Tuesday, July 05, 2005

"Tortise" and "hare" strategies in a family of infectious agents

There's an interesting short review in the June Trends in Microbiology about different epidemiology in three members of the Bortadella family . The ancestral Bortadella, B. bronchosepta, is endemic in nasal tissue of mammals, and has given rise to two highly virulent descendants, B. pertussis and B. parapertussis, both of which are unbelievably contagious and cause whooping cough in human children.

What is pretty cool about this family is the apparent tradeoffs under which all three are operating. The infectious versions can stay in a kid for about 10 days, and get spread to the next kid through little droplets expelled during coughing. In contrast, the ancestor version can stay put in wildlife for years. The review proposes that the infectious versions have become specialists in becoming maximally contagious and sacrificing their ability to live in any individual host. This strategy, like the hare in the story, increases the speed of propagation at the expense of persistence, and also increases risk of extinction- once everyone in sight (actually, coughing range) is infected, they either die or become immune, and you're left high and dry. This strategy might have become profitable when humans started living in big cities, which meant that there would be a year-round supply of fresh kiddies to infect.

The wildlife in which the ancestral form dwells never congegate enough to support an outbreak-directed strategy. The ancestral form instead follows a "tortise" strategy, with lower virulence and long persistence, giving it time to infect another target in dispersed host populations.

Human urbanization thus opened up a big niche opportunity, so much so that the jump to the "hare" strategy, high virulence/short persistence happened not one but twice. B. pertussis and B. parapertussis use different strategies to evade the human immune system, and even require different vaccinations, suggesting that the hare strategy was under strong selection no matter how it was implemented. These fast guys have succesfully competed away the ancestral form away in humans, so that B. bronchosepta is typically observed only in infants, people in contact with animals, and immunocompromised people.

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