Antibiotic Treatment for Horses

When treating horses with antibiotics, veterinarians must consider several factors. These factors include determining what bacteria the antibiotic is to treat, the susceptibility and resistance of the bacteria to various antibiotics, the dose and frequency of administration, the type of administration that is most effective, how the antibiotic works to kill the harmful bacteria, and what other effects the antibiotic could have in the horse. Mark Papich, DVM, MS, Dipl. ACVCP, of North Carolina State University, discussed these issues in an in-depth presentation on "Current Concepts in Antimicrobial Therapy for Horses."

Appropriate antibiotic treatment for serious infections can prevent a chronic or life-threatening condition from developing. One of the challenges that veterinarians face is using these drugs appropriately so that bacteria does not develop a resistance. There have been new advances in bacterial identification and susceptibility testing. Traditionally, bacterial susceptibility (response of the bacteria to an antibiotic) has been tested with the agar-disk-diffusion (ADD) test, also known as the Kirby-Bauer test. This test only determines if an antibiotic will work against a certain bacteria, not how well it will work.

Laboratories now can measure the minimum inhibitory concentration (MIC) with the antimicrobial dilution test or the epsilometer test, which have improved antibiotic selection. These tests help veterinarians determine the dose and frequency of drug administration.

Papich explained how drugs can enter either the extracellular (outside the cell) space or the intracellular (inside the cell) space. Most bacterial infections are located extracellularly, and a cure can be achieved once concentrations of the drug reach adequate amounts in the horse's plasma or serum. Drug diffusion into an abscess or cavitated lesion might be delayed because the site of infection might lack an adequate blood supply and the geometry of the tissue might create a  challenge for drug diffusion. The presence of pus and fibrosis can also cause problems with drug diffusion. Such tissues include the central nervous system, eyes, and epithelial lining of the respiratory tract. For infections in these locations, drugs to consider are macrolides, fluoroquinolones, tetracyclines, trimethoprim, or chloramphenicol.

For intracellular infections, drugs must be carried into the cell or diffuse passively. Intracellular organisms include Rhodococcus equi, Chlamydia, Rickettsia, and Mycobacteria. Drugs to consider for these infections are rifampin, tetracyclines, and/or macrolides (erythromycin or azithromycin).

Certain factors can decrease antimicrobial effectiveness, and these should be considered before treatment is started. For instance, pus and necrotic debris, cellular material, foreign material surgically implanted in the body, an acidic or anaerobic (non-oxygenated) environment, or poorly vascularized areas (where blood flow might be restricted due to shock, sequestered bone fragments, and  endocardial valves) can present an extra challenge for the veterinarian.

Unfortunately, oral absorption is low for many drugs in horses. They are not absorbed as rapidly or to as great an extent as they are in small animals or humans, thus limiting oral administration of many drugs in horses. Drugs that are poorly absorbed by the oral route in horses include penicillins, and cephalosporins.

Antibiotics are either bactericidal (bacteria-killing), bacteriostatic (growth-inhibiting), or both, depending on the drug and the organism. The action for bactericidal drugs might be concentration-
dependent or time-dependent. If it is concentration-dependent, the amount of the dose is the determining factor. The higher the drug concentration, the greater the effect. If time-dependent, the drug should be administered frequently. For bacteriostatic drugs, the drug concentration should be kept above the MIC at the site of infection for as long as possible.

Veterinarians and researchers will continue to decipher the mysteries behind various bacteria and the role antibiotics play in the treatment of infections. As  further advances are made in veterinary medicine, veterinarians will be able to make better and easier decisions about the use of antibiotics in controlling equine diseases. The use of therapeutic drug monitoring (of antibiotics) is a relatively new area for veterinary medicine and rapid advances can be expected as more is learned about this area in equine medicine.

Infection Control Strategies

The importance of minimizing exposure to infectious agents and optimizing an animal's resistance to them cannot be overemphasized. In addition to vaccination and the use of antimicrobials (antibiotics), infection control strategies can help save the lives of horses whether the program is developed for a farm or veterinary clinic. During the General Medicine session, Josie Traub-Dargatz, DVM, MS, Dipl. ACVIM, presented "Infection Control Strategies for Horses in the New Millenium" about the need for a plan of action to prevent the spread of disease.

Preventing the spread of an infectious disease can save time and money. The economic impact of an outbreak is felt through expenses incurred while trying to control it and treat infected horses, lost  use of the animals, lost income for the operation if movement or value of the horses is impacted, and lost value of animals that die.

The first step in controlling the spread of infectious disease involves determining what diseases a program needs to control and learning the characteristics of those diseases (i.e., how they are spread).

Next, caretakers should group animals based on their infection status. For instance, horses which are sick from one disease should be grouped together, while horses at risk of shedding a virus (such as those returning from a show which have been exposed to other horses and possible pathogens) would be housed in another area. These animals should be monitored daily with someone recording rectal temperatures, thus making early detection of a problem easier. Healthy horses should be housed together, but as far away from infectious animals as possible.

A medical history of each horse should be kept. One should question whether the horse or the herd that any new horse came from could have been or was exposed to contagious disease agents such as Streptococcus equi. A physical exam might also be needed. In addition, screening tests might be desired if a horse has come from a herd that recently experienced an infectious disease outbreak, such as strangles. Once horses are grouped, a strict hygiene program should be followed in order to prevent the spread of disease among groups.

Communication between people handling the horses is essential. Some techniques for improving communication include:

  • Developing written and verbal communication between personnel;
  • Posting information on horses;
  • Creating a communication board;
  • Providing clothing to maintain optimal hygiene; and
  • Posting signs and providing materials for hand washing and foot dips.

An immunization program should be created for each horse on the property. It is also important to minimize stress factors (such as unnecessary transportation), optimize nutrition, and minimize treatments that make animals more susceptible to disease (such as the use of corticosteroids, which lower the immune response, or giving unnecessary antibiotics that alter the gastrointestinal flora and thus make the horse more susceptible to enteropathogens).

"It is important to remember that an infection control program might not eliminate infectious diseases in horses, but hopefully would limit the severity of the problem by minimizing the number of animals affected," said Traub-Dargatz.

Sunrise Session: Infectious Disease

"There's relatively little that we know about the immune system of the horse," said Peter Timoney, FRCVS, PhD, head of the University of Kentucky's Gluck Equine Research Center and one of the moderators of the session. "In the average life of the animal, it's exposed to multiple antigens, which they seem to be able to cope with."

Practitioners agreed that there has been a tendency to over-vaccinate horses, and this might cause more problems than it prevents. "We need to tailor a horse's program to what he really needs," said Josie Traub-Dargatz, DVM, Dipl. ACVIM, the other moderator.

Another vaccination concern is that the "strains" used to make vaccines have changed little over the years, perhaps leaving horses unprotected against new strains of the disease. "We need to make sure that the vaccine strains are relevant and consistent with those in nature," said Timoney. "(The vaccine companies) certainly are aware of the need to update strain content of their vaccines."

The group agreed that nothing is better for preventing disease--especially after the stress of travel--than good management. "You cannot expect vaccines to protect horses from management issues," said Traub-Dargatz.

The group also discussed West Nile virus (WNV), and veterinarians were curious about protection against it with the killed vaccine from Fort Dodge Animal Health (see article Quick Find #2827 at "There's no question that the most effective vaccine against WNV would be a modified live vaccine," said Timoney. He said we supported use of the killed vaccine, but follow the progress of two live vaccines under development. Timoney said WNV can thrive and multiply in a wider variety of conditions than Eastern equine encephalitis or Western equine encephalitis.

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