When you think “worms,” you may think summer, but the damage caused by these pests can last longer than the warm temperatures. As the summer’s load of bot-fly larvae grow and mature inside the horse, they can cause significant damage to the lining of the stomach or small intestine. After they detach at maturity, they leave behind raw areas that are susceptible to further damage from stomach acid or bile. In large enough numbers, bots can interfere with the passage of food through the digestive tract.
When the weather cools and the flies stop laying eggs, bot problems are just beginning inside your horse. Larvae remain attached to — and feeding from — the lining of your horse’s stomach or intestine for seven months before dropping off as mature larvae.
If you only do one bot deworming with ivermectin after the first frost, you may miss some of the pests. Bot eggs on the legs (those tiny, sticky yellow to whitish-looking specs) only hatch when stimulated by warmth and moisture from the horse’s tongue. The young larvae then penetrate the tissues inside the mouth and take three weeks to reach the stomach. Until they emerge into the cavity of the stomach or small intestine, they’re protected from the effects of the ivermectin.
Once fly season is over, remove any bot eggs from the horse’s legs with a bot-fly-egg removal tool, or stimulate them to hatch by rubbing with a cloth soaked in warm water, then wash the legs. Deworm the horse with ivermectin, and repeat in six weeks to allow enough time for all young larvae to have made their journey into the stomach and be destroyed by the dewormer drug.
Another villain that can cause problems well into the winter months is the tapeworm. Horses with tapeworms often are free of the poor hair coat, pot belly and general unthriftiness characteristic of other parasites, but they’re at risk for something worse: colic.
About 20% of spasmodic “gas” colics are related to tapeworms. Over 80% of impactions that occur at the junction of the small intestine with the large (ileocecal valve area) are caused by tapeworms attached in that location. And, virtually all ileocecal intussusceptions, a telescoping of the small bowel into the cecum that almost always requires surgery, is related to tapeworms.
Winter is a high-risk time for colic as it is, so don’t compound that by overlooking tapeworms. The advent of combination paste dewormers that contain praziquantel has made deworming for tapes easy. Use a combination ivermectin and praziquantel product for one of your bot dewormings.
Problems with threadworms, strongyles, pinworms and other parasites quiet down over winter because infective larvae are in short supply, except for cyanthostomiasis, AKA small strongyles.
The immature larvae of this parasite burrow into the wall of the large intestine where many will go into a dormant stage, curled up inside the walls of tiny cysts. They can remain in this state for months or even years, so numbers grow steadily over the high exposure seasons, often into the millions. When large numbers of the larvae awaken simultaneously and emerge from their cysts, they can cause colic, diarrhea and severe weight loss.
The triggers for these mass emergences are unclear. Some experts believe that dropping numbers of mature parasites in the colon may be cause the awakening. Other experts s think the maturation is in response to a “biological clock” that signals the parasites to resume their growth so that they are egg-laying adults in time to take advantage of optimal pasture conditions in the spring.
Even horses on frequent dewormings year round can run into problems with cyanthostomiasis because the life stages up to and including the encysted forms of the parasite are not affected by regular doses of any of the paste dewormers. Symptoms related to emergence vary from unexplained weight loss and depression to severe colic and/or diarrhea that may be mistaken for colitis or a serious infection (e.g. salmonella infection).
We believe that the best deworming treatment to kill the encysted forms before they get a chance to do their damage is fenbendazole (Panacur or Safeguard), given at double regular dose for five consecutive days.
If you’re using the Panacur Power Pak brand, follow product label because this already includes instructions for double dosing. If using regular Panacur or Safeguard, double the dosage instructions on the tube.
The timing of cyathostome emergence is unpredictable, but most horses run into problems late in the winter. Timing a larvicidal deworming with fenbendazole for late December/early January will usually kill these parasites before they have a chance to do their damage to the horse.
Adequately covering these deworming bases will help your horse hold his weight over winter better, avoid parasite-related intestinal problems and will also mean he isn’t repopulating spring pastures with large numbers of parasites.
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