Single-Animal versus Herd Treatment and Establishment of Withholding Time WhT

5 Single-Animal versus Herd Treatment and Establishment of Withholding Time (WhT) 

In poultry and porcine husbandry in particular, the use of AMDs in feed or in drinking water for prophylaxis, metaphylaxis (or control in the United States) or therapy is widely practiced (see Section 2.73.2). Prophylaxis involves administration of AMDs to healthy animals known to be at risk (as a consequence, e.g., of close proximity of animals housed together or predictable stresses caused by transport or adverse weather conditions). Metaphylaxis involves 

In contrast, the therapeutic use of AMDs generally involves treating animals individually with AMDs formulated for parenteral (usually intramuscular or subcutaneous) or oral dosing, with animals dosed on a mg/kg body weight basis. Here, dosing can be more accurate even if body weight is normally assessed rather than measured under clinical conditions. 

As drug residue science is concerned with metabolites, as well as parent molecules, it should be noted that most (especially phase II) metabolites are more polar, less lipid-soluble, and less biologically active than parent drugs. Hence, most metabolites follow the general rules on disposition (poor penetration of cell membranes) and elimination (high concentrations in urine and/or bile) applicable to poorly lipid-soluble drugs. 

---