Virtual Medical Centre

Bird Flu (Avian Influenza)

• What is Bird Flu?
• Who gets Bird Flu?
• Predisposing Factors
• Progression
• Probable Outcomes
• How is Bird Flu Diagnosed?
• How is Bird Flu treated?
• Bird Flu References
• Drugs/Products Associated with Bird Flu

What is Bird Flu?

Bird Flu or Avian Flu is an infectious disease of birds caused by type A strains of the influenza virus. Bird Flu, which was first identified in Italy more than 100 years ago, occurs worldwide.

Fifteen subtypes of influenza virus are known to infect birds, thus providing an extensive reservoir of influenza viruses potentially circulating in bird populations. To date, all outbreaks of the highly pathogenic form have been caused by influenza A viruses of subtypes H5 and H7.

Within a country, bird flu can spread easily from farm to farm. Large amounts of avian flu is secreted in bird droppings, therefore contaminating dust and soil. An airborne virus can spread bird flu from bird to bird, causing infection when the avian flu is inhaled.

Contaminated equipment, vehicles, feed, cages or clothing - especially shoes - can also carry bird flu from farm to farm. Avian flu can be carried on the feet and bodies of animals, such as rodents, which act as "mechanical vectors" for spreading the bird flu. Limited evidence suggests that flies can also act as mechanical vectors.

Droppings from infected wild birds can introduce avian flu into both commercial and backyard poultry flocks. The risk that the bird flu nfection will be transmitted from wild birds to domestic poultry is greatest where domestic birds roam freely, share a water supply with wild birds, or use a water supply that might become contaminated by droppings from infected wild-bird carriers.

As the above picture illustrates, so called "wet" markets, where live birds are sold under crowded and sometimes unsanitary conditions, can be another source where bird flu spreads.

Bird flu viruses do not usually infect humans, however, several cases of human infection with bird flu viruses have occurred since 1997.

Who gets Bird Flu?

There are several types of avian influenza, those that cause infection and even death in humans are called highly pathogenic avian influenza (HPAI). The strain of the virus that is currently causing outbreaks in humans is called the H5N1 strain.

Confirmed instances of avian flu viruses infecting humans since 1997 include:

• 1997: In Hong Kong, bird flu A (H5N1) infected both chickens and humans. This was the first time that an bird flu virus had ever been found to transmit directly from birds to humans. During this outbreak eighteen people were hospitalized and six of these eighteen died. To control the outbreak, authorities killed about 1.5 million chickens to remove the source of the bird flu virus. Scientists determined that the avian flu spread primarily from birds to humans, though a rare person-to-person infection was noted.
  
• 1999: In Hong Kong, cases of avian flu A H9N2 were confirmed in two children. Both bird flu patients recovered, and no additional cases were confirmed. The evidence suggested the main mode of transmission was from bird to human. However, the possibility of person-to-person transmission remained open.
  
• 2003: Two cases of bird flu A (H5N1) infection occurred among members of a Hong Kong family that had traveled to China. One person recovered, the other died. Another family member died of a respiratory illness in China, but no testing was done.
  
• 2003: Avian flu A (H7N7) infections among poultry workers and their families were confirmed in the Netherlands during an outbreak of avian flu among poultry. More than 80 cases of H7N7 illness were reported and one patient died (a veterinarian who had visited an affected farm). There was evidence of some human-to-human transmission.
  
  The image below illustrates a patient with suspected avian influenza undergoing treatment.
  
• 2004: The deadly H5N1 strain of bird flu has caused the death of 22 people (in mainly Vietnam and Thailand), and the cull of some 80 million chickens since an outbreak in early January that has affected 10 Asian countries. The United Nations has urged Asian countries not to relax in the war on bird flu as the epidemic is still spreading in Cambodia, China, Indonesia and Laos.
  
  Milder strains of the avian flu have also been recently reported in the US (Texas) and Taiwan, causing widespread culling of birds, however, these have not been shown to affect humans.
  
• 2005: Outbreaks of bird flu in 2005 include various cases of infected poultry in European countries including Turkey, Romania and Greece. Human infections of the H5N1 strain have been reported in Cambodia, Indonesia, Thailand and Vietnam. Cases are numbered at slighly over 100, with deaths from Avian Bird Flu counted at 62 (as of October 2005).

Predisposing Factors

Experts say bird flu is probably spread by migrating birds. The affected people are thought to have caught avian flu from direct contact with infected poultry.

Infected birds shed the bird flu virus via their saliva, faeces, or nasal secretions. Birds can become infected when they have contact with surfaces contaminated with excretions, or contaminated excretions themselves.

Apart from being highly contagious between birds, avian flu viruses are readily transmitted by mechanical means, such as by contaminated equipment, vehicles, feed, cages, or clothing. Highly pathogenic viruses can survive for long periods in the environment, especially when temperatures are low.

Fears persist that bird flu could combine with a human flu virus to become a deadly disease that could spread among people. There is also concern that avian flu may have crossed into other species after the deaths of a rare leopard and two domestic cats in Thailand.

Progression

Based on data from six of the cases of bird flu, the median time between exposure to avain flu and onset of illness is 3 days (range 2-4 days). In these ten patients, the mortality rate was high, with death occurring a mean of 10 days after onset of bird flu.

However, these cases were identified by alert clinicians in tertiary care hospitals and cannot be taken to be representative of the full range of illness that H5N1 may cause.