"I think we need a stronger penicillin, Doc....."
Not all antibiotics are penicillins - there are several 'families' of antibiotics. Some of the 'families' are a little related and work against bacteria in the same way. Others are a different race altogether with different methods of stopping the bacteria.
Overall five broad mechanisms allow antibacterials to work. I won't go into these in detail, as this isn't the information that a vet uses when choosing which antibiotic (or more than one) a particular cow needs.
Obviously, the choice of antibiotic is always easiest if a laboratory has confirmed exactly which bacteria is present and antibiotics to which it is already resistant. Giving an antibiotic before a sample has been collected decreases the likelihood of the lab being able to identify the bacteria involved to under 50 per cent, even if the antibiotic is the wrong one and isn't working in the cow.
While we wait for this information, we make an educated guess using the following to allow treatment to begin immediately:
1. Where is the infection.
Depending on whether an antibiotic is water or fat soluble, its pH (acidity and alkalinity), and its charge (positive or negative), different antibiotics are able to penetrate different organs. The first thing considered is where the antibiotic needs to target. One example of this is drugs that can cross the blood brain barrier. Drugs that are highly fat soluble penetrate better into the brain and spinal cord.
Others are great at achieving high lung concentrations, while another has almost no penetration into the lungs but has high levels of penetration in the liver and muscles. Identifying the target area for the antibiotic changes the choices available.
2. What bacteria does the vet suspect ù often this is a list of known possibles.
Generally, there are two main groups of bacteria and a few parasites like Coccidia and Crypotosporridia in addition to those.
Most bacteria are either gram positive (thick outer cell wall) or gram negative (thin outer cell wall).
For example, Staphlococcus aureus is a gram positive and Escherichia coli is a gram negative. So a different antibiotic might be chosen for each of these bacteria.
If the list of possible bacteria is large then a broad-spectrum antibiotic would be chosen over a narrow-spectrum antibiotic. Broad-spectrum antibiotics work against a larger range of bacteria.
3. When combining more than one antibiotic.
This is when the mode of action is considered. All antibiotics, regardless of the mechanism they use, are either bacteriocidal or bacteriostatic.
Bacteriocidal antibiotics directly kill bacteria. They usually need to act on an actively growing bacterial cell wall.
Bacteriostatic antibiotics do not kill the bacteria but they prevent it from being able to make the proteins and DNA to allow it to grow or replicate. This essentially puts it in a form of suspended animation.
Most bacteria replicate incredibly rapidly once inside the body, in order to overwhelm the immune system and cause infection. Preventing this growth allows the immune system to remove the bacteria before they can multiply out of control.
Knowing this is vital when combining different antibiotics. Generally adding a bacteriostatic to a bacteriocidal prevents the bacteriocidal drug from being able to work. In effect, they cancel each other out.
Other drug combinations, typically two bacteriocidals, work together additively and also increase the spectrum of activity.
There are exceptions to every rule as some antibiotics are bacteriostatic at low doses but bacteriocidal at high doses and dose rate can be used to ensure the combinations work optimally. Some combinations work in practice, when they shouldn't in theory, and know one is quite sure why.
4. Known local resistance patterns.
In different areas, the vet will know what types of resistance they tend to see and this is often geographical. Salmonella infection is a good example here. In one district where I worked, 90 per cent of the farm Salmonellas were sensitive to Oxytetracycline, yet in my current location 90 per cent of them are resistant to Oxytetracyclines.
However, even within one local area, two farms side by side can have opposite or different Salmonella resistance patterns and will require a different antibiotic ù this is best determined by submitting a sample for culture and sensitivity.
Before reaching for that leftover antibiotic, it is always worth a quick phone call to the vet to discuss what seems to be wrong with the cow and which antibiotics are on hand, before administering it. Not all antibiotics are compatible with each other when given at the same time. This is especially true of mastitis intra-mammaries when used with injectable antibiotics.
If in doubt ù ring the vet before you jab.
Until next time, good milking.D
*Sherri Jaques is a practising veterinarian and reproduction adviser in the West Gippsland region of Victoria. All comments and information discussed in this article are intended to be of a general nature only. Please consult a veterinarian for herd health advice, protocols and/or treatments that are tailored to a herd's particular needs.
Want to read more stories like this?
Sign up to receive our e-newsletter delivered fresh to your email inbox twice a week.