In the Past 5 Years I have seen awareness about parasite resistance to deworming increase, but, owners still seem unsure of what to do about it. Creating programs that rotate dewormers and require a high frequency of treatment feel reassuring. Unfortunately those practices are outdated, expensive and have caused the resistance issues we now face in large animal veterinary medicine. In all our large animal species we are currently facing the frightening results of parasite resistance. High levels of parasitism can cause decreased performance, poor growth, severe illness and even death. There are many times I have felt powerless as the veterinarian trying treatment after treatment on a struggling patient only to find that nothing we have available works and I know many owners have had this same issue. So, how does this happen and why? In order to understand how this happens first we have to understand how animals get parasites, why some individuals seem to be effected more often and what exposure to dewormers does to the parasite population.
Every animal is born with the natural ability to fight parasites. Wait, can that be right? You may be asking why you have to deworm ever and the answer is because of us. Humans have done a few things to make life difficult for our fuzzy friends. We have taken grazing animals that used to cover large tracks of land and put them in beautiful fenced areas. We don’t allow them to follow the old adage “Don’t poop where you eat.” OK so that’s the PC version but the message is the same. Our grazing animals are forced to retrace there steps and eat a larger concentration of parasite eggs than they evolved to deal with. The second reason is because we have selected our animals to do some amazing things such as grow fast, jump high have beautiful colors, but until recently, nobody has paid a lick of attention to kills parasites effectively. By not paying attention to this issue we have accidentally created some individuals that struggle with parasitism. The third reason is that we have been spoiled by easy to administer effective dewormers and stopped paying attention to management strategies that prevent parasite transmission. The good news is we can change! By using fecal analysis we can find those problem individuals and treat appropriately. The general rule is that 20-30% of your herd creates 80% of your parasites. In simple terms if you have 10 horses at a boarding facility 2-3 need frequent deworming. It also means if you have a herd of 100 goats you will only have to pay to deworm 20 on a regular basis, bonus! The other way you can make a big difference in parasitism is by changing management practices, which is wait for it, FREE! Simple things like only spreading the pasture when it is above 85 degrees all day and using pasture rotation.
So this may sound like a lot of work and your horse has always been fine being dewormed by the farrier each time he comes to trim. Why change now? Because unfortunately our herd animals like to hide there diseases until they are dying. It is one of those survival of the fittest things that has developed over the past few hundreds of thousands of years and this means you may not notice until it is too late. When an animal is repeatedly dewormed some of the bugs still live. More importantly those guys reproduce. Pretty soon the only parasites left are the ones that have resistance to our drugs. So the new goal of veterinarians is to maintain a very low but diverse population of parasites. By keeping the population diverse we continue to have worms inheriting the ability to be killed and that is what we desperately need to maintain. Even better if we maintain this diversity while implementing good management practices the total population of parasites on a property will continue to decrease until very little treatment is necessary to keep your herd healthy!
Hopefully at this point you believe that parasite resistance is real and scary and unfortunately a wee bit complicated. There are some different strategies for different species so call your veterinarian and make a program that is right for you or check out the resources that explain all this in much greater detail: equine parasite guidelines AAEP and dewormer resistance.