Using antibiotics for viral illnesses such as colds simply doesn’t do anything to get rid of the cold, but it may create drug-resistant bacteria that are much harder to kill because they no longer respond to typical doses of regular antibiotics. Using large doses of less commonly used antibiotics may not treat the problem either or may simply cause more resistance.

Both bacteria and viruses are germs and can make dogs or cats (or people) sick. The two types of germs can cause diseases with similar symptoms, but each one has a different way of multiplying and spreading disease. Diarrhea, for example, can have several different causes, some of which are bacterial, and some are viral.

The difference between the two types of germs is that bacteria are living organisms and viruses are not. Bacteria are only single-cell organisms, but they are still considered alive. Viruses are essentially a collection of molecules that work together to replicate, so they only grow and reproduce after they’ve invaded living cells. Like all other species, their main goal is to replicate. Antibiotics fight living organisms – the bacteria – by killing them or stopping their growth and reproduction, but they can’t do that for viruses.

Your own immune system can successfully fight some viruses, but sometimes, such as after catching a common cold, viruses must simply run their course and there is little anyone can do to shorten the time frame. You can treat symptoms such as coughing and headache, but not the disease itself. Your pets are the same way.

Many diseases veterinarians see are only viral in nature and that’s the point at which you do not want to give your pet antibiotics. Such viruses include:

1. Distemper
2. Upper respiratory infection (rhinitis or bronchitis, such as kennel cough)
3. Viral infections in the eye (keratitis or conjunctivitis)
4. Parvovirus
5. Influenza
6. Rabies
7. Feline immunodeficiency virus (FIV)

Some bacterial diseases develop after a virus has made it easier for bacteria to infect. Examples include:

1. Upper respiratory infections
2. Eye infections
3. Pneumonia (after, for example, distemper)
4. Bacterial septicemia (bacteria in the blood) after parvovirus

Many organs can be infected with bacteria. Bacterial infections include:

1. Ear infections, usually combined with yeast infections
2. Leptospirosis
3. Tick-borne infections such as Ehrlichia, Lyme’s disease or bartonellosis
4. Clostridial diarrhea
5. Tetanus
6. Gingivitis or stomatitis
7. Pyothorax (infection in the chest cavity)
8. Respiratory infection, including pneumonia
9. Urinary tract infections in the bladder (cystitis)
10. Pyelonephritis (kidney infection)
11. Enteritis or Colitis
12. Bacterial conjunctivitis
13. Pyoderma (infection of the skin)
14. Deep wound infections

Taking antibiotics too often or for the wrong reasons can change bacteria so much that antibiotics don’t work against them. Giving antibiotics when they are not indicated can have two effects that can profoundly impact the health of your patient.

The first is killing healthy bacteria. Not all bacteria are bad, in fact most are probably beneficial. Beneficial bacteria protect from disease-causing bacteria, promote a healthy immune system, and help the body with metabolism. Bacteria in the gut are particularly important, however, bacteria are crucial to the health of many organs.

The second effect of antibiotic use is the potential for antibiotic resistance. This means that when a pet has a bacterial infection that could have been treated by antibiotics, the drugs may not be able to help end the infection; your pet will still be sick despite treatment. This can be true if the pet already has received antibiotics, even a different antibiotic.  One example is skin infections.  In some cases, pets can end up with methicillin-resistant Staphylococcus pseudintermedius (MRSP or MRSA). MRSA and MRSP are resistant to numerous drugs and adapt quickly to the immune system, so they are a big concern in the veterinary community. Another common organism that rapidly develops resistance is Escherichia coli, (usually called E. coli), which usually infects the urinary bladder.

Your veterinarian understands these risks and may suggest that antibiotics not be used if alternative treatments might be better. Examples include: 

1. Mild skin infections (bathing).
2. Ear infections (cleaning the ears).
3. Feline bite wound abscesses (opening the wound).
4. Gingivitis (cleaning the teeth).
5. Asymptomatic bacteriuria (bacteria in the bladder but your pet does not act like they have an infection).
6. Most diarrheas (Kaopectate in dogs or cats or bismuth subsalicylate [Pepto-Bismol] in dogs might be better).

Thankfully, you can help your pet avoid antibiotic resistance in several ways:

1. Let milder illnesses (especially those caused by viruses) run their course.
2. Take antibiotics for the entire time as prescribed by the veterinarian. Otherwise, the infection may come back stronger than it was, and it will be harder to get rid of than it was in the first place.
3. Don’t give your pet antibiotics longer than prescribed.
4. Do not use leftover antibiotics or save extra antibiotics for next time. The reason bacteria are cultured is to find out which antibiotic has the best chance of fighting the specific bacteria.
5. Don’t give your pet antibiotics that were prescribed for another pet or person.
6. Use the same caution about antibiotics for pets for everyone in your family. Antibiotic resistance affects people too.