Avian Influenza (Bird Flu)

Highly infectious viral disease of avian species. H5N1 is a particular strain of highly pathogenic avian influenza.

Disease Detail

Avian Influenza

Avian Influenza (AI) is a viral disease caused by virus belonging to family Orthomyxoviridae and genus Influenza virus serotype A. The virus is worldwide in distribution. It occurs in many forms form asymptomatic to respiratory disease and a drop in egg production, to systemic disease with mortality rate of up to 100%. Thus, the viral isolates are subdivided into low pathogenic (LP) and high pathogenic (HP). Sometimes strains of some subtype (to date, H5 and H7 can mutate from LP to HP. Highly pathogenic avian influenza is included in the list of disease reported by OIE similarly to Newcastle disease. Influenza A virus is classified into subtypes based on the serological reactions of surface glycoproteins, haemagglutinin (H) and neuraminidase (N). Sixteen H and 9 N antigens have been recognized. Many combinations of H and N antigen in virus subtypes have been reported in domestic and wild birds. Antigenic variations; antigenic shift is more common than antigenic drift have been reported in poultry and fewer in case of mammals.

Epizootiology

Waterfowl is the frequent source of AI virus especially ducks, geese, gulls and other wild birds which are the natural reservoirs. AI virus infection usually causes sporadic or no disease. In the past it has been isolated from domestic integrated commercial poultry, frequently from turkeys and chickens and other domestic birds. Birds from live poultry market system and backyard flocks have higher infection rates. However, outbreaks in commercial poultry have become more frequent last 2 decades. When AI infection occur it spreads rapidly through integrated poultry system from farm to farm causing epizootics of LP or HP AI. This virus is excreted by the respiratory and enteric routes thus, it is transmitted by direct contact among infected and susceptible birds or by indirect contact via aerosol droplets or exposure to virus contaminated fomites, equipment and people. Intraspecies transmission is frequent and easy; interspecies transmission may occur, being ever less frequent as the differences between the phylogenetic classes of the animal increase; but some exceptions have occurred. At present to limited extent H1N1 from swine to turkey in the USA and more recently, H5N1 and H7N7 from poultry to humans, respectively in Asian and The Netherlands. Transmission is generally horizontal and evidences of vertical transmission are lacking. However, the virus has been isolated from intestinal contents of eggs during natural outbreak of disease.

 Clinical signs and lesions

Clinical signs are variable depending on the pathotype of the virus and host species, age, concurrent infections and environmental conditions. LPAI virus infection in wild birds are asymptomatic however, in domestic birds it produces mild to severe respiratory symptoms like coughing, sneezing, rales along with malaise, anorexia and diarrhea; in breeders and layers there is decrease in egg production (10-80%). Sometimes high morbidity and mortality is low (3-5%) unless complicated by secondary bacterial infections in young birds.

The common lesions in respiratory tract are catarrhal fibrinous to purulent sinusitis, tracheitis, bronchopneumonia, more or less severe air sacculitis. Also depending on secondary bacterial infections, catarrhal to fibrinous enteritis and oviductitis are observed.

HPAI infection in wild birds and domestic ducks usually produces few or no clinical signs; however, a few cases have been reported with moderate to high mortality. In domestic poultry like turkeys, chickens, guinea fowl and quail, clinical signs varies according to damage induced by viral replication into specific organs and cardiovascular and nervous system. In many cases, disease is fulminating, in the absence of any clear symptom; if birds survive for few days, some may exhibit respiratory and nervous disorders. Respiratory signs are less prominent as compared to LPAI. Drop in egg production is rule. Morbidity and mortality is very high (50-90 to 100%). Edematous, hemorrhagic and necrotic lesions in visceral organs, often including the pancreas and non-feathered skin are seen. Hemorrhages are prominent in epicardium, proventriculus and Payer’s patches. In sudden death, no any gross lesions are seen, only congestion with higher severity are observed.

Diagnosis

Presumptive diagnosis is based on epidemiological and clinical data. A definitive diagnosis is established by direct detection of antigens or DNA of the virus in various specimens like tracheal and cloacal swabs, tissues. Isolation and identification of virus in embryonated eggs is done and diagnosis is confirmed by RT- PCR. Antibodies detection can be done by ELISA, HI, AGP, VN.

Control

Control strategies are mainly based on knowledge of how the virus has been introduced into the farm or an area and how it can spread. Wild birds should be considered the natural reservoirs for primary infection to commercial poultry. Strict application of biosecurity measures is the first line of defense which protects direct and indirect contact with virus. Contaminated poultry manures is major source of virus spread between flocks and farms.

When infection with subtype H5 and H7 is detected, response must be prompt and complete, even resorting to immediate culling or depopulation. As regards prevention by vaccination, inactivated vaccines in oily emulsion has been used, particularly in chicken and turkeys. The effectiveness of vaccines in preventing symptoms and mortality has been well documented. But, there is difficulty in developing the vaccine because there are 16 different H types of AI virus. When and outbreak of AI occurs and the virus subtype is identified, vaccination may be a useful tool, particularly in areas with high population densities, but the preparation of vaccine has to be quite rapid. Vaccination is a valid option in controlling the spread of infection, remarkably reducing the susceptibility of birds to the infection and the amount of viral shedding in the environment. The controlled used of vaccines in cases of LPAI H5 and H7 outbreaks might reduce the possibility of HPAI viral emergencies. But, their use continues to be debated in Europe and North America.

Heterologous vaccine has also been used for the N antigen, applying DIVA (differentiating infected from vaccinated animals) strategy, based on serological tests to highlight different specific anti-N antibodies. The trend seems to be in this direction considering the recent examples of results obtained in certain countries.

The judicious use of vaccine, making it possible to reduce transmission and spread of infection and the susceptibility of birds to the virus would favour eradication of the disease preventing it from being endemic.

Source: https://www.farm.com.np