Vaccination offers a cost-effective method to suppress
clinical outbreaks and reduce the impact of highly-pathogenic avian
influenza. By Simon Shane
Vaccination offers a cost-effective method to suppress clinical outbreaks and reduce the impact of highly-pathogenic avian influenza.
The traditional approach of detection and quarantine followed by rapid depletion and disposal of affected flocks is appropriate in industrialized countries with technical, logistic and economic resources to mount effective eradication campaigns.
Attempting to eliminate avian influenza by slaughter of exposed flocks in countries with large populations of non-commercial poultry is doomed to failure. It is not possible to confirm infection and then slaughter and dispose of numerous small flocks within a limited time period to achieve effective destruction of the virus in infected and contact birds.
Traditional opposition to vaccination is based on the following concepts:
"Vaccination obscures diagnosis since flocks have AI antibody titers."
Avian influenza is currently diagnosed by detecting antigen using RT-PCR and antigen capture ELISA. Demonstrating antibodies using conventional serology is no longer an acceptable diagnostic technique.
"Vaccination leads to persistence of virus."
In the absence of vaccination, susceptible flocks multiply virus at such an exponential rate that it is difficult to control. A preemptive vaccination lowers the proportion of susceptible flocks in a region to below the outbreak threshold. It is theoretically possible to reduce the proportion of susceptibles to below the endemic threshold and the infection can be eradicated over time.
"Vaccination inhibits detection of infected flocks."
Application of the DIVA principle in which a vaccine containing homologous H antigen with a heterologous N antigen is used, allows absolute differentiation between immunized and infected status of flocks. As a further precaution, non-vaccinated sentinels can be placed in a flock to indicate the presence of field virus.
"Vaccination will lead to loss of export markets."
Most of the nations currently affected by avian influenza are either self-sufficient or are net importers of processed poultry. Producers and consumers in these countries have more to lose by not protecting their flocks by using vaccine than countenancing susceptibility with the high probability of infection and catastrophic loss.
"Vaccination will allow AI virus to persist and result in potential mutation to a pandemic strain affecting humans."
Rapid multiplication of H5N1 virus in susceptible poultry populations will in fact increase the probability of mutation and will raise the level of exposure to human contacts throughout the chain of production and consumption. Vaccination will effectively reduce the quantum of virus in backyard and semi-commercial flocks and will limit transmission to free-living birds which disseminate infection.
"Once vaccination is introduced, it is necessary to continue a program forever."
Maintaining a high proportion of immune flocks requires expenditure for both vaccine and administration. Economic models have shown the financial benefits of a vaccination program compared to the realistic alternative of uncontrolled spread of highly-pathogenic virus. The continuation of poultry production in many countries in Southeast Asia and Eurasia where HPAI has become endemic may only be possible with protection provided by vaccination, paralleling industry practice with regard to velogenic Newcastle disease.
"Has vaccination ever been effectively applied to control an HPAI outbreak?"
In 1994 an outbreak of H5N2 HPAI in Mexico extended rapidly through a number of broiler-producing states. Since a traditional detection and slaughter program to eradicate infection was impossible due to logistic and cost restraints, vaccination was selected as the primary strategy for containment. Clinical cases of HPAI ceased within six months and the virus could not be isolated from commercial and backyard flocks after a year. Experience in the People's Republic of China has clearly demonstrated the efficacy of diligent immunization using potent vaccines in both commercial and subsistence flocks. Outbreaks of avian influenza during the past two years in this country have been ascribed to deficiencies in either quality of vaccines or their application.
"How can the effectiveness of a vaccination program be measured?"
All vaccination programs should be subjected to a structured surveillance program to detect the presence of virus in sentinel birds. The immune status of flocks can be monitored using serologic techniques including agar gel diffusion, ELISA or other procedures.
"International veterinary authorities including the OIE disfavor the use of vaccine and promote eradication."
Regulatory authorities advocated traditional eradication programs in response to the emerging epornitic(attacking many birds in a region at the same time) in 2001 without realizing the extent of infection and the lack of resources. Previous experience gained in Western Europe and the USA was inappropriate to the situation in Southeast Asia from 2002 onwards. Challenges included the extent of infection, widespread distribution of flocks, live-bird marketing, deficient diagnostic resources and the virtual absence of effective control measures in affected countries. Resistance to change among veterinary bureaucrats, lack of communication between the WHO and OIE, and conflicting pressures and lack of transparency by governments, integrators and exporters in some countries contributed to an uncoordinated and non-focused response.
With the experience gained since 2003, it is evident that intensive vaccination is an appropriate response to control HPAI in countries with extensive live-bird production and no export markets but only providing that an effective AI surveillance program can be implemented.
By: Simon Shane