Definitions and Criteria
A reservoir host is a vertebrate animal species that harbours a particular pathogen and acts as a long-term source of infection for other vertebrates or vectors (ticks in the case of LB). Ticks may also be infected by short term amplifying hosts and by co-feeding transmission. Reservoir hosts are usually the most significant species for the maintenance of the pathogen in the habitat. Detection of antibodies and of pathogen DNA in a host species or isolation of the pathogen from the host provide evidence that the animal has been exposed to infection, but are not sufficient criteria for the determination of reservoir status. This is best determined by the infestation of hosts by uninfected ticks in the laboratory (xenodiagnosis) and the subsequent demonstration of the pathogen in the ticks. Not only does this provide evidence of the reservoir capability of the animal involved but it is also possible to measure the degree of this capability. The limitations of the approach are, firstly that the particular animals involved may not be entirely representative of the wild population in a particular habitat, and secondly the impracticality of working with many animal species under laboratory conditions.
An alternative, or additional approach, is to harvest engorged larval ticks from wild animals and to determine their infection status after some period of development. Once again this involves the capture of animals and also depends on the assumption, apparently true in most habitats, that the infection rate of unfed larval ticks is very low.
One further approach is the detection of infection in unfed ticks in nature. This is the most indirect method of investigating reservoir host status, but can have value providing information is available on the hosts that they utilised as larvae. Its main advantage is that it permits a judgement on the long-term contribution of a particular host species to the circulation of the spirochaete in nature (reservoir role). So far this approach has only been useful in highly artificial habitats, such as farmland, where large hosts are feeding the majority of ticks.
The development of a sensitive test to identify the host species that unfed nymphal ticks have previously fed on as larvae, whilst simultaneously determining the infection status of that nymph (blood meal remnant assay), permits the identification of the host source (i.e. reservoir host ). Such a test, involving the detection of host and pathogen DNA has shown early promise (see Kirstein & Gray, 1996, Pichon et al, 2003) and has been used in the field with some success (see Gray et al, 1999, Pichon et al 2005).
The ultimate objective of studies on reservoir hosts should be to establish their overall role as reservoirs in particular habitats. This may be determined by the application of a simple formula which takes into account the infectivity of the host species for ticks (including the length of time this infectivity persists after infection), the abundance of the host in the habitat and the proportion of the tick population fed by that species (Mather et al. 1989 Am J Epidemiol 130: 143-50). However, this approach requires a large amount of work in different habitats and in different geographical areas to have a general application for any particular species. The values are likely to be highly variable between habitats and also between seasons and years.

European Reservoir Hosts
Many mammal species have now been determined to be reservoirs and to have significance as such in nature. The majority of these are rodents, the most important probably being mice (Apodemus spp), voles (Clethrionymus spp) and squirrels (Sciurus). Several insectivores are also involved including shrews (Sorex, Neomys) and hedgehogs (Erinaceus). The latter is an example of a species, which is probably very important as a reservoir in many habitats, including suburban and peridomestic situations, but is extremely difficult to study in a quantitative manner because of the problems in estimating their density in any particular habitat. Amongst the lagomorphs, hares (Lepus) have been shown to be reservoirs, but rabbits (Oryctolagus cuniculi) appear to have low reservoir capacity. These animals are not good hosts for the tick vectors in nature, the few larvae recovered from wild specimens are rarely uninfected and low percentage infection occurs in xenodiagnostic ticks fed on experimentally infected rabbits.
The role of carnivorous species is also probably limited. Although foxes (Vulpes vulpes) have been shown to carry antibodies to B. burgdorferi s.l. they are poor hosts for immature I. ricinus and infection rates of recovered larvae are very low. Dogs (Canis familaris) have been shown experimentally (in the USA) to be reservoir competent for a short time after infection but they rapidly become immune and in any case their significance in depositing fed ticks in the environment is probably very small. The same may not be true of cats (Felis domesticus) in view of the numbers of feral individuals in some areas, but so far no good data are available on this species.
Ungulates (deer, sheep, cattle, goats and pigs), feed large numbers of ticks in nature and anti-Borrelia antibodies have been detected in them. Spirochaetes have been isolated from local areas of tissue in some species and in sheep some co-feeding infection has been demonstrated, but most evidence, especially that obtained from field studies, suggest that they do not infect a high proportion of the ticks that feed on them. They are, however, crucially involved in the eco-epidemiology of borreliosis as maintenance hosts for the ticks.
Birds are more difficult to study than most mammals and it is not surprising that appreciation of their role as reservoirs of B. burgdorferi s.l. has lagged behind that of rodents and othert mammals. It appears that B. valaisiana is transmitted exclusively from birds, especially members of the thrush family, and most strains of B. garinii also transmitted from birds, especially pheasants (Phasianus colchicus) and blackbirds (Turdus merula) (it should be noted that there are also pathogenic strains of rodent-associated B. garinii). Certain seabirds can transmit strains of B. garinii to the tick, Ixodes uriae, but whether these strains are transmitted to other hosts is uncertain.
Lizards have been previously thought of as having a zooprophylactic role in that their blood may cleanse ticks of existing infections, but there is now good evidence that some species may serve as reservoir hosts of B. lusitaniae.

Closed enzootic cycles and bridge vectors
Closed enzootic cycles involving few hosts and host-specific ticks, have a role in maintaining pathogens in nature, and zoonotic disease does not appear until a bridge vector, such as I. ricinus, intrudes into such a cycle. The only obvious example of this in Europe is the circulation of spirochaetes between the European hedgehog (Erinaceus europaeus) and the hedgehog tick, I. hexagonus, Since I. ricinus frequently feeds on hedgehogs the potential is there for the I. hexagonus/hedgehog cycle to have a considerable impact on the eco-epidemiology of the disease in some areas. Another possible example is the rodent tick I. trianguliceps, but although B. burgdorferi has been found in this tick, its ability to transmit the spirochaete and therefore maintain it in rodents, has not been determined.