Selecting an Antimicrobial in Equine Practice Flashcards
What is MIC ?
The lowest drug concentration that inhibits bacterial growth.
* Commonly: concentration that inhibits 50% (MIC50) or 90% (MIC90)
What is the MBC?
Minimum bactericidal concentration (MBC): the lowest drug concentration that kills 99.9% of bacteria
When determining in vivo and in vitro resistance - does it always match up?
Not always
- Host defences are not predicted in the lab
- Tissues interact with different drugs differently
- Has your culture over/ underrepresented certain pathogens
- Infection envionrment
- In vivo synergism
What spcific tissue challenges?
- Need to cross BBB?
- Pus/ necrotic?
How can we deliver drugs directly to the local environment?
- Nebulisation of antimicrobials for pneumonia?
- Intra-venous regional limb perfusion?
- Intra-synovial administration?
What about intracellular bact?
e.g. rhodococcus equi
-> beta lactams (penicillin) don’t penetrate cells well so we go for Rifampin and Azithromycin
describe efficacy of TMPS
-TMPS works by blocking bacterial nucleic acid synthesis
- Sulphonamides substitute for PABA (para-aminobenzoic acid) blocking conversion to dihydrofolic acid
- So, they only work if sulphonamide concentration in tissue is high, and bacteria can’t get access to
PABA
- TMPS inactivated in pus and necrotic tissue (because bacteria get an alternative PABA source here)
What can affect drug dosing in foals?
- Immature hepatic metabolism
- Immature renal excretory mechanisms
- Less plasma protein
- Less body fat
- Increased body water
What potential Adverse effects of ABs?
-> Colitis (macrolides not suitable for adult horses)
-> anaphylaxis
-> urticaria
- >procaine reactions
- > IMHA
-> Local injection site reaction
-> Arthropathies in young animals (FluroQ)
-> Nephrotox (aminoG)
-> Collapse (TetraC)
-> hyperthermia (Macrolides)
What is licensed for horses?
Oral: TMPS
Injectable: penicillin, Ceftiofur, Cefquinome, Oxytet, TMPS, gentamycin
What else is available for oral use in horses?
-> Doxycycline
-> Metronidazole
-> Choramphenicol
-> enroflox
-> More in foals
What admin route considerations
- Enteral ABs abosprtion variable
- Palatability -> Metronidazole tastes liek arse - use per rectum?
- Struggle to inject IM? -> May be inappropriate of low muscle mass etc
Colitis and ABs - controversial
- AB tx routine for some causes
- e.g. C. difficile/perfringens – metronidazole, Potomac horse fever (not UK) - tetracyclines
- Otherwise, only treat if profoundly neutropaenic?
- Some clinicians widely use metronidazole anyway
Should we blanket give new-born foals penicillin cover?
Used to be - probs not now
What pathogens of concern in MDR infections?
- Extended-spectrum β-lactamase (ESBL) producing Enterobacteriaeceae
- Methicillin-resistant Staphylococcus aureus (MRSA)
- Salmonella Spp
- Other opportunistic pathogens with high intrinsic resistance and limited treatment options in adult horses
Records?
» Keep records on antimicrobial usage
* Incorporate into practice meetings and or M&M rounds?
* Discuss/review each case where protected ABs used
