Making the most of culture and susceptibility testing

In this article, Emma Chedgey, a veterinary surgeon at Lisadell Equine Hospital in Ireland, describes the importance of culture and sensitivity testing in reducing the use of antimicrobials. The use of C&S and AST was briefly described in step 2.3, where we explored the use of these tests in de-escalation and delayed prescribing.

Are antimicrobials indicated?

This is the first major question we ask every time we prescribe a treatment. Is a bacterial infection present? Are antimicrobials likely to improve the outcome of this case?

SAA is a useful tool to distinguish diseases with an infectious component, but it is non-specific. Bacterial culture is gold standard, and, in many cases, treatment can wait until culture is available. For example, uterine swabs/washes from oestrous mares or a tracheal wash from a coughing horse.

If treatment cannot be delayed, empirical treatment is indicated until culture results are obtained.

A BAL is most appropriate for diffuse lung disease and inflammatory airway disease (IAD). A tracheal wash such as this is more useful where infectious or localised disease is suspected.

Cytology offers a rapid indication as to whether a septic process is present and may indicate the type of bacteria present. Sampling at presentation in pneumonia cases such as this can help to guide antimicrobial selection.

What do we culture in practice?

• Swabs: uterine, wounds, corneal ulcers.
• Fluids & washes:
  • Uterine washes (higher surface area sampled & combine with cytology).
  • Tracheal wash (combine with cytology) useful in pneumonia.
  • Bronchoalveolar lavage (gold standard for diagnosing inflammatory airway disease).
  • Synovial fluid in sepsis cases.
  • Blood culture in septic neonates especially.

In a busy stud practice…

We culture uterine swabs in-house overnight. It’s important to remember that not every problem mare with uterine fluid has bacterial endometritis. Cases requiring antimicrobial treatment are rapidly identified with treatment of choice depending on the organism grown. This, combined with cytology results, provides vital information needed for diagnosis.

Which antimicrobial should I use?

This decision requires a knowledge of common pathogens, disease processes, host factors and understanding of the pharmacology of drugs available. You should start empirical treatment with an appropriate first-line drug, pending culture and sensitivity testing results, if possible. Obtaining a C&S on initial presentation avoids delays in choosing an appropriate treatment in cases which do not respond.

Combining your culture results with cytology allows interpretation of the significance of your findings:

• Are there intracellular bacteria, excess/ toxic neutrophils?
• Is the bacterial growth significant or likely to be a contaminant?
• Are there other processes at play? E.g. fungal component, allergic/inflammatory disease which may account for lack of response.

In this case (seen in the image above), of a severe melting corneal ulcer, swabs were taken for culture and cytology at presentation. Cytology revealed the presence of fungal hyphae which meant appropriate antifungal treatment could be immediately applied. Gram-staining of the sample also identified the likely bacteria present and guided antimicrobial selection. C&S was rapidly available in the case of a failure to respond.

In the past, we might have only swabbed after observing a poor response resulting in a delay in instigating appropriate treatment.

Wounds and septic synovial structures are cultured at presentation, sensitivity results are therefore rapidly available in cases of inadequate response. If the subsequent growth is found to be susceptible to first-line empirical antimicrobial choice but has failed to respond, other processes affecting healing may be suspected rather than simply switching antimicrobials.

Consider alternatives to systemic antimicrobials

Targeted local treatment:

• Am I reaching the source of infection? Am I reaching the MIC for this drug and pathogen?
• Intravenous regional perfusion (IVRP).
• Intra-synovial treatment.
• Nebulised antimicrobials for respiratory disease.

Non-antibiotic treatments:

• Lavage/ drainage.
• Debridement.
• Can I remove the source of infection?

Be brave enough to NOT prescribe antimicrobials

This requires understanding of disease processes and education of both veterinary professionals and staff. Think about whether surgical prophylaxis is required. Prioritise sterility over antimicrobials.

Remember that antimicrobials are not always indicated:

• Adult horses with diarrhoea.
• Uncomplicated viral disease e.g. influenza, herpes.
• Strep. equi – allow abscesses to burst!!!
• Inflammatory airway disease.

Protect ME BEVA initiative

• Conserve protected antimicrobials for cases of proven resistance or life-threatening conditions:
  • Quinolones (Enrofloxacin).
  • 3rd and 4th generation cephalosporins (Ceftiofur and Cefquinome).
• BEVA provides suggested protocols:
  • First line and alternative antimicrobials for different clinical conditions.
  • Suggested dose rates and routes for common antimicrobials based on best evidence (not just the data sheet – remember off-label use).
  • Protocol for use of protected antimicrobials.
  • All can be tailored to your own practice population and displayed for staff to reference.
  • Encourages a culture of responsible antimicrobial use.

Click here for more information.

Trust your first-line treatments

In 2019 we stopped using 3rd and 4th generation cephalosporins in hospitalised septic or critical care neonates. Most cases were instead treated with Beta-lactam and aminoglycoside combinations (as long as renal function was not compromised). Cephalosporins, which were previously widely used, were reserved for cases of proven resistance – but these were rare. There was no associated reduction in outcome!

Use the comments section to discuss your views on the importance of culture and sensitivity testing. Were you aware how important it was before starting this course? What have you learnt that you can use in practice?