Ryle
10-18-2008, 09:14 AM
First, understand that you should always involve your vet in planning your deworming program and that's even more important now because strategic deworming should be done rather than following the old "deworm every 6-8 weeks rotating dewormers" which will no longer provide effective protection for your horse and will only help to build resistance. Along with the development of resistance is the fact that we now know that 20% of horses carry 80% of the parasites and those are the ones that need a more stringent deworming regimen while the other 80% of horses will need less frequent dewormings because they have a better resistance to parasites and thus don't carry big burdens even without frequent dewormings. . Add in the fact that environmental conditions vary all over the world and they have a direct affect on environmental contamination with parasite larve and on when infection rates are going to be highest in each situation. All of these facts mean that there is no "one-size-fits-all" deworming program. Some horses may only need deworming twice yearly while others like foals require much more frequent dewormings. So, rather than just following the old plan it's now recommended to practice strategic deworming--plan a program based upon the specifics of each horse and use diagnostic testing to ensure that the program is appropriate or to determine when deworming is necessary. This is to help reduce the number of dewormings to help slow the development of resistance while still providing adequate deworming for each horse. This is important because there aren't any new deworming drugs that will be hitting the market anytime soonIt is important to take all factors into account and know which horses are more resistant and which are less resistant in order to plan a deworming program that is going to be effective for minimizing parasite loads, minimizing the frequency of treatments and also minimizing the risk/rate of parasite resistance developing to the drugs in use.
So, rather than just following the old plan it's now recommended to practice strategic deworming--plan a program based upon the specifics of each horse and use diagnostic testing to ensure that the program is appropriate or to determine when deworming is necessary. This is to help reduce the number of dewormings to help slow the development of resistance while still providing adequate deworming for each horse. This is important because there aren't any new deworming drugs that will be hitting the market anytime soon.
There are 4 classes of dewormer on the market:
benzimendazoles --fenbendazole, oxibendazole, other chemicals that end in -azole (there is a long list)
pyrantels---pyrantel pamoate (paste) and pyrantel tartrate (daily dewormer)
avermectins---ivermectin and moxidectin
praziquantel
Of those 4 classes, all but praziquantel are "broad spectrum" meaning that they kill several types of parasites. It is not necessary (or at least was not prior to the developement of parasite resistance) to rotate dewormers using these products to kill the most common parasites of horses. The "rotate to kill the different types of parasites" was necessary when we only had the much older drugs which were often only effective against one or two types of parasites. Praziquantel is the only one of the current drugs that is not broad spectrum and it kills tapeworms which the other drugs are not effective against unless you use pyrantel at twice the normal dose.
But, these drugs are not all as effective as they used to be because they have been over-used and mis-used for many years. Now we have parasites that are becoming and have become resistant to these drugs so we have to change our deworming strategies to help slow the build up of resistance while still minimizing the parasite load in our horses.
When planning a deworming program for adult horses, your main concerns are strongyles, tapeworms and bots in that order. (And this is where you can really see the big problem with deworming based on that link above.) Strongyles are the parasite with the most resistance issues--in more than 90% of areas tested these parasites are now resistant to fenbendazole and in more than 40% they are resistant to pyrantel. There has even been 1 study showing strongyles becomeing resistant to ivermectin. So, the standard rotation in that link you are likely not going to be effectively killing strongyles for 1, maybe 2 and even as many as 3 out of 3 dewormings.
For foals, your main concerns are ascarids, strongyles, tapes and bots. So you have the same concerns as with adults---resistant strongyles, but you also have the added concern of ascarids which are shown to be resistant to ivermectin in some areas of the country. Rotating is still a good idea in foals so that you balance possibly not killing ascarids with one treatment and then the next treatment killing ascarids but possibly not killing strongyles. However, it's probably best to stick to rotating either pyrantel or fenbendazole with ivermectin.
So, rotation options are limited at best and it's really not the rotation that is most important for preventing parasite resistance but appropriate dosing and treatment intervals.
You need to be SURE you are not under dosing your horses so always use a weight tape or measure your horse and calculate his weight. In studies even many vets were way off on weight estimations and they have the benefit of spending a few years working in situations where they get to walk horses onto scales daily to see what 900 lbs LOOKS LIKE.
Weight calculation information:
Body Weight Estimation of Horses (http://www.omafra.gov.on.ca/english/livestock/horses/facts/98-093.htm) KG calculation shown as well as a chart based upon heart girth measure (remember, the heart girth only assessment may be off by as much as 200 lbs just like weight tapes)
Horse Weight: Estimate It Easily (http://www.cowboyway.com/HowTo/HorseWeight.htm) How to calculate in pounds
As for appropriate dosing intervals, you want to treat according to the egg reappearance period so that you are dosing to prevent continual recontamination of your pastures. This method means that you will be cutting back on your horse's risk of parasite infestation significantly after a year appropriate dewormings because you will have cut the number of parasite larva on your pastures. It also means that you won't be deworming at a time when there are no parasites in the system that will be susceptible to the product you are using next which happens if you dose too soon after ivermectin or moxidectin with pyrantel or fenbendazole because at normal doses these two chemicals only kill adults in the GI tract and if you've dewormed with moxidectin last 8 weeks ago there aren't adults there to kill and you've missed the migrating larva which will then start shedding eggs 4 weeks after you treat with either of these drugs and shed for the 4 weeks until you deworm again. Or if you dewormed with ivermectin 6 weeks ago and treat with either pyrantel or fenbendazole you will start seeing egg shedding into your pasture in 2 weeks because you didn't have adults in the GI tract when you dewormed last but they will be there in a couple of weeks. Or in the case where you actually do have adults for the pyrantel or fenbendazole (which again is not likely to be working) to affect because it's been 8 weeks since your last dose of ivermectin you will again have parasite ova being shed in 4 weeks because that is how long after the use of either of those drugs you start seeing adult egg shedding parasites in the GI tract. So deworming based upon Egg Reappearance Periods, you would deworm and then deworm again based upon what drug you used last--4 weeks later for pyrantel (or Fenbendazole, though the next day would probably be more useful LOL), 8 weeks after ivermectin or 12 weeks after moxidectin. In this manner, you can significantly reduce the parasite load on your pastures in a year's time.
So, rather than just following the old plan it's now recommended to practice strategic deworming--plan a program based upon the specifics of each horse and use diagnostic testing to ensure that the program is appropriate or to determine when deworming is necessary. This is to help reduce the number of dewormings to help slow the development of resistance while still providing adequate deworming for each horse. This is important because there aren't any new deworming drugs that will be hitting the market anytime soon.
There are 4 classes of dewormer on the market:
benzimendazoles --fenbendazole, oxibendazole, other chemicals that end in -azole (there is a long list)
pyrantels---pyrantel pamoate (paste) and pyrantel tartrate (daily dewormer)
avermectins---ivermectin and moxidectin
praziquantel
Of those 4 classes, all but praziquantel are "broad spectrum" meaning that they kill several types of parasites. It is not necessary (or at least was not prior to the developement of parasite resistance) to rotate dewormers using these products to kill the most common parasites of horses. The "rotate to kill the different types of parasites" was necessary when we only had the much older drugs which were often only effective against one or two types of parasites. Praziquantel is the only one of the current drugs that is not broad spectrum and it kills tapeworms which the other drugs are not effective against unless you use pyrantel at twice the normal dose.
But, these drugs are not all as effective as they used to be because they have been over-used and mis-used for many years. Now we have parasites that are becoming and have become resistant to these drugs so we have to change our deworming strategies to help slow the build up of resistance while still minimizing the parasite load in our horses.
When planning a deworming program for adult horses, your main concerns are strongyles, tapeworms and bots in that order. (And this is where you can really see the big problem with deworming based on that link above.) Strongyles are the parasite with the most resistance issues--in more than 90% of areas tested these parasites are now resistant to fenbendazole and in more than 40% they are resistant to pyrantel. There has even been 1 study showing strongyles becomeing resistant to ivermectin. So, the standard rotation in that link you are likely not going to be effectively killing strongyles for 1, maybe 2 and even as many as 3 out of 3 dewormings.
For foals, your main concerns are ascarids, strongyles, tapes and bots. So you have the same concerns as with adults---resistant strongyles, but you also have the added concern of ascarids which are shown to be resistant to ivermectin in some areas of the country. Rotating is still a good idea in foals so that you balance possibly not killing ascarids with one treatment and then the next treatment killing ascarids but possibly not killing strongyles. However, it's probably best to stick to rotating either pyrantel or fenbendazole with ivermectin.
So, rotation options are limited at best and it's really not the rotation that is most important for preventing parasite resistance but appropriate dosing and treatment intervals.
You need to be SURE you are not under dosing your horses so always use a weight tape or measure your horse and calculate his weight. In studies even many vets were way off on weight estimations and they have the benefit of spending a few years working in situations where they get to walk horses onto scales daily to see what 900 lbs LOOKS LIKE.
Weight calculation information:
Body Weight Estimation of Horses (http://www.omafra.gov.on.ca/english/livestock/horses/facts/98-093.htm) KG calculation shown as well as a chart based upon heart girth measure (remember, the heart girth only assessment may be off by as much as 200 lbs just like weight tapes)
Horse Weight: Estimate It Easily (http://www.cowboyway.com/HowTo/HorseWeight.htm) How to calculate in pounds
As for appropriate dosing intervals, you want to treat according to the egg reappearance period so that you are dosing to prevent continual recontamination of your pastures. This method means that you will be cutting back on your horse's risk of parasite infestation significantly after a year appropriate dewormings because you will have cut the number of parasite larva on your pastures. It also means that you won't be deworming at a time when there are no parasites in the system that will be susceptible to the product you are using next which happens if you dose too soon after ivermectin or moxidectin with pyrantel or fenbendazole because at normal doses these two chemicals only kill adults in the GI tract and if you've dewormed with moxidectin last 8 weeks ago there aren't adults there to kill and you've missed the migrating larva which will then start shedding eggs 4 weeks after you treat with either of these drugs and shed for the 4 weeks until you deworm again. Or if you dewormed with ivermectin 6 weeks ago and treat with either pyrantel or fenbendazole you will start seeing egg shedding into your pasture in 2 weeks because you didn't have adults in the GI tract when you dewormed last but they will be there in a couple of weeks. Or in the case where you actually do have adults for the pyrantel or fenbendazole (which again is not likely to be working) to affect because it's been 8 weeks since your last dose of ivermectin you will again have parasite ova being shed in 4 weeks because that is how long after the use of either of those drugs you start seeing adult egg shedding parasites in the GI tract. So deworming based upon Egg Reappearance Periods, you would deworm and then deworm again based upon what drug you used last--4 weeks later for pyrantel (or Fenbendazole, though the next day would probably be more useful LOL), 8 weeks after ivermectin or 12 weeks after moxidectin. In this manner, you can significantly reduce the parasite load on your pastures in a year's time.