Facts at your fingertips - a guide to promoting milk bacteriology testing by Andrew Biggs of The Vale Vet Group

Bacteriology has moved on from testing the odd single sample simply to determine which antibiotic to use. More vets are realising the benefit of a total package for farmer clients, involving regular sampling and testing enough cows to get a representative answer, to monitor and control mastitis as part of a herd health plan.

It’s particularly relevant now because data from two years of samples sent to our lab by practices all over Britain, as part of Intervet’s Cephaguard® scheme, show that Strep. uberis is fast becoming the problem pathogen on farm (SEE CHARTS). It causes repeat clinical cases, persistent high cell counts and persistent infections which are often resilient to treatment. In addition, it creates problems with bactoscans. Research has recognised this bug is now commonly cow adapted, behaving in both a contagious and environmental manner.  Add to this the fact that it employs several survival tactics, and you get an insight into what we are dealing with.  Its changing and complex nature, makes Strep. uberis hard to deal with on-farm.  The old adage about knowing your enemy (ie. milk bacteriology) is worth remembering. At the practice we think the way forward is not only through developing better drugs for treatment but finding new ways to prevent mastitis. Our aim is to change management to try and restrict new cases of mastitis and target efforts in treatment and management. 

However, there are still many farmers who don’t see the benefit of undertaking bacteriology testing. Their classic grumble is that they tried it once and the results showed no culture, so it was a waste of time. Equally, there is the danger in the ‘we did cultures once, found bug X and so based all subsequent treatment programmes on that’ approach. Bacteria can change – and management needs to as well. Plus, you can’t just do one bulk milk and two cow samples and expect to solve a herd’s mastitis problem. If we have insufficient samples and struggle to read any significance into the results, the farmer soon gets disillusioned and won’t want to use lab sampling again.

The Cephaguard scheme involves filling in a questionnaire to give farm background, then taking 20 samples generally from five clinical cases and 15 high SCC quarters. Once the high SCC cows have been identified, using a rapid milk test (such as a CMT) will help identify the problem quarters to sample. This gives a good picture of what’s going on and, if this system continues on a regular basis, with today’s larger herds the greater numbers generate better data with significant trends and more robust answers. Long term, the best method is to regularly freeze clinical samples (prior to treatment) to build a database, so that when there is a mastitis outbreak, the last say 10 samples can be tested. High SCC samples can be collected fresh as these cows tend to be long term infected and freezing them might alter the picture as they may contain few bugs.

This scheme is subsidised, which makes it more attractive to clients and perhaps an easier ‘sell’ for us as vets? Apart from significant bacteriological data, an added feature is the individual management report which I write which interprets results and indicates what they mean.  I also suggest management techniques to help.  This report it is based on the information supplied in the questionnaire and any relevant history added by the farm’s vet. The more information we receive, the better the report. Ideally, vets would then get the opportunity to sit down and go through this report with their clients, highlighting problem areas and animals, looking at management and suggesting improvements.

Part of this includes explaining the relevance of no growths. Two common reasons are E. coli in a clinical sample and intermittent excretion of Staph. aureus in a high SCC sample. It’s another important reason to do enough samples to get something in addition to the no growths or contaminated samples.

Our results have traditionally shown a lot of Strep. uberis on farms in the south west of England in straw yard systems. And as far back as 1997, we knew it wasn’t just a housing problem – we would see a rise in Strep. uberis in July and August. Data showed a quite high incidence in heifers ‘camping’ under trees during hot weather, lying in dung and increasing infection levels. It seems able to survive on calving paddocks, though we’re not sure if it multiplies there too.

We do know, however, that the susceptibility to infection is higher in the dry period, with a dramatic increase in the first and particularly the last two weeks. Also the infection rate is higher during the first 75 days of lactation, compared with the rest of lactation.

We can make it better – often farmers seem to treat Strep. uberis with almost anything – although the constant challenge is maintaining the cure, particularly in chronic cases. Occasionally we simply see a sick cow, but a more common scenario is that when a cow is treated, she seemingly recovers but just as the milk is being put back in the tank, the mastitis returns. Cure rates are generally around 30% if treated early, but much less if allowed to become persistent.  Studies have shown that although 60% of infections last for fewer than 30 days, some 18% become chronic lasting over 100 days.

Strep. uberis is able to inactivate neutrophils, so they behave less phagocytically in its presence, as well as adhere to and actively invade bovine epithelial cells. Treatment therefore needs to be aggressive – often even more so than combination therapy, it frequently requires off-label extended treatment.

Our typical approach to a Strep. uberis problem protocol would be to introduce pre-dipping, concentrate on hygiene at housing and at pasture for both milking and dry cows and, of course, prompt identification of clinical cases with treatment and subclinical cases with treatment where appropriate. Where a cow-adapted strain is prevalent, leading to repeat cases and persistent high SCC cows, extended treatment might be six tubes at 12 hourly intervals. By extending treatment protocol, the drug is above MIC for a significant length of time. Another option is treating high SCC cows. Looking at the economics of the high SCC cow alone, it is difficult to justify on an individual cow basis as milk discarded isn’t paid back. However, when contagious bacteria (or contagious strains in the case of Strep. uberis) are involved, there is a cost benefit to the rest of the herd from reducing the risk of spread. Treatment of subclinical Strep. uberis will shorten the duration of infection and reduce that chance of spread.

Current research isn’t only looking at new drugs but, as with many diseases, a vaccine would be the holy grail. Strep. uberis doesn’t grow well in milk and tends to get easily flushed out but first we have to find out how all the strains work. At Compton, 250 mutants have been isolated – some are nasty, others would never be recovered because they would be flushed out before establishing. Strain variation in virulence appears to be linked to their ability to release nutrients from milk. These cow adapted strains can grow rapidly, adhere and internalise and therefore overcome the flushing effect of milking.

We hope the Cephaguard scheme proves to be a catalyst for vets and farmers to be involved in tackling Strep. uberis infections together as part of herd health planning. Taking regular samples (quarterly or annually) looking at what’s causing clinicals and high cell counts, and using milk recording management packages to monitor subclinical records for early warning, are all needed to monitor mastitis as a herd disease. We might deal with individual cases, but we need to stand back and see the bigger picture. The goal should be to prevent cows from getting mastitis, not just find new ways to treat them.