Seasonal changes in the level and composition of nematode worm burdens in young sheep

Abstract

Seasonal changes in the level and composition of nematode burdens in young sheep during their first year were studied in successive lamb crops over a 3-year period. Post-mortem worm count data were obtained from groups of 10 lambs slaughtered at 28-day intervals throughout each of the 3 years. Eighteen nematode species were recorded-14 Strongylidae, 2 Trichinellidae, I Metastrongylidae, and I Rhabditidae. Seasonal patterns of infection by most species were relatively constant during the period of study. Peak infections of Nematodirus occurred in the spring. Of the principal trichostrongyle species, the patterns of infection by Haemonchus contortus, Ostertagia spp., Trichostrongylus spp. of the small intestine, and Chabertia ovina presented similarities; infections rose rapidly to a peak in late summer/early autumn, followed by an abrupt decline to low numbers in winter. Duration of infection by these species was similar. Infection by Cooperia curticei and T. axei extended over a longer period and formed the major part of the worm burdens in animals entering their second year.Species that attained numerical dominance were, in order of succession, Strongyloides papillosus, Nematodirus spp., Ostertagia spp., Trichostrongylus spp. of the small intestine, C. curticei, and/or T. axei. In late autumn/early winter, after peak flock infection, a "self-cure" type reaction occurred which resulted in rapid elimination of the major part of infections by most species.Throughout the course of infection each year the patterns of increase and decline of mean strongyle egg counts (Nematodirus excluded) and mean total strongyle worm burdens were of similar shape.Considerable within-flock variation was recorded in faecal egg counts and in both level and composition of infections; this emphasises the need for adequate diagnostic samples and for caution in their interpretation.

The host immune response to gastrointestinal nematode infection in sheep

“…Immune competence can be observed through prevention of establishment of most incoming infective larvae, suppressed GIN growth (and therefore fecundity), the expulsion of adult worms, or a mixture of the above . Lambs start to demonstrate immune competence from 2 to 3 months of age , with regular exposure to larval challenge allowing the immune response to develop until a significant protective immune capability is developed by 10–12 months of age . Adult sheep tend to remain relatively resistant to infection, harbouring only a few adult worms, although regular exposure to some level of infection is required to retain immunity .…”

“…Consequently, infections are generally comprised of a mix of species, which infect both the abomasum and intestine. The species of infective larvae on pasture is dependent on a number of factors including temperature and moisture and therefore often displays a seasonal distribution . As GIN is highly aggregated within the host population, susceptible individuals can harbour thousands of worms, which in turn leads to increased pasture contamination.…”

Effect of anthelmintic treatment strategy on strongylid nematode species composition in grazing lambs in Scotland

“…vitrinus becoming more prevalent towards the end of the grazing season in the TST, SPT and MT groups, consistent with previous studies [ 17 , 22 ]. Seasonal shifts in species prevalence result from a combination of factors including the development of acquired immunity in lambs [ 23 , 24 ] and environmental shifts in temperature and rainfall impacting the hatching of nematode eggs on pasture [ 25 ]. Comparable species profiles were obtained from all treatment groups in the 2006 early season, showing that the populations were similar at the start of the experiment in both species composition and contamination levels.…”

Management effects on growth and roundworm burdens of autumn-born lambs

“…For unweaned lambs, strongyle infection levels of the control treatment were high to very high. In similar-aged spring-born lambs at Wallaceville, Brunsdon (1970a) recorded flock FECs of a few hundred eggs/g over 3 years (peak of 1600). For spring lambs between weaning and 1 year old, McKenna (1981) accepted 600-2000 eggs/g as a "moderate" level of strongyle infection.…”