Invasion Thresholds and the Evolution of Nonequilibrium Virulence

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from: Evolutionary Applications
Volume 1 Issue 1 Page 172-182, February 2008

Invasion thresholds and the evolution of nonequilibrium virulence

James J. Bull1 Integrative Biology and Institute for Cellular and Molecular Biology, University of Texas, Austin, TX, USA and
Dieter Ebert2 Zoological Institut, Evolutionary Biology, University of Basel, Basel, Switzerland

1 Integrative Biology and Institute for Cellular and Molecular Biology, University of Texas, Austin, TX, USA

2 Zoological Institut, Evolutionary Biology, University of Basel, Basel, Switzerland

J. J. Bull, Integrative Biology and Institute for Cellular and Molecular Biology, University of Texas, Austin, TX 78712, USA. Tel.: 512 471 8266; fax: 512 471 3878; e-mail: bull@mail.utexas.edu

[bolding added, ed.]

Abstract
The enterprise of virulence management attempts to predict how social practices and other factors affect the evolution of parasite virulence. These predictions are often based on parasite optima or evolutionary equilibria derived from models of host-parasite dynamics. Yet even when such models accurately capture the parasite optima, newly invading parasites will typically not be at their optima. Here we show that parasite invasion of a host population can occur despite highly nonoptimal virulence. Fitness improvements soon after invasion may proceed through many steps with wide changes in virulence, because fitness depends on transmission as well as virulence, and transmission improvements can overwhelm nonoptimal virulence. This process is highly sensitive to mutation supply and the strength of selection. Importantly, the same invasion principle applies to the evolution of established parasites, whenever mutants arise that overcome host immunity/resistance. A host population may consequently experience repeated invasions of new parasite variants and possible large shifts in virulence as it evolves in an arms race with the parasite. An experimental study of phage lysis time and examples of mammalian viruses matching some of these characteristics are reviewed.

full text at: http://www.blackwell-synergy.com/doi/full/10.1111/j.1752-4571.2007.00003.x

The virulence of influenza variants varies somewhat, although it is difficult to apportion mortality between the virus (its intrinsic virulence) and the host (whether it has any prevailing immunity to the strain). The case mortality rate of the H1N1 1918 flu that killed 20?40 million people worldwide was reported to be about 1%, somewhat more than a 10-fold excess of the typical mortality rate per infection of the viruses of today (Taubenberger and Morens 2006). The virulence of this strain appeared to wane over a few years. H1N1 disappeared in the 1950s, and its accidental reintroduction in 1977 to naive hosts did not lead to the high mortality rate of 1918, so it appears that the virulence per se evolved downward (Kilbourne 2006). Although mortality rates of the usual antigenic shift strains remain relatively constant, some high virulence exceptions are known (e.g., an H3N2 strain from 1997; O?Donnell et al. 2003), and there may well be many low-virulence variants that go unnoticed. However, the H5N1 strain (bird flu) that is circulating widely in birds has a case mortality rate exceeding 50% in humans, far in excess of the mortality rate observed in any strain that established itself in humans (Webster et al. 2006).
H5N1 has not established itself in the human population, but the obvious fear is that it can do so and maintain its high mortality rate, at least during its first round of global spread. The invasion threshold perspective is thus especially relevant here. Ewald has used the public media to argue that the virulence of H5N1 will quickly evolve to low levels, should it invade (Orent 2005); this view has not been widely accepted (Normile 2005), and the perspective of this paper is that, even if the H5N1 optimum in humans is low virulence, the epidemic could have devastating effects before it neared the viral optimum.

credits Aeolus