Monday, November 14, 2005

I should be too old for this...

Its been a particularly crappy past three weeks for me... what started as just a bad cold has turned out to be pertussis - and while I'm not burning through a box of tissues a day anymore, I could do without the coughing and that cracked rib I got as a result.

Like most people in the western world, I had always assumed pertussis (better known as whooping cough) was one of those childhood diseases our grandmothers used to worry about, but which had been efficiently dealt with through the miracle of vaccination. Unfortunately, pertussis likes to throw curveballs... over time vaccinated and previously infected people lose their immunity, creating a population of adults that are fully susceptible to the disease. The one saving grace is that usually these later infections are less severe than what is seen in children.

Whooping cough is caused by a nasty little bacterium called Bordetella pertussis, which glues itself to the ciliated cells of your respiratory tract and releases a cocktail of toxins to disable your immune system and damage your respiratory cells. Patients with pertussis go through a two week period of sneezing and runny noses, followed by 4 to 6 weeks of violent coughing. During this period they are highly contagious, but once they recover they can enjoy years of aquired immunity to the disease. B. pertussis evolved from the bacterium Bordetella bronchiseptica, a closely related bacteria that causes a mild but chronic respiratory infection in animals. We can also be infected by B. bronchiseptica, but it doesn’t happen often… B. bronchiseptica isn’t very contagious to begin with, and the immunity most of us have to B. pertussis confers immunity to B. bronchiseptica. In a small number of cases, whooping cough is caused by B. parapertussis… another descendant of B. bronchiseptica. The symptoms are the same (although milder), but significantly immunity to B. pertussis does not make you immune to B. parapertussis.

The evolution of this group of bacteria appears to be tied to the changes in human culture that led us out of the fields and into the cities, and may even be a result of the high density urbanization of the last half millennium. Genetic comparisons among the three bacteria place the divergence of B. pertussis within the past few thousand years, while the first records of the distinctive ‘whoop’ of whooping cough go back only 400 years.

Why is the emergence of whooping cough tied to human cities? The highly infectious nature of B. pertussis means that this disease needs a minimum population size in order to continuously have a supply of new hosts. If the population is too small (e.g., in a medieval town or rural village) the entire population would be infected and subsequently immune in a matter of months. In large cities, however, there are enough hosts present and a constant influx of new hosts (births and immigration) to keep B. pertussis circulating indefinitely. The ancestral bacterium B. bronchiseptica, in contrast, has to survive in animal populations that are at low density, dispersed, and seasonal in reproduction. Instead of trying to infect as many new hosts as possible in a short period of time, it lays low in the hosts nasal passages causing only a mild disease, waiting for the rare opportunities when uninfected hosts are encountered or it can infect its current hosts offspring.

Vaccination for pertussis began in the 1940's and in the western world at least, rapidly transformed whooping cough from a mothers nightmare into one of those 'old tyme' diseases your grandparents would tell you about. In a matter of years, the pool of young children which the pertussis bacterium depended on for its survival virtually disappeared. This could have spelt the end for B. pertussis, save for one factor: unlike many other diseases, immunity to pertussis isn't life-long. Instead, it starts to fade after a few years, and most adults who had been immunized or infected as children become susceptible again to the disease (albeit in a milder form). Adults have always been a reservoir for the disease to hide in when it ran out of children to infect, but the milder symptoms meant that adults were not as good at transmitting the disease. Without children, however, it appears pertussis may not have much of a choice. The last few years have seen a re-emergence in adults and older children and an increase in the severity of the infections. Pertussis, it appears, is evolving to become a significant disease of adults as well.

Something I found out the hard way, three weeks ago.