The Veterinarian's Role in Parasite Management

Mar 23, 2011

The Veterinarians Role in Parasite Management:

The role of the veterinarian in parasite management has decreased to a point where it is rarely even discussed with clients. With the availability of dewormers at every feed store and in every catalog, parasite management has fallen out of the veterinarians hands and therefore, left up to the owner to decide proper management. We carry a few dewormers on the truck and in the pharmacy for the occasional client who doesn't want to mess with the hassle of driving into town or paying for shipping. As an owner, why do I need to change what I have done for the last how many years? Do I have a horse that doesn't shed out its coat, looks a little rough, maybe a pot belly? Do I have a horse that is hard to keep weight on? What about a horse with recurrent colic. Maybe parasite management should be evaluated to make sure we are not missing something.

One of the big questions that we need to ask ourselves both as a veterinarian and/or owner, what are we trying to accomplish? Kills the worms? This may be true for some of the equine parasites, such as equine roundworms, but an emerging problem with more and more resistance is the small strongyle (cyathostome). Small strongyles exert the majority of their damaging effects before they are susceptible to many dewormers. With small strongyles we need to focus our efforts on preventing contamination of the environment with eggs. In young horses the equine roundworm is a very detrimental parasite. It leads to poor weight gain, rough haircoats, and pot belly appearances. They exert their effect on the young horse by robbing the nutrients as well as leading to intestinal inflammation. The only practical way to decrease future infection is by limiting the passage of worm eggs, and this can be accomplished by killing female worms before they reproduce.

With all of the so-called management practices out there, only one practice works most of the time. Dewormers are still the best tool that we have in parasite management. Most dewormers will cover the horse parasite load, but what about the small strongyles and large strongyles. Small strongyles have proven to be the most difficult in management. As I said in the last paragraph, small strongyles exert their damage before they are susceptible to dewormers. The small strongyle does this by forming cysts while in the larval stage in the gut lining. The cystic larvae can lead to alterations in the gut and the potential for recurrent colic. The large strongyle larva migrates through the vasculature of the intestines and cause scarring. This can lead to recurrent colic. There are only a couple of dewormers which have been shown to get encysted small strongyles and migrating larva. Those are Quest administered once and the Panacur PowerPak administered over five days. Use of dewormers that are labeled as larvacidal is pivotal in the proper management of the strongyle parasites.

What about resistance? Most people recommend a rotation of dewormers thinking that this will help prevent resistance. Rotations were implemented to cover the deficiencies of certain dewormers. Some dewormers may not cover all potential parasites, so we rotate. True resistance of dewormers against common parasites is a growing problem and concern. Rotation is not as important as making sure that the dewormer is effective.

I cannot take credit for this quote, but I like it.

"Parasites are plastic organisms with the ability to adapt to, and ultimately triumph over, virtually all man-made selection pressures. Because most of these adaptations have a genetic basis, future generations of worms may not be susceptible to the same interventions that would have killed their grandparents."

One of the most important aspects of parasite management is making sure the drug administered is effective in your herd. Well you ask, how do I make sure that the dewormer is effective in my herd. We at Conley and Koontz Equine Hospital can do quantitative fecal egg counts (FEC). This involves collecting feces from a group of horses in your herd prior to deworming. Once the proper dose of dewormer has been administered, we wait 10-14 days and collect feces from the same horses and perform another FEC. We want to see a reduction in FEC by about 90% to determine effectiveness. We can also use FEC to determine when to redose the dewormer. By collecting feces every 2 weeks after dosing, one can determine the egg reappearance period. This will allow us to determine if the time interval between dewormers is appropriate so that we can reduce the pasture contamination thereby, decreasing the infection. The sampling of individual horses in the herd can help us determine the spectrum of effective dewormers, the optimal timing of treatments, and the required expenses and effort needed for each individual horse in the herd.

One last thought about parasite management. Not every horse is created equal. Just because one dewormer works well on the majority of horses it may suboptimal for a member of your herd. By performing a FEC four weeks after the egg reappearance period you can determine which horses in your herd can handle strongyles on their own and which horses need the extra special attention.

By performing fecal egg counts, we can help you determine the optimal dewormer and dosing interval for horses in your herd.