Practical Antibiotic Selection in Avian Patients
Don J. Harris, DVM
The practice of avian medicine in many instances has become a repetitious act of employing the same antibiotic over and over again for administration to all patients without regard for the true nature of the patient's condition. While certain antibiotics fulfill a broad range of applications, never is an automatic, unregulated approach to antibiotic administration acceptable. Different circumstances prompt different considerations, and situations frequently occur in which a specific class of antibiotic, or even no antibiotic at all, is indicated.
Ideally, antibiotic use should be directed by appropriate diagnostic measures. Laboratory investigation of suspect cases can consist of hematology, cytology, Gram's stains, culture & sensitivity testing, etc. When clinical circumstances dictate an immediate need for antibiotics, several factors direct their selection. First, because of protein binding, ionization, molecular weight, etc. different antibiotics reach different organ systems to varying degrees. Second, similar infectious organisms consistently invade certain anatomic sites. For example, Staph aureus is a common pathogen in skin infections but rarely implicated in gastrointestinal infections. The site of the suspect infection therefore to some degree suggests which antibiotics are more appropriate. The practitioner can use historical data to anticipate the identity of the infectious agents in order to predict appropriate antibiotic selection. Historical data also illustrates the characteristic bacterial sensitivity to commonly employed antibiotics therefore allowing a statistical prediction of antibiotic sensitivity patterns. Rarely is it necessary to initiate antibiotics without at least some laboratory guidance. The combination of the above considerations with even the most basic of laboratory data will increase the chances of appropriate antibiotic selection from the outset. Other factors such as drug cost, drug availability, drug toxicity, method of administration, etc. certainly influence antibiotic selection. Ultimately, the clinician must consider many factors before employing an antibiotic in a clinical situation.
Before one can even remotely consider which antibiotic to use in a clinical situation, the need for antibiotics must be justified. Not every patient will require an antibiotic, and in some cases injudicious use of an antibiotic may even prove harmful. One of the first questions to ask when confronted with avian illness is "might the etiology of this presentation be infectious?" Certain clinical presentations clearly suggest either an existing infection or the potential for one. Contaminated wounds, purulent sinus discharge, odiferous feces, etc. suggest the presence of pathogenic or opportunistic organisms that may be controlled with antibiotics. Fever, as manifested by increased warmth of the feet or beak, will sometimes be a sign of "antibiotic responsive" illness. When clinical signs are vague, laboratory data may suggest the presence of an infectious process. Elevated white cell counts accompanied by a heterophilia or monocytosis support the possibility on an infectious disease. Certain electrophoretic patterns warrant the consideration of disease caused by chlamydia, fungi, etc. It is sometimes argued that all "sick" birds should be "covered with antibiotics". While there can be legitimacy to this philosophy, the veterinarian's creed must always be remembered: "...above all, do no harm".
When laboratory diagnostics fail to illustrate whether or not antibiotics will be useful, they still can provide insight into the patient's physiologic status, helping to avoid inappropriate antibiotic selection. For example, laboratory evidence of compromised renal function would strongly contraindicate the use of aminoglycosides. Clinical and laboratory evidence of dehydration would suggest that sulfonamides be avoided. At the very least, proper triage can therefore help recognize conditions that might be aggravated by antibiotic administration.
When a particular antibiotic is being considered for a suspected infection, it is important to know if that antibiotic typically reaches therapeutic levels in the target site. The route of administration, protein binding, biotransformation, etc all affect the concentration of the antibiotic in circulation. Achieving satisfactory blood levels however does not guarantee that effective drug concentrations will be achieved at the site of the infection. The blood brain barrier is commonly referred to as a limiting factor in antibiotic distribution, but other factors may impede penetration of antibiotics into a particular site. The extensive network of sinuses and air sacs comprising the avian respiratory system creates much "dead space" that antibiotics may have difficulty reaching. Direct infusion or nebulization is sometimes helpful in bringing effective concentrations of antibiotics to particular sites of infection. Avian abscesses are usually densely inspisated and completely unavailable to antibiotic penetration. Surgical excision may be the only way to "cure" such an infection.
Some consistency is seen in the types of organisms that are isolated from particular avian anatomical sites. These observations also represent the microbiological findings in patients from the geographic area sampled. It is highly advantageous for veterinarians seeing avian patients to establish databases from which to establish statistical references. Not every case seen at Avian and Exotic Animal Medical Center displays a clear association between the clinical presentation and the microbiological isolate obtained through standard culture techniques. It is critical to remember that organisms such as Chlamydia or Mycoplasma play a significant role in the cause of illness in pet avian species, yet these organisms are not easily detected through routine in-house screening. In certain presentations, especially those with vague clinical signs, these other pathogens must be considered when determining which, if any, antibiotic to use. The data serves to provide the practitioner with insight into the common culturable organisms that must be acknowledged should the need for antibiotics be determined.
In reviewing this data certain trends in antibiotic sensitivity patterns (as determined by Kirby-Bauer disk diffusion) become apparent. The data provides a relative index of which antibiotics are most often effective for each isolate. While the Kirby-Bauer method of assessing bacterial sensitivity to common antibiotics is not standardized for avian species, it still provides the practitioner with a scale by which to judge different antibiotics relative to each other. When a particular bacterial isolate is suspected, this statistical history of antibiotic sensitivity patterns guides the practitioner in making a preliminary decision regarding which antibiotic to employ. The practitioner can (to a degree) extrapolate from a presenting condition to a logical choice of antibiotics appropriate for that presentation. By including knowledge gained from clinical experience the practitioner can then select a particular antibiotic that is most likely to address the primary infectious suspects in a typical presentation. Simple in-house lab work such as cytology and Gram's staining may offer significant insight into which etiology or etiologies to target.
Whenever an antibiotic is being evaluated for suitability in a clinical presentation, all incidental considerations such as nephrotoxicity, route of administration, cost-effectiveness, etc. must be evaluated in light of the patient's overall condition and the owner's willingness to accept the therapeutic recommendations.
In reality, never should one index or statistic be used to dictate an entire course of action. The practitioner should employ all practical diagnostic techniques to direct primary care before initiating any therapy. While the practice of anticipating bacterial isolates and predicting antibiotic sensitivity patterns will sometimes produce erroneous conclusions, having a statistical reference for making preliminary choices is still better than blind repetition. While certain antibiotics may prove to be useful more often than others, certain situations dictate a change in routine and a more appropriate course of action, even when the course of action is to do nothing.
"Sometimes it is best to dispense reassurance- reassurance that self-limiting diseases do not require treatment."
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