Avian Influenza
The level of interest and concern about Avian Influenza (AI), especially the H5N1 virus has increased since the end of the year 2003. The Asian Highly Pathogenic Avian Influenza H5N1 virus has migrated from Asia to the Middle East, India, Africa and Europe. Infections have been reported in many species of wild birds, waterfowl, commercial duck, goose, turkey and chicken poultry farms, several species of mammals, and even people.
There are three broad Types of influenza virus – A, B and C. Types B and C only infect humans, whereas Type A, in addition to humans, also gives rise to disease in birds, horses, dogs, pigs and other animals. Avian Influenza, caused by a Type A virus, has been recognized for 125 years. The first description of the disease referred to it as “Fowl Plague”. Further classification refers to two enzymes, hemagglutinin (H) and neuraminidase (N), which are found on the surface of the virus. There are 16 hemagglutinin subtypes (H1-16) and 9 neuraminidase subtypes (N1-9), thus giving rise to the combinations, such as H5N1, that identifies different strains. AI strains also are divided into two groups based upon the ability of the virus to produce disease, also known as pathogenicity. These groups are called low pathogenic (LP) and highly pathogenic (HP). Low Pathogenic or "low path" Avian Influenza (LPAI) occurs naturally in wild birds and can spread to domestic birds. In most cases it causes either no signs of infection or only minor symptoms in birds. These strains of the disease pose little significant threat to human health, and are common around the world. Highly Pathogenic or "high path" Avian Influenza (HPAI) is often fatal in chickens and turkeys. HPAI spreads rapidly and has a higher death rate in most bird species than LPAI. LPAI viruses can mutate to HPAI. So far this has not been reported for subtypes H5 and H7. HPAI H5N1 is the subtype of the current Asian strain that has rapidly spread in some parts of the world.
That is not the end to influenza’s complexity; it is particularly adept at mutating and changing its genetic structure, for example by different strains exchanging genes. This property helps the virus to survive and remain circulating in the population, whether that population is birds, mammals or humans. One possibility with Avian Influenza is that such mingling of genes could take place in pigs, which can act as reservoirs for both bird and human influenza viruses.
The large number of subtypes coupled with the virus’s ability to mutate into different strains within a subtype makes influenza a particularly difficult vaccine target.
Control of Avian Influenza with Vaccination
The use of vaccination to control Avian Influenza is gaining increased support, and its use is being increased worldwide. However, vaccination alone cannot be used to replace other control measures. It is an additional tool in a comprehensive control strategy. Indeed, deciding upon an overall strategy to combat AI is complex. Biosecurity, surveillance, quarantine and eradication of infected birds are the traditional measures used to prevent or control an outbreak of Avian Influenza before it has the opportunity to spread to a large geographic area. The ability to effectively utilize these prevention and control components varies widely on a global basis. The current epidemic of the Asian HPAI H5N1 virus has, amongst other factors, grown so large due to the lack of an early diagnosis of AI and the associated delay in culling of infected birds. Moreover, the human and financial resources available at the local, country and international level to use eradication as the primary solution are limited.
The fundamental benefit of vaccination against AI is that less virus will be shed by vaccinated birds, thus reducing environmental contamination and the risk of infection both to other birds and people. Mortality and production losses are reduced. Vaccination can create buffer zones between infected and non infected areas, help protect disease free areas that are considered at high risk of infection, protect high value genetic stock, and protect poultry flocks intended for initial restocking in previously infected areas.
One strategy is to use prophylactic routine vaccination to blanket a geographic area by vaccinating every susceptible bird that can be found. Another strategy is to vaccinate targeted populations, such as “backyard” flocks, special breeds and the various species of birds housed in zoological gardens. Still another strategy is to use vaccination in areas of outbreaks, along with culling and depopulation, to strategically vaccinate the minimum number of birds required to stop the spread of the disease. Additionally, vaccination with economic compensation to the grower is gaining acceptance as a control strategy by governments in an effort to reduce the number of healthy animals that must be destroyed in order to control a highly infectious disease like AI. This approach has been called the “vaccinate to live” option.
Some have voiced concern that use of vaccination to control Avian Influenza may actually allow for the “silent” spread of the disease by increasing the risk of transmission between flocks prior to detection. While no vaccine confers 100% protection, Avian Influenza vaccines are highly effective and have been successfully used as part of programs to control and eradicate the disease. Such concerns highlight that vaccine must be appropriately administered and wherever possible coupled with other sound disease management techniques that incorporate biosecurity and surveillance, including the use of sentinel and/or other DIVA approaches[1], in order to allow for the rapid identification and eradication of infected flocks.
Regardless of the type of vaccine utilized, the products will give the optimum benefits only when the manufacturers’ recommendations are strictly followed. It is important that the vaccines are stored, handled and administered properly. Each bird must receive a full dose of the vaccine in order to build the highest level of immunity.
Avian Influenza vaccines have been used in control or eradication programs in Hong Kong, Viet Nam, Italy, China, Mexico and Central America.
Types of Avian Influenza Vaccines
Current commercially available vaccines for Avian Influenza include conventional inactivated oil emulsion vaccines, reverse genetics inactivated vaccines, and live recombinant vectored vaccines. Vaccines may be either homologous, containing the same haemagglutinin (H) and neuraminidase (N) subtypes as the field virus, or heterologous, containing the same haemagglutinin (H) but a different neuraminidase (N). Experimental data has shown that vaccinated birds are protected from Avian Influenza infection provided that the vaccine strain H subtype is the same as that of the field infection.
Conventional Inactivated Virus Vaccine
Conventional inactivated oil emulsion whole virus vaccines provide a long lived immune response when administered properly. These vaccines can, in principle, be given to all age groups. The maximum antibody response will be reached approximately two to four weeks after administration of a single dose of vaccine. Repeat injections are necessary in order to induce a long-lasting immunity.
Reverse Genetics Inactivated Virus Vaccine
Inactivated virus vaccines can also be created by a process called reverse genetics. With reverse genetics, scientists can custom make a flu vaccine by assembling components from many different viruses into a single vaccine virus. For example the HA gene from the Asian H5N1 virus can be combined with the genes of other influenza viruses that are known to be safe and grow well in eggs. The reverse genetics AI vaccine virus is inactivated and mixed with emulsifiers and adjuvants like a conventional killed vaccine. The technique is able to generate a new vaccine virus candidate much quicker to keep pace with the mutation of the pathogenic field virus.
Live Vectored Virus Vaccines
Live vectored Avian Influenza virus vaccines incorporates the Avian Influenza H subtype gene of interest into a non-virulent form of either Fowl pox virus, Infectious Larygotracheitis (ILT) virus or Newcastle disease virus. The live vectored vaccines can be used in one-day old chicks, even in the face of maternal antibodies.
The first commercial Avian Influenza vector vaccine for poultry used Fowlpox as the backbone. Other experimental viral vector vaccines have been constructed with Turkey Herpesvirus, Infectious Laryngotracheitis, and Adenovirus. The Fowlpox–Avian Influenza vector vaccine has been used to immunize chickens in several countries for more than eight years. The Fowlpox vaccine contains the hemagglutinin gene from a strain of Avian Influenza that stimulated very high levels of immunity to a broad range of AI subtype H5 viruses.
Recent announcements of the development of Avian Influenza vaccines using the Newcastle disease virus were made in China, Germany and the United States. Besides the advantage of immunizing birds against two important respiratory diseases of poultry, the Newcastle vector is well suited for mass vaccination of thousands of chickens at one time by spray or in the drinking water. The combined protection against both Avian Influenza and Newcastle Disease makes it an ideal vaccine for developing countries, which face both diseases. The inactivated and Fowlpox vector Avian Influenza vaccines must be injected one animal at a time.
Other Vaccine Technologies
Sub Unit Vaccines
Another type of experimental inactivated vaccine is the subunit vaccine. It contains a highly purified antigen with or without an adjuvant to increase the antigenicity and immunogenicity of the vaccine. An experimental AI vaccine using purified HA as the antigen has been demonstrated to be effective. However, there are currently no commercial subunit vaccines to combat Avian Influenza.
DNA Vaccines
DNA vaccines contain concentrated and purified DNA that is directly injected. Experimentally, they have demonstrated efficacy and the ability to stimulate cellular and humoral immune responses similar to live virus infection or vaccination. However, DNA vaccines are not currently practical for use in poultry due to the cost of manufacture. There are currently no commercial DNA vaccines to combat Avian Influenza.
Live Vaccines
The introduction of a cold-adapted live influenza vaccine for use in humans has stimulated interest in the potential for a similar product for use in poultry. There are many concerns over the use of a live influenza vaccine in poultry, including the circulation of the virus among birds or flocks and recombination with other AI strains. However, with advances in biotechnology, it could be possible to vaccinate in ovo (which is easy and cost effective) against Avian Influenza using live-attenuated or replication defective influenza virus. There are currently no such commercial live Avian Influenza vaccines.
Monitoring for Avian Influenza Infection in Vaccinated Poultry Flocks
Due to potential implications for disease surveillance and trade in poultry products, it is important to be able to differentiate vaccinated from infected birds. Such an approach is often termed a “differentiation of infected from vaccinated animals” (DIVA) strategy. Use of a DIVA strategy allows for the realization of the benefits of vaccination without interfering with efforts to implement other control measures, including eradication of birds exposed to “field” virus. Four different DIVA strategies have been described: Use of Sentinels, Use of Heterologous Neuraminidase Vaccines, Use of Subunit Vaccines, and Use of the Nonstructural Protein 1.
A simple approach that allows monitoring at the flock level involves regular monitoring of “sentinel” birds left unvaccinated in each vaccinated flock. As the current vaccines do not transmit from one bird to another, the “sentinel” birds will only develop positive serologic tests for Avian Influenza as a result of field exposure.
A more sophisticated approach uses vaccination strategies that enable the detection of exposure to “field” virus in vaccinated populations, and each of the other DIVA strategies are designed to do this.
The heterologous neuraminidase DIVA strategy for AI involves the use of a vaccine containing a virus of the same H subtype but a different N subtype from the field virus. Antibodies made by the bird to the N subtype of the field virus indicate exposure and act as natural markers that allow differentiation from vaccinated birds. This is accomplished by comparing the antibody profile of the bird to the known differing N subtypes of the vaccine and the field virus. Such an approach was utilized in Italy following the re-emergence of a low pathogenic AI H7N1 virus in 2000, where a H7N3 vaccine was used. A serological test for anti-N antibodies was used to differentiate vaccinated birds from those exposed to field virus. In 2002, Italy again used this approach to control a low pathogenic AI strain caused by H7N3 with an H7N1 vaccine.
This same approach can be used with subunit vaccines, such as the recombinant vaccines that contain only an H subtype. Vaccinated birds would have anti-H antibodies, but no anti-N antibodies, thus allowing the use of tests to detect anti-N antibodies for the differentiation of vaccinated birds from those exposed to field virus.
Another approach measures the differences in immune responses to the influenza nonstructural protein 1(NS1). NS1 is a protein that is created in large quantities in infected cells, but is not part of the final influenza virus. Therefore, a vaccine that uses a killed whole virus should create either a lesser host immune response to NS1 (depending upon the level of purification of the vaccine) or none when compared to the serologic response to natural infection, where large quantities of anti-NS1 antibody are created. Current killed Avian Influenza vaccines for use in poultry are only partially purified and contain small amounts of NS1 protein. Vaccinated birds will develop antibody to NS1, particularly with repeated vaccination. However, the level of anti-NS1 antibody is much lower in vaccinated birds than in birds naturally infected, hence allowing their differentiation.
About IFAH
The International Federation for Animal Health (IFAH) is the international body representing the global animal health industry. IFAH members are 13 international animal health companies and 26 national/regional animal health associations. A number of IFAH’s member companies are manufacturers of avian influenza vaccines with considerable expertise in dealing with the disease.
For more information on IFAH, please see: www.ifahsec.org.
November 2006
