Transmission across species
In order to be able to infect humans, avian flu viruses have to mutate to adapt to human cells. More specifically, the surface proteins have to alter their shape in order to attach to the receptors on human cells, because avian flu viruses are adapted to attach to bird cells, not human cells. H5N1 is one of the few avian flu viruses that have mutated to adapt to human cells.
People usually get avian flu from contact with infected birds, or from contact with surfaces contaminated by infected birds.
Often, this happens on chicken farms, to the people that are handling the chickens. It is very likely that they could inhale the virus, or touch surfaces that infected chickens have touched when they are working with the chickens.
In rural areas of countries like Vietnam, Indonesia, and Thailand, families commonly keep chickens in their house. The chickens are usually free to roam around the house or yard, so the family is around the chickens a lot, and often have contact with them. If the chickens got infected with avian flu, it would be very easy for the virus to spread to the family. This is a common situation with human avian flu cases in Vietnam and Thailand.
Example of Transmission: Cats
It had been previously thought that domestic cats could be at risk for avian flu infection from other cats (horizontal transmission) or from birds. In September 2004 researchers produced avian flu infections in cats in the laboratory but had not seen this as a wide spread phenomenon. However feline avian flu infection had been noted only to occur occasionally in Asia. For example in Thailand in February 2004, fourteen-fifteen cats died in one household and at least one cat had contact with dead chickens. The cat later tested positive for the H5N1 virus.
Picture of William attempting to hypnotize a cat.
The cat later on scratched him.
Feline infections in cats again resurfaced in February 2006 but this time in Europe. On a small island in Germany called Reugen, a domestic cat became infected and died from H5N1 infection. It was thought that the cat had consumed an infected bird. Previously over a hundred birds on the island had died of the H5N1 flu. In March 2006 three cats in an animal shelter in Austria were infected. The shelter had had chickens infected with H5N1the previous month. Although not confirmed as H5N1 infections many cats died in the areas where H5N1 infections were present in birds in Iraq and Indonesia between October 05 and February 06. Domestic cats are not the only felines at risk. Large cats can also be infected with the disease. In December 2003 two tigers and two leopards died in a zoo in Thailand after eating infected chicken carcasses. Although feline to human transmission is currently thought to be unlikely there are many viral diseases that can pass from one mammalian species to another.