Everyone dreads the idea of a pathogen that can rapidly spread across vast distances, especially if that pathogen causes high host mortality or morbidity. For today, I’m going to ignore host mortality/morbidity part, and just focus on the first part: how fast can pathogens spread across space, and what affects their rate of spread?
To start answering this question, we need to dig backwards in the literature more than a decade to an Ecology Letters paper by McCallum et al. (2003). They searched the literature for estimates of the rate of pathogen spread across space in marine or terrestrial systems, while excluding any pathogens whose spread was mostly facilitated by human activities. The pathogen with the fastest rate of spread was a herpes virus that infected pilchards (fish) in Austrailia. The virus spread at rates of up to 11,000 km per year!!
In general, pathogens in marine environments tended to spread faster than pathogens in terrestrial environments. Pathogens might spread more quickly in marine environments because marine habitats tend to be more connected (or more “open”), with higher dispersal rates among patches than in terrestrial environments. However, there was a lot of variation in the rates of pathogen spread in both environments, where some terrestrial pathogens spread rapidly and some marine pathogens spread very slowly. Some of that variation can be explained by host mobility: pathogens that infect sessile (or at least very slow) host species spread more slowly than pathogens that infect highly mobile host species. Neat!
I want to see this analysis done again with the data that have accumulated since 2003. Someone get on that!
McCallum,H., D. Harvell, and A. Dobson. 2003. Rates of spread of marine pathogens. Ecology Letters 6(12): 1062-1067.