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  Discussion on Rhodococcus resistance to azithromycin and/or rifampin?
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New Member:
sshorses

Posted on Tuesday, Aug 4, 2009 - 2:45 pm:

We've been treating a foal for rhodococcus since the end of May (filly was approximately 6 wks old). She only had lung involvement at diagnosis, but there was significant consolidation at that time. She's been on azithromycin/rifampin with dosage adjusted for growth. Three weeks ago, she had been completely asymptomatic for a couple of weeks and we had her rechecked (u/s). Her lungs were clearly improving, but disease was still quite evident. The vet suggested that we continue treatment and bring her back in a month. About a week ago, the filly started coughing periodically. Prior to the initial diagnosis, she had very weak and very sporadic coughing, no fever, no nasal discharge, no anorexia and only very slight lethargy. Now her coughing is more frequent and pronounced (several bouts of 3-4 coughs per day with or without exercise), and she has slightly 'wet' lung sounds. I am assuming we need to rule out resistance, but I was wondering if we should switch to SMZs or another drug in the meantime. Also, we are treating only every other day per vet instructions at last checkup - is it advisable to go back to daily azithromycin/rifampin treatments?
Moderator:
DrO

Posted on Tuesday, Aug 4, 2009 - 7:22 pm:

Hello Lisa,
I am sorry to hear about your foal this is a very tough disease.

A careful exam and lab needs to try to determine if the cause of the cough is due to resurgence of the Rhodococcus or other possible cause. If the exam/lab findings are inconclusive then it would seem to me the most reasonable course would be continued aggressive therapy for Rhodococcus.

TMP/SMZ is not effective against Rhodococcus as it does not penetrate abscess walls well. We list the other reported effective antibiotics in the article. If the choice is to stay with the antibiotic you are using by all means you should return to recommended treatment intervals but such a regimen as you have been using may have promoted resistance.
DrO
New Member:
sshorses

Posted on Tuesday, Aug 4, 2009 - 8:34 pm:

Thanks Dr. O, I agree - I was never all that happy with the every other day regimen, but that is what our specialist (who has dealt with many rhodococcus cases) recommended, so returning now to every day is only a return to the regimen of the first few weeks of treatment. My husband doesn't want to do erythromycin b/c he doesn't think switching to another macrolide will gain anything -so I was thinking aobut another class. I did read a research paper (I can't remember the citation now) that was guardedly optimistic about doxycycline. I saw your previous comment on its lack of effectiveness. I'll have to try to find that article again. Sure wish gallium would turn out to be the 'silver bullet' - fat chance :-/
Moderator:
DrO

Posted on Wednesday, Aug 5, 2009 - 9:09 am:

There was a study on the prophylactic use of azi against Rhodo and after some time resistance in the Rhodo population had not occurred so maybe returning would be OK. Rifampin resistance is reported however.

Rhodococcus is very easy to kill with a wide range of antibiotics the problem with the pulmonary form in foals is the abscess walls are difficult to penetrate. It has to be both effective against the rhodo and penetrate the abscess wall. Looking at the research rhodo was sensitive to doxycycline in vitro and oral administration was able to achieve MIC for rhodo in vitro. I don't know of any case studies however. The problem with the use of "tetracyclines" in young animals is that it may adversely effect teeth and bones but other than staining I unsure how serious this is, it is what we have always been told so I have always avoided it in horses less than 2 years old. Considering the serious nature of the disease it may be a reasonable choice if it comes down to it.
DrO
Member:
judyhens

Posted on Wednesday, Aug 5, 2009 - 12:59 pm:

Hi Lisa,
We are in Texas also (near Austin). For the first time in 15 years of breeding, we experienced Rhodococcus in a foal last year. The baby was diagnosed at about a month or so of age. Went from having mild respiratory symptoms to being very ill virtually over-night. He was hospitalized for about 10 days, then discharged home with aggressive treatment for months. Periodic x-rays were done to document progress. I think treatment was continued daily for several weeks after all abcesses had resolved. We ended up with a good outcome, but were told it was crucial to continue treatment. We contacted A & M - and consulted a Dr. Noah Cohen. You might want to contact him. He was willing to talk with us over the phone and work with our vet. I think gallium maybe did not end up being conclusively helpful. But not absolutely certain. He did suggest our vet go on and give this year's foals plasma against this infection. Said it may not prevent it, but it could help. So...this year we did that for all babies. No problems so far, and we are approaching weaning time. Also, it has been incredibly dry, the sand has been blowing, and the DrOught conditions we had last year are still with us. Don't know if that plays a role or not.

Anyway, my thoughts and prayers are with you and your baby. From our experience, this might be a long haul. Keep us posted. Where in Tx. are you?

Judy
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