Less Virulent Strains Of Avian Influenza Can Infect Humans

Crossing the species barrier is an importantstep in the development of a flu virus with pandemic potential.Previous studies have focused on the ability of highly pathogenicstrains, such as H5N1 “bird flu” circulating in Asia, to spread frompoultry to humans. The new study shows less pathogenic strains are alsocapable of jumping to humans, giving them the opportunity to swapgenetic material with human strains, which could result in a morevirulent virus.

Researchers in Italy studied outbreaks thatoccurred among poultry between 1999 and 2003, to determine the risk ofavian influenza virus transmission to persons in contact with theanimals. The outbreaks occurred in northern Italy in regions where themajority of the country's commercial poultry are raised on farms. Mostprevious cases of human infection with avian influenza viruses haveinvolved close contact with infected poultry, particularly ill or dyingchickens.

The investigators performed a serologic analysis ofindividuals exposed to infection during the outbreaks, collecting 983blood samples from poultry farm workers in the outbreak regions. Bloodwas tested using three different techniques to ensure the validity ofresults.

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 pathogenicH7N3 tested seropositive for H7N3. The infected persons came fromdifferent farms in two locations and had close contact with turkeys orchickens. No serious symptoms were reported in connection with theinfections.

The authors of the study, Dr. Isabella Donatelli ofIstituto Superiore di Sanità in Rome and colleagues, noted that theirwork provides the first serologic evidence of transmission of lowpathogenic strains of avian influenza virus to humans during anoutbreak in domestic poultry. Previous reports of human infection inAsia, Canada, and the Netherlands have been associated, in contrast,with highly pathogenic strains. The investigators emphasized that theirstudy probably underestimates the real infection rate of the twostrains among exposed individuals, since blood samples were consideredpositive only if they repeatedly produced unequivocal positive resultsusing several different serologic techniques. They commented, however,that very sensitive serologic techniques such as those developed in thestudy provide an efficient tool for preventing or controlling thespread of avian flu to humans.

The authors say their findingshighlight the importance of improving disease surveillance not onlyduring outbreaks of highly pathogenic avian flu, but also when lesspathogenic strains are circulating. To forestall genetic exchangebetween human and avian strains, as in mixed infections, theyemphasized that poultry workers should be systematically vaccinated,pointing out that such workers are identified as high-risk and includedin the annual Italian vaccination campaign.

In an accompanyingeditorial, Frederick Hayden, MD, of the University of Virginia andAlice Croisier, MD, of the World Health Organization concluded, "Thetransmissibility of avian viruses may increase as the viruses adapt tohumans. In affected countries, public education about simpleprecautionary measures for food preparation, poultry handling, andavoidance of contaminated water are essential until specific preventionmeasures such as vaccines become available."

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