Should Varroa and honey bees, left to their own devices, evolve towards a less harmful relationship?  How about AFB and bees? During my holiday I read a great article by an American evolutionary biologist named Paul Ewald (Scientific American, May 2001, 284: 32-33) that may shed light on these questions. Dr. Ewald has challenged a long standing theory that diseases and parasites necessarily tend towards being less harmful (virulent) to their hosts.  His theory suggests whether a condition will tend, over evolutionary time, to being benign or harmful depends the way the disease gets to its next host.  The theory suggest the "tendency of a parasite tending towards benignity is reserved for conditions passed directly from host to host.  Someone too sick to mingle with others would indeed be a dead end for the most dangerous infections, but Ewald showed that infectious agents that used intermediate vectors fpr transmission, such as malaria's mosquitoes and cholera's contaminated water, are free to evolve towards greater destructive power.

The theory predicts that a disease like Varroa, that spreads easily among colonies, will evolve towards being more benign to bees, while AFB, that can lay dormant in the environment, being vectored by next cavities, will tend to be more virulent.

The implications of Ewald's ideas on bee diseases was recently explored in a recent article and the abstract of that article is available online:

Fries, I. and S. Camazine.  2001.  Implications of horizontal and vertical pathogen transmission for honey bee epidemiology.
http://www.edpsciences.org/articles/inra-apido/abs/2001/03/fries/fries.html

The abstract reads:
The degree to which a disease evolves to be virulent depends, in part, on whether the pathogen is transmitted horizontally or vertically. Eusocial insect colonies present a special case since the fitness of the pathogen depends not only on the ability to infect and spread between individuals within a colony, but also on the ability to spread to new individuals in other colonies. In honey bees, intercolony transmission of pathogens occurs horizontally (by drifting or robbing) and vertically (through swarming). Vertical transmission is likely the most important route of pathogen infection of new colonies. Theory predicts that this should generally select for benign host-parasite relationships. Indeed, most honey bee diseases exhibit low virulence. The only major exception is American foulbrood (AFB). In light of current ideas in evolutionary epidemiology, we discuss the implications of horizontal and vertical pathogen transmission for virulence of AFB and other honey bee diseases.






__________________________________________________
D O T E A S Y - "Join the web hosting revolution!"
             http://www.doteasy.com