Antimicrobial therapy: regimen selection (Proceedings)


Antimicrobial therapy: regimen selection (Proceedings)

Nov 01, 2010

Recently, the use of antimicrobials in food animals has been scrutinized by the general public (see report on CBS,, by federal legislators (see AVMA issues brief at, and by public health organizations (see Infectious Diseases Society of American statements at Some of these concerns relate to the use of antimicrobials as growth promotants, while some relate to the use of antimicrobials in food animals in general. Regardless of the focus of the scrutiny, it is not likely to go away anytime soon. Food animal producers and veterinarians must get the message out about how they use antimicrobials appropriately, and they must also provide their own scrutiny to abolish imprudent practices.

Traditionally, we have assumed that if a bacterial pathogen is "susceptible" to an antimicrobial, we just use the dose on the bottle or in a formulary, and the infection will be eliminated. The increasing incidence of "resistant" pathogens, i.e., pathogens requiring high concentrations of antimicrobials such that they become untreatable, has focused attention on identifying ways to reduce the selection for resistant organisms. In addition, we are beginning to recognize more and more frequently that the ecology of bacteria is very complicated and that the use of an antimicrobial in one animal can affect the non-targeted bacteria in that particular animal as well as in any other species of animal (including humans) that come into contact (direct or indirect) with the treated animal. This understanding compels us to use antimicrobials only when needed and only in a manner which is most likely to result in therapeutic success – in other words, balance the benefits and risks of therapy.

The American Veterinary Medical Association has published and disseminated a set of principles which can be used to help guide decision-making about antimicrobial therapy. Those principles are listed below:
      - Preventive strategies, such as appropriate husbandry and hygiene, routine health monitoring, and immunization, should be emphasized.
      - Other therapeutic options should be considered prior to antimicrobial therapy.
      - Judicious use of antimicrobials, when under the direction of a veterinarian, should meet all requirements of a veterinarian-client-patient relationship.
      - Prescription, Veterinary Feed Directive, and extralabel use of antimicrobials must meet all the requirements of a veterinarian-client-patient relationship.
      - Extralabel antimicrobial therapy must be prescribed only in accordance with the Animal Medicinal Drug Use Clarification Act amendments to the Food, Drug, and Cosmetic Act and its regulations.
      - Veterinarians should work with those responsible for the care of animals to use antimicrobials judiciously regardless of the distribution system through which the antimicrobial was obtained.
      - Regimens for therapeutic antimicrobial use should be optimized using current pharmacological information and principles.
      - Antimicrobials considered important in treating refractory infections in human or veterinary medicine should be used in animals only after careful review and reasonable justification. Consider using other antimicrobials for initial therapy.
      - Use narrow spectrum antimicrobials whenever appropriate.
      - Utilize culture and susceptibility results to aid in the selection of antimicrobials when clinically relevant.
      - Therapeutic antimicrobial use should be confined to appropriate clinical indications. Inappropriate uses such as for uncomplicated viral infections should be avoided.
      - Therapeutic exposure to antimicrobials should be minimized by treating only for as long as needed for the desired clinical response.
      - Limit therapeutic antimicrobial treatment to ill or at risk animals, treating the fewest animals indicated.
      - Minimize environmental contamination with antimicrobials whenever possible.
      - Accurate records of treatment and outcome should be used to evaluate therapeutic regimens.
While these principles do not specifically address regimen design, they will help when deciding which drug to select, and when NOT to use antimicrobials. In order to develop a complete regimen, which includes dose, frequency, duration, route of administration, as well as withdrawal time in the case of food animals, the following list of questions will need to be addressed.