A mechanistic model of developing immunity to Teladorsagia circumcincta infection in lambs

Abstract

Acquired immunity influences the severity of parasitic disease, but modelling the effects of acquired immunity in helminth infections has proved challenging. This may be due to a lack of suitable immunological data, or to the perceived complexity of modelling the immune response. We have developed a model of T. circumcincta infection in domestic sheep that incorporates the effects of acquired immunity on parasite establishment and fecundity. A large data set from commercially managed populations of Scottish Blackface sheep was used, which included relationships between IgA activity and worm length, and between worm length and fecundity. Use was also made of a recently published meta-analysis of parasite establishment rates. This realistic but simple model of nematode infection emulates observed patterns of faecal egg counts. The end-of-season faecal egg counts are remarkably robust to perturbations in the majority of the parameters, possibly because of priming of the immune system early in the season, reducing parasite establishment and growth and, therefore, faecal egg counts. Lowering the amount of early infection leads to higher end-of-season egg counts. The periparturient rise in egg counts in ewes appears to have an important role in supplying infection for the priming of the immune response. This feedback in the immune priming suggests that nematode infections may be difficult to eliminate.

The transfer of IgA from mucus to plasma and the implications for diagnosis and control of nematode infections

“…Singleton et al (2011). One advantage of the nematode system is that it is possible to quantify variation among hosts in the number and size of the antigenic challenge.…”

Progress in the development of subunit vaccines for gastrointestinal nematodes of ruminants

“…<6 months old) and pregnant ewes; the latter, to test whether vaccination reduces the “periparturient relaxation in immunity” observed during late pregnancy and early lactation with this parasite . Such reductions in immunity result in a rise in trichostrongyle FECs in ewes around lambing, which then act as a critical source of pasture infection for parasite‐naïve lambs when they start ingesting grass later in the season . Mitigating the ewe source of pasture contamination with parasites via vaccination early in the season could have a substantial impact on production losses in lambs.…”

Exploiting parallels between livestock and wildlife: Predicting the impact of climate change on gastrointestinal nematodes in ruminants

“…immune response e.g. Singleton et al, 2011) is often represented implicitly by decreasing establishment rate as a function of the duration of infection (Grenfell et al, 1987), or explicitly as a single measure of resistance that increases and decays as a function of the duration of, or magnitude of exposure to, infective stages (Anderson and May, 1985; Roberts and Grenfell, 1991; Grenfell et al, 1995; Roberts and Heesterbeek, 1995). However, it is unclear to what extent the potential application of acquired immunity functions based on livestock ruminants can be extended to free-ranging ruminants.…”