Investigating interactions between coinfecting gastrointestinal nematodes and host immunity using meta-analytic, observational and experimental approaches

Abstract

Gastrointestinal nematodes (GIN) have enormous global impacts in humans, wildlife and grazing livestock. Natural infections are composed of co-infections with multiple GIN species, forming an ecosystem of interacting organisms within the host. In this thesis I used meta-analytic, observational and experimental methods to explore this ecosystem, the interactions within it, and the impacts it has on host health. The work focuses on GIN coinfections in sheep, both as a model for the evolutionary ecology of host-pathogen relationships, and to inform decision-making in sheep farming, where widespread anthelmintic resistance necessitates a holistic understanding of GIN coinfection epidemiology.

I first conducted a systematic review and meta-analysis of experimental GIN coinfections, which demonstrates that coinfecting GIN tend to interact antagonistically. This means that on average, fewer worms of one species establish when a coinfecting species is also present. This effect is dependent on the infectious dose and is more pronounced in sequential coinfections. The effect occurs across sites within the gastrointestinal tract, suggesting a common mucosal immune mechanism may drive the interactions.

I then applied an ITS2-sequence based nemabiome speciation technique to over 3000 faecal samples collected across four years from over 500 sheep living in an unmanaged population on St Kilda. The results show how parasite community diversity and the species-corrected faecal egg counts (sFEC) vary with season, reproductive status and age, in the absence of human intervention. This includes descriptions of the seasonal epidemiologies of relatively understudied species (Trichostrongylus axei, Chabertia ovina and Bunostomum trigonocephalum). In particular I found that Teladorsagia circumcincta and Nematodirus spp. conform to epidemiological patterns previously described in domestic sheep, whereas Trichostrongylus vitrinus is more abundant in lambs in the summer than domestic studies would predict. This epidemiological difference may be a consequence of the antagonistic interaction between Te. circucmcincta and Tr. vitrinus, combined with variation in sFECs in late pregnant females and co-grazing between multiple age classes.

I then go on to show that the epidemiology of Tr. vitrinus has important implications for the health and fitness of wild sheep in this system, with significant negative associations between Tr. vitrinus and both the bodyweight and overwinter survival of lambs. This demonstrates the power of the nemabiome approach, with these species-specific measures explaining more variation in bodyweight and survival than are explained by unspeciated faecal egg counts (FEC) or summary measures of parasite alpha- and beta-diversity. The approach also identified a robust, positive association between Tr. axei and August bodyweight, that suggests the impacts of parasites in natural coinfections may be more complex than simply the addition of their individual pathological effects.

Given that cross-protective immunity between sites within the gastrointestinal tract may be a key driver of the interspecies interactions in this GIN ecosystem, I then performed an experimental infection to identify the mechanism. This study demonstrated that infection with the intestinal GIN Trichostrongylus colubriformis induces IgG within intestinal and abomasal mucus that cross-reacts the abomasal-dwelling parasite Teladorsagia circumcincta, whereas cross-reactive IgA is restricted to the site of the primary infection. Proteomic analysis indicates that the cross-reactive IgG recognises conserved structural and metabolic proteins unlikely to be exposed in live nematodes and hence unlikely to be effective targets in vivo. However, Tr. colubriformis infection results in upregulation of other immune pathways within the abomasum that are associated with anti-parasite T-helper type-2 (Th2) immunity and are a more likely mechanism for inter-GIN interactions.

Together, these findings demonstrate that coinfecting GIN tend to interact antagonistically, most likely via cross-protective immunity that is mediated by immunological changes within the mucosa rather than cross-reactive antibodies. These interactions may be interrupted in domestic systems, with different seasonal epidemiologies for some GIN species observed in wild sheep. These differences may help guide management decisions in domestic flocks and the development of multispecies anti-GIN vaccines. The individual species within the GIN ecosystem also have specific significant impacts on health and fitness of their wild hosts, with demographic feedback on the epidemiology potentially driving host and pathogen co-evolution.