Figure 1.
Zoonotic transmission of influenza A virus.
The hemagglutinin of avian influenza A viruses (blue) preferentially bind to oligosaccharides that terminate in sialic acid–α-2,3-Gal (red), whereas the hemagglutinin on human influenza A viruses (green) prefer oligosaccharides that terminate in sialic acid–α-2,6-Gal (orange). Fatal viral pneumonia in humans infected with the H5N1 subtype of avian influenza A viruses is likely due to the ability of these viruses to attach to and replicate in the lower respiratory tract cells, which have sialic acid-α-2,3-Gal terminated saccharides. The horizontal arrows indicate interspecies transmission, including the transmission from an avian or porcine reservoir into the human species. Image credit: Bart Haagmans, Erasmus MC. Original images (left to right, from top to bottom) by Roman Köhler, Alvesgaspar, Anton Holmquist, Joshua Lutz, and CDC.
Figure 2.
Zoonotic transmission of SARS-CoV.
Genomic analyses provided evidence that genetic changes in the spike gene of SARS-CoV from bats (left) and civet cats (center) are essential for the animal-to-human transmission (horizontal arrows). Species-to-species genetic variation in the (thus far unidentified) viral receptor in bats and in the angiotensin converting enzyme 2 (ACE2) gene, encoding the SARS-CoV receptor in civet cats and humans also affects the efficiency with which the virus can enter cells (vertical arrows). The SARS-CoV that caused the epidemic evolved a high affinity for both civet (center) and human (right) ACE2 receptors (indicated by the single diagonal and the right side vertical arrow). Image credit: Bart Haagmans, Erasmus MC. Original images (left to right) by Dodoni, Paul Hilton, and Hoang Dinh Nam.