Emerging, novel, and known influenza virus infections in humans

Abstract

Influenza viruses continue to cause yearly epidemics and occasional pandemics in humans. In recent years, the threat of a possible influenza pandemic arising from the avian influenza A(H5N1) virus has prompted the development of comprehensive pandemic preparedness programs in many countries. The recent emergence of the pandemic influenza A(H1N1) 2009 virus from the Americas in early 2009, although surprising in its geographic and zoonotic origins, has tested these preparedness programs and revealed areas in which further work is necessary. Nevertheless, the plethora of epidemiologic, diagnostic, mathematical and phylogenetic modeling, and investigative methodologies developed since the severe acute respiratory syndrome outbreak of 2003 and the subsequent sporadic human cases of avian influenza have been applied effectively and rapidly to the emergence of this novel pandemic virus. This article summarizes some of the findings from such investigations, including recommendations for the management of patients infected with this newly emerged pathogen.

Fig. 1

Negatively stained transmission electron micrographs showing some of the ultrastructural morphology of (A) the recently emerged pandemic influenza A (H1N1) 2009 virus of swine origin (strain: A/CA/4/09), (B) the recreated 1918 pandemic influenza A (H1N1) virus grown in Madin-Darby canine kidney cell culture, and (C) 2 avian influenza A (H5N1) viruses (magnification ×108,000) showing the stippled appearance of the roughened surface of the proteinaceous coat encasing each virion. It should be noted that although these images show different views of these influenza viruses, electron microscopy generally cannot distinguish among the different influenza virus types, subtypes, or strains.

Fig. 2

Reassortment history of human pandemic influenza strains. Each influenza gene segment is represented by a colored horizontal bar. The 1918 Spanish flu influenza and classic swine influenza probably originated from an avian influenza virus population at some point in the past, but arrows indicating their origins have been omitted here because the exact species-crossing events cannot be defined for certain and remain controversial. The reassortment events generating the H3N2 Hong Kong flu pandemic strain have been simplified here because of space constraints. Sporadic bird-to-human transmission events are also shown in the bottom right corner.

Fig. 3

A series of consecutive chest radiographs showing the progression and final resolution of an adult woman aged 22 years infected with pandemic influenza A (H1N1) 2009. The initial appearance is suggestive of a developing viral pneumonitis, which then seems to resolve (A–C), but then the patient probably developed a secondary bacterial infection (although not proven conclusively) (D) that necessitated a transfer to the intensive care unit (E) before finally resolving (F). The patient was finally discharged feeling well, with no long-term sequelae.