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Roundworms such as stomach, gut and lung worms are cylindrical and vary in length from a few millimetres to several centimetres. They parasitise the small intestine, large intestine, stomach and lungs. 


Generally they are host specific, i.e. sheep nematodes do not infect cattle, however there are a couple of exceptions, these are the Barbers Pole Worm and the gut worm Nematodirus battus which can affect either host species.


Lifecycle of roundworms


The adult female worms lay eggs, which pass out in sheep faeces onto the pasture.  They can take as little as a few days to two weeks to hatch during the summer depending on whether conditions.


Eggs hatch to form L1 larvae which feed on bacteria and organic matter in the faecal pat, grow, and moult to become L2 larvae.  These continue the same way to become infective L3 larvae which migrate out of the dung onto pasture. Development from egg to third stage larvae takes approximately three weeks. 


The Infective larvae (L3) are the picked from the pasture and eaten, L3 larve continue development inside the rumen. 


Moulting twice through L4 and L5 the larvae develops into an adult worm around three weeks after being ingested. 


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