The implications of antibiotic use in horses

Currently, the primary focus of antibiotic usage has centered on food-producing livestock industries. Although equine owners have avoided this scrutiny, it is important to understand the implications associated with antibiotic usage and be proactive to maintain access to antibiotic treatments in the future.

R. Kraig Peel

It is important to understand how antibiotics work. Antibiotics either kill or impede the growth of bacterial populations by disrupting cellular processes essential for them to live.

By impeding the growth of a bacterial population, antibiotics allow for the host immune system to get ahead of the bacteria and clear the foreign cells. A key point to antibiotic function is the lack of effect on viral infections. The lack of living cell components prevents the interaction of antibiotics with viral particles.

Viral infections require a host cell for replication, removing the targets normally identified by antibiotics. The bottom line is that antibiotics do not work on a virus. It is important to identify the cause of the illness prior to administration of antibiotics. The use of antibiotics on a viral infection can lead to increased antibiotic resistance.

Antibiotic resistance has been called one of the most pressing public health concerns. As of 2012, the World Health Organization reported 450,000 cases of multiple-drug-resistant tuberculosis.

Although resistance is a natural phenomena associated with the use of antibiotics, there are ways to help slow the adaptation of bacterial populations. Each use of antibiotic challenges the bacteria’s ability to survive.

Improper usage can select for resistant bacteria by eliminating competition for space and nutrients. Surviving bacteria may experience physiologic changes that allow them to resist antibiotic effects, creating more opportunity for growth of selected or resistant bacterial strains.

The FDA has made moves to regulate the judicious use of antibiotics within the livestock industry. The most recent of these efforts was the release of the Guidance for Industry #213. This plan is designed to phase out the use of medically important antibiotics used in food-producing livestock for the purpose of food production and to transfer the oversight of antibiotics to veterinarians.

Horse owners are not the primary focus, but access to antibiotics may require more involvement from veterinarians in the future. Proper management will help maintain the access to and effectiveness of available antibiotics.

Proper use of any medication begins with an accurate diagnosis. Diagnosing the problem and the cause is the first step to treatment, and often this process must involve a veterinarian and lab work or cultures to isolate an infectious agent.

With this information, the decision to use antibiotics is made more judiciously and allows for a more accurate match of the antibiotic type to the bacterial population of interest. It is also important to determine the dosage, route of administration and duration of treatment.

These decisions are important not only for their implications to antibiotic resistance but potential side effects associated with antibiotics and the overall effectiveness of the treatment.

Side effects are a primary concern when administering antibiotics to a horse. The three primary side effects of concern are:

Antibiotic-induced colitis
Anaphylaxis
Toxicity due to compromised organ function

Antibiotics, especially broad-spectrum, can target commensal bacteria often involved in digestion. Some people think all bacteria is bad. This is not the case in grazing animals. There are healthy bacteria that assist in the breakdown of cellulose and produce volatile fatty acids as a byproduct that the animal can use as an energy source.

Antibiotic-induced colitis may occur in horses when the digestive bacteria become destroyed. Disruption of normal GI flora decreases the resistance to pathogenic bacteria, the fermentation of carbohydrates, and the absorption of water and sodium, resulting in mild to severe cases of diarrhea.

Without resolution, colitis can lead to severe dehydration, sepsis, laminitis and death with an estimated mortality rate of 50 to 80 percent. Matching infection bacteria population with a narrow-spectrum antibiotic can help prevent the incidence of antibiotic-induced colitis.

Allergic reactions are always a risk when introducing foreign substances into the body. Careful observation during treatment may identify any signs of a reaction.

In addition to the general side effects associated with most antibiotics, specific reactions range from injection-site sensitivity to neurologic alterations that are often fatal. Consultation with a veterinarian for dosage, route of administration and any unique requirements is most effective at avoiding and preventing adverse reactions.

Antibiotics serve as an integral part of health management for horse owners. Although effective, they serve as only one portion of the puzzle. All aspects of management including nutrition, vaccination, hoof care and oral health tie into animal wellness.

Decisions to use antibiotics should be made only after an accurate diagnosis. It is the responsibility of the horse owner to seek veterinary advice in order to use antibiotics correctly, which will limit the opportunity for bacteria to develop resistance.