Wednesday, November 05, 2008
Parasites and "Dewormers"....something new
Treating horses for parasites has been a staple of barn management since...oh I guess, forever. People have put just about any toxic substance into their horse to "git rid of worms". Virtually all of my clients use some form of rotational system whereby they use different products every 4-8 weeks thinking they are preventing drug resistance. "Unfortunately , parasites have risen to the chemical challenge. Anthelmintic resistant cyathostomes (small strongles) now are highly prevalent and even where drugs still are effective , the egg reappearance rate ERP following treatment has become significantly shorter"." Today horse owners continue to follow recommendations that are based on knowledge that is 30-40 years old and frequently use anthelmintics that have become totally ineffective due to the presence of drug-resistant parasites" It has been know for a long time that most of the parasites on a farm are actually in the ground as eggs and larvae (about 90-95%) and only a small percentage
are really in the horse at one time , but what if most are in only 1 or 2 horses that are shedding most of the eggs. A new approach has been published recently and it really isn't that new , it's just looking at the horse population as individuals instead of the herd. This has been published by Dr Ray Kaplan at the Univ. of Georgia. What Dr Kaplan suggests is to treat each horse as an individual and do fecal egg counts on each. You then can identify which horses are the ones that are responsible for most of the egg shedding.
Once these are targeted you can concentrate more of your treatment on those that are the main problem instead of treating the entire herd as one. This study showed that on 40% of farms there was small strongyle drug resistance to fenbendazole(Panacur), oxibendazole(Anthelcide) and pyrantel pamoate (Strongid) meaning that on almost half of farms only one drug class is effective (avermectin/milbemycin) which has been around for 25 years. This is a very disturbing trend that should scare every horse owner. What the current plan is now is to treat the right horse with the right drug at the right time. The key to this is the FEC (fecal egg count) and the fact that not all horse are as susceptible to parasites as others. About 20-30% of horses harbor about 80% of the worms and shed almost all of the eggs. We have to identify these horses to be successful. The current protocol in North Florida is as follows
SEPT 1 Fecal Egg Count
Treat all horses
Start treatment on all horses( regardless of FEC)
Ivermectin or Moxidectin with praziquantel for tapeworms
based on FEC you can categorize your horse as
Only if treated with ivermectin. If Moxidectin then wait until December to treat again
FEC on all horse >150epg
treat all horses regardless of FEC
ivermectin or Moxidectin plus praziquantel
FEC on all horses
Based on FEC,Treat if Ivermectin used in DEC, if moxidectin used then wait until
Based on FEC, only treat horses with FEC >150 epg or has been a high shedder.
treat if using Moxidectin , treat horses as for FEB with FEC >150 epg
based on FEC, only horses with >150 epg
No need to treat during the summer months unless obvious parasite issue presents on a case by case basis.
To hot for transmission, most worm eggs will not survive and develop L3 form and these will die rapidly.
As you can see the real key is doing fecal exams on the horses regularly to see which horses need treatment instead of just treating every month or so and actually promoting resistance. Not to mention the money wasted on dewormers that either are not needed or not effective. I really think this is the way to go and will be sending out info to every owner in the practice. I think we can cut down on the incidence of colic substantially by just monitoring the parasite loads these horses carry.