Relative abundance of Mycobacterium bovis molecular types in cattle: a simulation study of potential epidemiological drivers

摘要

Background: The patterns of relative species abundance are commonly studied in ecology and epidemiology toudprovide insights into underlying dynamical processes. Molecular types (MVLA-types) of Mycobacterium bovis, theudcausal agent of bovine tuberculosis, are now routinely recorded in culture-confirmed bovine tuberculosis cases inudNorthern Ireland. In this study, we use ecological approaches and simulation modelling to investigate theuddistribution of relative abundances of MVLA-types and its potential drivers. We explore four biologically plausibleudhypotheses regarding the processes driving molecular type relative abundances: sampling and speciation;udstructuring of the pathogen population; historical changes in population size; and transmission heterogeneityud(superspreading).udResults: Northern Irish herd-level MVLA-type surveillance shows a right-skewed distribution of MVLA-types, with audsmall number of types present at very high frequencies and the majority of types very rare. We demonstrate thatudthis skew is too extreme to be accounted for by simple neutral ecological processes. Simulation results indicate thatudthe process of MVLA-type speciation and the manner in which the MVLA-typing loci were chosen in NorthernudIreland cannot account for the observed skew. Similarly, we find that pathogen population structure, assuming forudexample a reservoir of infection in a separate host, would drive the relative abundance distribution in the oppositeuddirection to that observed, generating more even abundances of molecular types. However, we find that historicaludincreases in bovine tuberculosis prevalence and/or transmission heterogeneity (superspreading) are both capable ofudgenerating the skewed MVLA-type distribution, consistent with findings of previous work examining the distributionudof molecular types in human tuberculosis.udConclusion: Although the distribution of MVLA-type abundances does not fit classical neutral predictions, ourudsimulations show that increases in pathogen population size and/or superspreading are consistent with the patternudobserved, even in the absence of selective pressures acting on the system.