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Introduction Lungworm infection is one of the most important respiratory diseases of cattle in northern Europe. It is caused by lungworm (Dictyocaulus viviparus) which is a roundworm (nematode) parasite similar to gutworms. However, it completes its life cycle in the lungs rather than in the gastrointestinal tract. The clinical signs of infection include coughing and difficulty in breathing (especially when animals are being moved). The disease is described as a ‘parasitic bronchitis’ and is commonly known as hoose or husk and and can result in death where serious infections occurs. As deaths from hoose can occur with very little warning and at various times of the year, it is essential that farmers consult their own veterinary practitioner when drawing up their parasite control programmes. Lungworm Lifecycle

Figure 1: Lungworm lifecycle

Figure 1 shows the lifecycle of lungworm. When L3 stage larvae on pasture are ingested, they pierce the intestinal wall and move through the lymphatic system and blood stream to the lungs. In the lungs they leave the blood and penetrate the lung tissue, where they grow rapidly and where they mature into adult worms. Within24-28days of ingestion, eggs are laidby adult females in the large airways, coughedup and are then swallowedby the host animal. During the passage through the intestine, the eggs hatch and the immature larvae (L1) are excreted in the dung. The rateof development of the free-living stages depends on theprevailingenvironmental conditions. If theweather iswarm (approx. 20 0 C) andhumid, infective larvaemaybeavailableonpasturewithin sevendays or less of beingpassed in the faeces. Larvae may be dispersed from the dung pat by a fungus or by the splashing effects of rain. This means that pastures can become contaminated with infective larvae very quickly. Older animals (yearlings and adults)may serve as carriers over winter as some adult worms will survive in the lungs (either as fully mature or hibernating immature adults). Infective larvae are relatively short lived (a few weeks in hot, dry conditions) but may survive in pasture regrowth after silage and also over the winter on pasture in enough numbers to cause disease in susceptible animals turned out to pasture in early spring. The numbers of larvae present, their survival and their rate of development on pasture are very variable and unpredictable.

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