Geographic distribution

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.

Figure1: Distribution of Ixodes ricinus and Ixodes persulcatus in Eurasia and Northern Africa.

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.

Figure2: U.S. Lyme disease risk map with four categories of risk. Black = High risk, white = Minimal or no risk.

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.

Figure3: Map of established (at least 6 ticks or 2 life stages - larvae, nymphs, adults - were identified in the county) and reported (at least 1 tick was identified in the county) distribution of the Lyme disease vectors Ixodes scapularis and Ixodes pacificus, by county, United States, 1907-1996. Red = Established Ixodes scapularis; Blue= Reported Ixodes scapularis; Green = Established Ixodes pacificus;Yellow = Reported Ixodes pacificus.

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 send 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.

Seasonal distribution

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.

Figure4: Histogram comparing the seasonal distribution of dates (months) of clinical onset of human Lyme disease (erythema migrans: EM) and seasonal activity of the Deer tick nymphs, the primary vector in North America.