Emerging pathogens: the epidemiology and evolution of species jumps

Abstract

Novel pathogens continue to emerge in human, domestic animal, wildlife and plant populations, yet the population dynamics of this kind of biological invasion remain poorly understood. Here, we consider the epidemiological and evolutionary processes underlying the initial introduction and subsequent spread of a pathogen in a new host population, with special reference to pathogens that originate by jumping from one host species to another. We conclude that, although pathogen emergence is inherently unpredictable, emerging pathogens tend to share some common traits, and that directly transmitted RNA viruses might be the pathogens that are most likely to jump between host species.

Figure I

Impact of R₀ on the population dynamics of pathogen emergence. (a) Relationship between final epidemic size (log scale) and R₀ from Equation I with N =10 000, for I₀=1, 10, 100 and 1000. (b) Relationship between the probability of a major epidemic, P_(epidemic), and R₀ from Equation II, for I₀=1, 5, 10 and 25. Redrawn with permission from . (c) Approximate relationship (valid when μ≪1 and R₀ is <1 and not too close to 1) between the probability that the pathogen adapts during an outbreak so that R₀ becomes >1, P_(adaptation), and the original value of R₀ from Equation III for μ=0.0001, 0.001 and 0.01. Redrawn with permission from .

Figure 1

Preventing cross-species transmission. As part of a campaign to reduce the risk to public health in Matongo, Tanzania, local people in the region are encouraged to have their domestic dogs vaccinated against rabies. Reproduced with permission from T. Lembo.

Figure 2

Examples of overdispersed outbreak sizes. Plots show the cumulative fraction of all cases against the cumulative fraction of outbreaks ordered from the largest to the smallest for outbreaks of human infection with Ebola virus in sub-Saharan Africa from 1976 to 2004 [red line; 18 outbreaks, 428 cases; data from ProMed (http://www.promedmail.org), the World Health Organization (http://www.who.int) and the Centers for Disease Control and Prevention (http://www.cdc.gov)] and Escherichia coli O157 in Scotland from 1996 to 2003 (blue line; 63 outbreaks, 1008 cases; data from Health Protection Scotland (http://www.hps.scot.nhs.uk)). For both examples, most outbreaks are small (<10 cases) and only a few outbreaks are large (hundreds of cases), as indicated by the strongly convex shape of the plots. This is consistent with a general trend for disease outbreak size distributions to follow a power law with exponent >2, indicative of severe overdispersion .