A residual anthelmintic 2,6-diiodo-4-nitrophenol (Disophenol). Methods of testing its anthelmintic efficacy

A subcutaneous injection of 2,6-diiodo-4-nitrophenol (Disophenol) at 10 mg/kg sheep leaves a residue bound to serum albumin which is lethal to Haemonchus contortus for 3 months after treatment. In the larval anthelmintic test, susceptible worm free sheep are dosed so that either third stage larvae (L3), or fourth stage larvae (L4) or 5th and adult stages are present on the day of treatment but slaughter is delayed to allow these larval stages to develop to adults because the larger worms are more easily seen, identified and counted. The larval anthelmintic test in sheep had to be altered and sheep killed within a few days of treatment, because the residues may be more effective against subsequent stages of development. Disophenol was greater than 60% effective against L3 of Oesophagostomum columbianum and L4 of H. contortus in greater than 60% of sheep (Class B). It rose to greater than 80% effective against adult H. contortus in greater than 80% of sheep (Class A). Against H. contortus it maintained Class A for 32 days, fell to Class B from 45--76 days and Class C (greater than 50% effective in greater than 50% of sheep) at 91 days after treatment respectively. In the RSA a treatment in December followed by another in March would protect sheep adequately against H. contortus for the entire season.