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Fighting Infections with Antibiotics

Your veterinarian likely has an arsenal of antibiotics at the ready, both oral and injectibles. Some are better than others for dealing with specific kinds of infections.

Because of fears about developing antibiotic-resistant strains of bacteria, antibiotics are under ever-increasing scrutiny today. This is a valid concern, but it's important not to lose sight of how critical these drugs are. Let's consider how antibiotics work and the role they can play in keeping your horse healthy.

Just What are Antibiotics?
Antibiotics belong to the class of drugs called antimicrobials, which also includes agents that can kill viruses, protozoa, and fungi. Antibiotics either directly kill bacteria or they inhibit their growth and multiplication.

Antibiotics that kill bacteria are termed bacteriocidal. Those that only inhibit the growth of these microbials are called bacteriostatic. Both can be effective, but as a rule the bacteriocidal drugs work more quickly and need to be given for shorter time periods than the bacteriostatic antibiotics.

Many of the antibiotics we use are similar or identical to chemicals that bacteria produce themselves. They excrete these chemicals to kill off their competition. In fact, this is partly how the "friendly" bacteria in the bowel protect from infections like Salmonella.

When are Antibiotics Needed?
The world is not a sterile place. There are about 4 million bacteria sitting on 1 square inch of your horse's skin. The respiratory tract and digestive tract also are teeming with bacteria. Most bacteria are not harmful and will not invade healthy tissue. The bacteria that cause disease normally are crowded out by the huge numbers of harmless bacteria.

Dose of Wisdom

  • Use antibiotics only as prescribed by a veterinarian.
  • Follow instructions on dosage, route, and how long to continue antibiotics.
  • Avoid preventative use of antibiotics unless specifically instructed by your veterinarian.
  • Don't assume that pricey, "new" antibiotics are better than the tried-and-true older drugs.
  • Store antibiotics as recommended on the label.


Antibiotics are indicated in two general situations. The first is if your horse is directly infected by a disease-causing bacteria (such as a strangles infection that gets into the bloodstream). The other is if the protective barrier of skin, intestinal lining, or the respiratory tract has been damaged, allowing normally harmless bacteria to get in. Examples of this would be an infected wound or a bacterial pneumonia that develops after a viral infection.

In some cases, your vet may decide it is wise to use antibiotics prophylactically, as a prevention against infection. This may be best if your horse has an extensive wound, or one heavily contaminated by dirt or manure. Viral respiratory infections are not treatable with antibiotics, but your vet may decide to add them if symptoms worsen after a few days, or if the nasal discharge changes to a yellow color.

There are also times when antibiotics should not be used. When infections are confined to a single location and forming an abscess, the horse's body has built a thick layer of tissue around the infection that keeps the bacteria from spreading in the tissues or getting into the bloodstream. Common examples of this are a hoof abscess, a strangles infection in lymph nodes, and a pigeon fever abscess. In these cases, antibiotics don't get through the abscess wall in high enough concentrations to actually kill the organisms. But they might get in enough to slow down their multiplication. If this happens, it just takes longer for the abscess to open and drain.

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