Don’t ignore drug-resistant parasites

Dr. John Gilleard, the associate dean of research and professor of parasitology at the University of Calgary faculty of veterinary medicine says most of the studies about drug-resistant worms in cattle were done in the U.S. and Europe but the story they tell is applicable to Western Canada.

Dr. John Gilleard

Dr. John Gilleard

“What has happened in the past 10 years has been a recognition that the macrocyclic lactones (ivermectin and moxidectin — the endectocide pour-ons) are less effective now than they used to be,” he says.

When you treated animals with an ivermectin pour-on 15 years ago you would expect zero egg counts after treatment. “If you treat cattle today, likely the egg counts will still be moderately high afterward,” he says.

The reasons for drug resistance are complex, but there are two main elements. One is that there are several species of worms and they have different levels of innate drug resistance. The worm we have traditionally been most concerned about is the brown stomach worm, Ostertagia. This one is quite sensitive to ivermectin. Another group of parasites, the most important being Cooperia (in the small intestine), is intrinsically less susceptible to this drug. So what’s happened in the U.S. is a change in species balance and prevalence. There are less Ostertagia and a lot more Cooperia now.

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“By treating intensively with the ivermectins for many years, we have selected for species that are less affected by this drug,” he explains.

The other element is drug resistance within a species. “We’ve selected for mutations within that species that make them more resistant. Both of these things are happening, and this is why we are now getting poor responses to ivermectin.”

Will the resistance problem happen here? “There is no reason to believe it wouldn’t,” says Gilleard. In a recent study looking at egg counts after treatment in young cattle going into Alberta feedlots the findings were similar to what has been seen in the U.S.

An older class of drugs which includes fenbendazole known as the white dewormers have not been used as much in recent years, predominantly because they only kill worms in the GI tract and are not active against migrating grubs or external parasites like lice. Ivermectin and moxidectin have been considered much more useful because they could do all these jobs at the same time and were easy to use as a pour-on.

“In recent years the white drenches have been used less, and there hasn’t been much resistance issue in cattle. So we do have this class of drugs to fall back on, even though they are more difficult to apply. They may be more effective now than ivermectin or moxidectin against the GI tract roundworms,” he says.

Gilleard says roundworms are the major parasites of cattle and there are many different species.

Some producers are now combining different classes of drugs to get around the resistance. This is already happening in sheep. Deworming drugs have been used more intensively in sheep and the resistance problem is much further along than in cattle. Single drugs don’t work at all anymore. In Australia and New Zealand, most sheep producers now use combinations of three different drug classes. “This is the only recourse they have, and even that is beginning to fail,” says Gilleard.

“This is a warning for cattle producers. We may end up in the same dilemma. We need to become more intelligent about the way we use these drugs. Also, we need to monitor — to know whether or not the drugs are actually working,” he explains.

Most producers just treat their cattle and assume it works. But Gilleard says a followup fecal sample after deworming would provide a clue as to the degree of resistance in your pastures or feedlots.

“The problem right now is that we’ve been taking (these drugs) for granted and splashing them around,” he says.

“It’s important to continue to do strategic deworming, targeting the animals that need it, and when.”

Don’t think Canada’s cold climate will break the cycle for you. The larvae can survive freezing on pasture, and significant numbers of worms survive inside the animals. If cattle are not dewormed properly, worms begin laying eggs again the next spring and reinfect the pasture.

The major limiting climatic factor is dryness, since eggs and larvae don’t survive on dry pastures. Again, that is little help since the moisture in this part of the world is more than sufficient during grazing season to allow buildup of pasture contamination with infective larvae. “Moisture levels are actually higher here than in many parts of the U.S. where parasites are known to have significant production impacts,” says Gilleard.

There are ways you can break the parasite’s life cycle, however, such as rotating different classes of livestock since worms are species specific and won’t continue their life cycle in a different host.

“Also we need to realize that cattle grazing pastures from the previous year will have some worm burdens, and we need to make sure these animals don’t become a source of pasture contamination for the following year. Some people advise cattlemen to treat in the fall, at the end of grazing season. Transmission does not occur in winter so if you treat in late fall this will minimize pasture contamination the next year.”

The second time to treat is after those animals have been grazing in spring for about six weeks. By then they have picked up worm eggs from the pasture but the worms have not matured enough in the GI tract to begin shedding many eggs yet.

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