Re:Strongid C

[Trish Dowling <dowling@skyway.usask.ca>]

On Wed, 02 Sep 1998 16:04:51 -0700 (PDT) ridecamp-d-request@endurance.net wrote:
This subject comes up from time to time as we get more new folks on the list.
Many of us have seen endurance horses develop a rare type of colic called
anterior enteritis in which the small intestine literally shuts down, and it
almost always seems to occur in horses on Strongid C, the daily wormer.  Other
endurance horses on it have developed serious anemias which have resolved when
they were taken off of it--again, serious anemia is a pretty unusual problem
in our horses.  Since it also doesn't appear that a very high percentage of
endurance horses are on this medication, the correlation seems pretty
suspicious.  I don't recommend the product for endurance horses at all, and in
fact, strongly discourage its use in endurance horses.  I will hasten to add
that it is a very good product for controlling worms in growing youngsters,
and seems to be an invaluable tool in crowded situations during weaning, etc.,
and that I have never seen problems in any other class of horses other than
endurance horses with the product.  There should be some more posts on this
subject in the archives.

Heidi Smith, DVM--Sagehill Arabians (Oregon)

Heidi,

I think clinical observations are valuable, but I also think this subject needs a little more discussion before 
dismissing a good product and a valuable program for horse owners. I have practiced in Texas, Ohio, North 
Carolina and Alabama, before coming to Canada to teach pharmacology. I have had great success with Strongid 
C. During an internship, medicine residency and practice, I have seen numerous horses with anterior enteritis. I 
have seen several epidemiological studies on the syndrome, but I have never seen anyone associate anterior 
enteritis with Strongid C before. Several years ago, I first heard the rumours of anemia associated with Strongid C. 
I am on a USP veterinary committee, and at that time, so was Dr Joe DiPietro. He is now the Dean of Florida's 
veterinary school, but he is one of the world experts in equine parasitology and did initial trial work with Strongic C 
for Pfizer. So I asked him about these reports. He was amazed to hear that this was attributed to pyrantel tartrate, 
as it is minimally absorbed from the horse's GI tract and that is not its mechanism of toxicity. Recently, I  
checked with Dr Kathy Myers, the veterinarian who heads the USP's Adverse Drug Reporting Program, and she 
has not received reports regarding Strongic C. To cause toxicity in a horse, you have to give 100 mg/kg. Strongid 
C is dosed at 2.64 mg/kg (there's not a lot of veterinary drugs with a 40X safety factor!). When it does cause 
toxicity - it causes nervous system toxicity - it's a depolarizing neuromuscular blocker. So that means toxicity 
shows as increased respiratory rate, sweating, incoordination, muscle paralysis. At 20X doses in horses, ponies 
and foals, there were no changes seen in blood cell counts or liver enzymes. I can not think of a mechanism for 
this product to cause anemia, and we don't have any idea yet what causes anterior enteritis.

However if you believe that you see adverse drug reactions to Strongid C or any product, it is really important to 
report these cases. The best place to report is to Dr Myers at 1-800-4-USPPRN. She files reports with the FDA 
and the drug company and the problems are investigated. So if it's real, we can get a handle on the numbers. As 
an example of her work, she recently wrote an article for Journal of the American Veterinary Medical Assoc. on 
adverse drug reactions to albendazole, fenbendazole and metronidazole (albendazole DOES cause aplastic 
anemia in some dogs).

For owners, if you want to see the studies that were done to get a new drug approved by the FDA, you can go to 
http://www.fda.gov/cvm/  and follow their search instructions for the Freedom of Information Summaries - that's all 
the stuff that the drug company had to submit to the FDA to get their drug approved. There's also a database of 
adverse drug reactions, but they're not interpreted. For pyrantel, it does not distinguish between pryantel tartrate 
(Strongid C) and pyrantel pamoate (Strongid P). But under horse reactions, anemia is not reported.

Cheers,
Trisha

Trisha Dowling, DVM, MS, Diplomate ACVIM & ACVCP
Associate Professor, Veterinary Clinical Pharmacology
Western College of Veterinary Medicine
Saskatoon, SK S7N 5B4
306-966-7359/FAX 306-966-7376