At this time of year, sheep farmers in the UK start keeping an eye on a website linked to local weather stations making forecasts: not of rain or sun, but parasites.
Nematodirus or thread-necked worms, are parasitic roundworms that infect lambs when they start eating grass. The infection is usually mild, but lambs ingesting large numbers of worm larvae can suffer severely, with a mortality rate of up to 30%. Nematodirus hatching is triggered by specific weather conditions. A cold snap followed by several days of temperatures of 10C or more will produce mass hatching and extremely dangerous conditions. This tends to be in April or May in southern England, but might be in June in Scotland, with a local variation due to differences in microclimate.
The industry group SCOPS (Sustainable Control of Parasites) provides its online parasite forecast site to warn farmers exactly when there is a high risk in their area, based on the weather. There are still variations to watch out for though: south-facing fields have an earlier nematodirus hatch because they catch the sun, and each hundred metres of altitude brings cooling and slows the hatch by about seven days.
Once alerted, farmers can take protective measures, such as moving lambs to lower-risk fields not grazed by lambs the previous year.
