Biannual birth pulses allow filoviruses to persist in bat populations
by David Hayman
Pioneers of the application of mathematics to infectious disease biology posed three interrelated questions about infectious disease dynamics: can we predict and explain the size and periodicity of epidemics, how does the flow of susceptible individuals affect endemic persistence and do critical community sizes exist? Those pioneers applied their models to human infectious diseases, however most emerging infectious diseases are zoonoses from wildlife hosts. Thus, today disease ecologists often aim to address the same questions in wildlife systems.
Filoviruses Ebolavirus (EBOV) and Marburgvirus (MARV) cause hemorrhagic fevers with high mortality rates, posing significant threats to public health. To understand transmission into human populations, filovirus dynamics within reservoir host populations must be understood. Studies have directly linked filoviruses to bats, but the mechanisms allowing viral persistence within bat populations are poorly understood. Theory suggests seasonal birthing may decrease the probability of pathogen persistence within populations, but data suggest MARV may persist within colonies of seasonally breeding Egyptian fruit bats.
This study used stochastic modelling to explore fundamental questions relating to filovirus ecology: 1) can filoviruses persist within isolated bat colonies; 2) do critical community sizes exist; and 3) how do host-pathogen relationships affect spillover transmission potential? Data from serological surveys were used to test if the model findings were supported by field data.
The results showed that synchronous annual breeding and shorter incubation periods did not allow filovirus persistence, whereas bi-annual breeding and longer incubation periods, such as reported for Egyptian fruit bats and EBOV in experimental studies, allowed persistence in colony sizes often found in nature. Serological data supported the findings, with bats from species with two annual birth pulses more likely to be seropositive (odds ratio 4.4, 95% confidence interval 2.5-8.7) than those with one, suggesting biannual birthing is necessary for filovirus persistence.
Figure 1. Number of susceptible and infected juveniles and infected adults over the final years for 500 stochastic simulations of a 25 year simulation model of filovirus-bat infection dynamics. This model has biannual birth pulses, with the synchrony parameter (s) 14.3, a birth rate of 0.98 per female per year, and an incubation period (1/σ) of 21 days. (Hayman, 2015)
The results of this study can perhaps explain the periodicity of infection prevalence within the host, and suggest that filovirus peak prevalences may be found in juvenile bats and be seasonally peaked. Given current knowledge we might expect colony sizes of tens of thousands of bat for filovirus persistence, given the periodicity of birth and infection prevalence.
The results of this study provide a useful framework for future studies and offer insights into filovirus ecology that can be used to guide fieldwork, experimental studies and emerging infectious disease management.
This work has recently been published in Proceedings of the Royal Society B.
Hayman DTS. 2015 Biannual birth pulses allow filoviruses to persist in bat populations. Proc. R. Soc. B 20142591 http://dx.doi.org/10.1098/rspb.2014.2591