Nematode parasites of sheep in the UK
There are about 20 different species of nematodes of sheep commonly found in Britain, the more important of which are shown in the table below. Pathogenicity varies with species, the numbers of nematodoes present as well as host factors such as age (maturity), nutritional status and body condition.
|Abomasum||Teladorsagia (Ostertagia) circumcincta||‘Small brown stomach worm’ 0.8 – 1.5cm||H|
|Haemonchus contortus||‘ Barber’s Pole worm’ 1.5 – 3.0 cm long and stout. Very obvious to the naked eye.||H|
|Trichostrongylus axei||‘ Stomach hair worm’ 0.3 – 0.6 cm||M|
|‘Black scour worm’ 0.4 – 0.9 cm||M
|‘Thin-necked intestinal worm’1.0 – 2.3cm||H
|Cooperia curticei||‘Small intestinal worm’ 0.5 – 0.8 cm||L|
|Bunostomum trigononcephalum||‘Hookworm’1.2 – 2.6 cm||M|
|Strongyloides papillosus||‘Threadworm’ 0.4 – 0.6 cm||L|
|Capillaria longipes||‘Hairworm’ 0.1-0.2 cm||L|
|Large intestine||Oesophagostomum venulosum||‘Large bowel worm’ 1.0 – 2.4 cm||L|
|Trichuris ovis||‘Whipworm’ 4 – 8 cm||L|
|Chabertia ovina||‘Large-mouthed bowel worm’ 1.4 – 2.0cm||L|
|Lungs||Dictyocaulus filaria||‘Large lungworm’. Live in bronchi, 3 – 10 cm||M|
|Protostrongylus rufescens||Live in the small bronchioles. 1.6 – 4.0 cm||L|
|Cystocaulus ocreatus||Live in the small bronchioles. 4.0 – 9.0 cm||L|
|Neostrongylus linearis||Live in the small bronchioles. 0.5 – 1.5 cm||L|
|Muellerius capillaris||‘Small lungworm’. Form nodules in lung parenchyma. 1.2 – 2.2 cm||L|
The Typical Life Cycle
The life cycles of the gastrointestinal nematodes are all very similar, with one or two minor exceptions (for more information see the SCOPS Technical manual).
The worms do not multiply within the sheep and the life-cycle is direct (no intermediate host). Adult female worms in the sheep lay eggs that pass out in the dung and hatch; each egg releasing one first-stage larva (L1). These L1s develop and moult to second stage larvae (L2) and both of these stages remain in the dung feeding on bacteria.
At the second moult the third stage larvae (L3) appear and these are the 'infective' larvae, which migrate on to the herbage where they are ingested by grazing sheep, where they enter the wall of the stomach or intestines and develop into fourth stage larvae (L4) and then about 14 days later they mature into adult worms. The prepatent period (time between the L3s being eaten by the sheep and the appearance of eggs in the dung) is normally between 16–21 days.
Adult worms that are not expelled by the immune system of the sheep or killed by anthelmintics will dy naturally after short period (typically less than 12 weeks).