Assessment of Galleria mellonella as an in vivo virulence model for Brachyspira species associated with avian intestinal spirochaetosis.

Abstract

Several species of Brachyspira are associated with avian intestinal spirochaetosis (AIS), a gastrointestinal disease common in layer hens and broiler breeders. AIS costs the UK egg laying industry approximately £18 million per annum, resulting from reduced egg production and poor egg quality. Prevalence of Brachyspira is increasing, and due to a limited understanding of this pathogen, mitigation strategies have been largely unsuccessful. Therefore, improving the understanding of these organisms and subsequent immune responses is essential to inform treatment and preventative measures. The Galleria mellonella model has been established as a simple in vivo model to explore innate immune responses to a multitude of pathogens. However, to date it has not been utilised as a model to assess Brachyspira infection. In this study, the virulence of eight well characterised Brachyspira isolates was investigated by assessing larval mortality and melanin production, in addition to investigating the location and morphology of Brachyspira using histopathology. The mortality rates of infected G. mellonella varied markedly among the different Brachyspira species, with B. intermedia infection resulting in the highest mortality whereas B. pilosicoli resulted in highly variable mortality between isolates (p = 0.013). High melanisation scores were observed in G. mellonella infected with all Brachyspira spp. Brachyspira remained viable up to 48 h post infection and exhibited no signs of morphological changes. There was evidence that B. pilosicoli translocated to the muscle of the G. mellonella, with no evidence of Brachyspira presence in the gut.