A new semester...

...starts tomorrow, and with it we'll try once again to be a bit more active here.  This semester brings Biology of Fish, as well as an online Evolution class - a couple of topics that should lend themselves well to blogging.  Even if it means not collapsing on the couch at the end of the day.

We'll start with this an interesting study examining the evolution of disease.  That's a topic that my evolution students will look at in depth later in the semester.  This one's in birds, house finches to be specific.

Pathogenic organisms can find themselves in a bit of a quandary.  They're dependent on their host organisms for survival, reproduction, and dispersal.  But they run the risk of overdoing it.  As they reproduce, the host may become sick.  If the numbers are too exorbitant, the host may even die.  If a pathogen reproduces in such numbers that it kills or disables its host prior to spreading to new ones, it'll soon be out of the parasite business.  However, coevolution of parasite and host can result in just the right balance of "sickness".

Virginia Tech's Dana Hawley and her coauthors, in a paper published in PLOS Biology, explore the evolution of house finch eye disease, a form of conjunctivitis in the invasive house finch caused by the Mycoplasma gallisepticum.  The researchers expected to see the disease become milder with passing time, all the better to expedite its spread.  They were surprised to see it actually become more virulent. In two different locations.

The house finch, Haemorhous mexicanus,
carrier of house finch eye disease 

What is particularly interesting in this case involves the two regions that were examined.  The study focused on birds from two different areas, California and the Eastern Seaboard.  Samples taken from sick birds in each area from 1994 through 2010 showed increasing virulence.  However, the bacterial strain the spread from east to west across the continent was less virulent.

Apparently, to spread, the bacteria needed healthier birds.  Birds that could fly a little further and live a little longer.  Once established in a location, though, more virulent strains could evolve - sicker birds may not disperse as well, but they produce lots of bacteria.

This relates to a number of other studies that have revealed some truisms about human disease.  For example, a pathogen that can be transmitted through the air or by an insect vector can afford to be more virulent than one that requires the host to be actively eating or drinking.  More to come on that.