Structure Avian influenza A virus
Influenza viruses are spherical or filamentous enveloped particles 80 to 120 nm in diameter. The helically symmetric nucleocapsid consists of a nucleoprotein and a multipartite genome of single-stranded antisense RNA in seven or eight segments. The envelope carries a hemagglutinin attachment protein and a neuraminidase

Classification and Antigenic Types
Influenza viruses are members of the family Orthomyxoviridae (the Greek root myxo-, mucus, attests to the respiratory ramifications of disease caused by these viruses). These viruses are further classified, on the basis of antigenic properties of the nucleoprotein and matrix proteins, as influenza A, B, or C. Influenza A viruses have been isolated from various animal species, including birds, pigs,horses, and humans, where as influenza B and C viruses are primarily human pathogens. Influenza A viruses have much more variable surface glycoproteins than either B or C viruses, yet phylogenetic analyses show that influenza A and B viruses are more closely related to each other than to influenza C.
In wild aquatic birds and poultry around the world, influenza A viruses carrying 15 antigenic subtypes of hemagglutinin (HA) and 9 antigenic subtypes of neuraminidase (NA) have been described. Here we describe a previously unidentified antigenic subtype of HA (H16)

Spread
Virus concentration in nasal and tracheal secretions remains high for 24 to 48 hours after symptoms start and may last longer in children. Titers are usually high and so there are enough infectious virions in a small droplet to start a new infection

Clinical signs
· ruffled feathers
· soft-shelled eggs
· depression and droopiness
· sudden drop in egg production
· loss of appetite
· cyanosis (purplish-blue coloring) of wattles and comb
· edema and swelling of head, eyelids, comb, wattles, and hocks
· diarrhea
· blood-tinged discharge from nostrils
· incoordination, including loss of ability to walk and stand
· pin-point hemorrhages (most easily seen on the feet and shanks)
· respiratory distress
· increased death losses in a flock

Diagnosis

Field Diagnosis
· Highly pathogenic avian influenza is suspected with any flock where sudden deaths follow severe depression, inappetence, and a drastic decline in egg production. The presence of facial edema, swollen and cyanotic combs and wattles, and petechial hemorrhages on internal membrane surfaces increases the likelihood that the disease is HPAI. However, an absolute diagnosis is dependent upon the isolation and identification of the causative virus. Commercially available type A influenza antigen-capture enzyme linked immunosorbent assay kits designed for use in human influenza have recently shown promise as a possible rapid diagnositic test for poultry.

Specimens for Laboratory
· Specimens sent to the laboratory should be accompanied by a history of clinical and gross lesions, including any information on recent additions to the flock. Diagnosis depends upon the isolation and identification of the virus from tracheal or cloacal swabs, feces, or from internal organs (5). Specimens should be collected from several birds. It is not unusual for many of the submitted specimens to fail to yield virus. Swabs are the most convenient way to transfer AI virus from tissues or secretions of the suspect bird to brain and heart infusion broth or other cell culture maintenance medium containing high levels of antibiotics. Dry swabs should be inserted deeply to ensure obtaining ample epithelial tissue. Trachea, lung, spleen, cloaca, and brain should be sampled. If large numbers of dead or live birds are to be sampled, cloacal swabs from up to five birds can be pooled in the same tube of broth. An alternative technique is to place 0.5 cm3 of each tissue into the broth. Blood for serum should be collected from several birds. If the specimens can be delivered to a laboratory within 24 hours, they should be placed on ice. If delivery will take longer, quickfreeze the specimens and do not allow them to thaw during transit.

Laboratory Diagnosis
· Nine to 11-day-old embryonated chicken eggs are inoculated with swab or tissue specimens. Avian influenza virus will usually kill embryos within 48-72 hours. If the virus isolated is identified as a Type A influenza virus, through the AGP or ELISA tests, it is then tested using a battery of specific antigens to identify its serologic identity (HA and NA type).

· Sera from infected chickens usually yield positive antibody tests as early as 3 or 4 days after first signs of disease.


Literature
1. http://www.vet.uga.edu/vpp/gray_book02/fad/avi.php#
2. http://doacs.state.fl.us/ai/aiindex.htm
3. http://www.med.sc.edu/micro/mhunt/flu.htm
4. http://jvi.asm.org/cgi/content/full/79/5/2814