Avian influenza, commonly known as "bird flu," is a disease caused by multiple strains of influenza virus. The viruses can infect a wide range of animals, including people and pigs, but the natural reservoir lies in populations of wild aquatic birds such as ducks and gulls, according to an announcement from The Pirbright Institute in the U.K., which is conducting research to better understand avian influenza viruses to help prevent the disease in poultry.
Flu virus strains are categorized by the combination of the two proteins found on the outside of the virus: hemagglutinin (H) and neuraminidase (N) -- for example, H5N6 or H9N2. The H protein binds to cell receptors in order to initiate infection, while the N protein helps release new viruses from an infected cell, the institute explained.
Strains are also classified by severity. Low-pathogenic avian influenza (LPAI) viruses cause clinical signs such as mild breathing problems, decreased egg production and growth. Some birds, particularly ducks and geese, do not display any signs of LPAI infection and are still able to spread the disease, Pirbright said. When LPAI viruses circulate in high-density poultry areas, the viruses can mutate into highly pathogenic strains.
Highly pathogenic avian influenza (HPAI) viruses have severe and often fatal effects on some species of birds such as chickens, Pirbright noted, but some HPAI strains cause mild or no clinical signs of disease in wild birds and domestic waterfowl. These strains are notifiable, meaning government control measures must be put in place.
Avian influenza can be carried vast distances by wild migratory birds and can infect domesticated poultry through direct contact or through contaminated droppings and bodily fluids, Pirbright said. Because of this, there is a year-round risk of avian influenza, which can increase depending on the disease status in a region and the migration season of wild aquatic birds.
According to Pirbright, good biosecurity is essential in preventing poultry from being exposed to the disease and can be practiced on a commercial farm, with game birds or even with just a few backyard hens.
Why is influenza so difficult to prevent?
The institute explained that influenza viruses are constantly changing, and when they replicate, small errors called mutations are copied into their genome. Some of these will alter the virus’s proteins, allowing the virus to escape detection by the immune system.
All flu viruses also have the potential to undergo a process called reassortment, where two or more strains of influenza infect the same animal, allowing them to swap parts of their genome, Pirbright said. The resulting viruses can potentially have different characteristics than the original strains, such as the ability to replicate in different species, including people, the institute added.
Avian influenza viruses such as H5N1 or H7N9 have infected people, predominantly in Southeast Asia, but in these cases, the virus has been transmitted directly from infected birds to people, with no onward spread. There is some evidence that avian influenza can be passed from person to person, and while there are no records of efficient or sustained human transmission, there is a constant risk that one of the new strains may spread easily among people, which could result in a new influenza virus pandemic, Pirbright said.
Changes in the virus genetics from mutation or reassortment make it very difficult to anticipate how influenza viruses will spread through a population and whether current vaccines will be suitable for the strains that are circulating, Pirbright announced.
The influenza research programs at Pirbright are progressing understanding of the virus and finding better ways to prevent its spread.
Pirbright Institute group leaders Dr. Holly Shelton and professor Munir Iqbal are researching how the characteristics of influenza viruses can change the outcomes of disease. Studying flu at the molecular level allows them to find out how the virus evolves to overcome avian immunity and how changes in their genome can alter disease severity and their ability to affect different species, the announcement said.
Shelton's and Iqbal's teams are working on identifying genetic markers for specific disease traits that can be used in surveillance systems to provide forewarning of strains that could cause pandemics. For example, Shelton’s Influenza Viruses group is looking at which characteristics allow the viruses to become resistant to antiviral drugs that are currently used to treat human infections of influenza, allowing surveillance to provide an early warning of viruses that could cross into humans and be difficult to control, Pirbright said.
The team is also investigating how long it takes for avian influenza viruses to adapt to new mammalian species. According to the institute, establishing which strains can rapidly adapt and identifying the genetic changes made to enable infection of mammalian cells will help with surveillance efforts and potentially lead to the development of techniques that will help prevent these mutations occurring.
Vaccination and rapid diagnosis are very important for influenza prevention and control, but it is still not currently possible to protect against all influenza strains, Pirbright said, noting that Iqbal’s Avian Influenza group is exploring which sites on the virus trigger the chicken immune system to respond quickly and fight off infection. This will better inform scientists’ ability to match vaccines to strains in the field and could help to develop vaccines that are more effective.
Iqbal’s group is also developing multivalent vaccines that protect chickens and ducks from several avian influenza subtypes at once as well as test kits for rapid diagnosis of different subtypes at the flock-side, the announcement said.