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Nematodirus – Not just a spring phenomenon!

As we know, Nematodirus battus typically causes mass outbreaks of parasitic gastroenteritis in young lambs during the spring and early summer. This synchronized hatch is due to the environmental conditions needed to trigger a hatch – a prolonged period of chilling prior to at least ten days over 10C. Additionally, unlike most important roundworms, Nematodirus can develop to the infective stage protected inside the egg; this results in a concerted challenge to the lambs immediately at the time of hatching.

However, in recent years, Nematodirus outbreaks have been observed in autumn, and even in some cases in winter. This may be due to several factors. In some years, spring temperatures have been variable, and there may not have been a full ten days above 10C, meaning that not all eggs will have hatched in the spring. Additionally, it has been found that on some farms, eggs are able to hatch without the “priming” of a period of chilling, and so can hatch later in the year e.g. in autumn. It has also been suggested that we could even have selected for autumn hatching populations due to the effective spring anthelmintic regimes used on many farms to target worm populations which hatch in spring.

White drenches (1-BZ) remain the treatment of choice for Nematodirus, as they have a high safety margin for young stock, and use in Nematodirus outbreaks will “save” the other classes for the treatment of other roundworms, hopefully slowing the spread of anthelmintic resistance. Resistance to white drenches in Nematodirus remains rare, although it is widespread in other classes of roundworms. If a mixed infection with Nematodirus and other strongyles is present, then another class of anthelmintics should be used – in these cases worm egg counts are invaluable in evaluating which type of anthelmintic may be of best use. Worm egg counts can also be used as a post-dosing efficacy check.

In summary, keep in mind this autumn that Nematodirus may not just be a spring phenomenon, and please feel free to contact us with any queries about testing or treatment.

Anne Seaton, BVM&S MSc MRCVS
Veterinary Centre Manager - Perth

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