Abstract
In reproducing ewes, a periparturient breakdown of immunity is often observed to result in increased fecal egg excretion, making them the main source of infection for their immunologically naive lambs. In this study, we expanded a simulation model previously developed for growing lambs to explore the impact of the genotype (performance and resistance traits) and host nutrition on the performance and parasitism of both growing lambs and reproducing ewes naturally infected with Teladorsagia circumcincta. Our model accounted for nutrient-demanding phases, such as gestation and lactation, and included a supplementary module to manage the age structure of the ewe flock. The model was validated by comparison with published data. Because model parameters were unknown or poorly estimated, detailed sensitivity analysis of the model was performed for the sheep mortality and the level of infection, following a preliminary screening step. The parameters with the greatest effect on parasite-related outputs were those driving animal growth and milk yield. Our model enables different parasite-control strategies (host nutrition, breeding for resistance and anthelmintic treatments) to be assessed on the long term in a sheep flock. To optimize in silico exploration, the parameters highlighted by the sensitivity analysis should be refined with real data.
Highlights
Gastrointestinal nematodes (GINs) are one of the most pervasive challenges to the health and welfare of grazing ruminants
Animals may eventually develop immunity to gastrointestinal nematodes, but a periparturient breakdown of immunity to parasites is frequently observed in reproducing ewes and manifests as an increased number of nematode eggs released in the feces
To explore the influence of host resistance and nutrition on the infection and epidemiology of T. circumcincta in the long term, a model was developed based on the work of Vagenas et al (2007a, b) that accounts for host characteristics, host nutrition and T. circumcincta infection in growing lambs
Summary
Gastrointestinal nematodes (GINs) are one of the most pervasive challenges to the health and welfare of grazing ruminants. Animals may eventually develop immunity to gastrointestinal nematodes, but a periparturient breakdown of immunity to parasites is frequently observed in reproducing ewes and manifests as an increased number of nematode eggs released in the feces. This is the usual source of infection for the immunologically naive lambs and the main cause for loss of performance and death in these animals (Barger, 1993; Houdijk et al 2001). Gastrointestinal infection is caused by different types of parasites developing in different climatic conditions This leads to a diverse geographical distribution of the parasites threatening the sustainability of the sheep industry worldwide. Infection is controlled with anthelmintics; due to the development of parasite resistance to anthelmintics, and the potential impact of such products on the environment, finding alternative strategies is essential to control gastrointestinal parasitism
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