The importance of the aggregation of ticks on small mammal hosts for the establishment and persistence of tick-borne pathogens: an investigation using the R(0) model

Aggregation of parasites amongst hosts is important for the epidemiology of vector-borne diseases because hosts that support the majority of the vector population are responsible for the majority of pathogen transmission. Ixodes ricinus ticks transmit numerous pathogens of medical importance including Borrelia burgdorferi s.l. and tick-borne encephalitis virus. One transmission route involved is 'co-feeding transmission', where larvae become infected via feeding alongside infected nymphs. The aggregation of ticks on hosts leads to an increase in the number of larvae feeding alongside nymphs, increasing the transmission potential via this route. The basic reproduction number, R 0, can be used to identify whether a pathogen will become established if introduced. In the current study we use previously published tick, and pathogen, specific data to parameterize an R 0 model to investigate how the degree of aggregation of ticks on hosts affects pathogen persistence. The coincident aggregated distribution permitted the establishment of tick-borne encephalitis virus but did not influence whether B. burgdorferi s.l. became established. The relationship between the k-exponent of the negative binomial distribution and R 0 was also defined. Therefore, the degree of aggregation of ticks on small mammal hosts has important implications for the risk to human health in a given area.