A consideration of resistance and tolerance for ruminant nematode infections

Abstract

Debates on the relative merits of resistance (the ability of the host to control the parasite lifecycle) and tolerance (the net impact of infection on host performance) are often lively and unhindered by data or evidence. Resistance generally shows continuous, heritable variation but data are sparser for tolerance, the utility of which will depend upon the disease prevalence. Prevalence is a function of group mean resistance and infection pressure, which itself is influenced by mean resistance. Tolerance will have most value for endemic diseases with a high prevalence but will be of little value for low prevalence diseases. The conditionality of tolerance on infection status, and hence resistance, makes it difficult to estimate independently of resistance. Tolerance is potentially tractable for nematode infections, as the prevalence of infection is ca. 100% in animals grazing infected pasture, and infection level can be quantified by faecal egg count (FEC). Whilst individual animal phenotypes for tolerance are difficult to estimate, breeding values are estimable if related animals graze pastures of different contamination levels. Selection for resistance, i.e., FEC, provides both direct and indirect benefits from ever decreased pasture contamination and hence decreased infectious challenge. Modeling and experimental studies have shown that such reductions in pasture contamination may lead to substantially increased performance. It is proposed that selection goals addressing nematode infections should include both resistance and performance under challenging conditions. However, there may be benefits from exploiting large datasets in which sires are used across cohorts differing in infection level, to further explore tolerance. This may help to customise breeding objectives, with tolerance given greater weight in heavily parasitized environments.

Resilience in farm animals: biology, management, breeding and implications for animal welfare

“…Although Clunies Ross did not use the term, later studies described the latter phenomenon as disease resilience (Albers et al 1987;Bisset and Morris 1996;Bishop 2012;Doeschl-Wilson and Lough 2014), which is typically defined in terms such as a capacity to maintain high productivity in the face of ongoing infection (Bisset and Morris 1996;Bishop 2012). More recently, interest in the impact of non-infectious stressors on animal performance has used the term resilience to describe these broader aspects of an animal's response to environmental challenges (Hermesch and Dominik 2014).…”

“…In farm animals, disease tolerance provides a phenotypic measure of the net impact of a given level of infection on performance whereas disease resilience is a phenotypic measure of productivity during infection (Bishop 2012;Doeschl-Wilson et al 2012). The substantial biological significance of this minor semantic difference is explored in detail by Bishop (2012) and Doeschl-Wilson et al (2012). The biological mechanisms contributing to these phenotypic outcomes can differ between pathogens and are not considered further here.…”

“…The dynamics of adverse health outcomes can be highly situation-specific such that occurrence and distribution within a group can be strongly influenced by factors other than underlying resilience (Bishop 2012). We suggest therefore that function of the immune system is better assessed through measuring responsiveness to a standardised controlled challenge from vaccination with industry relevant vaccines rather than relying on the occurrence of disease challenges during the assessment period.…”

Host genetics of response to porcine reproductive and respiratory syndrome in nursery pigs

“…the traits are heritable) and phenotypes related to disease resistance, tolerance, or resilience can be collected on breeding stock. Here, resistance refers to the ability of an animal to prevent infection when exposed to a pathogen or to limit replication of the pathogen when infected (Bishop, 2012). Tolerance refers to ability of an animal to maintain performance at a given level of infection or pathogen load (Bishop, 2012).…”

“…Here, resistance refers to the ability of an animal to prevent infection when exposed to a pathogen or to limit replication of the pathogen when infected (Bishop, 2012). Tolerance refers to ability of an animal to maintain performance at a given level of infection or pathogen load (Bishop, 2012). Thus, at a given level of viremia, growth and performance of tolerant animals is less affected than that of less tolerant animals.…”