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, R0, 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 R0 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 R0 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.