As of Aug. 23, the World Health Organization had reported 241 confirmed cases of human avian influenza A/H5N1. Of this total, 141 people have died from complications of viral infection. Despite these documented diagnoses, there currently is no human pandemic influenza in the world. The system used to define and determine the six stages of a pandemic places risk to people at Phase 3: “Human infection(s) with a new subtype, but no human-to-human spread, or at most, rare instances of spread to a close contact.” Information indicates that people who have contracted avian influenza from A/H5N1 typically live and/or work in close contact with previously ill birds. The virus has yet to accomplish routine human-to-human transmission.
Three conditions must be met for pandemic influenza to emerge. First, a new influenza A virus must evolve and be found to infect humans. A/H5N1 is the current candidate. Second, infection with the new virus must cause serious disease in humans. This also has occurred with more than a 50 percent fatality incidence for human cases since 2003. The last criterion requires that the etiologic virus be spread easily from person-to-person on a global scale. This viral property has not yet emerged.
However, the virulent A/H5N1 viral subtype has infected millions of domestic birds in many countries, resulting in mass destruction of infected animals and other asymptomatic birds within the same flocks. In 1997, A/H5N1 emerged when highly pathogenic avian influenza resulted as a massive outbreak among poultry in Hong Kong. Coinciding with this epidemic was the finding that 18 people were also infected with this influenza A virus. Six of these people died from their illness. The public health response was the destruction of more than one million domestic chickens in the Hong Kong area. This seemed to curb the spread of A/H5N1 until it reappeared in 2003 in other southeast Asian countries. Since then, the virus has spread to birds in a number of Asian, African, and European countries with human cases found in nine of them.
There are differences between human cases of seasonal influenza with symptoms and severity of those individuals infected with A/H5N1. Seasonal influenza has a sudden onset after an incubation period of about one to five days. Symptoms sometimes lead people to think they only have a bad cold. But a person with the flu is far more sick, with a fever of 100 to 104˚ F, severe cough, sore throat, myalgia, and muscle aches. In many instances, a person is too sick to go to work, and spends at least a few days in bed. In contrast, infection with a highly virulent strain like A/H5N1 can result in a variety of symptoms, ranging from seasonal flu to severe, life-threatening respiratory disease. In most cases, the course of the disease is aggressive with rapid deterioration of patients and a high fatality rate. Data indicate the incubation period for A/H5N1 influenza can be longer than seasonal flu, ranging from two to eight days to as many as 17 days. High fever and other influenza-like symptoms are often accompanied by diarrhea, severe vomiting, abdominal pain, and chest pain. In many of the cases that resulted in death, bleeding from a patient’s nose and gums was an additional manifestation. The development of lower respiratory tract symptoms early during illness was a common feature noted with many of the patients.
The following list summarizes information published by WHO about pandemic influenza:
