The Department of Agriculture, Food and the Marine released a nematodirus warning last Monday. The warning, based on advice from the Nematodirus Advisory Group, states that peak hatching of nematodirus larvae is expected in the first week of April in the south and west, and from 9 April to 15 April in the rest of the country.

The prediction is based on mean soil temperatures in March being on or above their long-term average (hatch occurs following a sustained lift in temperature after a cold period). It adds that, nationally, the maximum larval occurrence is in line with normal conditions (where normal is the average between 1981 and 2010).

High-risk categories

Nematodirus is a severe disease, mainly affecting lambs from six to 12 weeks of age. The Department of Agriculture warns that early lambing flocks, where lambs are five to six weeks old and already grazing, are most at risk. This risk may be intensified where ewes and lambs have undergone a period of undernourishment, resulting in lambs relying on grazed grass due to a lower ewe milk yield at an early age.

The risk is avoided where lambs are kept off pastures grazed by lambs a year earlier (pasture contamination occurs a year earlier). But this is not an option for the majority of flock owners with dosing the best method of prevention and treatment for these.

The warning advises dosing of lambs in the south and westerly coastal areas with an anthelmintic from mid-April, with those in the rest of the country dosed from late April. This course of action, it says, will decrease the likelihood of clinical disease later and reduce pasture contamination for next year.

Flock-specific circumstances may vary from farm to farm and, in this case, it is advisable to seek veterinary advice in the event of clinical cases or for a flock control programme comprising specific measures. For example, where there is a drawn-out lambing period or a split in lambing, treatment may need to be delayed for younger lambs in the flock until they approach their greatest risk period.

The warning also recommends submitting any lambs that die to the Department’s regional veterinary laboratories to confirm the cause of death by post-mortem examination as nematodirus can cause death before clinical signs are apparent.

Clinical signs

Infection is characterised by profuse diarrhoea, wasting and deaths in untreated lambs. It is important to act in time as mortality rates in untreated lambs can quickly increase. In some cases, as mentioned above, death can occur before signs of scouring induced by adult worms are observed. This is due to large numbers of larvae invading the intestinal mucosa after ingestion.

White drenches (benzimidazoles) and yellow drenches (levamisole) are effective and recommended against larval and adult stages of nematodirus. There are no drenches with residual (lasting) activity, which means that lambs can continue to become reinfected with larvae again. This is why the timing of treatment is critical and veterinary advice is sought where you think your flock may be especially exposed to reinfection for any reason.

Nematodirus can sometimes be confused with other parasites that cause diarrhoea. Coccidiosis will also cause severe diarrhoea in young lambs but it differs to nematodirus in that the scour is dark grey in colour and can often be blood-stained.

It spreads easiest in moist conditions indoors or outdoors, such as around feeding troughs, drinking points or in areas prone to poaching. Moving troughs regularly will help to prevent coccidiosis.

Prevention and treatment is generally administered orally with Vecoxan. Deccox, an in-feed additive, can also be used to reduce the risk of disease occurring.