Page 4 - Clinical Connections - Spring 2022

RVC RESEARCH STUDY VETERINARY SERVICES RVC.AC.UK
Equine
RESPONSIBLE ANTIMICROBIAL USE IN
HORSES

Bettina Dunkel, Head of RVC Equine and Professor in Equine Internal Medicine, Emergency and
Critical Care
any diseases in horses are and neoplastic conditions – or following of negating a treatment success often leads
associated with bacterial infection. administration of corticosteroids – and is to prolong use of antimicrobials in clinically
M Unfortunately, viral or fungal therefore also of limited use. As immediate apparently recovered patients. In the past,
infection, bacterial toxins, tissue damage or return to normal haematologic parameters
neoplasia also cause systemic inflammation or normal concentrations of acute phase
and clinical signs are indistinguishable from proteins have been used as markers to
bacterial infection. safely discontinue antimicrobial treatment.
Although not effective against viral, Return to normothermia, improved appetite
inflammatory or neoplastic conditions, and return of normal demeanour might be
antimicrobials are often used in these better indicators that further treatment is not
cases as it can be difficult to conclusively necessary.
rule out a bacterial infection. Due to
increasing microbial resistance, pressure
on veterinarians is mounting to adhere to
responsible use of antimicrobial drugs.
Clinical examination findings are often A foal at the RVC Equine Referral Hospital
non-specific and while the presence of a
fever increases suspicion of an infection, treatment is usually required, clinicians
it is not conclusive as inflammation, need to make an educated guess whether
hyperthermia, neoplasia or tissue trauma bacterial infection is likely.
can also lead to an increased temperature. Many equine viral and some bacterial
Equally, the absence of a fever does not rule diseases can be diagnosed by polymerase
out even severe infection. chain reaction (PCR) with results often being RVC Equine Referral Hospital
Haematology and acute phase protein available the following day. Cytological
concentrations, such as fibrinogen or samples can be of great value when trying It is also common practice to initially
serum amyloid A (SAA), can be difficult to rule out an infectious aetiology, as results treat systemically ill horses with injectable
to interpret as increases are triggered by are available much quicker than culture antimicrobial drugs for 48-72h followed by
infectious and non-infectious inflammatory results. In cases where clinicians have continued oral treatment. In most cases, this
conditions 1, 2 . In adult horses, leucopaenia already initiated antimicrobial treatment, is not necessary. An alternative approach is
is commonly observed with inflammation, but PCR or cytology do not support stopping antimicrobial treatment after 48-72h
often originating from the gastrointestinal a bacterial infection or identify a viral and monitoring the patient for another 24h
system. It does not necessarily indicate cause, antimicrobial treatment should be closely. Should signs of infection re-occur,
the presence of infection or the need for discontinued immediately. such as recurrence of a fever, a decrease in
antimicrobials. The old concept that “a course of appetite or change in demeanour, treatment
antibiotics needs to be finished to avoid can easily be re-initiated – but this is rarely
development of resistance” is obsolete and necessary.
not supported by evidence. To the contrary,
reducing the length of treatment has a
significant effect on decreasing previously References
existing resistance while increasing the 1. Westerman, T.L., et al., 'Evaluation of serum
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length of antimicrobial use increases the risk amyloid A and haptoglobin concentrations as
prognostic indicators for horses with inflammatory
of resistance development . In most cases disease examined at a tertiary care hospital', Am J
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a short duration of antimicrobial therapy, Vet Res, 2015. 76(10): p. 882-8.
ranging from a single dose (for example 2. Long, A. and R. Nolen-Walston, 'Equine
Inflammatory Markers in the Twenty-First Century:
pre-operatively) to 24-72h, is sufficient, with A Focus on Serum Amyloid', A. Vet Clin North Am
long-term treatment being rarely required. Equine Pract, 2020. 36(1): p. 147-160.
Recent studies in people have shown 3. Dona, D., et al., 'Implementation and impact of
pediatric antimicrobial stewardship programs: a
impressive reductions in antimicrobial systematic scoping review', Antimicrob Resist
resistance with a reduction of not only Infect Control, 2020. 9: p. 3.
overall use but also with a decrease in days 4. Llewelyn, M.J., et al., 'The antibiotic course has
had its day', BMJ, 2017. 358: p. j3418.
A horse at RVC Equine of antimicrobial therapy, highlighting the
importance of shortening treatment . Limiting For equine referrals, please call: 01707
3
Leucocytosis in mature horses can be the duration of antimicrobial treatment in 666297
observed with inflammatory, infectious proven infections can be challenging. Fear Email:
equinehospital@rvc.ac.uk

4 Spring 2022