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The correct terminology is gastrointestinal nematode parasites for what is commonly referred to as worms. Gastrointestinal nematode parasitism is one of the most important production-limiting diseases of sheep in New Zealand.
Internal parasites can be thought of as 'silent thieves' as the loss in production caused by parasitism often goes unnoticed. Gastrointestinal parasitism can be classed as either clinical or sub-clinical.
Clinically affected animals are easy to identify as they show more obvious symptoms including illthrift, scours, anaemia and weight loss. It is the subclinically affected animals which pose the greatest challenge to farmers as they are more difficult to identify generally with decreases in weight gain and depression in wool production. While clinical parasitism is associated with losses in sheep, it is the subclinical animals bearing low levels of infection which result in the greatest economic losses.