Cost of Subclinical Parasitism in Ewes

Almost 10 years ago, a study was published in the Small Ruminant Research Journal, documenting the cost of subclinical parasitism in grazing ewes (in New Zealand). Five hundred ewes from a mixed aged flock of 3000 crossbred Romney ewes were selected for the experiment. Of these, 250 ewes were given controlled released capsules of either albendazole or ivermectin. The other 250 ewes served as untreated controls.

The initial worm burdens in January were low (mean: 34 epg), but in the untreated ewes rose to high levels before mating in March and reached a mean of 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 9.9 lb (March), 9.5 lb (June), 12. 5 lb (July) and 7.9 lb (November). There were more multiple pregnancies among treated ewes, identified at ultrasound scanning as 12.4 fetuses per 100 ewes. At weaning of lambs, on the 26th November, mean body weight of twin lambs born to treated ewes were on average heavier by 4.73 lbs.

According to this study, control of subclinical parasitism has the potential to improve ewe body weight by up to 13.2 lb, to increase the number of fetuses by >12 foetuses per 100 ewes and to increase weight of twin lambs weaned by >4.4 lb. These are significant benefits, therefore innovative ways to control subclinical parasitism, without increasing risk for development of anthelmintic resistance need to be adopted on pastoral farming systems.

The authors were not recommending repeated anthelmintic treatments, as performed in this trial, as it is highly unlikely to be sustainable. The design was carried out in this trial and was used as a model, only with a view to meet the specific objectives of the work.

Source: Small Ruminant Research Journal. October 2009. Read abstract