Fall Chores: Deworming Targets

We recommend using ?target? deworming instead of interval and/or rotation deworming. Target deworming means using fecal-egg counts to determine what parasites are infecting your horse and then choose the right drug to get them. This is because of the increasing evidence of parasite resistance to today?s dewormers. Ivermectin and moxidectin remain the only two drugs with no proven resistance problems.

The one drawback to target deworming, however, is in regard to parasites that don’t show up in fecal egg counts. The best time to eliminate these buggers, tapeworms, bot flies and small strongyles, is fall.

Tapeworms are primarily picked up on pasture because their infective stage is carried inside a tiny soil mite. When horses are pulled off pasture, or following the first good freeze when the mites become inactive, the risk of infection drops off and it is the ideal time to rid the horse of tapeworms.

Bots refers to bot flies, which lay their eggs on the horse’s legs or around the mouth. Larvae are activated to hatch by saliva, then either crawl directly into the mouth or are picked up on the horse’s lips and tongue when he is rubbing his lower legs.

Small strongyles also accumulate over the grazing season, with peak levels often found on fall pastures. Although this has not been studied in detail, there is some evidence to suggest they tend to go into dormant tissue stages over the winter, only to emerge en masse in late winter, in preparation for grazing season again.

Fecal flotations, the most frequent method of counting worm eggs, are not good at picking up tapeworms. In addition, the bot flies don’t actually lay eggs in the horse.

Small strongyle eggs may be detected on these counts, but the level doesn’t necessarily reflect the numbers present in the tissue. In short, fecals can’t be relied upon to direct deworming decisions for these parasites. Assuming they are there and deworming for them is a safer thing to do.


The only drugs that can kill both small strongyles and bots are ivermectin and moxidectin.

Moxidectin kills more immature life stages of small strongyles than ivermectin does, but if you’ve dewormed regularly with ivermectin over the high-risk seasons (spring and summer) there probably isn’t a buildup of encysted forms. Using a combination product of ivermectin or moxidectin with praziquantel will also kill tapeworms.

You can also deworm with plain moxidectin or ivermectin, followed by one month’s worth of a daily dewormer containing pyrantel tartrate. A one-month course of pyrantel tartrate will eliminate tapeworms. The advantage to the one-month daily dewormer is that it will also prevent reinfection with any small strongyles that may have still been present in the environment after the first deworming with moxidectin or ivermectin.

If you prefer, use plain ivermectin or moxidectin with a double-dose of pyrantel pamoate, given once or twice (one study showed return of tapeworm segments in the manure when only one deworming was used).

By the end of grazing season, the horse is at high risk for burdens of bot-fly larvae, tapeworms and small strongyles. Simple fecal flotations are not reliable in detecting these infections. Targeted deworming sends your horse into winter free of nutrient-robbing parasites and their colic risk.

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