Emerging and re-emerging infectious diseases: influenza as a prototype of the host-pathogen balancing act

Abstract

To respond to emerging and re-emerging infections, it is necessary to understand the interactions between microbial pathogens and their hosts and the impact of environmental and social factors on these interactions. The importance of understanding host-pathogen interactions is underscored by the emergence of virulent H5N1 avian influenza viruses and their transmission to humans, and the potential pandemic threat they pose.

Figure 1

Documented H5N1 Influenza Cases among Birds (Blue) and Humans (Orange) 2003–February 9, 2006

Figure 2

Possible Mode of H5N1 Avian Influenza Virus Transmission among Migratory Birds, Domestic Poultry, and Humans Migratory birds harboring a low pathogenic strain of influenza virus are generally without symptoms of disease but can excrete virus. If a low pathogenic avian influenza A virus is transmitted to domestic poultry through direct or indirect contact or by exposure to fecal droppings, the virus can progressively mutate following passage from one susceptible bird to the next. This is illustrated in the figure by the increase in the number of infectious virions (green) following each passage of the virus from bird to bird until a mutant virus emerges that replicates to high levels and can kill the infected bird. Poultry harboring low pathogenic virus can make a full recovery, but those with highly pathogenic mutant forms of the virus almost always succumb to the virus and die. A highly pathogenic strain of H5N1 avian influenza virus replicates more efficiently and can be transmitted among poultry and from poultry to pigs and other mammals, including humans, through direct or indirect contact and via transport of infected poultry to other farms and markets. Susceptible migratory birds can become infected with the highly pathogenic mutant forms of the virus. Recently, migratory birds have started to show signs of disease from infection with the highly pathogenic H5N1 strain and may be able to transmit the more virulent virus to other birds, both wild and domestic, along their flight path, leading to rapid spread of the virulent virus.