As the dairy industry faces increasing pressure to reduce antibiotic use, on-farm milk culturing could help in achieving this end. Louise Hartley reports.
Laboratory milk culture to identify mastitis types is not routinely undertaken by dairy farmers here because of the turnaround time, cost and inconvenience of submitting samples.
But in a bid to fine tune mastitis treatment and reduce intra-mammary (IMM) antibiotic use, an increasing number of North American farmers are culturing milk on-farm, according to the University of Minnesota’s Dr Erin Royster.
Speaking at the iFeed Forum, organised by Dugdale Nutrition, Lancashire, Dr Royster said on-farm milk culturing involved farmers collecting milk samples from cows with clinical mastitis and culturing them on media plates. After a short period of incubation, farmers look at the bacterial growth on the plate to make a more informed decision about how to treat the affected cow.
“Not all cases of mastitis should be treated the same and not all cows deserve IMM antibiotics for a case of mastitis,” said Dr Royster.
Just looking at the milk sample alone was not sufficient, as although some samples may look different they could all be from cases of mastitis caused by E.coli.
Dr Royster said: “You cannot diagnose the pathogen causing mastitis based on what the milk looks like – the only way to do this is by a diagnostic test.
“Cows which have severe mastitis, meaning they are systemically ill, need systemic antibiotic treatment, but not all cows with mild and moderate cases deserve antibiotics – this is where milk-culturing comes into play.
“I would advise culturing samples from cows with clinical cases of mastitis and repeatedly high somatic cell counts to find out what is happening. At the very least doing bulk tank cultures monthly or quarterly will help detect any contagious mastitis in your herd.”
With several types of culture plates on the market, Dr Royster gave details of the plates made at the University of Minnesota which are part of the MN Easy Culture System.
“A cleanly collected milk sample is put on to a culture plate and incubated at 37degC (body temperature) for 18-24 hours.”
With the MN Easy Culture System Bi-plate or Triplate, a farmer could diagnose infections as Gram-positive, Gram-negative, ‘no-growth’, Staph aureus, or Contaminated. The Tri-plate can also be used to distinguish Staph species from Strep species.
“Plates range from about £1.26 to £2.10 and have a shelf life of 10 weeks. One limiting factor is herd size – if you only get a couple of mastitis cases per month you may not be able to use the plates fast enough. Working with other farmers or vet practices is therefore an option.
Recognising this fact, an increasing number of vet practices are starting to operate a milk culturing service for their farmers.
At Lanes Vet Group in Garstang, Lancashire, the practice has four farmers regularly taking milk from mastitis cows to be cultured.
Vet Mike Bevan said: “We decided to invest in the culture system as it was a much quicker and cheaper way to identify individual cows’ type of mastitis than sending samples away to a lab.”
Farmers collect a milk sample, bring it to the practice and usually within 24 hours they have a result to make a more informed treatment decision. Mr Bevan estimates that his farmers using the culture system have reduced their antibiotic use significantly.
The practice uses the Minnesota Easy Culture System’s bi-plates, and results can either show a gram negative or gram positive infection, or ‘no growth’ where there was no bacterial growth on the media plate.
“No growths usually make up 20-40% of clinical mastitis cases,” explained Dr Royster.
“In rare cases this could be because the cow was shedding bacteria at such low rates it was not found, or the mastitis was caused by mycoplasma which does not grow in the typical culture conditions.
“However, most no growth results are caused because the cow’s immune system cleared the infection by the time the sample was collected, or that there was no real infection.
“Significantly, if there is no bacteria, the cow does not need antibiotics because its immune system has been effective.
“If ‘no growth’ results make up 20-40% of clinical mastitis culture results, it could mean a significant reduction in antibiotic use.”
Gram negative results can account for up to 40% of culture cases. According to Dr Royster, gram negative bacteria typically elicit a rapid, robust host immune response. The cow is often able to cure the infection on its own without antibiotic treatment.
“At this point, culture results can be combined with cow history to help decide if it warrants antibiotics. If she is mid-lactation, healthy and suffering from her first case of mastitis (gram negative), you probably do not need to give antibiotics as the cow will cure it on her own.
“Antibiotics will be needed if the cow has a chronic infection, repeated cases of mastitis, several months of high SCC, or if she is immune compromised, such as a fresh cow which might be suffering transition stress.
“Significantly, if you combine the no growth and gram negative culture results, it could mean up to 50-80% of mild or moderate mastitis cases which do not need IMM antibiotics treatment.
“If you are uncomfortable not treating gram negative cases, you could consult your vet about using antibiotics which are more effective against gram negative infections,” added Dr Royster.
Gram positives can range from 15-70% of cases, depending on the dairy, and these cows almost definitely need treating.
These cases do not initiate a strong immune response, have a low rate of spontaneous cure and usually need extra help and IMM antibiotics.
When mastitis is identified, Mr Bevan advises his farmers to tube the cow once before bringing the sample to be cultured.
“The approach of not giving any antibiotics until you know the culture results has been evaluated in the USA but not in the UK, and I would advise giving an initial antibiotic tube,” he said.
Whether culturing on-farm or taking a sample to your veterinary practice, Mr Bevan strongly recommend working closely with your vet on treatment protocols.