The pathogens of Lyme borreliosis are clearly distributed in the northern hemisphere. B. burgdorferi s.s. is spread over the USA and Europe up to Asia, whereas B. afzelii, B. garinii and supposedly also B. lusitaniae are endemic in the Eurasian area.
Lyme borreliosis is found in Germany,Switzerland, Czech Republic, Slovakia, Austria, France, Portugal, United Kingdom, Ireland, Scandinavia and many regions in Eastern Europe including Russia. It has also been confirmed in Asia (China, Japan, Korea) while it is unlikely and still questionable whether the disease is present in Australia, where ticks of the Ixodes persulcatus complex are not present. In Eurasia and northern Africa, the principal vectors are I. ricinus and I. persulcatus.
Infection rates of I. ricinus are relatively high (up to 36 %) in some European continental populations, but significantly less than in I. scapularis in the U.S. In Britain and Ireland the infection rates are much lower.
In the United States, Lyme borreliosis is mostly localized to states in the North-eastern, mid-Atlantic, and upper North-central regions, and to several counties in North-western California.
The trend of increasing incidence in some established endemic areas continues, as well as geographic spread of B. burgdorferi to new areas. The distribution of the principal vectors of B. burgdorferi s.s. in the United States (based on a comprehensive review of the literature in the United States from 1907 through 1996) mirrors the areas of high reported incidence of Lyme disease and identifies areas of potential disease emergence. The principal vectors are the Deer tick, I. scapularis, in the North-east and Upper-midwest states and I. pacificus along the West Coast of the United States. Although the Deer tick is widely distributed in the southern USA, it is not an established vector of Lyme disease to humans. One reason might be that lizards are the natural hosts for I. scapularis in that region and lizards have borreliocidal components in their blood that eliminate the bacteria.
One possible explanation for the rise of Lyme borreliosis is that populations of deer, raccoon, opossum, birds, and other wildlife have rapidly expanded in the last few years due to reduced hunting activities, and proximity of domestic animals and humans to these hosts has increased as suburban development has encroached on woodlands and people spend more time outside due to their recreational activities. Migratory birds may partially account for the spread of this disease since larval and nymphal black-legged ticks often feed on ground-feeding birds.
The risk of Lyme borreliosis infection depends on the seasonal activity of the transmitting ticks in the respective region. While I. ricinus is active from spring to fall with less intensity during hot and dry summer months, the peak months of the Deer tick are June and July.
Natural reservoirs for Borrelia burgdorferi s.l.
Ticks, small rodents, and other non-human vertebrate animals all serve as natural reservoirs for B. burgdorferi s.l.: the bacteria can live and grow within these hosts without causing a fatal disease. Larvae and nymph ticks typically become infected with B. burgdorferi s.l. when they feed on small animals that carry the bacteria in spring and summer. The pathogens remain in the tick during the developmental changes from larva to nymph or from nymph to adult in late summer or early fall. Infected nymphs transmit B. burgdorferi bacteria to other small rodents and other mammals through biting and sucking blood, all in the course of their normal feeding behaviour.
Borrelia spp. multiply in mice, though mice do not get diseased nor show severe clinical signs. In Europe, the yellow-necked mouse (Apodemus flavicoliis), the wood mouse (A. sylvaticus) and the bank vole (Clethrionomys glareolus), to name the most common once, function as reservoirs. In the USA, the white-footed mouse (Peromyscus leucopus) is the primary animal reservoir for B. burgdorferi and the preferred host for larval and nymphal forms of I. scapularis, the Deer tick. Deer are the preferred host for the adult ticks in the USA; in Europe it is the roe dear (Capreolus capreolus). Other mammals including humans and dogs can be incidental hosts and can develop Lyme disease.
There seems to be a host specifity for the different Borrelia species: B. burgdorferi s.s. and B. afzelii e.g. usually prefer rodents as reservoir, whereas B. garinii and B. valaisiana mainly occur in birds. Furthermore, the tick hosts seem to be differently competent as Borrelia reservoir. In case of a high prevalence of non-competent hosts in an area, the general risk of infection is massively reduced.