Nematodirosis - Sheep

Several species of Nematodirus occur in the small intestine of sheep, although only one (N. battus) causes an acute and fatal disease in the field. N. battus may also cause disease in first season calves. Adult Nematodirus are 2cm long and similar to cotton wool in gross appearance when present in large numbers.

Disease (N. battus)

Affects lambs at 6-10 weeks old at the beginning of the first grazing season (late April - June)
Clinical signs
- Sudden onset profuse diarrhoea (black-green, pale yellow then colourless and scanty)
- Rapid dehydration
- Death (<30%)

Recognition of Eggs

Nematodirus species eggs, including N. battus, are much larger than strongyle eggs
N. battus eggs are brownish in colour and have straight sides

Epidemiology of Nematodirosis

N. battus eggs deposited on pasture in the spring develop slowly to the L3 stage which remains inside the egg (capable of surviving up to 2 years on pasture)
Hatching only takes place after

A prolonged period of chill (winter); followed by
A mean day/night temperature >10°C (spring)

Majority of L3 only hatch the following spring (12 months after deposition on pasture); however, an increasing number of cases are being seen in the autumn of the same year.
Disease occurs if large numbers of L3 ingested by susceptible lambs (reduced risk if increase in L3 occurs early while lambs still suckling, or later when lambs are better able to resist challenge)
Eggs are rarely passed by ewes (even during the PPRI). Therefore, they do not play a significant role in the epidemiology of disease.

Diagnosis of Nematodirosis

Clinical signs
Season
Faecal egg count:
- Often zero (as clinical signs are seen after 11-12 days whilst the prepatent period is 15 days)
Blood pepsinogen levels:
- Normal (useful for differential diagnosis in calves eg. Ostertagiosis)
Post-mortem examination:
- Enteritis
- >10,000 adult worms (examine male worms to identify species)