Avian influenza viruses are usually type A viruses found mainly in birds, but infections can occur in humans. Not all bird flu strains produce serious illness, but the current H5N1 strain is unusually pathogenic and has been shown to be transmissible to humans.
Unlike most bird flu viruses, the H5N1 virus can be passed easily from birds to humans without first incubating in other animals such as pigs. Most cases have been traced directly to contact with sick poultry, and human-to-human transmission appears to have occurred in at least one instance. According to the World Health Organization, as of July 26, 2006 there have been 232 cases of flu worldwide, mostly in Southeast Asia. More than half the cases have been fatal. These statistics include only cases in which diagnosis has been confirmed by lab tests. However, the true incidence may be higher, and the true death rate lower, since cases of severe illness or death are most likely to be diagnosed and reported, while milder cases go unnoticed and unreported.
Infected birds shed influenza virus in their saliva, nasal secretions, and feces. Domesticated birds may become infected through direct contact with other birds or through contact with contaminated surfaces, water or feed. There are two main forms of avian flu that are distinguished by low and high extremes of virulence. The “low pathogenic” form can go undetected and usually causes only mild symptoms in birds, such as ruffled feathers and decreased egg laying. However, the highly pathogenic form spreads rapidly through poultry flocks, affects multiple internal organs and has a mortality rate of up to 90-100% within 48 hours. The virus can persist for more than a month in infected materials such as bird feces.
Symptoms of avian influenza in humans have ranged from typical human influenza-like symptoms such as cough, fever, sore throat, and muscle aches to eye infections, pneumonia, severe respiratory distress, and other severe and life-threatening complications. The symptoms of avian influenza may depend on which virus caused the infection. In many patients, the disease caused by the H5N1 virus follows an unusually aggressive clinical course, with rapid deterioration and a high risk of death.
The incubation period of avian flu in humans is longer than the usual seasonal flu, perhaps up to 18 days, though usually less than a week. Initial symptoms include a fever higher than 38 degrees C, and the systemic malaise typical of influenza. Unlike typical flue, symptoms can also include watery diarrhea, vomiting, abdominal and chest pain, and bleeding from the nose and gums. In two Vietnamese patients, the clinical diagnosis was acute encephalitis, as neither patient had respiratory symptoms when first evaluated by medical staff. In a case from Thailand, the patient had fever and diarrhea, but no respiratory symptoms.
However, most commonly patients have symptoms in the lower respiratory tract when they first seek treatment, and almost all develop viral pneumonia unresponsive to antibiotics.
Evidence suggests that some antiviral drugs, such as oseltamivir (Tamiflu), can reduce the duration of viral replication and improve survival, provided they are administered within 48 hours following onset of symptoms. However, given the high mortality rate of H5N1, and evidence of long-lasting viral infection in this disease, administration of the drug should also be considered later in the course of the illness.
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