Estimating the cost of subclinical parasitism in grazing ewes

The aim of the study was to quantify the cost of gastrointestinal nematode parasitism in a typical New Zealand sheep flock. On the 9th January 2007, 500 ewes from a mixed age flock of 3000 Romney cross-bred ewes were selected. Of these, 250 were given a controlled release capsule releasing a continuous dose of either albendazole or ivermectin over a 100- day period. The other 250 ewes remained as untreated controls. Further capsules were administered to the treatment group on the 12th March, the 12th June and the 26th July. At each of these times, the ewes were weighed and faecal samples were collected forwormegg counts and larval cultures. The initialwormburdens in January were low (mean: 34 epg), but in the untreated ewes rose to high levels before mating in March and reached ameanof 1416 epg. Haemonchus was identified as the principal parasitic genus. In June, epg were reduced to a mean of 526 epg, with Trichostrongylus being the principal genus (48%), whilst in late July mean epg was 826 with Cooperia being the principal genus (60%). Epg of treated ewes were zero throughout that period. Throughout the study, there was a significant difference in body weight of treated ewes over controls, which was 2.7 kg (March), 4.3 kg (June), 5.7 kg (July) and 3.6 kg (November). There were more multiple pregnancies among treated ewes, identified at ultrasound scanning as 12.4 foetuses per 100 ewes (p < 0.05). At weaning of lambs, on the 26th November, mean body weight of twin lambs born to treated ewes were on average heavier by 2.15 kg (p < 0.05). Assuming that in treated ewes no adverse effects were observed from subclinical parasitism alone, the benefits of controlling the problem could be clearly evaluated. Innovative ways of controlling subclinical parasitism on pastoral farming systems need to be considered. It should be taken into account that the authors do not recommend repeated anthelmintic treatments, which was performed in the trial, only with a view to meet the specific objectives of the work.