A parasite population can develop resistance to a chemical through:
Pour-on applications can be susceptible to under-dosing (see points above) plus caked mud, lesions, wet skin etc can reduce chemical uptake. Social licking may also decrease the actual dosage received.
Everyone working in the rural industry has a ‘duty of care’; a legal obligation to provide a safe workplace for everyone on the property.
The chemical works either by direct contact with the parasite or by ingestion of the active as the parasite feeds on the treated host. Depending on the chemical, the active ingredient is transported by spread across the surface of the skin via the oil layer (e.g. synthetic pyrethroids), or via absorption through the layers of the skin to blood vessels and tissues (e.g. macrocyclic lactones). If the formulation affects internal parasites the chemical is transported via the blood stream to cells and capillaries lining the gastrointestinal tract. There may also be re-secretion of the active ingredient into the lumen of the intestines where internal parasites reside.
All pour-on products have a degree of persistent activity which is specified on the label. Formulations combine active ingredient and solvent components (variable dependant on product) to produce chemistry that moves across the skin, or from skin to bloodstream, to the site of activity. This is followed by metabolism and a decaying profile until all the active is removed from the animal’s system. Some more recent formulations give prolonged release and metabolism of chemical, providing parasite control over extended periods.
Chemical actives can be formulated at different concentrations in different products. Historically, the concentration of pour-on products was two and a half times that of oral or injectable versions containing the same active ingredient.
Some formulations are made twice as concentrated as older products and are marketed as ‘low volume’. It is important that the dose rate is checked on the product label to ensure that it is appropriate for the application for which it is being used.
Social licking or allo-licking refers to licking between family groups in cattle herds. It presents an uncontrolled mechanism of exchange of chemical between individuals when the pour-on formulation is used. Transfer of chemical between individuals in this way is unlikely to be a problem if all animals in the mob are treated. However, if only a percentage of the mob have been treated then licking is thought to be responsible for a reduction in the applied product dosage to individuals, potentially leading to under-dosing. Without holding animals separately, little can be done to prevent social licking.