Evidence for transferrin allele as a host-level risk factor in naturally occurring equine respiratory disease: a preliminary study.

REASONS FOR PERFORMING STUDY: Recurring respiratory infections can contribute to prolonged burdens of disease, especially in younger horses and better knowledge of factors and effective interventions, such as vaccines, should improve therapeutic and preventive strategies.
OBJECTIVES: To identify factors and infections associated with naturally occurring respiratory disease in recently weaned Welsh Mountain ponies maintained at pasture and to determine whether ponies vaccinated with an experimental inactivated bacterial vaccine had lower burdens of disease and infection compared to nonvaccinated controls. Transferrin allele, which may influence the ability of pathogenic bacteria to acquire iron, was examined specifically as a host-level risk factor.
METHODS: Twice weekly clinical evaluations and weekly microbiological samplings over a 10 week period were conducted in 29 ponies, of which 12 received an experimental bacterial vaccine, 12 received placebo and 5 were untreated. A multilevel modelling approach suitable for analysing longitudinal data containing repeated observations was used to identify factors associated with disease and to evaluate any effect from vaccination.
RESULTS: Analyses demonstrated significant variation in clinical disease between ponies that possessed different alleles of iron binding transferrin protein but no significant effect from vaccination. Of the 29 ponies monitored, 14 possessing homozygote or non-F2 heterozygote transferrin D alleles demonstrated significantly less clinical disease (P < 0.001), whereas 14 possessing the F2 transferrin allele demonstrated significantly more clinical disease (P < 0.001). These effects remained apparent even after the significant effects of tracheal bacterial isolates, clinical score the previous week and repeated observations from the same ponies were accounted for. CONCLUSIONS AND POTENTIAL RELEVANCE: Results provide evidence for a potential genetic basis for variation in susceptibility to clinical equine respiratory disease of bacterial origin, although more work is required to corroborate this conclusion.