The pattern of infection with gastro-intestinal nematodes in young fattening lambs at pasture.

The pattern of infection with gastrointestinal nematodes in young fattening lambs was determined during the period 1965-69 in 10 counties of the UK. On 25 farms with a history Nematodirus disease, 193 lambs were killed, mostly in pairs, at intervals of three weeks from the end of April to mid-July. In three of the seasons similar studies were extended to mid-September on 33 farms which had a history of parasitic gastroenteritis. Worms were counted 361 parasitic gastroenteritis. Worms were counted in 361 lambs killed at intervals of four weeks. The degree of infection and incidence of different genera covering the same period were identical in both series. The division into Nematodirus and parasitic gastroenteritis farms proved artificial and the results were combined to obtain an overall picture based on average worm burdens during weekly periods from the last week in April to the second week in September. An average total of 2500 worms was present in four-week-old lambs at the end of April and this rose steadily to a peak of 10, 000 during the first week in June. A gradual fall to 5500 in mid-July was followed by a rapid rise which had reached 22, 000 at the end of observations in mid-September. The first wave of infection up to mid-July consisted almost entirely of Nematodirus and Ostertagia in the approximate proportions 3;1, 2:1 and 1-5:1 midway through the months of May, June and July respectively. The second rise from mid-July onwards consisted principally of Ostertagia and Trichostrongylus. Nematodirus worm numbers remained at a lower level while a few Cooperia and Haemonchus had appeared. Unidentified third and fourth stage larvae in the abomasum and small intestine reflected the prevalence of Ostertagia and Nematodirus respectively. The significance of this infection pattern, which was common to all geopgraphical areas, in relation to the epidemiology of parasitic infections in lambs is discussed.