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Strategies for reducing parasite resistance

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We heard years ago about insect resistance developing with with fly tags. I believe the first ones on the market were called Bovaid. With no other competing products, after a few years researchers noticed resistance developing in pests the tags were supposed to repel.

Soon other companies were making tags using different chemical families as active ingredients, so producers could rotate tags and chemicals to reduce the risk of resistance developing.

Horn flies are visible on the backs of animals as they continually feed, so if the tags or other fly-control methods become ineffective flies are immediately noticeable. This is especially true on bulls as they attract more flies and hundreds to thousands will be visibly feeding if control is inadequate.

In the last several years a product called Cylence was developed as a pour-on for flies. Its effectiveness lasts about two-thirds as long as leading fly tags.

The pour-on product is used lots because it was easy to apply — cows didn’t have to caught and held. Timing-wise it could be applied when cattle were turned out to pasture or later in the season if cattle were being processed during the summer.

Resistance Developing

But we are now starting to see some resistance to Cylence. For producers who have used it several years in a row, flies can now be seen on the backs of cattle way short of the 60-day 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 and Cylence.

I am sure there are other products or trade names 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.

This rotation prevents resistance from developing and the products are therefore more effective. Cattle weight gains are improved, which is one of the main reasons for using the products. Next time you are out in a pasture, use binoculars and check on fly numbers.

When processing cattle flies are very easy to spot surface feeding over the backs of cattle especially on the herd sires. If you do nothing else in a pest control program, treat breeding bulls on turnout.

Internal Parasites

With internal parasites (worms), resistance to endectocides has been shown in the U.S. in a few instances. Most of that is in areas of the southern states where treatment for internal parasites is administered several times a year. However, we are starting to see these similar results in Canada as researchers are finding resistance to a fair degree. So what do we do as conscientious producers? There are several things.

First, don’t over-treat. If cattle need treating treat them, but that doesn’t mean everytime they go through the chute a product like Ivermectin is applied. Just because it is cheaper now some producers are treating more often than they used to. At the same time you don’t want to underdose either, as that under dose in fact leads to resistant pests. This may have been the case in some instances before when the endectocides were very pricey. Producers apply product according to animal weight — some ranches have scales so accuracy of dosing is very good, while others can estimate weight very well.

If you are not sure if treatment is necessary, consult your herd veterinarian. He or she may perform a few fecal tests to determine the worm load. With fecals, some tests such as the modified Wisconsin are more accurate at detecting a lighter worm load. If just worms are the problem, changing 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 of which fenbendazole (SafeGuard) belongs and a drench that contains albendazole (Valbazen).

SafeGuard comes in numerous formulations so cattle can be drenched or the product added into grain or the minerals by prescription for treatment on pasture in mid-summer. Resistance to SafeGuard does not appear to develop in cattle because of the speed with which the product 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 treatment. Tapeworms in cattle have not been deemed significant at reducing production so if this species is found in fecal matter it will also be secondarily eliminated by the SafeGuard treatment.

Winter helps the cause

Fortunately in most of Canada, winter puts parasite transmission on hold making controlling them a lot easier than in the more temperate climates.

But producers need to use their diligence and not overuse the good products we have. It is important to alternate or use products in combination (a combination of Ivermectin and SafeGuard for example) to get a complete kill and hopefully we an avoid resistance from developing.

Checking fecals occasionally on the group and especially the poorest-doing animals will identify whether internal parasites are a problem and what species or group of worms are present.

Work with your veterinarian to devise a treatment protocol and proper timing to control both internal and external parasites. Removing these troublesome parasites will lead to much better production and with today’s higher cattle prices these treatments should yield a good economic return. †

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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