Lyme disease is classed as a zoonosis; an infection transmitted to humans by a bite from a tick which feeds on both humans and animals which form a natural reservoir of bacteria. The zoonotic nature of Lyme disease has been understood for many years but the accurate classification of those ticks responsible for infection in a variety of geographical locations is still under investigation. The hard ticks which transmit Lyme disease are those of the Ixodes family, with several types of Lyme disease tick distributed across the globe. Most infections are caused following the bite of a nymphal tick, although this varies between tick types, with some only usually biting humans when in the adult stage of the tick lifecycle.
Sheep Ticks in Europe
In Europe Ixodes ricinus is the main vector of Lyme disease and this hard tick is also known as the sheep tick or castor bean tick. In China and other parts of Asia the taiga tick (Ixodes persulcatus) is the key vector, whereas North America has both the black-legged tick or ‘deer tick’ (Ixodes scapularis/dammini) and the western black-legged tick (Ixodes pacificus). Some of these ticks may also be infected with other disease-causing organisms such as Babesiosis, Ehrlichiosis, and Rickettsia.
Until recently it was thought that the western black-legged tick was less likely to cause infection as these ticks frequently feed on host species resistant to Borrelia bacteria such as lizards. The susceptibility of lizards to Borrelia infection is now under question however and ideas regarding bio-engineering of the environment to disrupt host populations of zoonotic diseases such as Lyme disease may not be as advantageous as previously thought.
The specific animals that form the host reservoir for a zoonosis such as Lyme disease vary from place to place with younger ticks tending to feed on smaller animals close to the ground such as mice, squirrels, shrews, and other small vertebrates. These nymphal ticks may then climb up to the tips of tall grasses where they can be brushed off onto the skin or fur of a larger passing mammal such as a deer, sheep, dog, or human.
Abundant Food Sources and Tick-Hosts
White-footed mice are a key infection reservoir for Lyme disease bacteria and there are those who monitor for likely Lyme disease outbreaks based on the likely increase of a resident population of such animals. A particularly large harvest of acorns, for example, may serve to increase the number of host animals in the area and provide an abundant food source for the ticks carrying Lyme disease bacteria. In the following years as these nymphs and larvae turn into adult ticks the rates of Lyme disease infection in the nearby human population may then increase. The CDC monitors cases of emerging zoonosis such as Lyme disease and many researchers are looking at the effects of climate and biodiversity on this type of disease.