Efficacy of pyrantel pamoate against a macrocyclic lactone-resistant isolate of Parascaris equorum in horses.

The expanding prevalence of Parascaris equorum populations that are resistant to macrocyclic lactone (ML) anthelmintics makes it desirable to identify dewormers which remain effective. The objective was to evaluate the efficacy of pyrantel pamoate in 14 suckling foals that had been infected orally with approximately 600 larvated eggs of a P. equorum isolate selected for ML resistance (ML-R). Seventy days after inoculation, foals were weaned, housed individually, and fecal samples were examined frequently to detect the onset of patency. Between 73 and 80 days post-inoculation, all 14 foals developed P. equorum egg counts>or=150 eggs per gram (EPG). An initial cohort of eight foals was treated orally with ivermectin paste (200 microg/kg) 84-91 days post-inoculation. Egg counts were reduced by only 47% at 2 weeks after ivermectin treatment, confirming the ML-R status of the isolate. A second cohort of six foals was not treated with ivermectin. Within each cohort, eligible foals were allocated randomly to treated (pyrantel pamoate; n=7) or untreated control (n=7) groups. Treated foals were dosed orally on Day 0 with a paste formulation of pyrantel pamoate at 13.2mg/kg. Mean ascarid egg counts of treated foals were reduced by 96.0% and 98.8% at 1 and 2 weeks post-treatment, respectively. On Day 14, foals were euthanatized and specimens of P. equorum were recovered from the gut contents, preserved in 10% formalin, and counted. Mean numbers of P. equorum adults recovered postmortem were significantly lower (P=0.0031) in foals treated with pyrantel pamoate (X=1.7; range 0-16) compared to control foals (X=63.0; range 0-320). A paste formulation of pyrantel pamoate, at a dosage of 13.2 mg/kg, was 97.3% effective against a ML-R isolate of P. equorum.