by Janet Grischy

Created on: February 02, 2009   Last Updated: February 10, 2009

Water birds, such as ducks, geese, gulls and shorebirds, are the natural hosts of avian influenza. They harbor a form of the virus that does them no harm. Migrating birds pick it up in the lakes or on the shores of Canada or Russia in the northern spring, when they feed in infested waters, and they carry it all year. Probably, the birds and the virus evolved together, to the point where each does the other no harm.

The virus appears to be stable in the water birds. The two life forms are in balance, as far as science can tell. The virus reproduces efficiently within the birds digestive systems, transmitted by an oral-fecal route, and the birds stay well. Often they have shed the virus by fall. However, when wild birds transfer the virus to domestic fowl or other livestock, trouble may come.

It doesn't usually come fast. The form of avian flu wild birds generally transmit to domestic fowl is called LPAI, low pathogenic avian influenza. It could happen when a domestic bird uses water that a wild bird has contaminated, or when it comes in contact with wild bird feces. Infected domesticated birds don't get really sick, as a rule. They may lay fewer eggs, or gain weight more slowly, but the difference is often inconsequential in the first generation.

However, the virus tends to mutate in domestic birds. The local environment is different in the new hosts, and this difference may prompt changes in the virus. Influenza virus, especially avian influenza virus, is highly adaptable by nature.

The influenza virus is essentially bits of RNA packed inside a coat, the capsid. The RNA is eight or so single strands of instructions for building more viruses. The capsid covering is studded with two kinds of protein: haemagglutinin (HA) and neuramidase (NA). The HA helps the virus get into a specific host cell that matches it. (A virus is a parasite and not really alive unless it is in a host cell). Once inside, the RNA takes over the operating system of the cell, and has it mass-produce copies of the virus. Then the NA enables the new virus cells to leave the host cell. They proceed to invade other cells. This process kills the host cells.

RNA is single-stranded in influenza, and more fragile than double stranded human DNA. In influenza RNA there are few checks against copying errors, and other virus mechanisms even seem to encourage errors. Errors cause changes in the way the virus works; they are mutations.

When avian virus is reproducing in a new environment, say a chicken intestine

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