Written by Dr. Emily McNally and appeared in our July/Aug. 2016 Issue

At the first sign of a runny nose it can be tempting to automatically put your horse on antibiotics. We want our horse to get better soon and what harm can it do? Unfortunately, the over-administration of antibiotics in all species, horses included, can have a major impact on the future of medicine, both human and veterinary. The job of antibiotics is to help natural immune systems clear bacterial infections through the killing or weakening of the bacteria responsible. In recent years there has been a rise in antibiotic resistance in bacterial populations. A lot of the resistance is a result of using antibiotics inappropriately, leading to strains of bacteria that are no longer mitigated by classes of antibiotics that used to be effective.

Bacteria are living organisms and there are a myriad of ways that they can react and evolve in the face of drug treatment. Bacteria replicate quickly and have very short generation intervals leading to a high incidence of mutation in their DNA. At times this mutation can give them an advantage and keep them from being killed by a drug. To see how this can lead to widespread antibiotic resistance, consider this example. Let’s say there are 100 bacteria in a population. An antibiotic that targets and destroys their cell wall is administered. If there are two bacteria in the population who have a mutation in their DNA that gives them a stronger cell wall that isn’t destroyed by the antibiotic, they will survive while the rest of the bacteria around them die. These bacteria are able to replicate and pass on their mutated DNA to future generations. This propagation will occur even more quickly than usual because the resistant bacteria no longer have to compete for space or nutrients because all the non-resistant bacteria were killed. These two resistant bacteria can quickly become an entire population of resistant bacteria, meaning we can no longer use that antibiotic in treatment against that organism.

Antibiotics aren’t always called for in cases of equine illness. Unless there is evidence or a strong suspicion that an illness is caused by a bacteria, antibiotics should not be used. Antibiotics only work against bacteria and have absolutely no effect on viruses. When a horse has a fever with a mildly snotty nose, the culprit is often a virus such as Influenza. Giving antibiotics in a case of Influenza would be of no benefit and may contribute to antibiotic resistance.

Most antibiotics are relatively safe, however there are still side effects to be aware of. Horses have a natural ecosystem of bacteria in their gastrointestinal system that aid in the digestion of their food. Antibiotics can kill these “good” bacteria leading to colitis and diarrhea. When your horse is sick, further diagnostics, such as bloodwork, performed by your veterinarian can help diagnose if the cause is bacterial, viral or fungal and if antibiotic treatment is warranted in your specific case.

When the culprit is a bacterial infection, your veterinarian will often also elect to perform a culture and sensitivity to ensure the correct antibiotic is used in treatment. Certain antibiotics work best against certain bacterial organisms. To perform a culture, we swab the area of concern and incubate the bacteria collected, in a laboratory. The grown bacteria are then exposed to a variety of antibiotics to see which ones are effective against it. Although this takes some time, knowing that the antibiotic chosen is appropriate means the horse will recover more quickly and will be less likely to contribute to resistance. Obviously, there are some cases where the horse is very sick and immediate treatment required. In those cases the veterinarian makes their best guess as to which antibiotic to use.

Veterinary on a farmIf your veterinarian decides your horse needs antibiotics, it is very important to administer them exactly as directed by your veterinarian. The adequate dosage and timing of all antibiotics have been studied thoroughly and the veterinary recommendation has been proven to be effective. The goal in treatment is to create a steady stream of therapeutic levels of antibiotic in the blood so that the bacteria are constantly exposed to the drug of choice. Some drugs need to be given once a day to achieve this, while others need to be given up to four times per day. To give an antibiotic once a day generally means to administer once every 24 hours, twice a day generally means once every twelve hours, and three times a day should ideally be given every eight hours. It is crucial to also give the course of antibiotics for the entire time frame recommended. If your horse is prescribed antibiotics for two weeks, then be sure to administer  even if your horse appears clinically better. Stopping the antibiotic course too soon can not only lead to a relapse of the illness, but also directly leads to antibiotic resistance.

In conclusion, antibiotics are amazing medications to have in our arsenal in the fight against infectious disease. However, their use should be targeted against appropriate infections  and to be cavalier in their use has repercussions that reverberate across all aspects of human and veterinary medicine. As a quick reminder, prevention is often the best medicine. Make sure to follow proper biosecurity protocols by quarantining new introductions to your herd for two weeks and limit nose to nose contact with unknown horses while at horse shows and on trail rides. If you have questions about the use of antibiotics, call our office at 540-854-7171.