It has to be below freezing for at least 10 days to break the parasite life cycle in winter, and over 85 degrees for at least 3 days to break it in the summer.
This isn't quite true.
Sm. strongyles eggs don't mature into infective stages below 45 degrees, so when the temps are below 45. reinfection isn't a huge issue. And at temps over 85 degrees the infective stage of small strongyle larva don't survive long on pastures. So just waiting 3 days isn't going to "break the life cycle" because it very much depends on when each egg is shed into the pasture in the summer and shedding is going to be pretty continual in horses that aren't dewormed appropriately. And in the winter, you don't EVER kill the parasites---there is no such thing as a killing frost where small strongyles are concerned. These parasites over-winter very well on pastures and then hatch out into infective stages once the temperatures start getting over 45 degrees.
Shinakonga, I would strongly recommend that you watch the free webinar available through www.thehorse.com
This is an hour long presentation made by one of the top research vets in equine parasites. He lays out the reasons why we need to NOT follow the kind of deworming program you were talking about above and how to set up an appropriate deworming program for your horse. The information that is presented in this webinar is the same that was presented at the American Assoc. of Equine Practitioners meeting in 2009. You should of course also talk to your vet, but understand that many of them are way behind when it comes to what is currently recommended in parasite control for horses especially if you are dealing with a vet that doesn't concentrate only on treating horses.
When you get your new horse, be sure to find out the name of the product last used for deworming and the date. Then use that information to determine when you should have a fecal sample tested and then have a fecal egg count run at the appropriate time.
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 “’The recipe’ no longer represents an acceptable program for strongyle control” according to Dr.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)