HYPOBIOSIS IN THE OSTERTAGIDS OF RED DEER AND THE EFFICACY OF IVERMECTIN AND FENBENDAZOLE AGAINST THEM

An abattoir survey was carried out during the 1991 to 1992 slaughter s eason to examine the structure of the ostertagid populations in red de er, Most of the animals were 15- to 22-month-old stags, but there were also hinds on two occasions and calves on one, Ostertagids of mixed s pecies were present in all of the animals, and 43 of 120 (35.8 per cen t) stags and 10 of 17 (58.8 per cent) hinds, but no calves, also carri ed small numbers of Trichostrongylus axei. Most animals carried adult and developing ostertagids and after September they all contained hypo biotic larvae (EL4), EL4 were present in three of nine stags in Septem ber and they increased in number through the autumn, In stags killed b etween November and February the group mean numbers of EL4 represented 62 to 94 per cent of the total burdens present, compared with 37 and 55 per cent on April 1 and 7, respectively, In the hinds, the number a nd the structure of the ostertagid populations was similar to that in the stags, Of 14 calves killed on October 9, only two contained a few EL4, Subcutaneous ivermectin at 400, mu g/kg removed 100 per cent of t he adult and developing worms, but although it also removed 95 per cen t of EL4, some larvae remained in every treated animal, The efficacy o f fenbendazole at 15 mg/kg given over five days was 75 per cent agains t EL4 and although the long interval after treatment obscured the resu lt, it was probably much higher against rater stages.