Patterns of Beak and Feather Disease Virus (BFDV) prevalence and effects on individual fitness in threatened island birds

Emerging infectious diseases (EIDs) threaten biodiversity but their long-term impact on recovering species remains unclear. To investigate patterns of pathogen spread, we examined the phylogeny of the Psittaculidae parrot family and evaluated the distribution of Beak and Feather Disease Virus (BFDV), in order to see whether there is any evidence of a host phylogenetic pattern in the observed incidences of documented infection. Our evaluation revealed that the global pet trade has played a major role in shaping host movements and, consequently, the worldwide spread of BFDV. However, geographical gaps in wild sampling suggest that the true extent of viral spread may be underestimated and should be considered in management plans. Building on this global perspective, we conducted a case study investigating the prevalence and diversity of BFDV in the endemic Mauritius parakeet (Alexandrinus eques). We assessed the effects of viral strains on recruitment into the breeding population and their interaction with host inbreeding. We further compared immunocompetence between Mauritius parakeets and invasive ringneck parakeets (Alexandrinus krameri) occurring nearby. Using PCR screening and haplotype analyses of 231 rep gene sequences and 331 cap gene sequences collected over three decades, we evaluated recruitment rates and associations with host inbreeding. Immunocompetence was assessed using phytohaemagglutinin (PHA) challenge and haemolysis-haemagglutination (HL-HA) assay. Recruitment probability was lower in birds carrying more recent viral haplotypes compared to those from the initial outbreak, while inbreeding probability was higher in the same individuals. When compared to ringneck parakeets which had a strong immune response in both assays, Mauritius parakeets exhibited reduced immunocompetence, suggesting that the invasive species may act as a viral reservoir. Together, these findings show that global processes, such as the pet trade, have shaped the spread of BFDV across the Psittaculidae family, while local dynamics in Mauritius reveal ongoing viral evolution towards greater virulence. Reduced recruitment in the endemic population is further exacerbated by inbreeding and amplified by the potential reservoir role of invasive ringneck parakeets, highlighting a Red Queen dynamic in which the host struggles to keep pace with viral evolution.