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Influenza viruses that primarily affect birds are called “avian influenza viruses” or popularly known as bird flu viruses. Of the three bird flu virus types (A to C), only influenza A can infect birds.
The virus multiplies in the intestines of certain birds, usually waterfowl, and is later shed. Infected birds, on the other hand, shed the virus from other sites including saliva, nasal secretions, and feces. Wild birds are the natural hosts of all influenza A subtypes but they typically do not get sick when infected. Certain avian influenza A virus subtypes, however, can cause extensive disease and death among some domesticated birds. One very pathogenic bird flu virus of importance is the H5N1 strain which has been the causative subtype for the recently reported bird flu outbreaks in Asia.
Bird flu infections among humans
From 1997 to early 2005, several outbreaks of avian influenza A among humans have been reported in several countries all over Asia. The recent outbreak reported in Vietnam was caused by the avian H5N1 strain. Outbreaks caused by this particular strain are closely monitored due to its unique capacity to cross species barriers and infect the human population.
Transmission from birds to humans can occur:
- Directly from birds or from avian-contaminated environments (bird droppings, contaminated dust and soil, contaminated equipment, feeds, or cages in affected farms);
- Through an intermediate host, like pigs (direct contact and respiratory droplets).
What is significant about this transmission is the gene exchange which can occur if the human infected with both avian and human influenza virus which may give rise to a completely new subtype to which few may have any immunity to. Human to human transmission is possible. At present, there is no confirmed case of human to human transmission. Majority of human cases and deaths are in Vietnam and Thailand, countries with very widespread outbreak in poultries.
Symptoms of avian influenza among humans may include fever, cough, sore throat, muscle pains and eye infections. Complications, however, may be sudden and severe leading to pneumonia, acute respiratory distress, encephalitis, and other severe and life-threatening conditions.
Generally, the diagnosis is made on clinical grounds. A thorough history of exposure is sought. Suspicion of a possible avian flu outbreak is raised when there is an unexplained high number of mortality among domestic fowl (chickens, ducks).
The most important precautionary measure for now is prevention of human exposure to the avian flu, hence the need to quickly detect any outbreaks among poultry. In the Philippines, 14 wetlands and 2 dams which serve as sanctuaries for migratory ducks have already been identified as critical areas for surveillance. Residents and wildlife bureau workers in the perimeter of these areas together with poultry workers have also been identified as target risk groups. Finally, a key preventive measure which cannot be overemphasized is frequent hand washing.
The current influenza vaccine will not protect against disease caused by the H5N1 strain but this vaccine will protect against currently circulating human strains and subsequently reduce the risk for high risk individuals to be infected with human and avian viruses at the same time.
As the influenza virus is capable of antigenic exchange (mutations), vaccines are prepared annually to match the predicted circulating strains of human influenza A and B.
Vaccination in young adults is 70-90% effective. This efficacy, however, although significantly diminished among the elderly, still reduces the number of confinements and mortality. In the Philippines, the influenza vaccine employed is the Southern hemisphere formulation, which is generally made available starting February of each year.
Excerpt from Health Alert, a bulletin from the Infection Control Service, St. Luke's Medical Center.