Parasite control in sheep requires plan

RED DEER — Worm control in livestock is an ongoing battle.

Parasites are showing more resistance to chemical treatments so alternative controls are needed, said Michel Levy and John Gilleard of the University of Calgary faculty of veterinary medicine.

Speaking at the recent Alberta sheep symposium held in Red Deer, different approaches were discussed to help producers stay ahead of troublesome parasites that can affect small ruminant livestock.

Few drugs are approved for sheep but a local veterinarian can request a special permit for treatments.

“The role of the local vet is important,” said Levy.

The local practitioner knows regional pastures, worm species in an area and the condition of livestock.

There is only one anthelmintic drug licensed for use in sheep in Canada.

Ivermectin is licensed both as a drench and an injection for sheep. Another product, Flukiver, was recently approved to get rid of barber pole worm in sheep and lambs.

Levy said producers must be careful about the amount used to treat animals and they need to remember goats often need a higher dosage because they met-abolize the drugs faster.

“We sometimes find people are using the dose that is not recommended,” he said.

Levy recommends a drench for sheep. An injection could leave low levels of parasites untreated. Avoid pour-on products for sheep because the absorption is variable.

“Using concentrations that are not adequate is the way to speed up resistance,” he said.

A combination of different classes of drugs may reduce more of the troublesome worms. In the last three to four years, people have combined drugs because the effects are additive. This is becoming more common in Australia and New Zealand.

Judicious use of treatments is recommended where anti-parasite drugs known as anthelmintics are only used to reduce fecal egg counts when necessary.

If animals are treated they should look better afterward. A fecal egg count should show a decrease of at least 95 percent 14 to 16 days after treatment.

If there is 90 percent reduction in the next fecal egg count, by definition the treatment did not work, said Levy.

Besides resistance, reduced efficacy may be linked to insufficient dosage because the animal’s weight was underestimated.

Other causes are faulty equipment, poor administration technique and inactive medications that are out of date or were incorrectly stored.

“Be careful about using drugs where you don’t know their source. You don’t know what the compounds are,” he said.

Producers who have reduced parasite loads are taking a more integrated approach to keep sheep from ingesting eggs while on pasture.

Safe pastures like harvested hay fields, tilled fields or areas that were not grazed or were left idle for 90 days in summer or 180 days in winter can help reduce parasites.

Farmers are also encouraged not to treat every animal. They should also find out what kind of worm is present so the treatment can be targeted.

It is recommended producers treat ewes in spring when few worms are present and they shouldn’t assume once a year treatment is enough.

Lambs may need treatment later in the season based on fecal egg counts.

When a ewe or lamb dies, producers should work with a veterinarian to do a necropsy and look in the fourth stomach to search for the presence of worms.

Producers could also try the concept of refugia, which is a way to dilute the resistant stages by having susceptible worms make up most of the parasite population.

Introducing refugia can slow resistance. This means decreasing the frequency of dewormers and requires producers to follow several steps:

  • Do not treat every animal.
  • Leave animals in a dry lot for 48 hours after treatment so they can shed what remains of the worms before moving to a clean pasture.
  • Do not deworm when there is a low level of pasture infestation or parasites in the sheep.

Levy and Gilleard have been running ongoing resistance tests on flocks in Alberta and Saskatchewan since 2014 and have seen a lot of variability in resistance to treatments.

Their 2017 results showed more farms are reducing their parasite levels.

Further research is trying different chemistries because scientists know the population of worms can change on a farm.

Other work is investigating the role of deer in translocation of sheep roundworms.

A coccidiosis study is also proposed to learn prevalence, species and the effectiveness of current treatments.

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