Convention: Managing Parasites in an Age of Drug Resistance

Managing Parasites in an Age of Drug Resistance

The Shepherd

Dr. Richard Ehrhardt of Michigan State University told the ASI Production, Education and Research Council that gastrointestinal nematodes – especially Haemonchus contortus (barber pole worm) – are “blood sucking, egg-laying machines” that are a major burden to profitable sheep production in many areas of the United States.

These parasites are found throughout the traditional “farm flock” sheep range in the United States, where there is a combination of warm and humid conditions, including irrigated pastures in the West, and warm valleys in colder climates. Although stocking rate is a major factor in determining infection risk, there is concern that the parasite might be moving north and perhaps adapting to other conditions, as well.

Drug use to control them has led to resistant parasites and poor control, Ehrhardt said, but combination drug treatments will slow the rate of resistance and give much better control.

Deworming medications:

Macrocyclic lactones (ivermectins)

Benzimidazoles (fendbenazole like Valbazen)

Imidazothiazole (levamisole like Tramisol)

Ehrhardt recommends that combination drug treatments need to be used in addition to refugia maintenance practices, and treating according to risk and grazing control methods.

Dewormers act additively to achieve good control when the efficacy of individual products are suboptimal. Treatment with approved drugs in sequence is within use guidelines in the United States. Producers need to give all drugs at a full dose. He cautioned that mixing drugs (compounding) is a bad idea if they form a suspension, and this practice is not within use guidelines.

Farming parasites

The goal of farming these parasites is to maintain refugia without compromising health, performance or welfare of the sheep.

Ehrhardt recommends partial or selective flock treatment while pastures are “clean,” such as turn-out in spring or in previously ungrazed areas, annual crops, late fall or winter.

He noted contaminated pastures have a huge parasite refugia population in the grass relative to that in the sheep flock, so refugia can be maintained even when all sheep in the pasture are treated.

Combination drug treatments

Producers should obtain veterinary guidance when using drugs off-label, and should not rely on combination product therapy as their only control approach.

Instead, producers should utilize an integrated approach to have a substantial control program, including:

• The use of refugia maintenance strategies;

• Elimination of unnecessary drug treatments, replaced with treatment according to risk;

• Reduced pasture contamination (use multi-species grazing, machine harvest, use of annual crops, alternate grazing of susceptible animals with animals having lower infection levels and greater immunity).

Genetic selection

Heritability is high for post-weaning, GIN fecal egg count under a standardized, field challenge such as weaning lambs and treating with an effective anthelmintic, then exposing them to infected pasture and monitoring, obtaining individual FEC on each lamb. Progress will be faster in breeds with higher resistance as a starting point, and NSIP has fecal egg count EBVs as an index of parasite resistance in several breeds.

Ehrhardt ended his presentation by noting that the United States sheep industry needs combination drug formulations and licensing of new drugs that already exist in the rest of the world.

Skip to content