Unfortunately although this article was newly posted, it is sadly out of date. It is a repeat of the same of 40 year old recommendations and doesn't take into account that the dewormers in use have changed and that resistance of parasites to some of the commonly used dewormers is a HUGE issue. Heck, let me just post the long-winded thing without having to retype it all.
Here are the main points and you can pick out they "why's" from the longer explanation:
1. We are using different drugs now than they were 40 years ago and they are effective against a broad range of parasites so rotation is not necessary to target specific parasites except in the case of spring and fall when you want to use an ivermectin/praziquantel or moxidectin/praziquantel to target bots and tapes along with the other GI parasites.
2. Deworming every 6-8 weeks is no longer necessary because the parasite of most concern now is not the same one as when that recommendation was started 40 years ago and we are using different drugs. Many many horses only need to be dewormed twice a year--spring and fall--to control parasite populations.
3. Rotation is not truly an option any longer due to the widespread resistance issues with the commonly used drugs. The only drug currently in use with NO resistance issues is moxidectin. Fenbendazole is highly likely to be worthless in adult horse deworming programs and pyrantel is moderately likely to be of little use both due to the development of resistant strongyles to these drugs in many areas of the US.
4. We need to focus on deworming appropriately and limiting the number of times we deworm yearly to help slow the development of resistance in parasites to the drugs that we have for deworming. 2-4 dewormings a year is the new recommendation and not deworming during the time of year when envirnmental conditions are not conducive to parasite reinfection. This means not deworming during the summer months in the southern US and in the winter in the northern US. The newest recommendations as of June 2009:
For 40 years now, veterinarians have recommended that horses be dewormed every 8 weeks all year round and rotation of dewormers has been recommended for nearly that long. This is considered by many vets and horse owners to be “the recipe” for adequate and appropriate deworming of horses. However when you look at the research that has been done in the last 15 years you really have to question this practice. “’The recipe’ no longer represents an acceptable program for strongyle control” according to Dr. Craig R. Reinemayer, DVM, PhD of East Tennessee Clinical Research, Inc during a webinar presented via www.thehorse.com
. This means that deworming recommendations for adult horses need to be reconsidered because strongyles are the only significant nematode pathogen of mature horses. Continuing to use the same old deworming program will likely lead to heavily parasitized horses and further drug resistance. Instead, each horse and each situation should be evaluated to determine the reinfection rates and an appropriate deworming program for the individual.
It's now known that all adult horses living in the same situation do not necessarily need to be dewormed on the same schedule. 50% of horses in a herd will control parasite loads on their own due to natural resistance. Only about 20-30% of horses carry heavy parasite loads.
Thus each horse should be dewormed based upon an understanding of his own personal resistance to parasites. The best recommendation is now 2-4 dewormings a year based upon knowing which horses carry lots of parasites and which tend to carry little parasite load.
Rotational deworming is no longer an adequate or appropriate deworming program for adult horses. There are too many issues with strongyles developing resistance to 2 of the 4 most commonly used dewormers on the market---fenbendazole (more than 90% of areas tested have resistant strongyles) and pyrantel (around 1/2 of areas tested have strongyles resistant to this drug). And resistance is starting to be seen in strongyles to ivermectin---1st study showing it was done in KY in the last couple of years.
All adult horses in the continental US/Canada should be dewormed spring and fall with ivermectin/praziquantel or moxidectin/praziquantel. Other than those 2 standard dewormings, the rest of the deworming program should be based upon location and the horse's own resistance to parasites. The new recommendation is 2-4 dewormings per year based upon fecal egg counts used to determine the normal amount of egg shedding each horse does during the time of year when the weather in your area is most conducive to strongyle larva development and environmental survival. In the northern states in the US and in Canada, this means running a fecal egg count in the middle of summer (3 months after spring dosing if you used ivermectin or 4 months after spring dosing if you used ivermectin). In the southern US and Mexico you would be looking at testing in the middle of the winter (same time after spring deworming as listed above). Then based upon the number of eggs per gram of feces you can determine if you need more than the spring/fall dewormings and if so if you need 1 or 2 more dosings.
In the northern US and Canada, deworming should be discontinued during the winter months because the environmental conditions are not conducive to reinfection---that time of year has been proven to have extremely low reinfection rates. In the southern US and Mexico the opposit is true....deworming can be discontinued during the heat of summer because temps over 85 degrees lead to the infective strongyle larva dying quickly in the environment so the reinfection rates are lowest then.
For more detailed information check out the deworming webinar that was aired via The Horse magazine's website in April. Be prepared to sit for a while because it is an hour long presentation, but it's well worth the time. The veterinarian gives you all the information on strongyles and deworming in adult horses that you've always wanted to know and then some. It is a wonderful lecture. (And have plenty of paper and a pen.) http://www.thehorse.com/Video.aspx?vID=1…
(Craig R. Reinemeyer, DVM PhD --parasitologist)