SKINNING THOSE ECTOPARASITES
AS with anthelmintics, there is a wide choice of veterinary ectoparasiticides available with varying activity against the ectoparasites described. For cattle products, see Table 1 and sheep products turn to p16.
These can be divided into groups based on their chemical structure and mode of action. Some groups such as the organochlorines, organophosphates and pyrethroids have broad spectrum activity against ectoparasites.
The avermectin and milbemycin compounds have activity against both nematodes and ectoparasites (including warbles and sheep scab) but not against tapeworms and liver flukes. Drugs within this family are available as drenches, pour-ons, injectables and as a bolus.
These drugs have some persistent activity, particularly when administered in the injectable formulation and consequently have relatively long meat withdrawal periods. These range from four weeks for the pour-on, to six weeks or more for the injectables and around six months for the bolus.
They cause a flaccid paralysis in worms which results in their being expelled during normal gut contractions. Chemicals include the avermectins (ivermectin, doramectin, avermectin) and the milbemycins (moxidectin).
The organophosphates, fenthion and phosmet, the pyre-throids cypermethrin, delta-methrin, fenvalerate and permethrin, together with the avermectins, milbemycin, and amitraz are all active against lice.
Their activities against the different types of biting or sucking lice vary. Fenthion, phosmet, ivermectin, abamectin, dora-mectin and moxidectin are active against warble fly larvae. Cypermethrin, deltamethrin, fenvalerate and permethrin are active against hornflies and other biting and nuisance flies. Phosmet, ivermectin, abamectin, doramectin, moxidectin, and amitraz have variable activity against mange mites. Amitraz is active against ticks on cattle.
Table 1: Cattle ectoparasites
1% inject. (UK)
Table 2: Makers recommendations for prevention of parasitic gastro-enteritis (roundworm) and lungworm*
Treatment(Weeks after turnout)
DectomaxAt turnout and 8
Ivomec injectionfor cattle3, 8 and 13
Ivomec superinjection for cattle3, 8 and 13
Panomec3, 8 and 13
Ivomec pour-on 3, 8 and 13
Ivomec slowrelease bolusOne at turnout
*In first season growing cattle turned out in late April or May using avermectins.