Genetically resistant sheep avoid parasites to a greater extent than do susceptible sheep

Abstract

Livestock breeding programmes have created resistant (R) and susceptible (S) sheep that differ in their ability to control parasites through their immune function but potentially also their grazing behaviour (i.e. parasite avoidance). Using the Perendale genetic lines, we tested the hypothesis that R-sheep avoid parasites more effectively, reducing their parasite exposure/challenge, compared with S-sheep. However, in grazing systems, parasite-rich areas are also forage rich, suggesting that parasite avoidance behaviours are associated with nutritional penalties. We first created a naturally heterogeneous sward structure of gaps and tussocks and then used focal behavioural observations to quantify the sward selection of R- and S-sheep. Tussock swards were more nitrogen rich (41%), offered increased forage intake rates (32%) and contained 17 times more parasite larvae than gap swards. All the animals avoided grazing the tussock swards. However, the R-sheep grazed the tussock swards to a lesser degree than the S-sheep. We conclude that selection for genetic resistance has resulted in animals that, despite being well armed to fight parasitism through improved immune function, adopt parasite avoidance strategies with associated nutritional disadvantages. This experiment highlights the role of host behaviour in the control of parasitism and suggests that animals can be bred to avoid disease.

Disease ecology meets ecological immunology: understanding the links between organismal immunity and infection dynamics in natural populations

“…1999; Behringer, Butler & Shields 2006), presumably to reduce their own risk of infection. These avoidance behaviours could covary with other resistance mechanisms: in one of the few studies to date to explicitly examine genetic covariation between host susceptibility and exposure, sheep from lines that were resistant to gastrointestinal helminths (as produced by a selective breeding program) also avoided foraging in parasite‐rich areas of habitat more effectively (Hutchings et al. 2007).…”

“…A final key challenge in the merger of ecoimmunology and disease ecology is that exposure will vary alongside immune responses, and in some cases, the two may covary if susceptible individuals are also most likely to be exposed (Hutchings et al. 2007).…”

“…In some host-parasite systems, healthy animals actively avoid infected conspecifics (Kiesecker et al 1999;Behringer, Butler & Shields 2006), presumably to reduce their own risk of infection. These avoidance behaviours could covary with other resistance mechanisms: in one of the few studies to date to explicitly examine genetic covariation between host susceptibility and exposure, sheep from lines that were resistant to gastrointestinal helminths (as produced by a selective breeding program) also avoided foraging in parasite-rich areas of habitat more effectively (Hutchings et al 2007). Infected animals can also behave in diverse ways that alter the potential for pathogen transmission, including alterations of foraging behaviour, activity levels, and investment in sexual reproduction (reviewed in Moore 2002).…”

Phenotypic expression of parasite susceptibility to Haemonchus contortus in Pelibuey sheep

“…However, continued drug use has led to development of anthelmintic resistance. An option to avoid massive use of anthelmintic drugs is to breed animals with greater ability to respond to GIN infections (Hutchings et al, 2007;Stear et al, 2007). Selection for low and high faecal egg counts (FEC) has been applied in wool breeds to generate divergent lines of animals (high and low responders, respectively) (Woolaston et al, 1990).…”

Variability in the fecal egg count and the parasitic burden of hair sheep after grazing in nematode infected paddocks

“…Selection of sheep that are genetically resistant to gastrointestinal parasitic nematodes is another possible strategy that has been explored in recent decades (Eady et al 2003, Hutchings et al 2007, Stear et al 2007). This alternative has been used in combination with other control methods, especially when there is the problem of anthelmintic drug resistance (Molento 2009).…”