Past, Present, and Possible Future Human Infection with Influenza Virus A Subtype H7

CDC Emerging Infectious Diseases
Volume 15, Number 6–June 2009

Perspective

Excerpt:

If an influenza virus with an HA against which the human population had little or no immunity crossed the species barrier and was efficiently transmitted among humans, a pandemic could result. Three pandemics occurred in the 20th century: in 1918 (H1N1), 1957 (H2N2), and 1968 (H3N2). However, none of these pandemic strains possessed the HA cleavage site mutation characteristic of HPAI viruses (3). Thus, the HPAI phenotype is not required for an influenza virus to cause a pandemic. Three HA subtypes, H1–H3, subsequently established stable lineages in humans; 2 subtypes, H1N1 and H3N2, cause seasonal epidemics today, which result in ≈36,000 deaths in the United States annually (4). Although the severity of a pandemic virus cannot be known in advance, attack rates could reach 25%–35%. The resulting surge in the number of persons requiring medical or hospital treatment would undoubtedly overwhelm the healthcare system. Within the past decade, HPAI and LPAI viruses have been found to be associated with human infection, primarily as a result of direct transmission from poultry to humans (2,5,6). However, none of these viruses have yet acquired the ability to be transmitted efficiently among humans. LPAI viruses of the H7 and H9 subtypes have caused mild respiratory or conjunctival infections in humans. However, some HPAI subtype H5 and H7 viruses, which cause a high proportion of deaths in experimentally infected chickens, have been associated with severe human disease and death (5,6).

Due to an unprecedented geographic expansion of subtype H5N1 viruses since 2003 and continued sporadic human subtype H5N1 infections, much emphasis has been placed on the potential pandemic threat posed by subtype H5N1 viruses. In contrast, subtype H7 infection in humans has not been as extensively studied. In this perspective, we will discuss the epidemiology of subtype H7 in humans, current research that explores the pandemic potential of these viruses, and ongoing measures to prevent future human infection.