Guide to Pet Medications: Antibiotics

Antibiotics are substances that kill or inhibit the growth of bacteria. The first antibiotics were naturally-occurring chemicals derived from microbes like mold or other bacteria.  Today most antibiotics are semi-synthetic, which means that they are manufactured from natural substances that have been artificially modified, or they are totally synthetic.

These medications are fairly new to the field of medicine. Physicians have only been using antibiotics since the 1940s, when they were first used by the United States military to treat sick and wounded soldiers.

How Do Antibiotics Work?

There are three different mechanisms that antibiotics employ to work their magic. They can disrupt the integrity of the cell membrane (which breaks the bacterial cell open), they can prevent them from making proteins (which they need to do to survive), or they can interfere with bacterial DNA metabolism. 

To simplify this discussion, assume there are four basic categories of bacteria. These include gram-negative aerobic, gram-positive aerobic, gram-negative anaerobic, and gram-positive anaerobic. The composition of the cell wall is responsible for whether a microbe is termed gram-negative or gram-positive. Whether or not the bacteria grow well in the presence of oxygen determines its status as either aerobic or anaerobic.

These categories matter because the doctor chooses an antibiotic based on which type of bacteria is present. For example, many wounds or surgical infections are populated by gram-negative bacteria. Therefore, the veterinarian will choose a drug that is known to be effective against gram-negative strains, such as a fluoroquinolone antibiotic. If the bacteria are likely to be gram-positive, the doctor might choose a drug from the penicillin family because they are very effective at punching holes in gram-positive cell walls. 

Common families of antibiotics include penicillins, cephalosporins, fluoroquinolones, sulfonamides, aminoglycosides, tetracyclines, and macrolides. Common brand names of specific drugs you might recognize include Clavamox, Baytril, Cefa-Drops, and Flagyl. 

What Happens Next?

When prescribing an antibiotic, there are several questions that a doctor asks: 

• Is this infection treatable with antibiotics? Viral infections such as colds are not!
• What is the location of the infection?  Different drugs penetrate better or worse depending on the location in the body.
• What is the most likely pathogen?  This is the most important question because this is how the doctor chooses which drug to use.

Ideally, the doctor will take a sample from the infected site for culture in a laboratory to confirm the type of bacteria present. This isn’t always possible but is the best way to ensure that the correct drug has been prescribed. Your veterinarian might schedule you for a follow-up visit approximately two weeks later, with or without more laboratory tests.

What Should I Do if the Antibiotic Isn’t Working?

If the prescribed medication doesn’t seem to be working, call your veterinarian right away. The most common reason for antibiotic failure is that an incorrect drug/dose/frequency is being used. Other reasons include not giving the drug enough time to work (most will appear to be working within 72 hours), an animal with a compromised immune system (some drugs rely on the immune system to help clear the infection), or a biological barrier preventing the drug from getting to the site of infection (for example, poor blood flow).  

What About Antimicrobial Resistance?

The topic of drug-resistant bacteria is a controversial one.  Many in the medical community feel that antibiotics are currently overprescribed, which can contribute to the emergence of resistant bacteria. A 2001 human medical study reports that antibiotics were prescribed in 68% of acute respiratory tract visits but were unnecessary 80% of the time.  

Resistance occurs because when an antibiotic is prescribed, it only kills the bacteria that are sensitive to it. Unfortunately, there might be a few cells in the infection that are not sensitive to that drug. For example, if 99% of the bacteria are killed, the remaining 1% can flourish and propagate with no competition for resources. The result is a full-blown infection that is completely resistant to the originally prescribed drug. This new infection can be spread to other animals or humans, who now have fewer options regarding which drugs will work against this strain.

It is important to remember that antibiotics don’t cause resistance, they select for resistant strains. Antibiotics are not bad, they are only inappropriate if used inappropriately. 

How to Use Antibiotics Responsibly:

• Finish your pet’s prescription – don’t “save some pills for the next infection.”
• Follow the instructions on the label so that you are dosing your pet appropriately and at the correct frequency.
• Don’t pressure your veterinarian to prescribe antibiotics if she believes that the infection is caused by a virus!

What Are the Potential Side-Effects of Taking Antibiotics?

When dosed appropriately, most antibiotics are safe.  Ask your veterinarian to go over the side-effects of the particular antibiotic he or she is considering for your pet. The most common side-effect reported is that of gastrointestinal upset such as vomiting and diarrhea.