Equine influenza immunity in the Special Restricted Area (Purple Zone) of New South Wales, Australia.

Abstract

The 2007 equine influenza (EI) outbreak in New South Wales (NSW) consisted of a central infected area that extended south from the lower New England Tableland, across the Hunter Valley, Central Coast and the Sydney basin, and a series of isolated clusters outside this area across rural NSW. The central area was assigned the status of a Purple Zone (Special Restricted Area, SRA) approximately 1 month after the outbreak commenced. Within this SRA, the eradication program's focus was to increase the proportion of horses immune to EI via vaccination, thus reducing the susceptible proportion to a level below the critical threshold for EI spread. An estimated 84-87% of all horse-owning premises achieved immunity to EI by the end of the outbreak. A high proportion (60-90%) of premises within most parts of the SRA became naturally infected with EI, and the rate of newly infected premises fell to low levels before EI vaccination commenced. Immunity to EI from very rapid natural spread appears to have been the most important factor in disease eradication within the SRA.