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Are The Worms Winning?
The Lowdown on Rising Parasite Resistance
At Horsemen's Laboratory,
helping horse owners understand the most effective way to use fecal egg counts (FECs) is part of
our mission; as such, we're constantly on the lookout for new information and
research we can share with our customers.
For several decades, horse owners have been advised to
deworm their horses regularly, with the goal being complete eradication of
parasites. This is no longer the case; parasitologists around the globe have
become alarmed enough about perceived 'resistance' to current classes of
deworming medications that they've revised the recommended guidelines. Now,
instead of getting rid of all of the worms, we're instead supposed to live with
a small population of them. It's a change that some horse owners are finding
difficult to swallow.
Below are some terms and key points from Dr. John's
presentation titled 'Are The Worms Winning? The Lowdown on Rising Parasite
Resistance,' which he gave at the November, 2009, Equestrian Lifestyle Expo at
the Lake County Fairgrounds in northern Illinois. The event organizers felt it
was a topic worth spreading around, so they've asked him to repeat the
presentation for the Illinois Horse Fair audience on Sunday, March 7th. We'll
cover more of this information in-depth over the next few months, but in the
meantime if you have questions, please email us; we'll answer questions in
upcoming issues of Worm Control News.
Terms:
FEC = Fecal Egg Count: Test used to count, under a
microscope, the number of parasite eggs and larvae present in a manure sample.
EPG = Eggs per Gram of Fecal Material: Method of
approximating severity of a horse's worm infection and the rate at which that
horse is contaminating its pasture.
ERP = Egg Reappearance Period: The length of time between
deworming and when eggs start reappearing in a horse's stool. This is often
used to determine how effective a drug is at killing encysted larvae.
FECRT = Fecal Egg Count Reduction Test: May be used to
check for resistance. Perform an FEC before deworming, and an FEC a few days
after, to determine the effectiveness of the deworming medication.
Refugia: In this application, it's the portion of a
horse's parasite population that remains sensitive to deworming medications,
passing on anthelmintic-sensitive genes to offspring and thus slowing the rate
of resistance to medication.
Key points:
- Resistance to dewormer medications has been identified
in both small and large strongyle (bloodworm) populations.
- Dewormers were first introduced in 1940 with
phenothiazine; the most recent deworming medications have been ivermectin
(introduced in 1983), and moxidectin (introduced in 1997).
- Drug-resistant parasites have been identified with all
deworming medications, typically within about a decade after first use. The
exception is ivermectin, but after 17 years of use initial reports of parasite
resistance occurred in the early 2000s.
- There are currently no new deworming medications in the
drug development pipeline.
- The deworming protocols most horse owners are using
stem from a 40 year old plan designed to control the harmful Strongylus
vulgaris or large strongyle. Large strongyles have been nearly eliminated for
years, but we're continuing to deworm our horses as if they're still a threat.
- To slow the development of parasite resistance on your
own property and with your own horses, you can use FECs and FECRTs to identify
how best to deworm your horses. The goal is to maintain a 'dewormer sensitive'
population of parasites (refugia) that pass their genes along, preserving the
effectiveness of available dewormer medications.
- FECs are also used to determine which horses are low,
medium, or high-shedding horses. This doesn't refer to the horsehair on your
jacket; it's how many parasite eggs horses are 'shedding' in their manure.
Deworm high-shedders more frequently, about 4x per year; low-shedders might
only need deworming once or twice annually, to maintain refugia.
- It's important to combine proper pasture management
strategies with your revised deworming schedule, including removal of manure
before eggs hatch into infective larvae.
- Remember the objective of an effective worm control
program is not the killing of adult worms and some larvae in the digestive
tract of the horse, but more importantly, reducing that horse's exposure to
infective eggs and larvae in the first place.
Coming
next month: Refugia - what it is, why it's important now.
Return to the March issue of Worm Control News
Visit the Horsemen's Laboratory Web site
Worm Control News is written and produced by KempEquine.com
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