Deworming beef cattle is standard recommended practice, but dewormers may not be doing their job.
Resistance has been confirmed with many deworming products in the United States. Studies need to be done in Canada to find out if money is going down the drain.
Producers used to deworm only when worm loads in their cattle were at critical levels and damage was already extensive. In the 1980s, the importance of light parasite loads was revealed. A small number of worms was shown to reduce reproductive efficiency, rate of gain, carcass quality and milk production and boost death and disease through the worms’ effect on the immune system.
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A U.S. study calculated that parasites add $190 to the cost of raising a beef animal. It’s probably lower in Canada because this country’s cooler climate inhibits worm reproduction, but it’s still significant.
Fewer parasites are needed to trigger economic losses as production systems become more efficient, which is why it’s now the norm to deworm at strategic times of the year, using products that are highly efficacious.
Strategic deworming prevents economic loss and reduces parasite contamination. It is more than just applying a dewormer. Timing is based on season and type of grazing.
The key is for the medication to stop worm egg shedding in the early part of the grazing season when it has the most impact. Parasites continue to cause damage if they are not killed by a dewormer, but it also ensures continued contamination of the environment and future infestations.
The failure of pour-ons to eliminate worm egg shedding was identified shortly after their introduction to the U.S. market. Producers were seeing continued shedding of eggs and these worms grew up to develop resistance to the macrocyclic lactone (ML) compounds, including ivermectin, doramectin, eprinomectin and moxidectin.
Resistance builds up as worms are exposed to chemical residues in the feces and in the animals. These persistent residues are the problem and recent data show that parasite resistance is a real threat where ML pour-ons have been used for several years.
ML pour-ons are also less efficient because of a lack of consistent absorption into the bloodstream when compared to injectable products. For example, one study showed that while injectable doramectin was 90 percent absorbed, the pour-on formulation was only 15 percent absorbed. This poor absorption means animals may not receive a therapeutic dose, the parasites could survive or they could develop resistance.
Further studies suggest that if parasites are resistant to the first ML treatment, worms are far more likely to be resistant to the second treatment.
In 1999, researchers in Louisiana checked fecal samples for eight weeks after treatment. They found that efficacy of doramectin was 50 to 79 percent while ivermectin was 43 to 85 percent. Both were pour-ons.
More recent data from 2008 show an overall efficacy of 66 percent with pour-ons. Ivomec was 72.3 percent, ivermectin (generic) was 59.7 percent, Dectomax 78.9 percent and cydectin 67.2 percent.
Injectables fared a little better with an overall 72.5 percent efficacy. Ivomec came in at 76.2 percent, Ivomec Plus at 42.6 percent, Dectomax at 89.9 percent, Cydectin at 98.1 percent and generic ivermectin at 50 percent.
Safe-Guard/Panacur drench had more than 99 percent efficacy. Combining this product with an ML injectable or pour-on of Ivomec, Cydectin or Dectomax boosts efficacy to 99.9 percent.
