In recent times these outbreaks have been occurring with increasing regularity. Outbreaks are typically of a low pathogenic form of avian influenza (LPAI). Past experience indicates that in a susceptible domestic poultry population, circulating LPAI (especially H5- and H7- subtypes) has the ability to mutate into a more devastating high pathogenic avian influenza (HPAI) resulting in Fowl Plague.
Vaccination as an additional control tool has been used with success in controlling LPAI5 outbreaks as well as HPAI outbreaks in the past (1995 - Utah, USA; 2000 - Italy3 & California, USA; 2001 - Hong Kong; 2002 - Colorado, USA). At the Fifty-Second Western Poultry Disease Conference held in Sacramento, California during March 2003 Capua and Marangon2 proposed a scheme, summarised in Diagram 1, whereby vaccination is included in the control strategy in certain scenarios.
The concept of vaccination in the face of HPAI is however met with much resistance based on the arguments that it is not in line with OIE or EU control strategies, would have an negative impact on export trade agreements and potentially masks the symptoms of the disease thus removing the most significant early warning signal for HPAI - acute increase in mortality.
The DIVA strategy is based on the use of an inactivated oil emulsion vaccine containing the same haemagglutinin (H) subtype as the field virus, but a different neuramidase (N). The homologous H group ensures protection while it is possible to differentiate vaccinated from infected birds based on the serological response to the N group using an indirect immunofluorescence test. Vaccinated birds should only test positive to the N group used in the vaccine, a positive reaction to the N group of the prevailing infective virus would indicate a field challenge.
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