With the dairy industry facing increasing pressure to reduce antibiotic use, one alternative could be on-farm milk culturing. Louise Hartley reports.
After dry cow therapy, the treatment of clinical mastitis during lactation is the second biggest use of antibiotics on dairy farms.
In a bid to fine tune their mastitis treatment and reduce intra-mammary (IMM) antibiotics use, an increasing number of North American farmers are using on-farm milk culturing, 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 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.
She cited the picture of milk samples (pictured, below right). Although the milk samples looked very different, they were all 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) on the plate for 18-24 hours.”
“Plates range from about £1.26 to £2.10 each 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.
The Bi-plate in the MN Easy Culture System selects for gram negative and gram positive bacteria. Results can either show a gram negative or gram positive infection, or ‘no growth’, where there is no bacterial growth on the media plate.
“No growths usually make up 20-40 per cent of clinical mastitis cases,” said 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 of the 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 it is immune system has been effective. If ‘no growth’ results make up 20-40 per cent of clinical mastitis culture results, it could mean a significant reduction in antibiotic use.”
Gram negative results can account for up to 40 per cent of culture cases.
Dr Royster said: “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 deserves antibiotics. If she is mid-lactation, healthy and suffering from her first case of mastitis (gram-negative), you probably do not need to put antibiotics in to that cow as she will cure it on her own.
“Antibiotics will be needed if the cow has a chronic infection, repeated case 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 per cent 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 per cent 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.”