Problems arise in control of external parasites in beef cattle

External parasites, such as biting or sucking lice, create an obvious problem with cattle.

All lice cause irritation and this irritation causes rubbing, licking, itching and hair loss. Itchy cattle will rub on fences, buildings, trees and other fixtures and can even damage these structures.

It seems to be occurring more frequently despite the frequent use of pour-on endectocides. It is no wonder that beef producers are focused on external parasites when setting up their parasite control programs.

External parasites may be gaining greater resistance to pour-on products. As well, there may be issues with how we apply the treatments. It’s an important and emerging problem in cattle herds.

A less obvious problem is internal parasites known as gastrointestinal nematodes (GIN) or roundworms, which are widespread in grazing cattle populations and can cause subclinical production losses that go unnoticed.

It is challenging to determine the economic benefit of treating cattle for the control of roundworms, but one study suggested that the economic benefit of GIN management to the cattle industry is 2.5 times greater than the economic benefit of the use of growth promoters.

The pour-on endectocides that most producers use provide control against both internal and external parasites but our strategy of treating in the fall is largely targeted against external parasite control.

Most producers don’t use any form of diagnostic strategies to evaluate the GIN burden in their herds and simply blanket treat all animals. This may also be part of the reason for a gradual rise in the levels of anthelmintic resistance in the GIN population.

A study published in the most recent edition of the Canadian Veterinary Journal sheds light on how western Canadian cow-calf producers use parasite control products.

The study published by Dr. Felicity Wills, Dr. Fabienne Uehlinger and other researchers from the University of Saskatchewan, surveyed producers enrolled in the western Canadian cow-calf surveillance network. In this study, 95 percent of the surveyed producers treated cows and replacement heifers with parasite control products. However, only 46 percent of producers treated calves.

Almost all producers relied on a single fall treatment with a pour-on endectocide with only four percent using other parasite control products, such as oral drenching or in-feed products (usually in combination with pour-on endectocides). Most producers estimated weights in order to calculate dosages for the pour-on products.

This data is similar to other surveys published in 2016 from Alberta and in recent data from the National Animal Health Management Survey in the United States that showed similar high percentages of producers relying on parasite control products administered in the fall. The vast majority of cow-calf producers relied on the use of a pour-on endectocide, which is a macrocyclic lactone drug, in the fall of the year.

This recent study also asked producers about why they made these treatment decisions. The primary motivation was to control external parasites. There was much less emphasis on trying to control internal parasites, such as roundworms.

Few producers used fecal egg counts to evaluate the internal parasite load in their cattle herd or to monitor the effectiveness of treatment. Only 32 percent of producers had a fecal egg count performed in their cattle in the three years prior to the survey.

Fecal egg counts are an inexpensive way to evaluate the need for internal parasite control. It also helps to evaluate the level of resistance within the GIN population in the herd and the effectiveness of the treatments being used.

We know that the optimum timing for the control of internal parasites is a spring application of a parasite control product. Several species of roundworms are showing increased levels of resistance to the pour-on products (macrocyclic lactones) that most producers use. The combination of blanket treatment, lack of diagnostics and targeting treatments entirely based on external parasite control may be making the resistance problem worse.

Conducting fecal egg counts may be a first step to determining if you need to alter your parasite control program to better control roundworms.

Fecal samples from 20 cows in the spring and from 20 calves in the fall can provide useful information about the internal parasite burden. You may want to discuss with your veterinarian whether a spring treatment with a product such as fenbendazole is warranted in your herd to better control internal parasites.

We haven’t been precise in our management of internal parasites. Consult your veterinarian about an appropriate parasite control program for your herd.

Remember to follow the 5 Cs as described by Dr. Gilleard at the University of Calgary:

  • Use the correct product.
  • Apply to the correct class of animal.
  • Apply at the correct time.
  • Use the correct dose.
  • Check for efficacy using fecal egg counts.

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