Avian influenza viruses are highly contagious, extremely variable viruses that are widespread in birds.
Wild birds in aquatic habitats are thought to be their natural reservoir hosts, but domesticated poultry are readily infected.
Most viruses cause only mild disease in poultry and are called low pathogenic avian influenza (LPAI) viruses. Highly pathogenic avian influenza (HPAI) viruses can develop from certain LPAI viruses, usually while they are circulating in poultry flocks.
HPAI viruses can kill up to 90-100% of the flock and cause epidemics that may spread rapidly, devastate the poultry industry and result in severe trade restrictions.
Avian influenza viruses occasionally affect mammals, including humans, usually after close contact with infected poultry.
While many human cases are limited to conjunctivitis or mild respiratory disease, some viruses tend to cause severe illness.
A virus is defined as HPAI or LPAI by its ability to cause
severe disease in intravenously inoculated young chickens in the laboratory.
In
birds, avian influenza viruses are shed in the feces and respiratory
secretions. Fecal-oral and aerosol transmission are the predominant means of spread in
aquatic wild bird and domestic poultry.
There
is no specific treatment for influenza virus infections in animals. Poultry
flocks infected with HPAI viruses are depopulated.
Prevention involves: good
biosecurity and hygiene, All-in/ all-out flock management and vaccination.
Vaccines may be used as an adjunct control measure
during an outbreak or to protect valuable species such as zoo birds. Although some vaccines can increase
resistance to infection and decrease virus shedding, birds that are
clinically protected may still become infected and shed these viruses. Thus,
vaccination can mask infections if good surveillance programs are not used
simultaneously. Vaccine- resistant isolates can also emerge.
Dr. Moses Bwana
Post-graduate Student
University of Nairobi
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