With the ongoing discussion about deworming intervals, we find a number of opinions advocating regular intervals or the rotation of different drugs. In the not too distant past, the common practice was to deworm on a regular basis. About every two months horses were treated to kill Strongylus vulgaris (large blood worms). In time, however, another class of worms (small Strongyles) became the most popular worm and almost immediately began to develop resistance to that treatment plan so conventional wisdom changed.
Veterinarians began using a rotational approach, alternating different drugs with each treatment as it was found the worms grew resistant with more frequent use of the same drug. This strategy worked for some time until it was determined that large Strongyles were more or less no longer a problem. This did not eliminate the problem of worms entirely; of course, it merely caused a need to focus on small Strongyles.
Another option is to treat for worms daily as a preventative measure. This seemed to control the spread of worms for the time being but now many think may be one of the biggest causes of resistance in small Strongyles?
Cyathostomins (small Strongyles) seem to develop resistance more quickly than the large Strongyles, which came before them. With this knowledge, it is time to consider a new approach. While children and horses are obviously not the same and the diseases that affect them are not the same, we still might take a page from our parenting skills booklets and consider an alternative treatment plan.
With the knowledge that horses are not all identical in their susceptibility to worm infestation and vary in the passing of eggs in their stool, it stands to reason each case (whether it be one horse or an entire herd) should be treated individually. It may also be important to take special concerns with horses not yet of adult age as they are more susceptible to worm infestations (3). This fact supports the concept of having a Fecal Egg Count done at regular intervals and providing treatment as necessary.
I spoke with Dr. John Byrd, DVM about his thoughts (Dr. Byrd is the owner of Horsemen’s Laboratory in Mahomet, IL.). While the previously used methods each had merit, he believes it may be best now to consider treating for worms according to the horse’s parasite egg shedding rate. Like with your child, you don’t run them to the doctor monthly to give them a dose of cold prevention medication. Gone are the days when a spoon of castor oil every week was thought to be a preventative measure.
Absolutely, vaccines will prevent certain viral and infectious diseases; worms are not the same and cannot be treated as such. However, if, as seen in historical data, the worms grow resistant to over use of drugs, Dr. Byrd believes we cannot depend on symptoms to determine when a horse has a parasite problem. Because most horses exhibit no symptoms of worms even when their fecal egg count is very high. By doing a fecal egg count Dr. Byrd can determine the effectiveness of a horse’s worm control program. Ideally, rather than treating regularly, you should test regularly, at predetermined intervals and once egg counts exceed a set number (generally accepted 200 eggs/gm), you would then treat based on the measure of infestation. If a horse’s egg count remains negative or under 200 eggs/gm for a year Horsemen’s Laboratory recommends deworming once a year in late fall or early winter with an Ivermectin and praziquantel combination. This program appears to be best method of slowing the development of resistance in worms to the medication approved for use in horses today. Since there are not any other medications being tested for approval at this time it is critical to slow the development of resistance to the medication we have presently available.
The next question should be, how to treat to prevent worms from becoming a problem at all in the future, or to greatly reduce the risk to a tolerable level.
This article was written by Stephen Flynn for Horsemen’s Laboratory. Stephen Flynn is a freelance writer who served in the U.S. Army and is now retired. He graduated from American Intercontinental University with a Master’s Degree.
(1) 40-Year-Old Rotational Deworming Strategy Has Led to Resistance, 7/8/2015
Abby Strawder, Senior Reputation Manager, Sullivan Higdon & Sink
http://www.equinefecaleggcount.com/ The Horseman’s Laboratory Blog
(2) Symptoms of Parasite Infestation, Dr. Madalyn Ward, DVM
Holistic Horsekeeping with Madalyn Ward, DVM
(3) AAEP Parasite Control Guidelines, Developed by the AAEP Parasite Control Subcommittee of the AAEP Infectious Disease Committee, Revised 2013