HAVE YOUR SAY – Q & A

Question from an Ontario farmer about a dairy farm issue:

“Why are the drugs we’re using to treat diseases like mastitis not as effective?”

Answer from Dr. Simon Dufour, professor, Department of Pathology and Microbiology, Faculty of Veterinary Medicine, University of Montreal, and scientific director of the Canadian Bovine Mastitis and Milk Quality Research Network:

This is a very short question, but it requires a very long answer. There are two main problems with antibiotic treatment of intramammary infections:

First is the mastitis pathogens. These are, in general, extremely specialized in invading and persisting in the mammary gland. To achieve this, they have developed all sorts of mechanisms that help them persist and evade the immune system and antibiotic treatments.

Some can penetrate and hide inside the immune cells that are responsible for detecting and killing them. Could you think of any better strategy? The cow’s immune system cannot “seek and destroy” them anymore, and they are also protected from antibiotics, which do not readily penetrate immune cells.

Others can switch to a latent form (like an opossum pretending that he is dead) so that the immune system will not be concerned about them anymore and antibiotics, which often aim at interrupting vital bacterial functions, won’t affect them.

In this latent form, bacteria are more or less frozen in time, like Walt Disney or Han Solo, and they don’t have to maintain vital functions. When conditions are favourable (i.e., when the antibiotic is gone), bacteria can migrate back out of the cells or come back from their latent form and cause problems again.

Finally, others are resistant to specific antibiotics, sometimes too many antibiotics (i.e., multi-resistance). So bacteria have many tricks, and they usually use them concurrently to achieve survival in the mammary gland.

To defeat them, both a strong immune system and an appropriate antibiotic treatment are required, and sometimes it is still not enough.

The second problem is the difficulty for most antibiotics to achieve high concentration in the mammary gland and to reach all of the bugs hiding in the different infected areas. The inside of a mammary gland is more or less like a grape.

When administering a tube of antibiotics intramammary, especially in a high-producing cow, it can be difficult to achieve a good concentration in all parts of the mammary gland. If an infection persists in a focal zone, it can later spread back to the whole gland.

To make the matter worse, many of the antibiotics that we could possibly administer in the vein or muscle to circumvent this problem don’t migrate well into the mammary gland (i.e., concentrations will be high and sufficient in the blood, but relatively lower in the milk.)

This is the mastitis puzzle. And this is why, for this disease, prevention is so important.  PD

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Dairy Farmers of Canada (DFC) is the national policy, lobbying and promotional organization representing Canada’s farmers. DFC works to support sustainable dairy production; facilitate solutions to provincial/national challenges; provide credible research of dairy products on a national basis; and create innovative ways to grow the market.