Bird flu is caused by a virus. There are several different subtypes of the virus that have been known to cause the condition in people, including H5N1, H7N7, and H9N2. The letters H and N in the subtype name stand for proteins found on the surface of the virus that are used to distinguish between different subtypes.
Pathogenicity is a measure of how likely a virus is to cause disease. In the case of the bird flu virus, it can have either low or high pathogenicity. Different subtypes of the virus cause either a mild form of the condition or an extremely contagious and highly dangerous form that spreads quickly.
Influenza viruses are constantly changing their genes. This process is called mutation. It can occur by one of two methods:
- An entire gene from one subtype moves to another subtype.
- An existing gene can change within a subtype.
These genes determine whether the virus will cause a mild condition or a deadly condition in the infected person or animal. The bird flu virus that affects birds has at least 15 different subtypes and usually only affects the bird population. The most dangerous subtype is the H5N1 subtype.
When the virus is found in humans, it is said to have "jumped the species barrier." This means that the virus has mutated in a way that allows it to cause the condition in humans. Because humans have no natural protection or immunity to the virus, they are likely to become ill very quickly and possibly die due to complications of the bird flu.
Bird to human transmission: Birds such as turkeys, geese, and domestic chickens come in contact with the virus from food, water, or particles contaminated with the virus. The virus can be shed in the droppings of migratory birds since they are natural carriers and is able to survive for 3 months in cool temperatures. It can also survive in water at 0°C for more than 30 days and at 22°C for up to 4 days. Transmission of the virus from birds to humans occurs when a person working closely with these animals inhales dust particles containing the virus or by other means.
In countries where live birds (e.g., chickens, geese, turkeys) are sold in markets along with pigs or raised near pigs, the possibility of the virus recombining with other subtypes is greater. This is because both human and avian viruses can infect pigs. If a pig is infected with both viruses at the same time, different parts of the avian and human viruses can mix with each other. Later, the avian virus that has picked up some genes from the human form of the influenza virus is able to more easily cause the condition in humans.
Human to human transmission: Although the vast majority of human cases of bird flu are the result of direct contact with an infected bird, rare cases of direct human-to-human spread have been reported.