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The bird flu, or Avian influenza, affects normally only birds, less commonly affects pigs. It is a contagious disease of animals that is caused by viruses. On rare occasions, Avian influenza has crossed the species barrier and has infected humans. There are two main forms of Avian influenza which are distinguished by low and high extremes of virulence. The low pathogenic form causes only mild symptoms such as ruffled feathers or a drop in egg production. This version of the disease often goes undetected. The more highly pathogenic form spreads rapidly through flocks and causes disease that affects multiple internal organs. It has a mortality that can approach 100% within 48 hours.

Influenza A viruses have 16 H subtypes and 9 N subtypes. The only viruses that are known to cause the highly pathogenic form of the disease are the H5 and H7 subtypes. Even these viruses are introduced to the poultry in low pathogenic forms. If they are allowed to circulate, however, they can mutate within a few months into the highly pathogenic form. This is why it is so important to be concerned about the virus even when the symptoms are mild.

The symptoms of bird flu in humans range from typical flu-like symptoms, such as fever, cough, sore throat, muscle aches; to eye infections; pneumonia; severe respiratory diseases such as acute respiratory distress; and other severe and life-threatening conditions. Studies in laboratories show that prescription medicines prescribed for human flu viruses should work in preventing bird flu in humans. However, these medications may not always work since flu viruses can become resistant to drugs.

Molecular Structure and Genome of the Bird Flu

Influenza A viruses belong to a family called Orthomyxoviridae, which also includes Influenza viruses type B and C along with Salmon Anemia virus, and insect viruses Togotho and Dhori. The term myxo refers to the fact that these viruses interact with the mucus of slime of cell surfaces. These are enveloped viruses in which the viral RNA is present in the virion in separate pieces. Influenza virus is a single-stranded, negative-sense, helical RNA genome surrounded by an envelope made up of protein, a lipid bilayer, and external glycoproteins. In diameter, Influenza A viruses are 80-120 nm. The virus has no defined shape because of the way that the virus buds as it leaves the cell so it is said to be polymorphic. These viruses are readily unstable in the environment and can be readily deactivated by heat, extreme changes of pH, nonisotonic conditions, or dryness. Influenza A viruses have an envelope with a host-derived lipid bilayer and is covered with about 500 projecting glycoprotein spikes. One type of spike is hemagglutinin which causes the agglutination of red blood cells. Antibodies directed against the hemagglutinin prevent the virus from infecting the cell. The other type of spike is neuraminidase which is an enzyme that breaks down the sialic acid component of the cytoplasmic membrane, which is a derivative of neuraminic acid. It functions primarily in the virus assembly process. Influenza A virus also includes two other enzymes, RNA-dependent RNA polymerase, which is involved in the conversion of a negative to a positive strand, and an RNA endonuclease, which cuts a primer from capped mRNA precursors.

Influenza virus enters a cell through the process called endocytosis. The nucleoplasmid becomes separated from the envelope once inside the cell and migrates to the nucleus. Then, replication of the viral nucleic acid occurs in the nucleus. The virus RNA polymerase is activated by uncoating. Then the mRNA molecules are transcribed in the nucleus from the virus RNA, using oligonucleotide primers cut from the 5'-ends of newly synthesized capped cellular mRNAs. So, the viral mRNAs have 5'-caps. The virus mRNA molecules move to the cytoplasm after the poly-A tails of the viral mRNAs have been added.

Influenza virus proteins are synthesized in the cytoplasm. The eight segments of the virus genome encode ten virus proteins. Some of the proteins made are involved in virus RNA replication, and others are structural proteins of the virion. The formation of the complete enveloped virus particle occurs by a budding-out process, as also seen in rhabdoviruses. The overall strategy of virus RNA synthesis in Influenza A resembles that of the rhabdoviruses, with primary transcription resulting in formation of plus-strand templates for the formation of progeny minus-strand molecules.

Single-stranded RNA, the genetic material of Influenza A virus, is arranged in a highly unusual manner. The Influenza virus genome is segmented and has genes found on each of the eight distinct fragments of its single-stranded RNA. These eight molecules range in size from 890 to 2341 nucleotides. This arrangement allows for the rapid and constant reassortment of genes between different strains of Influenza virus because more than one strain of Influenza virus can infect a cell at one time. An antigen shift results when the genes are reassorted. Antigenic shift refers to modifications in the protein coat of the virions, especially to the two proteins important to the attachment and eventual release of virus from host cells, the hemagglutinin and neuraminidase. The hemagglutinin and neuraminidase antigens also develop minor antigenic changes because of genetic mutations that result in the change of one or more amino acids. This is known as antigenic drift. Pandemics are the results of antigenic shifts.

How could the virus be transferred to humans?

Humans can become infected with Avian influenza through direct contact to infected poultry, or surfaces or objects contaminated with their feces. Most human cases up until now have occurred in rural areas where many households keep small poultry flocks which roam freely. Birds infected with the flu shed large quantities of the virus in their feces, which leads to a greater chance of infection.

In the current outbreak of bird flu, there have not been many people infected with the disease. Only about 100 cases have occurred, which is a small number when compared with the huge number of birds infected. It is still not completely understood why some people become infected after contact and other people do not.

What is the risk for a pandemic?

A pandemic can occur when three conditions have been met: a new influenza subtype emerges; it infects humans, causing serious illness; and it spreads easily and substantially among