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, (high path) form that causes high levels of mortality and morbidity. High path AI has only been described for subtypes H5 and H7. The low path 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 of the eradication efforts in the State of Virginia and the two outbreaks in Northern Italy have raised questions about the cost benefit of depopulation strategies as the only way of handling large outbreaks of thousands of birds.

In the United States inactivated oil emulsion vaccines have been used occasionally to control small outbreaks of AI in turkeys along with an epidemiological monitoring program. In 2000 and again in 2002 Italy was granted permission to vaccinate birds with an inactivated AI vaccine manufactured using a strain of subtype H7.

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 subtypes H7N3 and H7N1 have been produced to control outbreaks in Northern Italy and Pakistan. 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 to control the spread of high path AI H5 in Mexico. More than 750 million doses have been sold in the countries of Mexico, El Salvador and Guatemala. 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.