Figure 1.

Avian influenza transmission dynamics between domestic flocks. Infected premises (IPs): a farm is first susceptible (S) and may become infected by infectious farms. Once infected, a farm becomes latent (L) during δ days then infectious but not reported (NR) during γ days, becomes infectious and reported to authorities (R) before being depopulated (D) within τ days. ring-culled premises (RCPs) and dangerous contacts (DCs): farms (susceptible or infected with no specific clinical signs) that are in the vicinity or linked to an IP just revealed infected (R) would be depopulated within τ days also. The number of RCPs depends on the radius rpolicy of neighbourhood control measures and the number of depopulated DCs depends on the ratio of pre-emptive culled farm to infected IPs.