At the herd level, culturing milk samples is a really useful practice. The type of mastitis being dealt with can greatly influence control and prevention strategy.
Let's say for example that a farm finds out it has a Staph aureus problem. This bug likes to hide in infected udders and is contagious, spreading from cow to cow at milking time.
To control it, the farmer needs to find the infected cows and either treat or get rid of them, using an antibiotic that works on this bacteria. Even with appropriate treatment, cure rates can be low.
On the other hand, if the farm has a Pseudomonas aeruginosa issue, we know that this type of bug likes to hide in contaminated water and hose linings. So to fight it, the farmer will need to replace cracked old linings and check the water supply. Sometimes we find out that a milker has been warming up intramammary tubes in warm water before putting them in teats - stopping this practice would be the quick first step towards solving the problem.
So different bugs on the farm will all have different stories with different endings. In an outbreak situation (or any situation), it's a good idea to know what is being dealt with so as to not waste time chasing the wrong thing.
Cow mastitis cultures Using milk culture to investigate herd mastitis outbreaks has been done for decades. Usually, 20 or more samples are taken from selected cows, the courier called, the milk sent away to the laboratory and then wait for the results. While the milk is in transit, everyone hopes that the bugs being looked for don't die and that contaminants don't grow.
But now with new advances in technology, many vet practices can do milk cultures in the clinic. Instead of sending the samples away to the city and having to wait several days for the result, samples can be turned over in a 24-hour period.
Why is this important?
Thanks to this quick turnaround, we have the ability to delay treatment until we're sure that the cow needs antibiotics. This only applies to mild cases (changes seen in milk) and moderate cases (changes seen in milk and udder). Severe cases where the cow is sick (has a fever or is not eating or is depressed) should still be seen and/or treated immediately.
By only treating cases which we know will respond to antibiotics, we can: Save money on drugs. Throw out less milk. Have fewer cows on the bucket - reducing risks of residue violations and time wasted on unnecessary treatments. Use antibiotics in a responsible way that reduces the risk of antibiotic resistance.
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How do culture results change our approach to treatment?
The reduction in antibiotic use will depend on the types of bugs commonly seen on the farm.
Research suggests that mastitis caused by gram-positive bacteria usually responds well to intramammary antibiotics. These include bugs such as Streptococcus uberis, Streptococcus dysgalactiae, Streptococcus agalactiae and coagulase-negative Staphylococci species.
Mild and moderate mastitis caused by most gram-negative bacteria, however, such as E coli and Klebsiella species, tend to self-cure. Using intramammary antibiotics on these cows makes little difference to cure rates, and poses a risk of introducing new infections during administration.
We sometimes see problem bacteria like Serratia species and non-bacterial agents like Candida species (which is a yeast). These cases often have poor outcomes, and will not respond to antibiotic treatment -- there is little value in using them in these instances. Supportive treatment like anti-inflammatories may have a bigger impact.
Finally, we can also identify cases where there is no bacterial agent. If the samples do not grow bacteria, there is little use in treating with antibiotics -- it may be that cow is not actually infected, or else the cow is likely to self-cure on its own and/or with stripping. We have had some farmers treating a mysterious mastitis infection in heifers with trisoprim. Culturing no growths gave them the confidence to stop treatment - and all 13 of the untreated heifers were perfectly normal.
In short, in-house mastitis culture can be a great way to save money and reduce our usage of unnecessary antibiotics. Anyone who thinks there might be a place for in-house mastitis culture in their management system, should give their local vet a call about it to find out more.
*Ee Cheng Ooi is a dairy veterinarian and fertility researcher in Northern Victoria.
All comments and information discussed in this article are intended to be of a general nature only. Please consult the farm's vet for herd health advice, protocols and/or treatments that are tailored to a herd's particular needs.