Fighting Infections with Antibiotics

With the increasing threat of new super bugs, consider using antibiotics to treat infections in your horse

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.

Giving Intramuscular Injections

If your horse needs an antibiotic that cannot be given orally, your vet may want you to do intramuscular injections yourself. This certainly saves on vet calls. Have your vet demonstrate the proper technique and site(s). The locations most often used are the neck, hindquarters, and chest.

Neck: The muscle is located in a triangle formed by the cervical spine, the front edge of the shoulder, and the nuchal ligament of the neck. Make sure you understand exactly where the safe area is located by having your vet show you.

Hindquarters: The middle of the semi-membranous muscle, the long “butt” muscle that flanks the horse’s tail, works well.

Chest: Use the paired pectoral muscles between the front legs when viewing the horse from the front.

Other areas can be used, but these are the most common. Injections should be rotated between sites, usually never administering more than 10 mL per site. If the coat is very long, injection sites should be clipped. If the coat is dirty, the area should be thoroughly washed and rinsed as a first step. And while some vets do injections “dry,” the usual practice is to wipe the injection area with alcohol on a gauze sponge to remove any surface dirt before injecting.

Injections are most painful for the horse if you are hesitant about getting the needle through the skin. A quick, forceful thrust can get the needle in position so quickly most horses don’t even react. (A good size horsefly bite hurts more!) It’s easiest to get the needle itself in place first, then attach the syringe. Syringes that slide rather than screw into place on the needle are preferred.

For your own safety, always have someone handling the horse who is experienced enough to control him and keep you safe. The handler should stand on the same side as you. The horse’s head should always be pulled toward you so that if he acts up, his body swings away from you. Always stand to the side when giving an injection, never directly behind or in front of the horse (in line with the legs).

Don’t Self-Prescribe for Your Horse
The decision to use antibiotics is a judgment call best left to your vet. There are other things to consider as well:

• The effectiveness of the antibiotic will vary depending on the type of bacteria involved. The best way to choose an antibiotic is to culture the infected area, grow the bacteria in a laboratory, then test the ability of several antibiotics to kill them. (This is known as sensitivity testing.) Your veterinarian also has been trained to know what types of bacteria are most common in various conditions (such as pneumonias, skin infections, uterine infections, etc.) and is the best person to decide which antibiotic to use.

• Dosing of antibiotics depends on the species and is not determined only by body weight. Different species will metabolize drugs in different ways. In other words, your horse may be 10 times your size, but that doesn’t mean he should necessarily get 10 times your dose.

• Many antibiotics routinely given to people in pill or capsule form can’t be taken orally by horses without risk of severe colic or death.

• Drugs safe for adult horses may not be safe for foals or elderly horses, which may have immature or compromised kidney or liver functions.

• Although modern antibiotics are certainly safer than their prototypes, toxicity can occur with overdosage. Underdosage results in poor effects and increases the chance of resistance developing.

What About Super Bugs?
“Antibiotic resistance” simply means that a type of bacteria is resistant to being killed by the antibiotic used. In some cases, this happens simply because the action of the drug does not influence that particular type of bacteria. For example, sulfa antibiotics interrupt a specific vitamin pathway that some bacteria just don’t have. In other cases, exposure of bacteria to an antibiotic, especially if the concentration isn’t high enough, will kill only some, prompting the rest to develop mutations that protect them from that antibiotic. Even when drug concentrations are adequate, genetic differences within the population of bacteria being treated may already mean that some are resistant to the antibiotic, even if they have never been exposed to it before.

Genetic mutations occur all the time. The more frequently genes divide, the more mutations there are. Bacteria typically divide every few hours. Anytime there’s an infection, we’re talking about millions-if not billions-of bacteria. It’s easy to imagine that a few of them may be resistant to the antibiotic by sheer chance alone. This is probably the case more often than not. But greatly reducing the number of bacteria is enough to let the body’s own immune system eliminate the rest, or the antibiotic-resistant bacteria may have some other type of weakness that means they won’t survive.

Antibiotic resistance sometimes occurs while a horse is being treated for an infection. If your veterinarian sees that the horse is not responding as he should be, the vet will switch to a different antibiotic.

The most serious type of antibiotic resistance occurs if the horse becomes infected with a “super bug,” a bacterial strain with resistance to most, and in rare cases, all, available antibiotics. Antibiotic-resistant bacterial strains were once confined to hospitals, but they have also “escaped” into the community at large.

Reducing the number of antibiotic-resistant bacteria boils down to responsible use of antibiotics. The bottom line is that every time a population of bacteria is exposed to an antibiotic, we are encouraging survival of the fittest. Use of antibiotics in food-producing animals is being phased out to prevent manure from those animals encouraging resistant bacterial strains in the environment. Veterinarians and physicians are also more aware of how we need to use antibiotics responsibly.

Antibiotics Used for Horses
Oral antibiotics routinely used in adult horses (except for some EPM drugs that only kill protozoa) are doxycycline and combinations of trimethoprim and a sulfa drug. Other types of oral antibiotics carry a higher risk of causing colic, severe diarrhea, and even death. This doesn’t mean that drugs that are safe to give orally are “weaker.” They’re just easier on the “friendly” gut bacteria and/or they’re better absorbed before they reach the large intestine. Some antibiotics given orally also produce gut problems because they are metabolized by the liver and cycled back into the intestines again in the bile.

Antibiotics that are given intravenously or by muscular injection can reach higher concentrations in the blood and tissues more quickly than oral antibiotics. This is because there’s a limit to how much the horse can absorb from the intestines, and intestinal absorption is a slower process.

Intestinal absorption also can be influenced by individual variations in how quickly contents pass out of the stomach and through the intestines. Absorption is probably influenced by the type of diet as well. For these reasons, your vet will sometimes prefer to start antibiotic therapy with intravenous or intramuscular drugs. Additionally, some types of infections need to be treated by antibiotics that are not safe orally, such as penicillin for strangles.

Antibiotics are also sometimes used locally. They may be used to flush out infections in joints, the uterus, or gutteral pouches, or to bathe wounds. Infections in the lower leg and feet are sometimes treated by “regional limb perfusion.” In these cases, a tourniquet is used to isolate the infected area and blood vessels supplying it are injected with antibiotics in high concentration. Special antibiotic-impregnated beads that release the drug slowly may be used in hoof infections.

Several “oldie but goodie” antibiotics are still being used in horses. They include tetracycline (IV only), penicillin (IV or IM-different forms for IV), gentamicin (IV or IM), ampicillin (IV only), amoxicillin (IM only), and erythromycin (IV only). The familiar thick, white form of penicillin you can get in most livestock stores is Procaine Penicillin G, the intramuscular form, which should never be used intravenously.

New antibiotics include the cephalosporin class (e.g., Ceftiofur) and the fluoroquinolones (e.g., Enrofloxacin). These antibiotics, which target many different species, were developed to overcome drug-resistant infections. But evidence suggests that use of these “broad spectrum” antibiotics may actually hasten the development of strains resistant to many antibiotics. They also may not work any better than older antibiotics like penicillin, gentamicin, combinations of penicillin and gentamicin, or tetracycline. In fact, penicillin is still the drug of choice for strangles infections (Streptococcus equi), and tetracycline works the best for Lyme disease. The decision to use the newer drugs should be left up to your vet.

Side Effects & Safety
Anytime a drug is used, it’s a matter of risk versus benefit, and there’s a possibility for drug allergy and adverse reactions. Antibiotics as a rule are very well tolerated. Some intestinal upset is the most common side effect. Much more rarely, bone marrow suppression or full blown allergic reactions may occur.

One fairly common reaction is called “penicillin allergy,” but it isn’t a true allergy. If intramuscular Procaine Penicillin G is accidentally injected into a blood vessel inside the muscle, it can cause a dramatic reaction that usually occurs before the injection is even completed. The horse begins to tremble, gets unsteady on his feet, and often collapses to the ground with seizure-like activity. Within a few minutes, the episode will be over.

Serious, even fatal, reactions may occur anytime a drug is accidentally injected into an artery rather than a vein, although specific information for individual antibiotics is not available since proper technique by veterinarians and technicians makes this highly unlikely to occur.

As already mentioned, use of the newer antibiotic classes increases risk of antibiotic-resistant strains developing. The fluoroquinolone class of antibiotics also has the potential to interfere with normal metabolism of cartilage and tendon tissues.

Are There Alternatives?
Optimizing nutrition is the single best way to minimize your horse’s chance of getting an infection, but they can still happen. Ancient cultures have been using natural substances for thousands of years. Many types of current antibiotics are actually based on naturally occurring chemicals. However, the safety and effectiveness of today’s antibiotics are far superior to the older treatments. Also, many alternative treatments that are touted as being proven to work by research, like garlic or silver, work only when in direct contact with the organisms at high concentration

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