Nematodirosis - Sheep

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

1. A prolonged period of chill (winter); followed by
2. 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)

Control of Nematodirosis

Clean Pasture

• Ideal but requires no lambs having grazed that pasture the previous year

Contaminated Pasture

• Dose lambs two or three times at 3-weekly intervals in May or June
• Optimum timing may be predicted by a forecasting system based on soil temperatures during March

Alternate Grazing of Sheep and Cattle

Good worm control is possible by alternating the grazing of fields on an annual basis with each host due to the relative insusceptibility of cattle to sheep nematodes and vice versa

Tannin-Rich Forages

Condensed tannins, found in clover, lucerne, trefoil, etc. protect plants against microbial and parasitic attack and have recently been found to

1. Protect dietary protein against rumen breakdown
2. Posses some anthelmintic activity

To date, trials investigating the anthelmintic activity of tannin-rich forages have shown variable results between studies; faecal worm egg output was reduced in most studies and worm numbers in some. Clearly, further research is required, though such forages do offer potential as a long-term epidemiological tool and may reduce farmers' dependence on anthelmintics.