Introduction

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.

HPAI is an OIE List A disease, hence the accepted control measure is implementing a "stamping out" procedure. All poultry that are infected, suspected of being infected or suspected of being contaminated are culled. This is accompanied by severe restrictions on the movement of poultry, personnel and related industry activities within the designated quarantine area. However in areas with high poultry density these stringent control measures may not be sufficient in curtailing the spread of the virus as has been experienced with the current HPAI outbreak in The Netherlands.

During a nine-week period (01/03/2003 . 02/05/2003) the virus has spread from an initial outbreak involving six poultry farms to a total of 243 confirmed cases, with new cases being confirmed daily. This is despite culling more than 21 million poultry during this time period and all efforts at strictly controlling movement within the industry. With the virus now jumping the border into Belgium questions must be asked whether there are not more effective strategies to control such an epidemic.

Avian Influenza Vaccination

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 - Italy & 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 Marangon proposed a scheme, 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.

Trade Regulations

As a list A disease (OIE) an HPAI outbreak has serious trade implications, such as a possible export ban on poultry products. Article 2.1.14.2. of the document ‘International Animal Health Code (2002)’ of the ‘Office International des Epizooties’(OIE) defines a country as HPAI free:

• when it has been shown that HPAI has not been present in the country for the past 3 years, or

• six months after the slaughter of the last affected animal for countries in which a stamping-out policy is practised with or without vaccination.

Claiming this free status is achieved by the lack of fresh outbreaks and sufficient proof that repopulated flocks remain AI seronegative. AI vaccinated flocks test seropositive, thus effectively complicating the surveillance required to declare a region free of HPAI. However, by vaccinating with a heterologous vaccine and applying the “DIVA” (Differentiating Infected from Vaccinated Animals) monitoring strategy it is possible to demonstrate that there is no circulating virus in the vaccinated population. This principle was successfully used to lift trade bans in Italy during the 2000 Italian AI outbreak (Decision 2001/847/EC^**).

^** Official Journal of the European Communities (1.12.2001). Commission Decision of 30 November 2001 amending for the third time Decision 2000/721/EC to modify the Italian avian influenza vaccination programme and current trade restrictions for fresh meat originating from vaccinated turkeys

DIVA Strategy

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.

Reduction of Virus Excretion

What is the advantage of vaccination? The most significant benefit of vaccination is the dramatic reduction in virus shedding from infected birds, reducing the load of environmental contamination and consequently containing the spread of the virus.

In a published article by Swayne et. al., SPF chickens vaccinated at day old or 3 weeks of age with an inactivated whole AI vaccine (H5N2) were challenged 4 weeks later with the HP A/Hong Kong/156/97 (H5N1) influenza virus. Two days post challenge Swayne demonstrated a reduction in the re-isolation rate of the challenge strain from vaccinated birds in comparison to unvaccinated controls.

Of more significance though was a significant reduction in the titre of virus re-isolated from vaccinated chickens in comparison to unvaccinated controls.