NADIS disease bulletins are written specifically for farmers, to increase awareness of prevalent conditions and promote disease prevention and control, in order to benefit animal health and welfare.

Farmers are advised to discuss their individual farm circumstances with their veterinary surgeon.

Nematodirosis

Nematodirosis is usually seen in young lambs and is characterised by acute onset profuse watery diarrhoea, lethargy, abdominal pain rapid weight loss, dehydration and sudden deaths.

SCOUR AND DEATH IN A WELL GROWN 8 WEEK OLD LAMB DUE TO NEMATODIROSIS

SCOUR AND RAPID WEIGHT LOSS IN 8 – 10 WEEK-OLD LAMBS

The clinical signs of nematodirosis are associated with severe catarrhal inflammation of the small intestine caused by the presence of 2000 or more immature or adult Nematodirus battus worms.

SEVERE CATARRHAL INFLAMMATION OF THE SMALL INTESTINE ASSOCIATED WITH TANGLED KNOTS OF N. BATTUS WORMS

Risk assessment

The life cycle and transmission of nematodirosis differs from that of the other important sheep worms. Infective larvae develop within the N. battus egg and can survive freezing and very low temperatures on pasture for up to two years. Hatching and release of infective larvae only occurs after a period of cold exposure followed by a period of daily maximum temperatures above 10^oC. Infective N. battus larvae generally don’t survive for long on pasture when weather conditions are warm and dry, but can survive for several months during cool and damp weather. Disease outbreaks are seen when the presence of large numbers of larvae on pasture coincides with the grazing of susceptible lambs. Most nematodirosis outbreaks are therefore seen in May and June, the precise timing being influenced by local weather conditions and sheep grazing management. While the warm weather during March this year would have encouraged an early hatch of N. battus eggs, subsequent cool and wet weather will have enabled prolonged larval survival on pasture, which April-born lambs are now beginning to eat.

The life cycle of N. battus essentially involves transmission from one season’s lamb crop to the next and the accumulation of infection on pasture takes place over a period of grazing by susceptible lambs (and occasionally dairy calves). Typically, lambs which are grazed on pastures used for young lambs during previous years are at greatest risk. However, during recent years, nematodirosis has also been seen in older lambs during late summer. This is probably associated with the effect of cold and wet summers on larval survival and means that some pastures may now be heavily contaminated as a result of grazing by older scouring lambs during the previous summer.

IN RECENT YEARS NEMATODIROSIS OUTBREAKS HAVE BEEN SEEN IN OLDER LAMBS DURING LATE SUMMER

Very young lambs are at low risk because they do not graze significant amounts of pasture, while older lambs develop some degree of immunity, which is partly dependent on previous exposure.

Diagnosis of nematodirosis

The diagnosis of nematodirosis is based on knowledge of the local disease pattern, history of susceptible lambs grazing pasture used by young lambs in previous years (or pasture where nematodirosis was identified in older lambs during the previous summer) and clinical signs of scour, weight loss, or sudden death. The diagnosis is supported by the identification of Nematodirus sp. eggs in faeces, although the absence of eggs in faecal samples does not exclude a diagnosis of nematodirosis, because disease can occur during the prepatent period of infection.

The diagnosis of nematodirosis can be confirmed by post mortem findings of catarrhal enteritis and the presence of slender 2 cm long white worms in the lumen of the intestine. N. battus worms are sometimes seen in tangled knots with a characteristic ‘cotton wool ball’ appearance, although in acute cases these may have been expelled before death.

TANGLED KNOTS OF N. BATTUS WORMS

Most of the modern anthelmintics are effective against N. battus although none afford persistent protection. During high risk seasons two or three anthelmintic treatments may be required before weaning. While anthelmintic resistance by N. battus is not currently recognised as a problem in the UK, its appearance is inevitable and should be monitored.

Spring teladorsagiosis

During recent years there have been several reports of teladorsagiosis in 8 – 10 week-old lambs, associated with the overwinter survival of high burdens of Teladorsagia circumcincta (brown stomach worm) larvae on pasture. Most spring teladorsagisois outbreaks have occurred on pastures which were grazed by older lambs during the previous winter months.

In several of the nematodirosis outbreaks which have been investigated this year, both T. circumcincta and N. battus infections have occurred concurrently. Unfortunately, benzimidazole (white drench) resistant T. circumcincta are present in many UK flocks. Therefore, white drenches should be avoided for nematodirosis control where benzimidazole resistance has been identified and the risk of spring teladorsagiosis exists. Irrespective of the anthelmintic used, checking the faecal egg counts of about 7 lambs, 7 – 10 days after treatment can provide useful reassurance about the efficacy of the drug.

NEMATODIRUS EGG (LEFT) AND TELADORSAGIA EGG (RIGHT)

Your vet can provide advice about the nematodirosis risk in your flock, monitoring for resistance and most appropriate choice of wormer.

Neil Sargison BA VetMB DSHP FRCVS