Tackling nematodirus in your flock is a key part of the thrive of your lambs in their first few weeks on the farm.

It is a severe parasitic disease of lambs aged six to twelve weeks which become infected by ingesting large numbers of infective larvae from grazing on contaminated pasture.

The Department of Agriculture, Food and the Marine (DAFM), in conjunction with Met Éireann, predict when nematodirus eggs will hatch and every year produce a forecast predicting the peak hatch and advise when farmers should treat to prevent disease due to this parasite.

With lambing underway, and even completed on some farms across the country, preparing a dosing plan for your flock will be important as lambs become susceptible to the parasite.

When can an infection occur?

Unlike that of other roundworms, Nematodirus battus eggs deposited on pasture do not hatch until the following year to release the infective larvae.

This happens during a mass hatching event in spring when soil temperatures increase after a period of cold weather. Disease typically occurs in April, May and June.

See the map below for expected timing of peaks in nematodirus egg hatching on pasture:

Estimated dates in March and April 2024 when peak nematodirus egg hatching is expected to occur on pasture Image: Met Éireann

 When this occurs, eggs can hatch en masse very quickly, providing a sudden and very high infection challenge.

This is known as the ‘spring flush’ and often coincides with lambs becoming more reliant on grazed grass at 6 – 8 weeks old.

Identifying nematodirus in the flock

Clinical signs usually consist of sudden onset, profuse, watery scours, usually affecting multiple animals within the flock.

Faecal staining of the fleece is frequently seen, and lambs can be dull, off feed and lose weight rapidly.

In outbreak scenarios, lambs can be seen congregating around water troughs due to the severe thirst that develops, and death can occur due to dehydration.

Adult sheep are unaffected by the parasite.

Source: Chanelle Pharma

Mortality rates can be high from nematodirus and up to 5% of the lamb crop may die within days.

Alongside the cost of treatment, recovered or less severely affected animals may have severely reduced growth rates and feed conversion efficiency, and may take many more weeks to reach a marketable weight, eroding into profit margins.

To identify nematodirus in the flock, worm egg counts are not useful in diagnosing nematodirus as larvae cause clinical signs before eggs are produced.

Diagnosis can be made on clinical signs, post-mortem examination and likelihood of disease based on forecasting.

Treatment and prevention

Treatment with an appropriate Group-1 white drench is usually very effective and is recommended by Sustainable Control of Parasites in Sheep (SCOPS).

Appropriate products include Albex 2.5% (Albendazole), Bovex (Oxfendazole) and Zerofen 2.5% (Fenbendazole).

Treatments are usually given three weeks apart through May and all lambs in a group should be treated.

Clinical symptoms of nematodirus and coccidiosis can be similar and in addition the diseases can often occur at the same time of year. Treatment for both conditions may be required and as such it may be worth contacting your veterinary surgeon for further information. Coccidiosis can be successfully managed by using products such as Dycoxan or Chanox.

The need for treatment, and timeframes, are highly variable year on year so always check with your vet.

Source: Chanelle Pharma

Be prepared and keep an appropriate product in stock if a hatch is anticipated as outbreaks can occur very quickly.

To avoid selecting for resistance, it is vital to weigh lambs and dose appropriately.

To prevent disease, do not graze lambs on pasture used for lambs the previous year. If this is unavoidable, pay close attention to parasite forecasts.

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