The H5N1 strain of bird flu, which has claimed the lives of at least 241 people worldwide, is much more severe than ordinary human flu viruses. Now scientists believe they have discovered why that is the case.

The explanation lies in the patient's viral load and the subsequent inflammatory response, say researchers from the Oxford University Clinical Research Unit in an article published in Nature Medicine.

Prior to this study, it has been unclear why H5N1 is virulent in humans.

Dr. Menno de Jong and colleagues assessed 27 people at the Hospital for Tropical Diseases in Ho Chi Minh City, Vietnam. Eighteen of the patients were infected with H5N1; the other nine had contracted common human influenza viruses.

The investigators learned that those who had H5N1 were carrying substantially higher viral concentrations in their blood and throat than their counterparts who had subtypes of human flu. In addition, the H5N1- infected patients who died had much higher viral loads than the H5N1-infected survivors.

The higher the level of H5N1, the team concluded, the greater the likelihood of death. The severity of lung damage and the increased risk of dying probably were associated with high levels of cytokines, low levels of lymphocytes, and the resulting intense inflammatory responses in H5N1 patients, the scientists suggested.

Early diagnosis and effective treatment with antivirals -- such as Tamiflu and Relenza -- to prevent an intense cytokine response ought to be the primary focus of clinical management, they advised.

Most cases of avian influenza infection in humans have resulted from contact with infected poultry, such as domesticated chicken, ducks, and turkeys, or surfaces contaminated with secretion or excretions from infected birds, according to the US Centers for Disease Control and Prevention.

The spread of avian influenza viruses from one ill person to another has been reported very rarely, and transmission has not been observed to continue beyond one person.