Infestivity of Psoroptes ovis on ivermectin-treated cattle.

Psoroptes ovis was not transmitted by natural contact to susceptible cattle which were exposed to infested, ivermectin-treated cattle 6, 12, 14, 16, and 18 days after treatment was given. However, clinical scabies did develop in 2 calves naturally exposed to P ovis-infested, ivermectin-treated calves at 10 days after treatment was given subcutaneously (200 micrograms/kg). Psoroptes ovis was transmitted to stanchioned cattle manually exposed to 200 to 300 ml of hair and skin scrapings from infested, ivermectin-treated cattle at 6, 10, 12, 14, and 16 days after treatment was given subcutaneously (200 micrograms/kg). Scabies did not develop in cattle exposed to skin scrapings obtained from infested, treated cattle at 18 and 20 days after they were treated with ivermectin. The 14-day isolation of P ovis-infested, ivermectin-treated cattle from susceptible cattle recommended by the US Department of Agriculture, although marginal, is adequate under natural conditions to prevent transmission of scabies from treated to noninfested cattle.