Jan 26, 2006 (CIDRAP News) – The latest World Health Organization (WHO) fact sheet on avian influenza shows that the complex mysteries of the H5N1 virus, while compelling, make it difficult to anticipate what the virus will do next.
On one hand, the virus has spread aggressively along migratory bird routes in the last several months, sparking poultry outbreaks and culls in places such as Russia, Romania, and Turkey. Its spread in Turkey led to a fast-developing human outbreak, which is now up to 21 cases with four deaths, but appears to be ebbing, WHO officials have said.
On the other hand, WHO says in its Jan 20 publication, "the virus does not easily cross from birds to infect humans." Tens of millions of poultry have been infected in the past 2 years, yet "fewer than 200 human cases have been laboratory confirmed."
The lengthy overview discusses current knowledge of H5N1 disease in birds, the role of migratory birds in spreading the virus, and the disease in humans. It identifies a number of areas of concern and gaps in knowledge, including:
- Containing poultry outbreaks – WHO suggests culling as the first line of defense, but adds that vaccinating poultry is a "supplementary emergency measure, providing quality-assured vaccines are used and World Organization for Animal Health (OIE) recommendations are strictly followed."
- The role of poverty – WHO describes how poverty can hamper efforts to control H5N1, especially when the virus infects backyard flocks. "In situations where a prime source of food and income cannot be wasted, households frequently consume poultry when deaths or signs of illness appear in flocks," the report says. Such deaths in flocks are common for other reasons, so H5N1 infections may not be suspected. "The frequent absence of compensation to farmers for destroyed birds further works against the spontaneous reporting of outbreaks and may encourage owners to hide their birds during culling operations."
- Migratory birds – Evidence is growing that wild birds are carrying the lethal form of H5N1 long distances and spreading it to poultry flocks along their flight paths. Among the details supporting the theory: viruses from Turkey's first two human infections were nearly identical to viruses found in birds around Qinghai Lake in China, site of a massive migratory bird die-off that began in late April of 2005.
- Who is at risk for infection – Most human cases have occurred in households that kept small poultry flocks. For unknown reasons, very few cases have been found in presumed high-risk groups, such as workers in live-poultry markets, poultry cullers, veterinarians, and health workers caring for patients without wearing adequate protective equipment.
- Differences between human H5N1 cases and ordinary influenza – The incubation period in H5N1 infections may be longer, and watery diarrhea without blood appears more often in H5N1 cases in people. Many patients also have lower respiratory tract symptoms by the time they first seek treatment. In addition, clinical deterioration is rapid in H5N1 cases.
- Treatment with antiviral drugs – WHO is working on an "urgent review" of recommendations on the use of oseltamivir (Tamiflu) and other antivirals in H5N1 cases. Clinicians should consider increasing the dosage and duration of oseltamivir treatment beyond the standard 150 mg per day for 5 days in severe cases, the report says.
This flu virus, WHO says, is of the most pressing interest to human health for two reasons: it has caused the most human cases of severe disease of any avian virus, and it could evolve to allow easy spread among people.
"H5N1 avian influenza in humans is still a rare disease, but a severe one that must be closely watched and studied, particularly because of the potential this virus [has] to evolve in ways that could start a pandemic," the report concludes.
WHO fact sheet on avian flu