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Workarounds for dewormer resistance in sheep and goats

Dewormer resistance

As I learned the hard way, you can use all kinds of holistic methods to keep sheep and goats healthy but at some point you'll end up having to rely on pharmaceuticals. Although it wasn't our issue last year, the most common problem requiring that step in our region is internal parasites --- worms.

The trouble is, dewormers have been used with such wild abandon in the U.S. that many don't work any longer. The worst offender --- barberpole worm --- was found to be resistant to three of the four classes of dewormer in every farm studied last year in Virginia and Georgia. Only the nicotinic agents (Levamisole) were still effective in that study, and even that class only worked in 30% of Georgia farms and 50% of Virginia farms. The benzimidazoles (such as Safeguard) and the macrocyclic lactones (such as Ivomec and Cydectin) are now useful only for lesser parasites such as tapeworms.

Luckily, there are a few methods available to make supposedly ineffective drugs more effective. First of all --- don't use them! The alternatives mentioned last year are still a good first step, along with pasture rotation. And, of course, you should use a FAMACHA test so you'll only treat individuals that have a high worm load.

Black-faced sheep

But what if you have a sheep or goat that is dangerously anemic due to barberpole worms...and your farm is showing resistance to all of the available drugs? First of all, make sure you're dosing properly. Even though it doesn't say so on the bottle, goats have a higher metabolism than sheep and should be given twice as high a dose on a body-weight basis as sheep should (or 1.5 times as much for Levamisol). Using less just speeds up dewormer resistance on your farm.

Second, you can prolong the efficacy of certain drugs even if there's already some resistance to the drug on your farm in a few different ways. Keeping your animal off feed for twenty-four hours before dosing will make the drug more effective since food dilutes the dewormer. Second, you can give a full dose of Safeguard then repeat in twelve hours, which was shown in one study to increase the kill rate of the dewormer from 50% to 92%. (With Levamisol, you'll want to wait 24 hours before repeating, while repeating didn't prove effective for the other classes.)

Finally, you can double up different classes of drugs to kill off a lot more worms. For example, if parasites on your farm are resistant enough to three different drugs so each one only kills 60% of the worms, dosing with all three drugs at once will eliminate over 90% of the problem --- pretty good for dewormers that you probably considered useless on your farm!

I hope the worms never get bad enough on our farm that I have to use this information...but I'm posting anyway in case either I (or you) do go there. Good luck...and please never deworm willy-nilly without testing each animal first to see if he or she needs it.

(Information courtesy of Dr. Dahlia O'Brien, Small Ruminant Specialist for the Virginia Extension Service...and really good speaker. If you get a chance to attend one of her workshops, I highly recommend it!)

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An excellent example of how evolution takes place as the environment exerts selection pressure to alter gene frequencies in the population: if a mutation conferring resistance to Agent A occurs in 1 in 10 individuals, and to Agent B also 1 in 10, then only 1 in 100 (ie- 0.1 x 0.1 = 0.01) will be resistant to both agents. Using two agents at once won't eliminate the tendency to increasing resistance, but it will greatly slow down the process.
Comment by doc Mon May 1 07:37:33 2017

Did they say anything about the usage of copper?

I was amazed that goats aren't harmed by having copper boluses in their innards.

While humans can process and need copper in the form of organic compounds, metallic copper is a heavy metal and actually toxic to humans when ingested. Or when some kinds of food are cooked in copper cookware. (In both cases you need long-term expoure to do harm; touching a brass doorhandle or drinking water transported by copper tubing won't produce enough exposure.)

Comment by Roland_Smith Mon May 1 18:29:46 2017

Good question since I share your concerns! That said, their studies have been very much in favor of copper, which has been shown to be 80% or more effective. They do warn against using copper sulfate, which is more bioavailable than the COWP they recommend (and which we use).

I specifically asked them this time around what kind of signs to watch for to see if your animals are getting too much copper. They said the first sign is a very red, coppery urine that looks almost like blood.

What I didn't ask because I suspect the studies are too hard to be worthwhile in animals is --- are there milder negative repercussions that might not have been picked up by their study? How does copper affect the animals' guts long term, and does it impact the intestinal flora?

My minor misgivings aside, it is true that goats have very high copper needs that usually aren't met nutritionally in our climate. And studies so far have supported giving COWP to goats.

Comment by anna Tue May 2 07:19:08 2017

At a guess I'd say that it would be hard for the intestinal flora not to be affected, since copper is such an effective biocide.

OTOH, if a copper bolus would kill a significant fraction of gut bacteria, I would think that you'd notice that in fodder coming out practically undigested.

Comment by Roland_Smith Tue May 2 16:10:24 2017