Combination anthelmintics effectively control ML-resistant parasites; a real-world case history

Area of Science:

• Veterinary Parasitology
• Anthelmintic Resistance
• Cattle Health

Background:

• Macrocyclic lactone (ML) resistance in cattle parasites is an emerging global concern.
• Ill-thrift in grazing cattle prompted an investigation into potential resistance issues.
• Previous research had not confirmed ML resistance in cattle within the USA.

Purpose of the Study:

• To identify the source and prevalence of macrocyclic lactone (ML) resistance in cattle parasites in the USA.
• To evaluate the efficacy of different anthelmintic treatments and management strategies in controlling ML-resistant parasites.
• To confirm the utility of fecal egg count monitoring and PCR for nematode species identification in production settings.

Main Methods:

• Investigated an ill-thrift situation in cattle in Wisconsin.
• Traced the origin of resistant parasite populations to southeastern USA pastures.
• Compared the efficacy of single-active ML treatments versus combination therapy (ML + oral levamisole).
• Utilized fecal egg count reduction tests and PCR analysis to monitor parasite populations.

Main Results:

• The first documented case of ML resistance in USA cattle was identified.
• Resistant Cooperia and Haemonchus spp. were found to originate from southeastern USA pastures and were transported to Wisconsin.
• Single-active ML treatments were ineffective against resistant parasite populations.
• Combination therapy with ML and oral levamisole significantly reduced the transport and survival of ML-resistant parasites.

Conclusions:

• Macrocyclic lactone (ML) resistance in cattle is present in the USA, originating from specific geographic regions.
• Combination anthelmintic therapy (ML + oral levamisole) is a valuable strategy to control the spread of ML-resistant parasites.
• Fecal egg count monitoring and PCR are essential tools for managing parasite resistance in cattle production systems.