisers and farmers to communicate meaningfully about the interpretation of culture results, and to be able to identify when collection techniques on farm may be an issue needing attention.

Table 1 summarises the results –

 Table 1: Summary of Clinical Mastitis Culture Results


%    overall

% excluding No Growth & Contam

Streptococcus uberis 1013 33.0 54.3
Staphylococcus aureus 276  9.0 14.8
Escherischia coli 219 7.1 11.7
Streptococcus dysgalactiae 166 5.4 8.9
Corynebacterium bovis 48 1.6 2.6
Nocardia species 37 1.2 2.0
Klebsiella species 20 0.7 1.1
Serratia species 19 0.6 1.0
Pasteurella species 16 0.5 0.9
Arcanobacter (Actinomyces) pyogenes 15 0.5 0.8
Candida species/Yeasts 14 0.5 0.8
Streptococcus agalactiae 4 0.1 0.2
Pseudomonas aeruginosa 3 0.1 0.2
Salmonella species 3 0.1 0.2
Listeria monocytogenes 2 0.1 0.1
Other Staphylococcus spp 1 0.0 0.1
Other* 9 0.3 0.5
Contamination 495 16.1 -
No growth 713 23.2 -
TOTAL 3073 100 1865

*Other includes: Bacillus spp, Citrobacter spp, Gram negative bacillus, Hafnia alvei, Kluyvera intermedia, Kocuria kristinae, Pantoea sp, Prototheca sp, Raoultella ornitholytica


Clinicals website 


1. Stage of Lactation

The stage of lactation during which samples were collected appears to have had little influence on which of the four major pathogens were isolated during this study (Figure 1).

Strep uberis remained the dominant pathogen at all stages of lactation. No clear trends of increasing or decreasing isolation rates during the course of lactation were evident for Staph aureus, E. coli or Strep dysgalactiae.


Figure 1: Major Pathogens Isolated During Lactation



2. Geographical Region

Whilst Strep uberis was the dominant pathogen in all regions, it was less dominant in Gippsland than the other regions (Figure 2).

Also of interest was a difference in the proportion of Staph aureus & E. coli results in Northern Victoria, which had a higher proportion of E. coli results and a lower proportion of Staph aureus results compared to the other regions.

In fact, E. coli was the second most likely pathogen to be isolated in Northern Victoria.


Figure 2: Major Pathogens Isolated By Region



3. Clinical Mastitis by Parity

The frequency with which each major mastitis pathogen was isolated from cows of different parity was generally consistent for the four major pathogens (Figure 3).

Strep uberis was isolated most frequently from heifers in their first lactation, whilst cows in their third lactation had the lowest isolation rate for Strep uberis; however this isolation rate was still far higher than any of the other major pathogens.


Figure 3: Major Pathogens Isolated By Parity



Subclinical Mastitis Submissions

The pattern of pathogens in the subclinical samples was different to the pattern seen in the samples from clinical cases.

In the subclinical mastitis samples, Staph aureus was the organism most frequently isolated from subclinical mastitis samples at 41.8% of the positive samples (Figure 4), followed by Strep uberis at 31.2%.

E. coli was only cultured on 10 occasions (2.2%) from the subclinical samples.

The summary of subclinical mastitis culture results by region is represented in Table 2.

Table 2: Summary of Overall Subclinical Mastitis Culture Results


%    overall

% excluding No Growth & Contam

Staphylococcus aureus
189 17.5 41.8
Streptococcus uberis
141  13.1 31.2
Streptococcus dysgalactiae 36 3.3 8.0
Corynebacterium bovis 34 3.2 7.5
Eschericia coli 10 0.9 2.2
Other Staphylococcus spp
9 0.8 2.0
Nocardia species 8 0.7 1.8
Candida species/Yeasts 7 0.6 1.5
Pseudomonas aeruginosa 6 0.6 1.3
Serratia species 4 0.4 0.9
Streptococcus agalactiae 1 0.1 0.2
Other* 7 0.6 1.5
Contamination 439 40.7 -
No growth 188 17.4 -
TOTAL 1079 100 452

*Other includes: Bacillus spp, Citrobacter spp, Gram negative bacillus, Hafnia alvei, Kluyvera intermedia, Kocuria kristinae, Pantoea sp, Prototheca sp, Raoultella ornitholytica

The proportion of No Growth results for subclinical mastitis samples (17.4%) was broadly similar to the rate seen in the clinical mastitis samples (23.2%).

However, the frequency of contaminated samples from cows with subclinical mastitis (40.7%) was far higher than that seen in the clinical mastitis samples (16.1%).

As these were composite samples from all four quarters in one sample tube, it probably reflects the substantial increased risk of contamination when sampling four quarters versus only one.


Figure 4: Subclinical Mastitis Pathogens Isolated




Antibiotic Sensitivities

Comprehensive analysis of antibiotic sensitivities has yet to be completed, however a quick look at the data for one antibiotic revealed an interesting story.

One of the most commonly used intramammary antibiotics used in the Australian dairy industry for more than 30 years is cloxacillin – how did it fare?

Table 3: Cloxacillin Sensitivity in Clinical Mastitis Culture Results

Cloxacillin Sensitivity



Strep uberis

Strep dysgalactiae

Staph aureus



Resistant 0 1 0 1 0.07
Sensitive 1013  165 276 1454 99.93


Table 4: Cloxacillin Sensitivity in Subclinical Mastitis Culture Results

Cloxacillin Sensitivity



Strep uberis

Strep dysgalactiae

Staph aureus



Resistant 0 0 0 0 0.00
Sensitive 138  31 180 349 100.00


These are very significant results for an antibiotic that has been used extensively for mastitis control in the industry for so long.

This and other early results suggest that little resistance has developed to the antimicrobials used to treat mastitis in Australia.

This is a great story for the Australian dairy industry as it suggests that responsible use of antibiotics in the industry is not contributing to increased resistance.

Further Information

Further analysis of the results and data is continuing and it is the intention of Pfizer Animal Health to run information sessions in all dairy regions of Australia when this is completed.


Pfizer Animal Health and Dairy Focus gratefully acknowledge the assistance given by Gribbles Veterinary Pathology and the participating veterinary practices, dairy farmers and herd improvement organisations involved in this survey.


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