Fall Wormer Timing: How cold is cold enough?

We are expecting our first bit of freezing temps this weekend. Bar changes in the forecast, we are anticipating temps to drop to 32 for 3-4 hours.

Is that enough cold weather to proceed with fall worm treatment? Or should I wait for more cold weather.

I used Quest Plus in the spring (April) and have Equimax at the ready for this round.

I live in a warm climate so it might be while before we get more cold weather.

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I usually wait until the first hard frost or thereabouts…

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What qualifies as a hard frost? Visible frosting on grass etc in the am?

I’m in FL so my ideas of hard frost maybe warped

To me, a hard frost is when the frost remains on grass after sun is up.
So, here in the Midwest, we’ve already had a week or more of frost overnight, still on the grass when I feed around 7A.
But by the time I reopen the big pasture (closed overnight) it has mostly melted.
Ground is still soft too, another indicator.
In FL do you ever meet these criteria?
I’d ask my vet for timing if I didn’t have my markers.


In my neck of the woods, NW FL, yes. Maybe not a week of consecutive frost days though… and more likely to have cold weather in Jan/Feb

We had a wet summer (pretty normal) so no extended period of hot, dry weather to knock back worm populations.

Vet’s general advice is worm spring / fall @ 6 months apart and do a FEC 8-12 weeks post worming if any doubts.

That’s what my vet said too.

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The point about frost is to kill off the larvae on the grass. If the horse is not in grass or frost comes very late or not at all, it messes with the worming schedule. I do think it’s important to worm in early Spring when the encysted larva are ready to burst off the abdominal walls so it makes sense to do October /March regardless of the weather.

Hard freeze is what you want. That’s generally 27F or lower, for a few hours. Long enough to kill sensitive vegetation that doesn’t croak with just an hour at 30

the point of the hard freeze is to pretty much kill botflies :slight_smile:

Strongyle larva survive very very cold conditions. And once temperatures get below around 45*F, the strongyle eggs won’t morph into infective larva (which are easily killed by cold)

Cold does do a decent job on tapeworm mites, so that’s another consideration

If it’s going to be another 1-2 months before you get a really hard freeze, then 6-7 months after the QP is good timing for the Equimax.

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It probably will be and horses will be 7 months from QP in 10-14 days. So I’ll probably go ahead and deworm them post “not so hard frost”.

If it matters, I’m over seeded with winter grazing and horses graze year round. No evidence of bot fly eggs on horses this year. I plan to do FEC around new year to get a reading on where we are at.

totally appropriate. While it’s nice to be able to kill internal bots after all the botflies are gone (and bot eggs off the horse), as long as you’re killing them every 6-ish months give or take, that’s all that’s really needed.

Lucky you! Some years we don’t have many, but they’re really in full swing here lately

Make sure the FEC is at least 10 weeks, even 12, after this Equimax, to get the best reading

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10-12 weeks! Got it!

I’m tempted to do a pre deworming FEC on one or both as well. For educational purposes

ETA same here. I guess 3/4 years ago we had the worst bot fly season in my memory. I was scraping the eggs off the horses like crazy :face_vomiting:

As long as it’s been 3-4 weeks past the “good for” duration of your last drug, this is the perfect time for a FEC.

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No other wormers since April. I live around the corner from the vet clinic so I can drop them some samples off easy peasy.

I’m curious to see 1. If one horse has a higher worm load than the other and 2. If post worming FEC shows effective reduction as resistance to ivermectin is noted in my area

I never wait for a hard frost either but I dewormed this year after we had one because finding any dewormer lately is not easy to do.

I bought my equimax early and have been hoarding it in my kitchen.

That’s real world farmhouse decor lol


What is your area? We’ve only just published a study definitively showing cyathostomin macrocyclic lactone resistance in the US in a single population, so if there is indeed another population with known resistance, it would be great to collaborate with local veterinarians to do a formal study.

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NW FL. I’m not sure if it’s officially studied or whatever.

I’m in Middle Tennessee, where a hardfrost is by whose definition; being from the OH/PA border, that means January down here:)

We had a couple days of good frost last week but experienced a tank top/shorts warm-up immediately following. I held off working and sure enough there were bot eggs on the horses.

My horses are home on big pasture, therefore only get de-wormed twice yearly unless the fecals indicate otherwise. That means I’m waiting until December, maybe even January.

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The formal definition of “hard freeze” is along the lines of:

" when the temperature reaches 28°-or-lower for at least a few hours. It usually means that many types of plants and most seasonal vegetation will be destroyed ."
(from weather.com)

The Almanac defines it as
“A moderate freeze (between 28 and 25 degrees F), sometimes called a hard freeze, can cause wide destruction to most plants.”

(and then there’s severe freeze, below 25)

It’s not a subjective thing. It’s not just frost (and if you’re dry enough, there may not even be any frost at all)

The point really is about the botflies - what does it take to (mostly) kill them off for the season.

This is all aimed at timing the Fall deworming to maximize the benefits.

Some years we don’t get a hard freeze until well into December . I don’t wait until then to use Equimax, because at that point, we’re in the same category as those who don’t get a hard freeze, so at most I wait about 7 months from the Spring deworming (usually April) to do the Fall deworming.

We had 2 hard freezes in a row about 10 days ago - 27 here. And while it has since been in the 60s to even upper 70s for most of the last 9/10 days, and most of those nights above to well above freezing, it’s still as good as done, and I’ll be Equimaxing everyone this coming week, especially since the next few nights will be in the freezing to slightly below, and with cooler days ahead.

We will still get an occasional botfly who found a cozy hidey hole, will see a few more bot eggs on for another few weeks. But I don’t want to wait 9+ months do kill what’s accumulated since the Spring.


Looks like we will stay above freezing tonight. My plan is to run fecal samples over to vet Monday or Tuesday and then deworm as soon as samples collected.

My old guy is on SMZs (for his scratches that just won’t die). I guess I better double check with vet to make sure it’s ok to give him dewormer on top of that.