In most cases of parasitic gastro-enteritis (PGE, gut worm disease) it is not carrying a worm burden in their stomach or intestines that affects lambs most, but having to fight off constant fresh infection from grazing worm-contaminated fields, writes the Borders Sheep Health vets.
The foremost question when working out a worm control plan is how can we avoid lambs grazing worm-contaminated pastures and how can we do this sustainably (i.e. without selecting too much for resistance)?
After midsummer, worm control is often based on a combination of grazing management and anthelmintic treatments (guided by faecal egg counts) to ensure that weaned lambs are grazing only low-contamination pastures. If lambs are on wormy pastures, then you can dose them as often as you like, but they will not perform well if you don’t move them.
Finishing lambs off the ewes before midsummer means that they are not exposed to the heavily worm-contaminated pastures that can build up in the second half of the grazing season.
If warm/wet conditions continue into July, then an increased PGE risk can be expected this summer. These conditions will also increase the fluke risk in the autumn.
Dry periods reduce the immediate risk of PGE, although when rain follows a dry spell of several days, there can be an immediate increase in worm challenge. If the summer as a whole is dry, then the overall level of worm disease can be expected to be lower, although autumn and winter PGE can be a problem when the wet weather eventually arrives.