Evolutionary Medicine and Polio

The following quote is from  Paul Ewald, an expert in Evolutionary Medicine, regarding eradication of diseases:

“The completion of the campaigns often requires that the last holdouts are persuaded to cooperate or forced to submit.  All eradication campaigns suffer from this Achilles’ heel.  Some people want their houses fumigated, others resist.  Some prefer to have their contagious illness treated, other refuse.  If we are to win some of the more winnable wars- those against measles and polio, for example- we may need to violate more rules than we have to date” (Ewald, 2002).   

What is Evolutionary Medicine? 

Evolutionary Medicine views disease from the perspective of the disease causing pathogen.  By doing this, we gain a better understanding of how the pathogens are transmitted, the rate at which a disease spreads throughout a population and the factors that contribute to the virulence of the pathogen.  Instead of just looking for a cure or reduction in symptoms,  evolutionary principles focus on control of disease through reducing transmission and decreasing the virulence of  particular pathogens. 

Examples of evolutionary biology's influence on polio and the eradication effort:

• Sporadic cases of polio continue to occur throughout the world but in highly immune areas these small outbreaks burn themselves out quickly and do not become epidemic (Bull, 2001).  Even though they burn themselves out very quickly, it is still important to find out why they continue occur. The answer can be found through phylogenetic analysis of the offending virus.   Phylogenetics is the study of evolutionary genealogies and is now a standard part of any disease epidemiology which utilizes a branching tree that represents the evolutionary history of the lineages being studied and it has become indispensible in identifying disease reservoirs and in tracking step-by-step transmission of some viruses of which polioviruses are included (Bull, 2001).   In most cases the biologists have been able to trace the roots of these sporadic outbreaks to an imported wild type or a vaccine derived virus (Bull, 2001). 

• Since at least the mid to late 1990s, it had been theorized that HIV, the virus that causes AIDS, may have crossed into humans as a result of contamination of the OPV.  The claim was that a simian immunodeficiency virus (SIV) was transmitted to humans when chimpanzee tissues were allegedly used in the preparation of OPV (Hooper, 1999 as cited in Worobey, 2004).   Using evolutionary methods, specifically phylogenetic analysis, it was determined that the SIV was genetically distinct from all strains of HIV-1 which provided direct evidence that the chimpanzees were not the source of the AIDS pandemic (Worobey, 2004).  

Oral Polio Vaccine (OPV)  and Vaccine Associated Paralytic Polio (VAPP):

• VAPP occurs because OPV contains live attenuated virus that replicates in the intestines and is then shed in the feces increasing the risk of polio transmission via a VDPV to a non-immune host.  In areas where vaccine coverage is inadequate, the viruses can recombine with a wild type poliovirus or other closely related enteroviruses to form a neurovirulent Vaccine-Derived Poliovirus (VDPV) which can then cause VAPP. (Nathanson, 2002) It is highly probable that VDPVs occur because like their wild type counterpart they exhibit a high evolutionary rate and a high rate of mutations which contributes to the reversion of the virus to a neurovirulent type (Kew, 2005).  Considering all of this and with eradication so close, perhaps the time has come to look at a new type of vaccine- one that focuses on virulence factors in addition to ease of use and cost.  Everyone is acutely aware that times change and things like vaccines may need to change with them.

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