14 May 1999

Its essential to stay

on top of worming

By Jeremy Hunt

LAMBS forced to graze closely cropped pastures following poor April grass growth in many areas are more likely to have ingested higher levels of worm larvae.

Michael Clarkson, of Liverpool Universitys Leahurst Veterinary Hospital, says a strict dosing programme is essential this spring to avoid the impact of severe worm problems in lambs later this summer.

"An effective programme of worm control hinges on starting dosing at the right time and ensuring it is sustained at correct intervals throughout the early part of the spring and summer.

"If you can successfully get on top of the worm cycle at the right stage of the lambs life you will reap benefits from continuous growth rate."

He warns of a serious and costly check in development when lambs are subjected to the seasonal peak of infective larvae on pasture during July and August.

"The priority at this current stage of the sheep management year must be to prevent the July/August peak of infective worm larvae.

"These larvae develop from eggs laid this year by worms in ewes and lambs guts. The key to successful worm control is to reduce the number of these eggs to as few as possible."

Prof Clarkson says sheep producers must fully understand the life-cycle of worms.

"The time taken for eggs on pasture to become infective third stage larvae depends on temperature. Eggs passed in April take about two months to become infective, while eggs passed in June only take about two weeks.

"This means that all eggs passed between April and June become infective larvae about the same time in July."

Infective larvae eaten by sheep take three weeks to develop into the adult worm stage when they start to lay eggs. Control depends on dosing sheep sufficiently frequently to kill worms before they reach the adult stage. Unless a wormer with residual action is used it is essential to adhere to a strict policy of dosing every three weeks, he says.

"The usual strong immunity of ewes to worms begins to fade around lambing time and for a few weeks after that. This means its important to dose them before turnout and also three and six weeks later by which time most will have regained their immunity."

Prof Clarkson says that as lambs start to eat more grass – from about four-weeks-old – they begin to ingest over-wintered infective larvae, particularly where they are grazing closely cropped pastures. Dosing must start at six to seven weeks old and continue at three weekly intervals.

"Since most over-wintered infective larvae die off in May and June it is safe to stop worming after one treatment into the month of June. The intervals can be extended from three weeks when using a wormer with a residual action. When the wormer has five weeks residual action the interval can be extended to eight weeks."

Prof Clarkson says that an understanding of the worms life cycle enables sheep producers to tailor their programme to fit a particular worm treatment. Vet advice can help formulate a policy to achieve sustained growth rates in lambs.

"If you allow the July/August peak of infective larvae to occur by not dosing early in the season, wormers will not be as effective as lambs start to consume dangerous numbers of larvae every day."

"But I must stress it is vital to worm at three weekly intervals if a non-residual wormer is used to achieve the level of control early in the season and avoid the July/August peak. A slip back to worming at four weekly intervals is just too long and will lead to problems later in the summer."


&#8226 High levels of larvae.

&#8226 Regular dosing.

&#8226 Avoid growth check.


&#8226 High levels of larvae.

&#8226 Regular dosing.

&#8226 Avoid growth check.