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Introduction and Transmission of Disease

Wild birds, especially waterfowl, are natural reservoirs for the influenza viruses.  That is, these birds can be infected by the virus, yet show no clinical signs of the disease.  These birds are hypothesized to be the primary culprit in introducing the virus to domestic poultry. 

The disease can also spread to birds through contact with infected birds and poultry products, and through manure and litter containing high concentrations of the virus, for example through contaminated clothing and footwear, vehicles and equipment (egg flats, crates), feed and water.  That is why it is of paramount importance to have stringent guidelines about the domestic and international movement of poultry, poultry equipment, and people who may have come into contact with the virus.  

Avian Flu is obviously a big problem on poultry farms where many birds are in close proximity to each other facilitating fairly easy transmission of the virus from bird to bird.  The problem compounds itself as the high density of birds may produce a stressful “crowding effect” that has been found to suppress the bird’s immune system, making its natural defense against the virus less effective.     

A study conducted by the World Health Organization (WHO) and St. Jude Children’s Research Hospital (Memphis, U.S.) on ducks to investigate the effect of highly pathogenic Avian Flu virus subtype H5N1.  Researchers found that the virus infects, colonizes, and replicates in cells constituting the respiratory and gastrointestinal tracts of the experimentally infected ducks.  High quantities of the virus (103.5 – 105.5 particles per ml) are excreted via the respiratory route as well as in feces.  Infected ducks shed the virus for at least 10 days and some for a longer period, up to 17 days and longer.  One gram of contaminated manure can contain enough viral particles to infect 1 million birds.