Why did the vet prescribe… antibiotics?


If there’s one thing that can make a vet cringe, it’s not blood, guts or abscesses. It’s the misunderstanding and confusion surrounding antibiotics. Antibiotics are one of the most common prescriptions we make, with the average vet prescribing 2-3 courses every day. But what are antibiotics, and how do they work?

What is an antibiotic?

Antibiotics are a group of drugs that fight bacteria. There are hundreds of antibiotic drugs, divided into several classes depending how they work. There are antibiotics that kill bacteria, and antibiotics that simply prevent them from reproducing themselves. This allows your body’s immune system has a chance to catch up. They all have different uses, depending what they do, what type of bacteria they work against, and where in the body they go after being absorbed.

Where do antibiotics come from?

The earliest known antibiotic was penicillin. It was discovered by Alexander Fleming, who was carefully growing bacteria in his lab. Unfortunately, he left the window open and some mould blew in, infecting the petri dish and killing the bacteria. He was initially cross that his experiment had been ruined. However, he soon realised the amazing properties of the mould he’d just discovered. Before penicillin was discovered, a simple cut could end up proving fatal. His discovery would change all that.

Why are bacteria bad?

Let’s say your dog gets a grass seed under the skin. Hitching a ride on that grass seed are thousands of microscopic bacteria. Let’s imagine for a second that there’s just one single bacteria on that grass seed. Bacteria don’t need other bacteria to reproduce – they clone themselves. And one bacterium can make an identical clone in just 30 minutes. Now there are two bacteria, and they both take 30 minutes to clone themselves, so now there are 4 bacteria. In just 24 hours, there are over 100 trillion bacteria. They will have spilled away from the infection site, into the blood, and into every major organ, causing septic shock and death. 

Of course, an infection doesn’t actually happen quite like this. Your dog has a wonderful immune system that is constantly on the lookout for hitchhikers. It will quickly move to destroy anything that isn’t meant to be there. But when their immune system is impaired, the bacteria is particularly good at avoiding them. Or if there’s a huge infection, the immune system can’t keep up. And although it won’t happen as quickly as within 24 hours, the infection can build and build until eventually the result is the same.

So, what do antibiotics do?

Antibiotics are there to give your dog a fighting chance. Some types of antibiotic kill the bacteria, others just stop them from cloning or damage them in some way. Either way, your dog’s immune system has a chance to catch up and get rid of the infection. There are a lot of different antibiotics, but as an example we’ll pick one of the most commonly used – amoxycillin-clavulanic acid. This antibiotic is often the first one your vet (or your GP!) will reach for in an uncomplicated infection.

Amoxycillin-clavulanic acid is an antibiotic made of two parts. Amoxycillin is the actual drug. It’s closely related to the original penicillin discovered by Fleming, and it’s been around since 1974. It works by stopping bacteria from building their cell wall. Without it, they can’t replicate- they leak fluid, and die. 

What is antibiotic resistance?

Unfortunately, this is an arms race, and despite the antibiotic amoxycillin working well for years, bacteria started evolving to fight it. We humans think of evolution happening very slowly. But when you can double in number every 30 minutes, and have 48 generations in a day*, it’s much easier to evolve. This is especially true when you apply a selection pressure- something that forces the change. Every time amoxycillin was used, all the bacteria died – apart from, eventually, the one that didn’t. And when that one cloned itself, it passed the useful DNA down to its progeny. 

Bacteria also have a way to teach other bacteria, by sharing useful bits of DNA even to unrelated bacteria. So actually, any DNA that gives a distinct advantage is quickly spread down the generations. Across to other families until the amoxycillin-beating knowledge is known by everybody. And the antibiotic stops working.

In the case of amoxycillin, bacteria started producing an enzyme called beta-lactamase, which destroyed part of the amoxicillin and made it ineffective. Luckily, somebody invented clavulanic acid. Although the clavulanic acid can’t harm the bacteria on its own, it stops the beta-lactamase from working, meaning the amoxycillin is able to work effectively. Unfortunately, beta-lactamase is only one of the ways in which bacteria can become resistant. New resistances are evolving every day. Multi-drug resistance is one of the big challenges vets and doctors are facing. Where some species of bacteria are resistant to most of the known antibiotics. Around 5000 humans die each year in England because their infections cannot be cured.

How do vets know if bacteria are resistant to antibiotics?

Vets can collect some of the bacteria onto a cotton swab. Then store it properly and send it to the lab. In the lab they will spread the bacteria onto an agar plate, which contains food for the bacteria and dyes to make it more obvious where the bacteria are growing. They then watch them grow, and find out what type of bacteria they are. 

They also do the same thing with a fresh agar plate. Except this one has antibiotic-impregnated dots of paper on it. If the bacteria don’t grow near the paper, the antibiotic is still effective. If they don’t notice the paper is there and continue to grow right up to it. You can tell that they aren’t damaged by the antibiotic. This is called a ‘culture and sensitivity’ test, and ideally should be done before every single antibiotic use to ensure that the right antibiotic is chosen every time.

How can I stop antibiotic resistance?

If you want antibiotics to keep working to protect your dog, your cat, and your family, then you have a part to play in stopping antibiotic resistance. Firstly, you should only give your pet antibiotics if prescribed by a vet. This means not digging through your medicine cupboard hoping to find an old packet you can use. Different infections need different antibiotics and different doses. Even if you think the infection is the same as last time, that’s up to your vet to determine. 

The second thing you can do to reduce the spread of antibiotic resistance is to use the antibiotics for the full course your vet gives you. Even if it seems to have worked. This is because some antibiotics will kill bacteria more slowly than others. If one or two survive, they’ll be stronger next time. 

Lastly, consider culture and sensitivity testing when your vet asks to do it. It costs a bit more than not doing it – but it costs a lot less than repeated courses of antibiotics that don’t work, and it’s essential for some infections, such as recurrent ear infections. 

So why did my vet prescribe antibiotics?

So, your vet will have prescribed antibiotics because they feel there’s a bacterial component to your dog’s disease, and that your dog is struggling to shift the infection on their own. They may also have been prescribed antibiotics preventatively, something we don’t do often but may do when an infection would be catastrophic (like after bone surgery), or when infection is very likely (like after a cat bite). Regardless of the reasons for prescribing, it’s essential that you follow your vet’s instructions, complete the whole course, and do further testing if recommended by your vet.

*A human generation is about every 25 years. So, it takes us 1400 years to have the same number of generations as bacteria do in a day. If you think about how far we’ve come since 620AD, the middle of the Dark Ages, you can see how quickly it’s possible for bacteria to evolve and change.

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