Avian Influenza (AI) or Fowl Plague is an endemic disease in poultry and other birds around the world caused by type A influenza virus. It is a devastating disease that can cause high mortality in infected flocks and causes even more dramatic economic effect on the trade of poultry products between countries. Avian Influenza exists in two forms. The very virulent, (Highly Pathogenic) form that causes high levels of mortality and morbidity. Highly Pathogenic AI has only been described for subtypes H5 and H7. The Low Pathogenic form of the disease occurs with infections of all subtypes of AI. Mild respiratory signs, depression and a reduction in egg production in commercial layers characterize infection with the low path form.

The standard procedure for eradicating outbreaks of AI is depopulation of the birds on the infected premises followed by a period of cleaning and disinfection. Eradication is time consuming, expensive to conduct and usually requires an indemnification payment to the farmer for the value of the birds. The experiences with eradication have raised questions about the cost benefit of depopulation strategies as the only way of handling large outbreaks of thousands of birds.

The OIE and the FAO have recognized the that the use of vaccination is an important tool for the control of avian influenza along with target culling of infected flocks, surveillance and epidemiological monitoring of the spread of the disease.

Merial has a strong tradition with the manufacture of inactivated vaccine and is the only manufacturer of a vector vaccine for AI. The Merial facilities in France and Italy can produce inactivated AI vaccines. The vector vaccine is produced in the USA. Vaccines containing different subtypes have been produced to control outbreaks in countries around the world. Emergency vaccines containing other local AI isolates can be requested. Merial was the first company to develop a vector live vaccine for AI using a fowl pox virus as a vector and expressing the hemagglutinin gene from a subtype H5 virus. The TROVAC AI H5 vaccine was licensed by the USDA in 1998 and is being used widely to control the spread of AI in Mexico. More than 2 billion doses have been sold in the countries of Mexico, El Salvador, Guatemala and Vietnam. A temporary marketing authorisation was issued for France. TROVAC AI H5 is intended for use in one-day-old chicks or older birds that have not received a primary inoculation with a live pox vaccine. Development continues for similar vaccines containing subtypes H7 and H9.