Know more about 'Avian Flu' 

Introduction

  • Avian influenza, also known as ‘fowl plague’, is an infectious disease of birds caused by type A strains of the influenza virus.

  • The disease, which was first identified in Italy more than 100 years ago, occurs worldwide in many domestic birds, including chicken, turkey, ducks etc.

  • In the wake of Avian influenza ( AI ) spreading in Asian countries, the latest reports from neighbouring country Pakistan has put India on red alert

  • The high mutation rate of low pathogenic influenza virus can result in the generation of extreme virulent forms with high propensity for inter species transmission. This was highlighted by the recent human casualties in Thailand and Vietnam.

  • It is highly imperative that some concrete measures be taken at national level to prevent a major catastrophe and we veterinarian's have a vital role to play.

Some Important Facts about Avian Influenza

  • The etiological agent belongs to the Influenza virus A genus of Orthomyxoviridae family and are negative stranded segmented RNA virus.

  • The influenza A virus can be divided into 15 subtypes on the basis of haemagglutinin (H) antigens . Neuraminidase is the other type of antigen present (9 in No.). All H and N combinations have been isolated from birds.

  • Migratory waterfowls have proved to be the natural reservoir for this disease which are more resistant to avian influenza than domestic poultry.  

  • Among domestic poultry species, turkeys are more commonly infected than are chickens.

  • Avian Influenza, as a disease is classified into 2 categories viz Highly Pathogenic Avian Influenza (HPAI) which causes high mortality and Low pathogenic Avian Influenza (LPAI) which is less severe.

  • HA 0, the precursor of haemagglutinin antigen, requires post translational cleavage by host proteases before it becomes functional and infectious.The presence of multiple basic amino acids at the cleavage site of HA0 molecule in HPAI enables the virus to replicate throughout the body, damaging vital organs which results in disease and death. LPAI viruses on the other hand have only two basic amino acids in the cleavage site motif enabling them to replicate in only limited tissues and organs

  • Antigenic shift and antigenic drifts are very common in AI virus which can lead to production of HPAI from LPAI types. So control of LPAI is also very important.

  • Transmission of disease between avian species : direct and indirect contact; contact with wild birds are often incriminated as a source.

  • No evidence of vertical transmission in birds but viruses have been isolated from the outer shell of eggs.

  • Important clinical signs in birds: Sudden death; Nasal discharge, sneezing and diarrhea; Swelling of head, wattle, eyelids, combs and hocks

  • Transmission to humans: There has been reports that H5N1 strain of AI virus to infect humans during the 1997 outbreaks in Hong Kong and the recent outbreak in Vietnam. In 1997 outbreak, 6 out of 18 infected people died and 16 deaths have been registered in recent pestilence as on 5th Feb'2004.

  • MISSING LINK?  Although very rarely avian and human virus go for genetic reassortment, a mixing vessel is required for evolution of more virulent viruses. Pigs were identified as this missing link in 1993-94 and it therefore underscores the impact of disease in India than in Pakistan.

  • Till now it is not very clear regarding the HUMAN to HUMAN transmission of disease, although   W.H.O has indicated its possibilities.

  • Symptoms of H5N1 in humans are very similar to other influenza viruses, typically with fever, malaise, myalgia, sore throat and cough.Conjunctivitis is also noticed in some cases.

Control Measures:

  •  Immediate destruction and disposal of infected and exposed birds. An alarming situation for poultry industry.

  • Strict quarantine and movement control.

  • Total ban on marketing of slaughter birds from infected farms must be enforced.

  • Decontamination to remove and reduce the virus. One gram of chicken manure can contain enough viral particles to infect one million birds with AI

  • Improved surveillance to estimate the spread of disease.

  • Finally vaccination programme should be implemented using inactivated homologous or heterologous vaccines.

    Suggested Reading:

    Capua I and Marangon S (2000). The avian influenza epidemic in Italy, 1999-2000: a review. Avian Pathol. 29: 289-294.

    Capua I and Marangon S (2003). The use of vaccination as an option for the control of avian influenza. Avian Pathol. 32: 335-343.

    Claas E C J, Kawaoka Y, De Jong J C, Masurel N, and Webster R G (1994). Infection of children with avian human  reassortant influenza virus from pigs in Europe. Virology. 193: 50-506.

    Claas E C J (2000). Pandemic influenza is a zoonosis, as it requires introduction of avian-like gene segments in the human population. Vet. Microbiol. 74: 133-139.

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