Avian Influenza

Avian influenza (AI) viruses infect both ,domestic poultry and wild birds . In domestic poultry, AI viruses are of low pathogenicity (LP), generally causing subclinical infections. However, some of AI viruses can cause severe systemic infections which can induce high mortality. This highly pathogenic (HP) form of the disease used to be called fowl plague. In wild birds, AI viral infections are usually subclinical.


Avian influenza viruses are type A orthomyxoviruses characterized by antigenically homologous nucleoprotein and matrix internal proteins, which are identified by serology in agar gel immunodiffusion (AGID) tests. AI viruses are divided into 15 hemagglutinin (H1-15) and 9 neuraminidase (N1-9) subtypes on the basis of hemagglutinin inhibition and neuraminidase inhibition tests, respectively. Most AI viruses (H1-15 subtypes) are of Low Pathogenicity, but some of the H5 and H7 AI viruses are High Pathogenicity for chickens and other domestic poultry.

Epidemiology and Transmission:

Low Pathogenicity viruses have spread all over world and are recovered usually from clinically normal shore birds and migrating waterfowl. The viruses may be present in backyard flocks and other birds sold through live-poultry markets, but most of commercial poultry in developed countries are free of AI viruses. The HP viruses arise from mutation of some H5 and H7 LP viruses and cause devastating condition. Killing of bird is only solution to quickly eliminate HP viruses.

The incubation period is variable and ranges from a few days to 1 wk. Transmission between individual birds is by ingestion or inhalation. Transmission between farms is the result of negligence in bio-security practices. It is generally by movement of infected birds,contaminated feces and respiratory secretions on equipment or clothing. Spread of disease through air  may be there, but over limited distances.

Clinical Findings and Lesions:

Clinical signs, severity  and mortality rates are variable depending upon AI virus strain and host species.

Low Pathogenicity AI Viruses:

These AI viruses  cause respiratory signs like ocular and nasal discharge and swollen infra-orbital sinuses. Lesions in the respiratory tract  lead to congestion and inflammation of the trachea and lungs. In layers and breeders, there may be decreased egg production or fertility, ova rupture (evident as yolk in the abdominal cavity) ,involution,  mucosal edema and inflammatory exudates in the lumen of the oviduct. In some cases of layer and breeder acute renal failure and visceral urate deposition (visceral gout) is also noticed. The morbidity and mortality is  low unless accompanied by secondary bacterial or viral infections or aggravated by environmental stress factors.

High Pathogenicity AI Viruses:

Even in the absence of secondary pathogens, HP viruses cause severe, systemic disease with high mortality in chickens, turkeys and other birds. In peracute cases, clinical signs or major lesions may not be there before death. However, in acute cases, lesions may include cyanosis and edema of the head, comb, and wattle; edema and discoloration of the shanks and feet due to subcutaneous ecchymotic hemorrhages; petechial hemorrhages on visceral organs and in muscles; and blood-tinged oral and nasal discharges. In severely affected birds, greenish diarrhea is common. Birds that survive the fulminating infection may develop CNS involvement evident as torticollis, opisthotonos, or incoordination. The location and severity of microscopic lesions are highly variable and may consist of edema, hemorrhage, and necrosis in parenchymal cells of multiple visceral organs, skin, and CNS.
Avian influenza, hemorrhagic skin, chicken Avian influenza, hemorrhagic skin, chicken


AI viruses can be readily isolated from tracheal and cloacal swabs. They grow well in the allantoic sac of embryonating chicken eggs and agglutinate RBC. The hemagglutination is not inhibited by Newcastle disease or other paramyxoviral antiserum. AI viruses are identified by demonstrating the presence of 1) influenza A matrix or nucleoprotein antigens using AGID or other suitable immunoassays, or 2) viral RNA using an influenza A specific RT-PCR tests.

Differential Diagnosis:

LP AI must be differentiated from other respiratory diseases or causes of decreased egg production including: 1) acute to subacute viral diseases such as infectious bronchitis, infectious laryngotracheitis, lentogenic Newcastle disease, and infections by other paramyxoviruses; 2) bacterial diseases such as mycoplasmosis, infectious coryza, ornithobacteriosis, turkey coryza, and the respiratory form of fowl cholera; and 3) fungal diseases such as aspergillosis. HP AI must be differentiated from other causes of high mortality such as velogenic Newcastle disease, peracute septicemic fowl cholera, heat exhaustion, and severe water deprivation.

Prevention and Treatment:

Vaccines can prevent clinical signs and death. Furthermore, viral replication and shedding from the respiratory and GI tracts may be reduced in vaccinated birds. Specific protection is achieved through autogenous virus vaccines or from vaccines prepared from AI virus of the same hemagglutinin subtype. Antibodies to the viral neuraminidase antigens may provide some protection. Currently, only inactivated whole AI virus and recombinant fowlpox-AI-H5 vaccines are licensed in the USA. The use of AI vaccine requires approval of the state veterinarian. In addition, use of H5 and H7 AI vaccines in the USA requires USDA approval. Treating LP-affected flocks with broad-spectrum antibiotics to control secondary pathogens and increasing house temperatures may reduce morbidity and mortality. Treatment with antiviral compounds is not approved or recommended. Suspected outbreaks should be reported to appropriate regulatory authorities.

Zoonotic Risk:

Avian influenza viruses exhibit host adaptation and rarely infect humans, usually as isolated individual cases without human-to-human transmission. In the 1997 Hong Kong outbreak, the risk factor for human infection was direct contact with infected poultry, but not the handling, cooking, or consumption of poultry meat. In 2004, HP AI of strain H5N1 infected poultry and wild birds in 9 Asian countries. In Thailand and Vietnam, 37 human cases were confirmed, with a case fatality rate of 68%.

Reference :- Merck Veterinary Manual

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