Ivermectin once considered ‘wonder drug’ | Researchers study parasite species to find new treatments
Roundworm control has been changing for the last five years be-cause of increased resistance to common treatments such as ivermectin, said a parasitologist from the University of Calgary.
New ways of identifying the different types of worms are needed so more precise treatment can be used, said John Gilleard of the university’s faculty of veterinary medicine.
“Our understanding of the epidemiology of these organisms is very limited, and we really need to move toward more evidence-based control,” he told the university’s annual beef conference held June 19-20.
No significant studies have been done in Western Canada in the last 20 years that could help gain a better understanding of a large group of parasites affecting livestock.
The livestock industry typically relies on ivermectin to control internal and external pests.
“They were the making of their own undoing,” he said. “They were truly wonder drugs at the time.”
No treatments have been produced since the 1980s that worked as well as a broad spectrum treatment on internal and external parasites.
Gastro intestinal nematodes of cattle, or roundworms, inhabit the abomasums and small and large intestines. They all follow the same life cycles, in which they are picked up in pasture, move through the animal and are eventually passed back into the grass via feces.
Diagnosis still relies on fecal egg counts, in which feces are collected and the eggs counted. However, this method does not identify species that may be found in sheep, horses or cattle. It is a slow, insensitive and variable test for cattle.
“The issue of parasite species has been neglected for a long time,” Gilleard said. “It does matter what species we are talking about. The impact on production varies a lot.”
It is worthwhile knowing which among the four predominant species are infesting an animal so the best treatment can be made.
Merck collected samples in Nov-ember 2012 from farms across Canada: 23 in Alberta, one in British Columbia, seven in Saskatchewan, two in Manitoba, 16 in Ontario and one in Quebec.
They found 15 to 20 eggs per gram of feces, which is enough to affect an animal. A PCR test identified the species with genetic sequencing.
A good mix of species was found.
Ostertagi ,or brown stomach worm, is the dominant species in Canada.
One-third of Alberta cases did not respond well to treatment, meaning ivermectin is no longer guaranteed to remove all roundworms from Canadian cattle.
Most producers treat in the fall, but two to three doses may be required.
The current treatment practice is driven by convenience because ectoparasite control for lice and grubs works well on internal parasites such as roundworms.
“We are locked into using these drugs for ectoparasite control, but the problem with that is we are selecting for resistance for roundworms at the same time,” Gilleard said.
Survivors contaminate pastures the following year but the incidence of worms may vary from farm to farm and year to year.
“There has been no real research on the epidemiology of overwintering nematodes in pastures in Western Canada for 25 years,” said Doug Colwell of Agriculture Canada.
Fecal samples from calves in fall will assess how well parasite control is working. Fecal samples from cows in spring will assess how well the fall treatment worked.
It is not known how well these pests survive in a western Canadian winter, but researchers know the weather has an impact.
“These elements, the larvae and the eggs, are susceptible to environmental conditions,” Colwell said. “Moisture is one of those key conditions.”
Past data show there are fewer worms in dry weather.
More recently, researchers developed a geographic map of Alberta by collecting blood and feces samples from calves assembled at 20 auction markets as well as one feedlot from 2008-10.
The samples were frozen, which affects worms.
Blood samples were subjected to an ELISA test to assess the antibodies the animals produced in response to infestations.
This information was added to the map along with land use and environmental conditions, which provided a clear picture of the temperature, precipitation, humidity, growing days and evaporation from each location.
Three risk categories — low, moderate and high — were developed.
Infestation levels were low in Alberta during the study period with the exception of high risk areas in the middle of central Alberta and a small region south of Calgary that was relatively moist in 2010. Southeastern Alberta and the Grande Prairie region were relatively low risk.
Risk changes from year to year.
Researchers also found that egg counts did not correlate with environmental factors. Antibody levels correlated well with environmental parameters, but more information is needed.
An unexpected result was a large difference between egg count data and ELISA data. The antibody test indicated the cattle had more infestations than previously thought.
“A lot more calves are exposed to nematodes than is picked by the fecal egg counts,” Colwell said.
Vaccine development is far off because nematodes and their interaction with the host are complicated, he added.