Anthelmintics (wormers) are vital to control worm infections and protect against productivity losses. At present, resistant and multi-resistant worms are threatening the efficacy of existing anthelmintics.
- Resistance is the ability of worms to survive the normal dose of a wormer and pass that ability on to its offspring.
- Worms can be resistant to one class or several classes of wormer.
- Even low levels of resistance can leave enough worms behind to affect growth rates. Indeed, when the number of resistant worms is more than 10%, we start to gradually lose lamb performance because the product is not performing at its optimum level¹.
- A low to moderate worm burden can reduce growth rates by up to 50% without any obvious clinical signs¹. This is the reason why we need to act now and put a sustainable worm control programme in place.
- In the last 10 years the reports of resistance to the three older classes has been increasing.
There are five main groups of wormers available to sheep farmers in the UK.
The worm lifecycle
The lifecycle starts on pasture where stage three infective larvae wait to be eaten inside water droplets on blades of grass. Once ingested larvae travel into the gut and develop into stage four larvae and then adult worms which then begin to produce eggs.
Nematodirus battus is a real cause for concern in the UK. Changing climatic conditions have meant that disease caused by this parasite has been a real challenge for sheep over the last few years. The Nematodirus lifecycle takes much longer than other roundworms – up to nine months. Eggs laid in spring develop slowly to the third larval stage inside the egg, after which they survive on pasture for up to two years. Hatching takes place after a prolonged cool period (over winter), followed by an average daily temperature of 10˚C or more. If weather conditions are right, the majority of larvae will hatch at the same time (‘mass hatch’), which means that lambs are faced with a short, sharp burst of damage to their intestines, causing acute disease. If hatching is over a longer period of time, disease will not be so evident, but growth rates can suffer greatly. White drenches eg Rycoben™ have high activity against nematodirus and there have been very few confirmed cases of resistance to white drenches in this parasite in the UK.