RachelM

Alizon et al. in their review “Virulence evolution and the trade-off hypothesis: history, current state of affairs and the future” investigate the problems associated with the trade-off hypothesis in measuring and quanitfying factors such as virulence (and various definitions). Below is a outline of the history of virulence.

History of virulence
Pasteur and Koch (1881) looked into parasite virulence but this data was overlooked–concept of strain specific virulence.

After Pasteur’s experiment, the avirulence hypothesis was formed along with other theories to explain parasite virulence

Topley (1919) hypothesized that populations in high density had a greater propensity to replicate and migrate more rapidly.

Kostitzin (1934) challenges avirulence hypothesis, stating that virulence may occur from a completely mutualistic interaction.

Ball (1943) old host-parasite interactions are still virulenty

G C Williams, J Maynard Smith, and W. D. Hamilton (1960/1970) field of evolutionary ecology founded

Hamilton (1964) challenged avirulence hypothesis

Levin and Pimentel (1981) evidence that virulent and avirulent strains can coexist

May and Anderson (1983) avirulence hypothesis which was universally accepted although there was lack of evidence due to the fact that old host-parasite associations do not tend to be virulent. May and Anderson found a connection between recovery and virulence.

Ewald (1983) virulence depends on how the mechanism is transmitted.

So, the long accepted “common knowledge” avirulent hyypothesis went down the drain as lack of evidence supported the trade-off hypothesis. The trade-off hypothesis basically states simply that more transmissible parasite strains must remain with the host for a longer period of time. In the abstract, Alizon states that “the trade-off hypothesis states that virulence is an unavoidable consequence of parasite transmission.”

There is a lack of evidence for the trade-off hypothesis which is one reason why it is increasily challenged.  However, on the same note, the authors argue that lack of evidence is often times not due to lack of study.  Rather, investigations that fail to see a significant trend are not published.  Furthermore, defining virulence in its most crude state is also a debate among biologists.  Many factors are involved and the trade-off hypothesis may be “too simple” to encompass virulence.  Indeed, the trade-offs that exist may not be the trade-offs that are “usually assumed.”

Ultimately, Alizon et. al argue that although this statement may be considered a sweeping generlization for parasites, it is an important theory for predicting virulence.