October/November 2016 Horse Around New Mexico – That Was Then, This is Now (Parasite Management).
That Was Then, This is Now: Parasite Management
by Stacie G. Boswell, DVM — originally published in Horse Around New Mexico, October/November 2016
This article is provided for general information only. It is not a substitute for veterinary examination, diagnosis or treatment. Please consult a licensed veterinarian before beginning any treatment or making decisions about your horse’s care.
The text below was read from the printed magazine using optical character recognition and lightly edited for the web, so that this article can be found and read online. The original article as it appeared in print is reproduced at the foot of this page and is the version of record — where the two differ, the printed version is correct. If you spot an error, please let us know.
that was this is THEN,
BY STACIE G. BOSWELL, DVM, DACVS-LA
Up until relatively recently, veterinary medicine had very few tools. If you are familiar with All Creatures Great and Small by James Herriot, you know that he began his veterinary medical career without three of the major medication mainstays that we have now: steroids, antibiotics, and anthelmintics. Anthelmintics are more commonly known as dewormers or parasiticides. The word is from Latin, "ant"meaning against, and "helminth", referring to a parasitic worm. The only tool Dr. Herriot had available was to manage manure to reduce exposure to parasite eggs.
One of the earliest dewormers was levamisole. Ivermectin came on the market in 1971. These dewormers seemed a miracle cure for killing parasites. However, the worms quickly outsmarted the drugs, and it became clear to scientists, veterinarians, and stockmen that they did not kill 100% of the parasites. Worms that were resistant were able to survive longer, and reproduce better.
Many horsemen then tried "rotational deworming"schedules that were intended to prevent parasite resistance. The thought was that if some worms were not killed by Dewormer A, they could be killed using
Dewormer B, or Dewormer C. Again, parasites developed resistance to multiple dewormer strategies. Daily deworming came into practice, but it turns out that the absolute worst-case scenario of parasite resistance to dewormers is administration of daily dewormer pellets. This type of program quickly selects for worms that survive despite administration of these chemicals.
Inappropriate dosing also contributes to parasite resistance. Another method is the "Panacur
PowerPac", which is a double-dose of fenbendazole administered once daily for five days. It kills some stages of parasites that other dewormers do not. The PowerPac should be used only after fecal evaluation and consultation with your veterinarian.
In light of the history of parasite resistance, scientifically, we have come full circle: Now veterinary medicine recommends manure and pasture management as an important part of appropriate parasite control. These strategies are meant to reduce exposure an animal has to parasite eggs. This is critical
Brand of Sodium Nitrate Solution
Fecasol® is a standardized flotation medum with a specific gravity of 1.200 for use in performing feca analysis. Fecasol may be used for all flotaton techniques. → Vétoquinol NET I GALLON (128 FL. OZ – 379 L)
26 ml =
for ensuring that internal worm numbers (a horse's parasite burden) are minimized.
The bottom line
Deworming horses on a rotational basis four or six times a year is not supported by current science, and is harmful to the environment. Parasite resistance to dewormers is a common problem. Manure and pasture or paddock management is critical for modern parasite control. Fecal egg counts and veterinary consultation should be used to develop an appropriate deworming strategy. It will take into consideration the horse's age, climate, possible parasite exposure, and any special circumstances. Directions and dosage on the dewormer label should be followed precisely because inappropriate dosing is less effective for parasite control,
and may contribute to resistance.
So why did everything change?
In addition to the problem of resistance, dewormers pose two significant concerns for the environment. One is that farm dogs that eat horse feces will ingest the parasiticide that is excreted. Ivermectin is passed out of the body, virtually unchanged in chemical structure, in about 24 hours. Breeds that are highly sensitive to ivermectin (collies, shepherds, and sheepdogs) can ingest a toxic dose of the dewormer by eating the horse's feces. Dewormers also can affect natural, beneficial insect populations and water-dwelling animals. Experiments have shown suppression of insects in and delayed degradation of cattle feces after treatment with parasiticides. Runoff from pastures or paddocks containing feces from recently-dewormed horses can be lethal to a pond or stream's fish population and other water-dwelling creatures.
Testing to worm strategically
The McMaster's fecal egg count method effectively measures the parasite eggs per gram of feces (epg) [see inset]. In horses, The McMaster's fecal egg count consistently detects large and small strongyles and most other intestinal parasites. It is useful for determining which horses actually should be dewormed, and it is an excellent tool for identifying individual horses with heavy infestations. Using the McMaster's test before and after strategic deworming can show the effectiveness of the dewormer and identify farms or horses that harbor
parasites that are resistant to certain dewormers. However, no tool is perfect. Some para-sites shed eggs only intermittently. The bot lifecycle involves the fly laying its eggs directly on the horse, so those eggs will not show up in a fecal evaluation. The Mc- Master's does not reliably detect pinworms (photo 7), tapeworms, or bots; however, other tests for these parasites are available.
Special cases and considerations
Some age groups are more prone to parasite problems. Horses less than three years old need a more intense parasite-control program. Younger individuals are more prone to heavy, life-threatening infestations. Older horses are also at greater risk, possibly due to lower immunity.
Habronema (photo 8) is prevalent throughout the southwestern United States and is more commonly known as the stomach worm. Aberrant migrating larvae or abnormal egg deposition can cause skin lesions, conjunctivitis (inflammation around the eye), or other problems that can be very painful to the horse. While the larvae of Habronema have been identified in fecals, it often is only noted as "summer sores."These sores or eye problems need more than just a dewormer dose, and examination by your veterinarian is advised.
Good news for New Mexico
New Mexico does catch one break when it comes to parasite control. Ultraviolet light and dry conditions destroy parasite eggs. New Mexico's dry high-plains, mountainous, and desert environment is very hostile to equine parasites. The good news is that it is possible to virtually eliminate certain parasites by keeping paddocks and stalls manure-free. Manure management is probably the single most important factor in effectively reducing parasite populations.
Stacie G. Boswell, DVM, DACVS-LA is an equine veterinarian at Western Trails Veterinary Hospital in Edgewood, New Mexico. She may be reached at stacieboswell@gmail.com.
The article as it appeared in print:
Here is a link to the whole magazine on Issuu: https://issuu.com/ceciliakayano/docs/horse_around_new_mexico_octnov_2016.