RESIDUAL EFFECT OF INJECTABLE MOXIDECTIN AGAINST LUNGWORM AND GASTROINTESTINAL NEMATODES IN CALVES EXPOSED TO HIGH PASTURE INFECTIVITY LEVELS IN THE NETHERLANDS

The residual effect of a 0.2 mg kg(-1) injectable formulation of moxid ectin against lungworm and gastrointestinal nematodes in cattle was st udied in a grazing experiment in the Netherlands. Five groups of four calves were grazed between May and October 1991 and one similar group was used as permanently housed control group for the evaluation of the development of immunity against lungworm by challenge infections with 5000 larvae of all six groups. The main parameter used to determine t he residual effect for lungworm was faecal larval counts. Additional i nformation was derived from pasture larval counts, enzyme-linked immun osorbent assay (ELISA), respiration frequency, coughing score and, par ticularly for evaluating development of immunity, worm counts. For gas trointestinal nematode infections faecal egg counts and larval differe ntiation of faecal cultures were the main parameters used. Pasture lar val counts and an ELISA for Ostertagia and Cooperia were used as addit ional parameters. In three treated groups lungworm larvae (re)appeared in the faeces after 67, 95 and 119 days, respectively. This implies t hat a 100% residual effect did not last longer than 67-21 = 46 days. T he treated group with patency starting on Day 95 was exposed to extrem ely high infection pressure and the ELISA indicated some host-parasite interactions from 2-4 weeks after treatment. Thus some interaction be tween moxidectin treatment and high infection pressure delayed the ons et of patency in comparison to another treated group under much lower infection pressure. In all treated groups, including the one under hig h infection pressure, lungworm disease was prevented and the worm coun ts demonstrated development of immunity. In contrast, severe lungworm disease occurred in two control groups grazing together with the 'high infection pressure' treated group. The faecal egg counts and differen tiation of larvae from faecal cultures demonstrated a 100% residual ef fect of at least 3 weeks and indicated a high residual effect of appro ximately 5 weeks against Ostertagia. Moxidectin suppressed Cooperia fa ecal egg counts for over 98% and the results indicated a more than 95% residual effect on faecal egg output during 2-3 weeks. The ELISA resu lts were indicative for a delay of 2 weeks in the acquisition of gastr ointestinal nematode infections following moxidectin treatment.