  With a single circulating vector-borne virus, the basic reproduction number
incorporates contributions from tick-to-tick (co-feeding), tick-to-host and
host-to-tick transmission routes. With two different circulating vector-borne
viral strains, resident and invasive, and under the assumption that co-feeding
is the only transmission route in a tick population, the invasion reproduction
number depends on whether the model system of ordinary differential equations
possesses the property of neutrality. We show that a simple model, with two
populations of ticks infected with one strain, resident or invasive, and one
population of co-infected ticks, does not have Alizon's neutrality property. We
present model alternatives that are capable of representing the invasion
potential of a novel strain by including populations of ticks dually infected
with the same strain. The invasion reproduction number is analysed with the
next-generation method and via numerical simulations.
