Page 264 - The Veterinary Laboratory and Field Manual 3rd Edition

Microbiology 233

exist between unrelated chemicals (for example, accuratE dIaGnoSIS
erythromycin-lincomycin). Any recommendation to prescribe antibiotics
should be based on the clinical evaluation of the
EPIdEMIoLoGy oF rESIStancE animals under the care of the prescribing veteri-
The type of antibiotic resistance patterns seen in nary surgeon and, where possible, on the results
animals, especially farmed livestock, are largely of laboratory tests. When it is not possible to
affected by antibiotic exposure but will also vary make a direct clinical evaluation, the diagnosis
according to: should be based on knowledge of the farm epi-
demiological status and on ongoing sensitivity
• the size of the population of microorganisms testing. Antibiotic therapy should only be used
• pre-exposure prevalence of resistance genes if it will help to shorten duration and sever-
• the fitness of the selected population of ity of the infection or reduce risks of systemic
microorganisms in competition with other complications.
microorganisms present in the environment
which have not been exposed to antibiotics. antIbIotIc SEnSItIvIty tEStInG
Antibiotics should only be used when it is known
that the bacteria involved is likely to be suscep-
How to select an antimicrobial drug
tible to the therapy and that the correct dose
The selection of the appropriate antibiotic of the antibiotic can be given over a sufficient
treatment should take a number of factors into duration to be effective. However, in certain
account. The clinical experience of the clini- circumstances, for example, a disease outbreak
cian or veterinary officer attending the case involving high morbidity and/or mortality or
will influence the choice of antibiotic as will where the disease is spreading rapidly among
the availability of a suitable formulation of the contact animals, precautionary treatment might
drug for the animal(s) to be medicated. In many be started on the basis of clinical diagnosis prior
cases the attending veterinarian will prescribe to determining the likely efficacy based on labo-
a course of treatment before receiving micro- ratory tests. The effect of the drug in vivo (that
biology culture and sensitivity results from the is, in the animal) depends on its ability to reach
laboratory. In some cases, samples will not be the site of infection in a high enough concentra-
collected because testing may not be considered tion, the nature of the pathological process and
necessary. However, if samples are submitted to the capability of the host’s immune response to
the laboratory there will be an expectation that limit the infection. The response in vivo cannot
laboratory staff can provide advice to submitting always be predicted from the laboratory results
veterinarians and extension staff on the selec- in vitro due to the wide range of factors that can
tion and appropriate use of antibiotics. influence the host–pathogen interaction.
The choice of the right antibiotic to treat
an individual animal or a population should be trEatMEnt SELEctIon and EFFIcacy
based on the following: The choice of the right antibiotic for a given spe-
cies or disease is dependent on host and pathogen
• accurate diagnosis involved as well as the pharmacokinetic parameters
• antibiotic sensitivity testing of the available antibiotics such as bioavailability,
• treatment efficacy tissue kinetics and distribution and the drug half-
• duration and feasibility of treatment life to ensure that the selected antibiotic reaches
• drug withholding period. the site of infection at a sufficient dose.

Vet Lab.indb 233 26/03/2019 10:25