"We work really hard to get the cell count down, and then it just takes off on us, and we don't seem to be able to stop it."
The frustration was clearly obvious at a meeting with the farm team on this 300 cow farm.
"How can it spread so quickly?"
We had already established that the bacteria causing most of both the clinical and sub-clinical mastitis on this farm were cow associated bacteria which means that most of the spread of mastitis on this farm was occurring in the dairy during milking.
Countdown tells us that for every 100,000 cells/ml in a herd's Bulk Milk Cell Count (BMCC), there is likely to be about 10% of the herd affected.
So in this herd with a BMCC of around 300,000 it is likely that about 30% of the herd, or about 90 cows, will have a mastitis infection.
Generally, the most common method for bacteria to spread from one cow to another during milking is via a liner that has been contaminated by an infected cow.
We also know that when cups are removed from an infected cow, each liner that was on an infected quarter can potentially infect up to the next 5 - 6 cows that it is applied to.
Without making things too complicated, this means that the 300 cow farm we were talking to will have about 540 opportunities for spread of infection just via this method during each milking!
Now this does not mean that there will be 540 new infections at every milking, but it does mean that there is a huge opportunity for this farm to reduce both the number of opportunities for the spread of infection, and also the risk of each opportunity creating an infection.
However not all infected cows are equal.
Consider a cow with a clinical case of mastitis - it is likely that she will leave a far higher number of bacteria on the liner that was applied to that quarter than a sub-clinically affected quarter would.
This is why highly effective disinfection of cups after they have been on a cow with a clinical case of mastitis is so important.
Take into consideration that on this farm, 40% of teats were found to have some level of teat end damage (a not uncommon finding) and you can see that at each milking there is a regular occurrence of a high risk contaminated liner being applied to a high risk teat!
Obviously that means there is a very high risk of spread of infection.
For this farm, it means a change in some milking routines - high quality teat disinfection with good spray coverage, milking infected cows last, rigorous attention to disinfecting cups that have been on a clinical case, and a change of liner to reduce teat end damage are a few of the main items, plus a number of less important changes.
Now consider a farm of the same size with a BMCC of 60,000 cells/ml.
Without any extra action, the number of opportunities for spread of infection via a contaminated liner is only about 108 each milking.
If this herd has a relatively low level of teat end damage with only 15% of teat ends having some level of damage, the chance of a high risk contaminated liner being applied to a high risk teat is vastly lower, meaning a much lower risk of spread.
For this discussion we have assumed that for each infected cow only one quarter was infected. Naturally when multiple quarters are infected as is commonly the case, the risks become substantially greater.
What this all means is that if you combine a high risk of infection with a high number of opportunities for spread, spread will indeed be rapid!
But if you can control both, life is likely to be a lot easier - it is all in the power of the numbers!