Nematodirus (sometimes called thread-necked worms) is a parasitic roundworm that can infect young lambs, typically in spring when lambs are around 6-8 weeks of age. There are several species of Nematodirus that contribute to economic losses in lambs. Most of the losses in the UK occur as a result of Nematodirus battus.
Signs of infection include reduced appetite, reduced production, acute yellow/green diarrhoea and subsequent dehydration.
The ingested infective larvae can disrupt the normal integrity, structure and function of a lamb’s small intestine. Intensity of the infection varies between lambs. Whilst the majority will recover within a month, mortality rates can be as high as 10-30% of the lamb crop in which animals suffer intense scouring and succumb to dehydration quickly. Other clinical signs include sunken eyes and rough wool.
Acute disease occurs when synchronous hatching of worm eggs into infective larvae coincides with the grazing of young lambs. The timing of development and hatching generally depends on climatic conditions (chilling followed by a mean day/night temperature of greater than 10°C for 10 consecutive days) and hence most larvae will overwinter within the egg and hatch in the following year. Recent climatic changes and alteration of parasite behaviour have resulted in a second hatching event in autumn in some N. battus populations, thus providing the potential for a second generation per annum.
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