What are the main nematodes that infect sheep? What are the parasitic gastro-enteritis (PGE) diseases that they cause?
Ostertagia circumcinta (aka Teladorsagia) - Ostertagiosis Types I & II
Trichostrongylus axei & Trichostrongylus spp - Trichostrongylosis
Nematodirus battus - Nematodirosis
Haemonchus contortus - Haemonchosis
NB: all of the above are nematodes that belong to the Trichostrongyloidea superfamily of round worms, so they all lay strongyle eggs of about 80 µm.
Where does Ostertagia circumcinta do its damage in the sheep GIT?
What age of sheep is most suscpetible?
Abomasum of New Lambs
What is the most predominant form of Ostertagiosis in sheep?
What is the pathogenesis of the disease?
Type I Ostertagiosis caused by Ostertagia circumcinta, mainly.
Like bovine Ostertagiosis, the developing worm larva attack the gastric glands of the abomasum, destroying the epithelial cells that produce HCl (parietal cells) necessary for activation of the zymogen pepsinogen. There ends up being an increase in permeability to macromolecules such as plasma proteins, which end up osmotically drawing fluid into the lumen and contributing to diarrhoea.
When are new lambs most susceptible to Ostertagia circumcinta infestation?
During their first grazing season in the summer - July to September.
When are lambs most susceptible to Type II Ostertagiosis?
Lambs older than one year after their first grazing season, January to May (winter grazing).
These would be yearlings/shearlings.
Ostertagia circumcinta has a similar life cycle to ostertagia ostertagi that affects calves. What is the pre-patent period?
Three weeks, unless there is arrested development (hypobiosis) in winter
What is the mnemonic HOT?
All nematodes that contribute to PGE complex in sheep. Should also remember Nematodirus battus
Where does trichostrongylosis occur?
In very young lambs less than a year old.
Trichostrongylus axei prefers the abomasum, so the condition is similar to ostertagiosis.
Other Trichostrongylus species prefer the small intestine.
What is Periparturient Relaxation in Immunity? When does it occur and what impact does it have on the faecal egg count of Ostertagia circumcinta?
PPRI occurs around lambing in breeding ewes. Their immunity is suppressed due to lack of nutrition and maybe poor housing conditions. With their suppressed immune systems,
Immediately after lambing their faecal egg count is very high as there could be lots of L3 on the ground to eat, EL4 resume development and there are more eggs produced per worm (not enough IgE maybe to fight infestation).
What is "black scour" a sign of?
Trichostrongylosis. It's foul-smelling, dark faeces.
What type of lamb (age) does Nematodirus attack, where in the GIT and what are the clinical signs of Nematodirosis?
Very young lambs only 6-10 weeks of age. Considering their age, it's obvious that the worm is eaten during at the start of the first grazing season, April-June.
Attacks their small intestine.
Sudden onset profuse diarrhoea:
black-green →pale yellow → colourless
Even though Nematodirus is part of the Trichostrongyloidea superfamily, its eggs differ in appearance. What are the differences?
Nematodirus eggs are larger than strongyle eggs so they are more than 80-100 µm long and they are brownish with straight sides.
How does the life cycle of Nematodirus differ from Ostertagia?
Larvae develop to L3 on pasture but stay INSIDE the big brown egg, surviving up to two years on pasture.
Hatching only occurs in Spring. Most hatch in Spring, a full 12 months after they were dropped as eggs on pasture.
What are the characteristic features of the Haemonchus contortus adult worm?
Adult females = “barber’s pole” when fresh as blood-filled intestine is intertwined with uterus
Blood-sucking worm with piercing lancet in buccal cavity in L4+ adult that cuts down to the capillaries in abomasal mucosa
What is special about Haemonchus contortus in terms of its life cycle?
It has a very high biotic potential, producing up to 10,000 eggs per day in the abomasum of the sheep and lamb.
Also undergo hypobiosis in dry periods (this worm is adapted mainly to wet, tropical weather). Only becomes arrested in ewes, not lambs.
Prepatent period is very short - just 15 days, thus at least 6 and up to 9 generations per year.
Do lambs typically get Type I (early, summer) or Type II (later, autumn) disease with Haemonchosis?
Usually Type II.
In temperate regions, lambs ingest a lot in late-grazing period that were in hypobiosis in gut of other lambs over drier summer.
What is the pathogenesis of haemonchosis?
Moderate infection of about 2000 worms in the abomasum leads to:
increased RBC turnover
loss of Hb & Fe
Depleted Fe reserves leading to anaemia and death
What are the three forms of haemonchosis?
Hyperacute - mainly in tropics, severe anaemia, sudden death in 0-7 days
Acute - oedema, loss of condition, lethargy, death, anaemia, hypoalbuminaemia, death in 1-6 weeks
Chronic - progressive weight loss, death in more than two months
What is the succession of Sheep nematode/PGE diseases, starting early to late?
Nematodirus battus - April-June
Ostertagia circumcinta - July-September
Haemonchus contortus - late July-October
Tristrongylus axei - November-December