Crossing the species barrier is an important step in the development of a flu virus with pandemic potential. Previous studies have focused on the ability of highly pathogenic strains, such as H5N1 "bird flu" circulating in Asia, to spread from poultry to humans. The new study shows less pathogenic strains are also capable of jumping to humans, giving them the opportunity to swap genetic material with human strains, which could result in a more virulent virus.
Researchers in Italy studied outbreaks that occurred among poultry between 1999 and 2003, to determine the risk of avian influenza virus transmission to persons in contact with the animals. The outbreaks occurred in northern Italy in regions where the majority of the country's commercial poultry are raised on farms. Most previous cases of human infection with avian influenza viruses have involved close contact with infected poultry, particularly ill or dying chickens.
The investigators performed a serologic analysis of individuals exposed to infection during the outbreaks, collecting 983 blood samples from poultry farm workers in the outbreak regions. Blood was tested using three different techniques to ensure the validity of results.
The outbreaks involved two serotypes of avian influenza: one low and one highly pathogenic H7N1 and a low pathogenic H7N3 virus. Seven individuals exposed to the more recent outbreak of low pathogenic H7N3 tested seropositive for H7N3. The infected persons came from different farms in two locations and had close contact with turkeys or chickens. No serious symptoms were reported in connection with the infections.
The authors of the study, Dr. Isabella Donate lli of Istituto Superiore di Sanit?in Rome and colleagues, noted that their work provides the first serologic evidence of transmission of low pathogenic strains of avian influenza virus to humans during an outbreak in domestic poultry. Previous reports of human infection in Asia, Canada, and the Netherlands have been associated, in contrast, with highly pathogenic strains. The investigators emphasized that their study probably underestimates the real infection rate of the two strains among exposed individuals, since blood samples were considered positive only if they repeatedly produced unequivocal positive results using several different serologic techniques. They commented, however, that very sensitive serologic techniques such as those developed in the study provide an efficient tool for preventing or controlling the spread of avian flu to humans.
The authors say their findings highlight the importance of improving disease surveillance not only during outbreaks of highly pathogenic avian flu, but also when less pathogenic strains are circulating. To forestall genetic exchange between human and avian strains, as in mixed infections, they emphasized that poultry workers should be systematically vaccinated, pointing out that such workers are identified as high-risk and included in the annual Italian vaccination campaign.
In an accompanying editorial, Frederick Hayden, MD, of the University of Virginia and Alice Croisier, MD, of the World Health Organization concluded, "The transmissibility of avian viruses may increase as the viruses adapt to humans. In affected countries, public education about simple precautionary measures for food preparation, poultry handling, and avoidance of contaminated water are essential until specific prevention measures such as vaccines become available."