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 moxidectin against lungworm and gastrointestinal nematodes in cattle was studied 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 the residual effect for lungworm was faecal larval counts. Additional information was derived from pasture larval counts, enzyme-linked immunosorbent assay (ELISA), respiration frequency, coughing score and, particularly for evaluating development of immunity, worm counts. For gastrointestinal nematode infections faecal egg counts and larval differentiation of faecal cultures were the main parameters used. Pasture larval counts and an ELISA for Ostertagia and Cooperia were used as additional parameters. In three treated groups lungworm larvae (re)appeared in the faeces after 67, 95 and 119 days, respectively. This implies that a 100% residual effect did not last longer than 67-21 = 46 days. The treated group with patency starting on Day 95 was exposed to extremely high infection pressure and the ELISA indicated some host-parasite interactions from 2-4 weeks after treatment. Thus some interaction between moxidectin treatment and high infection pressure delayed the onset of patency in comparison to another treated group under much lower infection pressure. In all treated groups, including the one under high infection pressure, lungworm disease was prevented and the worm counts 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 differentiation of larvae from faecal cultures demonstrated a 100% residual effect of at least 3 weeks and indicated a high residual effect of approximately 5 weeks against Ostertagia. Moxidectin suppressed Cooperia faecal 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 results were indicative for a delay of 2 weeks in the acquisition of gastrointestinal nematode infections following moxidectin treatment.