Treatment and eradication of murine fur mites: I. Toxicologic evaluation of ivermectin-compounded feed.

Fur mite outbreaks remain a persistent problem in laboratory mouse colonies. All currently published treatment methods are labor-intensive, expensive, or unreliable. During a recent outbreak with Myobia musculi and Myocoptes musculinus in a large colony (approximately 30,000 cages), we developed a feed-based treatment regime in which ivermectin was the active ingredient. Rodent feed was compounded with 3 different concentrations of ivermectin (12, 24, and 48 ppm) and γ-irradiated. Postcompounding analysis revealed loss of ivermectin during manufacturing, but the remaining drug was stable for at least 6 mo. In an 8-wk toxicity study in a C57BL/6NTac mouse breeding colony, ad-libitum feeding of the 3 diets yielded estimated doses of 1.3, 2.7, and 5.4 mg/kg. Adult mice lacked adverse clinical effects, except that 1 of the 144 mice in the 48-ppm group developed tremors and ataxia and was euthanized. No significant differences between doses were revealed by CBC, serum chemistry, body weight, or gross necropsy. Plasma drug concentrations plateaued at a dose-dependent level 7 to 10 d after initiation of treatment and decreased to undetectable levels 6 to 9 d after its discontinuation. Fertility of the P0 generation was unaffected. Pup mortality was higher in the 24- and 48-ppm groups, reaching 100% at the higher dose. Animals exposed to ivermectin as neonates had normal weaning weights, but mice receiving 24-ppm feed had lower adult weights. Our results indicate that using feed containing 12 ppm ivermectin (estimated ingested dose, 1.3 mg/kg) was safe in a C57BL/6NTac breeding colony.