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Preventing parasite resistance to worms and flies

Developing and using good protocols will keep these valuable treatments working for you and your herd

Preventing parasite resistance to worms and flies

We heard years ago about resistance with fly tags.

I believe the first one was called Bovaid and with no other tags on the market, researchers noticed resistance developing after a few years. Soon other companies were making tags with a different family of chemicals in them, so producers could rotate them and not allow resistance to develop.

The great thing about fly resistance is the horn flies are visible on the back as they continually feed. So if the tags or other fly control methods become ineffective, flies are immediately visible on the back. This is especially true on bulls as they attract more flies — hundreds to thousands will be visible feeding if control is inadequate.

In the last several years, a new product called Cylence was developed as a pour-on for flies. Its effectiveness lasts about two-thirds of the time as fly tags, but was used a lot because it was easy to apply. The cows did not have to be caught and it could be applied at turnout to pasture (or if cattle were processed during the summer for some reason).

We are now starting to see some resistance to the Cylence. For producers who have used it several years in a row, flies can be seen on the backs of cattle way short of the usual effectiveness window. We are not getting the bang for the buck so to speak.

If producers do see flies present much sooner than expected, it is time to change products. The best remedy is to keep cycling through the different tags and other pour-on products (such as Saber or Boss as well as Cylence). I am sure there are other products or trade names out there I am not aware of and new ones are always being worked on. Your veterinarian can advise which product is best depending if lice, flies, or even ticks become the significant parasite.

Length of efficacy varies, so timing is always critical in applying these products. Rotation prevents resistance from developing and weight gains are improved on the cattle, which is what you as producers strive for. Next time you are out in your pastures, use binoculars and check for fly numbers. If processing, flies are very easy to spot when surface feeding on the backs of cattle, especially on the herd sires. If you do nothing else, treat bulls on turnout for breeding.

With internal parasites (worms), resistance (or lack of efficacy) has been shown in the U.S. and Canada to the pour-on and injectable endectocides. This has been a bigger problem in areas of the southern states where internal parasites are treated several times a year, but in Canada we are starting to see the same thing. Researchers are currently looking at this in Canada and a fair amount of resistance is being found.

So what do we do as a conscientious producer? There are several things you can do.

First, don’t overtreat. If cattle need treating, then treat them. But that doesn’t mean applying a product such as Ivermectin every time they go through the chute. Just because it is now cheaper, some producers are treating more often than they used to.

You also don’t want to underdose, as that can also speed the development of resistance as well. This may have been previously the case in some instances when endectocides were very pricey. Producers apply according to weight and some have scales, so accuracy of dosing is very good. Others can estimate the weight very well.

In cases where you are not sure if treatment is necessary, consult with your herd veterinarian. He or she may perform a few fecals to see what the worm load is. (With fecals, some tests, such as the modified Wisconsin, are more accurate at detecting a lighter worm load.)

If just worms are the problem, a change to a different class of dewormer is necessary. All the pour-ons belong to the macrocytic lactones (a family of dewormers). Another family is the benzimidazoles, which includes fenbendazole (such as Safe-Guard) and a drench that contains albendazole (such as Valbazen). Safe-Guard comes in numerous formulations, so it can be drenched or added to grain or minerals (by prescription) for treatment at pasture in the middle of the summer.

Resistance does not appear to be developing at this time to Safe-Guard in cattle because of the quickness with which it kills the parasite. As with most parasitic conditions, a very low level of infection elicits some natural resistance in the animal.

I personally have seen this with tapeworms. Young bison may have a heavy load and yet the adult bison are clean and this was without deworming them for the tapeworms. Tapeworms in cattle have not been deemed significant in reducing production so if this species is found on the fecal, it will also be secondarily eliminated by the Safe-Guard treatment.

Fortunately in most of Canada, winter puts parasites transmission on hold making controlling them a lot easier than in more temperate climates. If producers use diligence and don’t overuse the good products we have, alternate, or use products in combination to get a complete kill, we hopefully can avoid resistance developing.

Checking fecals occasionally on a percentage of the group (and especially the poorest-performing animals) will identify whether internal parasites are a problem and what species or group of worms are the problem. Work with your veterinarian to devise a treatment protocol and determine what period is best to deworm for internal parasites and treat for external parasites (primarily lice) in your region. Removing these troublesome parasites will lead to much better production and with today’s high prices, these treatments should yield a good economic return.

Think of treating for flies and worms midsummer if, for instance, you are needing to handle or move the cattle. Consider cattle oilers for flies and a script of a dewormer in the mineral as two almost labour-free ways to treat cattle for effective results. To measure worms, do fecals on calves or yearlings in mid- to late summer. To measure flies, watch them with binoculars to give you some idea how many the cattle have. Bulls as mentioned have the most so watch them. Using both these procedures if necessary should return economic benefits to the herd.

This article was originally published on the Alberta Farmer Express.

About the author


Roy Lewis is an Alberta-based veterinarian specializing in large-animal practice. He is also a part-time technical services vet for Merck Animal Health.



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