Nematodirosis - Sheep

Introduction

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.

The disease affects lambs at 6-10 weeks old at the beginning of the first grazing season (late April - June).

N. battus eggs are 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). The 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.

Clinical Signs

Signs include a sudden onset profuse diarrhoea (black-green, pale yellow then colourless and scanty), rapid dehydration and in some severe cases, death (<30%).

Diagnosis

Clinical signs and season are enough to make a presumptive diagnosis.

A faecal egg count should be performed, however, these often return with the result of zero eggs (as clinical signs are seen after 11-12 days whilst the prepatent period is 15 days). Nematodirus species eggs, including N. battus, are much larger than strongyle eggs. They are brownish in colour and have straight sides. Bloods should be taken. With this disease, blood pepsinogen levels are normal, which is useful for ruling out the differential diagnosis of Ostertagiosis. A post-mortem examination can be carried out on any sheep that may have succumbed to the disease. Findings at necropsy would include enteritis and the presence of >10,000 adult worms (examine male worms to identify species) .

Treatment and Control

In an outbreak of Nematodirosis on a farm, anthelmintic treatment should be given to all stock. If diarrhoea is very severe then supportive therapy, such as providing an electrolye solution may be given. Affected animals may need to be brought in and housed for a short period of time.

Clean Pasture would be ideal but this requires no lambs having grazed that pasture the previous year, and so is impractical in many cases.

This means in most cases, control strategies work around the presence of a Contaminated Pasture. You will need to 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.

Another way to manage worm control is to provide 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 are a new thinking into worm control. Condensed tannins, found in clover, lucerne, trefoil, etc. protect plants against microbial and parasitic attack and have recently been found to protect dietary protein against rumen breakdown and they 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.

References

Blood, D.C. and Studdert, V. P. (1999) Saunders Comprehensive Veterinary Dictionary (2nd Edition) Elsevier Science
Fox, M and Jacobs, D. (2007) Parasitology Study Guide Part 2: Helminths Royal Veterinary College
Merck & Co (2008) The Merck Veterinary Manual (Eighth Edition) Merial
Radostits, O.M, Arundel, J.H, and Gay, C.C. (2000) Veterinary Medicine: a textbook of the diseases of cattle, sheep, pigs, goats and horses Elsevier Health Sciences

Test yourself with the Small Ruminant Nematodes Flashcards

Small Ruminant Nematodes Flashcards