Methods to determine resistance to anthelmintics when continuing larval development occurs

Area of Science:

• Veterinary Parasitology
• Anthelmintic Resistance
• Ruminant Health

Background:

• The fecal egg count reduction test (FECRT) is standard for detecting anthelmintic resistance (AR) in ruminant gastrointestinal nematodes (GIN).
• Variations in FECRT methodology exist, impacting accuracy, particularly when larval development persists during testing.
• Commonly targeted GIN, like Haemonchus contortus and Cooperia punctata, often exhibit high biotic potential or re-infection rates, complicating AR assessment.

Purpose of the Study:

• To determine the most appropriate FECRT method for calculating AR under conditions of continuing larval development.
• To evaluate different FECRT designs and resistance-indicating rules using computer simulations.
• To provide guidance on accurate AR detection in pasture-based ruminant systems.

Main Methods:

• Computer simulations were employed to model three FECRT designs: post-drench counts only, pre- and post-drench counts for treatment groups, and pre- and post-drench counts for both treatment and control groups.
• Simulated fecal worm egg counts (FEC) were generated using negative binomial and binomial distributions, incorporating variables for worm burden, larval development, and drench resistance.
• Calculations of percent reduction and confidence limits followed Standing Committee for Agriculture (SCA) guidelines, with additional analysis using cumulative inverse Beta distributions for FEC.

Main Results:

• Simulations indicated that calculating AR from pre- and post-drench FEC of both untreated control and treatment groups is essential when larval development occurs during FECRT.
• Two resistance rules were assessed: (1) %reduction (%R) <95% and lower confidence limit <90%; and (2) upper confidence limit <95%.
• Applying both rules is recommended, particularly when %R falls between 90-95%, to enhance the detection of anthelmintic resistance.

Conclusions:

• For accurate anthelmintic resistance detection in ruminants, especially with GIN exhibiting continuous larval development, FECRT protocols must include pre- and post-drench FEC from both treated and untreated control groups.
• The study highlights the necessity of using robust statistical methods and potentially multiple resistance criteria to reliably identify AR.
• Adherence to recommended FECRT designs and interpretation rules is crucial for effective parasite control strategies in livestock.