What's new in foal medicine (Proceedings)

ADVERTISEMENT

What's new in foal medicine (Proceedings)

source-image
Aug 01, 2010

Rhodococcus equi

     What 's new in treatment ?

Standard treatment is erythromycin + rifampin
More recently replaced with:
     • -Azithromycin + rifampin
     • -Clarithromycin + rifampin

Resistance to macrolides or rifampin is now being encountered
A study compared MIC data between 2000 and 2006:
     • Over the 6 year period, the MIC of rifampin increased from 0.081 to 0.187 µg/ml
     • MIC of erythromycin increased from 0.258 to 0.583 µg/ml

Another study from 2008 (Giguere et al, ACVIM 2008)
           • 2 isolates were resistant to rifampin
           • 17 isolates were resistant to all 3 macrolides and rifampin

Overall prevalence of resistance was 3.7 % of isolates
Survival rate of foals infected with resistant isolates was lower than of those with susceptible isolates
     • 25 % vs. 70 %, P < 0.004

Resistant Isolates

We have found resistance in isolates from foals
Very concerning
Ask your micro lab to perform susceptibility testing of R. equi isolates
Not done routinely

Other Options
Other options for treatment?
Other macrolides ?
     • Tilmicosin (Micotil) ?
In vitro studies show concentrations achieved in serum, pulmonary fluids and cells is below the MIC
Clinical experience also suggest that it is not effective
Tulathromycin (Draxxin)

Unknown efficacy at this point
In one study comparing tulathromycin with azithromycin in the treatment of undiagnosed pulmonary abscesses:
It took significantly longer for pulmonary abscesses to resolve
Seems fairly safe, but high rate of self-limiting diarrhea in 30 % and IM swelling in 32 %.
Requires further study; cannot be recommended at this time

Doxycycline

Effective: 10 mg/kg q 12 h PO achieves serum and tissue concentrations > MIC of R. equi
It shows synergism with rifampin
Clinical experience suggests it is effective in treating foals with R. equi
Clinically the response appears slower as compared to response to macrolide + rifampin
Good option for macrolide-resistant strains and for foals that have adverse reactions to macrolides

Controversies

Is R. equi plasma protective?
7 published studies
     • 5 show improvement
          o 2 were natural infections on farms
          o 3 were experimental infections
     • 2 showed no difference in incidence of pneumonia
          o Both on farm setting
          o One showed reduced incidence of disease 19.1 vs. 30 %, but it was not statistically difference

Controversies

     • Prophylaxis with antibiotics?
     • One study showed benefit of azithromycin for 2 weeks
     • Concern over resistance to macrolides with indiscriminate use
     • Not recommended