Cutaneous borreliosis

1 - DEFINITION, BACTERIOLOGY, VECTORS, EPIDEMIOLOGY

Lyme disease is the most common tick-borne disease in the United States and Europe [1]. It is caused by several species of spirochetes belonging to the Borrelia burgdorferi sensu lato complex. In the United States, the disease is almost exclusively caused by Borrelia burgdorferi sensu stricto (B. burgdorferi sensu stricto), the first Borrelia species that was discovered. In Europe three species are predominant: B. burgdorferi, B. afzelii and B. garinii. In Asia, only the last two are regularly observed.

Cases of Lyme Borreliosis caused by B. bavariensis, B. bissettii, B. kurtenbachii, B. lusitaniae, B. spielmanii and B. valaisiana have also been described, but only anecdotally [1-5].

Additionally, in the United States at least two other species of Borrelia (B. andersonii and B. americana) seem able to cause erythema migrans, but their pathogenicity is only partially known [6, 7].

These bacteria have only very few proteins ensuring biosynthetic activities; they therefore cannot survive in the outside environment [1].

The natural reservoir of Borrelia sensu lato is very vast as it includes many birds (B. garinii), small mammals (mice, voles, field mice ...), and ticks. In contrast, deer and cattle are probably not reservoirs for these spirochetes [1, 5].

Their main vectors are hard ticks belonging to the Ixodes genus, which transmit the bacteria during a blood meal at the nymph stage, or more rarely at the adult stage [8]. A period of at least 36 hours seems necessary for a tick embedded in the skin to inoculate Borrelia to humans [9, 10].

Four species of ticks are involved in the majority of transmissions to humans: Ixodes ricinus in Western and Central Europe, Ixodes persulcatus in Eastern Europe and Asia, Ixodes scapularis in the Northern and Eastern regions of the United States and Ixodes pacificus on the West Coast of the United States[1]. 

Rare transmissions by other Ixodes species have been reported exceptionally. For example in Australia, Ixodes holocyclus has recently been identified as another vector for B. burgdorferi [11].

All age groups are affected by this disease. It is present on all continents, but predominates in forests and the humid regions of the northern hemisphere, and in areas where people practice outdoor activities with a risk of getting bitten. In Europe the most affected countries are Austria, Slovenia, Sweden and the Netherlands [12]. Regarding France, a recent study estimated the incidence of this disease at 42 cases per 100 000 inhabitants with significant regional disparities (from 0 to 184 cases per 100 000 inhabitants) [12].

Given the current climate changes, the increasing number of tourist trips and population migrations, the disease has spread to new areas such as Canada [13], Australia [11, 14], Mexico [15, 16], or Egypt [17].

Transmission is most often due to nymph rather than adult ticks. The nymphs of I. ricinus, I. pacificus and I. persulcatus are very active from early spring to mid-summer, reflecting the peak incidence of the disease at that time of the year. The nymphs of I. scapularis are mostly active from early summer to early fall [1, 12, 18].