My record is not good

by zunguzungu

When it comes to making jokes about the severity of upcoming public disaster type crises. But on the theory that homebrew cures both the vapors andH1N1 influenza, I suggested that we name the chunky black stout we brewed last night “Swine Brew.” Har! Har! If it becomes a pandemic and civilization collapses, we may consider alternate names for the beer.

To atone for my levity, I offer you this bit of expertise on the subject from a blogger with whom I once shared a Thanksgiving Tofurkey, to no one’s satisfaction:

“Simply put, to start, there is a cap on pathogen virulence. Pathogens must avoid evolving the capacity to incur such damage to their hosts that they are unable to transmit themselves. If a pathogen kills its host before it infects the next host it destroys its own chain of transmission. But what happens when the pathogen ‘knows’ that the next host is coming along much sooner? The pathogen can get away with being virulent because it can successfully infect the next susceptible in the chain before it kills its host. The faster the transmission rate, the lower the cost of virulence.

That’s the mechanism in the abstract. What of specifics? What objects or processes are responsible for increasing transmission rates in such a way as to ramp up a variety of influenzas to breathtaking virulence? Growing circumstantial evidence points to intensive livestock production or, in the more critical lexicon, factory farming.

While domestic populations can be divided into backyard and industrial, the former have been raised in one form or another for centuries without the now unprecedented outburst of newly pathogenic influenzas. On the other hand, the conditions for supporting such strains appear best represented in industrial animals. Otte et al. (2007) tabulated outbreaks on industrial and smaller farms for two recent outbreaks of highly pathogenic influenza. In British Columbia, 5% of the province’s large farms hosted H7N3 infections in 2004, while 2% of its small farms hosted outbreaks. In the Netherlands, 17% of industrial farms hosted H7N7 outbreaks in 2003, while 0.1% of backyard farms hosted clusters.

Even if these and other such influenza strains first developed on small holdings, industrial livestock appear ideal populations for supporting virulent pathogens. Growing genetic monocultures removes whatever immune firebreaks may be available to slow down transmission. Larger population sizes and densities facilitate greater rates of transmission. Such crowded conditions depress immune response. High turnover, a part of any industrial production, provides a continually renewed supply of susceptibles, the fuel for the evolution of virulence.