H7 Human Transmission Media Myth
Recombinomics Commentary
December 24, 2007
The H7N3 virus found in Vietnam, however, is classified as low-pathogenic, not readily transmitted to humans.
The above comment on low path H7N3 in Vietnam contains the popular media myth on H7 transmission to humans. High or low pathogenicity is based on a biological assay in chickens or the presence of a polybasic cleavage site. To date, only H5 and H7 have been shown to have a polybasic cleavage cleavage site, and more recent definitions of high pathogenicity state that the sequence alone is sufficient to classify avian influenza as highly pathogenic. Moreover, low path avian influenza can acquire a polybasic cleavage site via non-homologous recombination, so H5 and H7 are reportable diseases regardless of initial pathogenicity.
However, low and high path designations are not predictive of transmission to humans. H7, regardless of N serotype, or the presence or absence of a polybasic cleavage site, is readily transmissible to humans.
To date, only one death has been reported from H7 infections. The case was a veterinarian who was infected with H7N7. Although it was high path, the isolate from the veterinarian was significantly more complex than H7N7 isolated from cullers or infected birds, suggesting that H7N7 was formed via co-infections and recombination. The isolate also had PB2 E627K, a human polymorphisms that allows the virus to replicate more efficiently at lower temperatures. Cullers were also infected with H7N7 but had much milder symptoms, including conjunctivitis. However, testing of culler contacts indicated over 1000 have developed H7 antibodies, indicating H7 was readily transmitted to humans.
Transmission to humans from H7 infections is common, regardless of pathogenicty in chickens or the presence or absence of a polybasic cleavage site. Earlier this year there was an low path H7N2 outbreak in England. Although there were no reported fatalities, media accounts from some of the infected patients indicate that symptoms were severe. Moreover, the number of suspect infections in humans was similar to the number of infected birds. A full report on human cases linked to this outbreak has not been released.
Recently there have been additional H7 outbreaks in Saskatchewan, Canada (H7N3) and South Korea (H7N8). Reports of human cases are lagging, but the H7 assay is poor, and as noted for the outbreak in England, frequently delayed.
Co-circulation of H5 and H7 rauses concerns of a reassortant and/or recombinant that has the human pathogenicity of H5N1 and teh transmissibility of H7.
The media myth on poor human transmission by low path H7 is widespread, but has no scientific basis.
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