Lyme Disease, the clinical manifestation of Lyme borreliosis,
is a chronic, debilitating disease of humans,
and presently infects more humans than any other
tick - borne infection. Lyme borreliosis is caused by the
spirochaete Borrelia burgdorferi and is transmitted by hard ticks in the genus Ixodes . The important vector species vary geographically: I. scapularis and I. pacificus are the important species in the eastern and western regions of North America, respectively, whereas I. ricinus and I. persulcatus are the important species in Europe and Asia, respectively. Lyme disease now occurs in most temperate areas of the Holarctic region, and also from Australia and occasionally from South America and Africa. Lyme borreliosis has become increasingly important to humans in recent years due to increased contact between ticks borne on wildlife, and humans. Due to its increased prevalence, it is often designated an emerging pathogen.
The host range of Lyme borreliosis is impressive, with over 50 species of wild mammals known to support the pathogen. The important mammalian reservoirs in North America are eastern cottontail, Sylvilagus fl oridanus ; wood rats, Neotoma spp.; kangaroo rats, Dipodomys californicus ; white - footed mice, Permyscus leucopus ; eastern chipmunks, Tamias striatus ; black rat, Rattus rattus ; and Norway rat, Rattus norvegicus . In Europe, the important mammalian reservoirs include mountain hare, Lepus timidus ; squirrels, Sciurus spp.; wood mice, Apodemes sylvaticus ; yellow - necked mice, Apodemes fl avicollis ; bank voles, Clethrionomys glareolus ; and Eurasian hedgehog, Erinaceus europaeus . Also, a number of birds support the bacterium, including ring - necked pheasant, Phasianus colchicus ; wild turkey, Meleagris gallopavo ; Atlantic puffi n, Fratercula arctica ; house wren, Troglodytes aedon ; robin, Erithacus rubcula ; yellowthroat, Geothlypis trichas ; song sparrow, Melospiza melodia ; house sparrow, Passer domesticus ; and orchard oriole, Icterus spurious . Migratory birds are important in spreading the disease and ticks. Adult ticks feed on larger animals, and various deer are favored. Although the deer are not good hosts for the bacteria, they are important because they are the principal source of nutrition for ticks that are already infected with B. burgdorferi . Often overlooked is the potential role of lizards and other reptiles to support ticks. However, research in Hungary showed that lizards were common hosts for Ixodes ricinus . Further, Borrelia burgdorferi was found in several species of lizards. The Lyme disease cycle is shown diagrammatically in Fig. 8.2 .
Vectors of this disease are said to be three - host ticks because they take three blood meals, each from a separate vertebrate host. They feed as a larva, again as a nymph, and then as an adult. Fully fed (replete) female ticks detach from their fi nal host to oviposit, and larvae hatch from the eggs. Typically, larvae ingest the bacteria during their fi rst blood meal but are not capable of transmitting disease, whereas nymphs and adults developing from these infected larvae can infect new hosts. Nymphs and larvae usually feed on the same hosts; thus, nymphs are important in continuing the cycle because they inoculate vertebrate hosts that are fed upon by uninfected larval ticks. Rodents and some birds are persistently infected and fed upon by a high proportion of the immature tick population. In contrast, lizards and deer do not become persistently infected so do not act as such important reservoirs of the disease.
The primary site of infection is typically the skin, and a small infl ammatory response at the site is normal. The spirochaetes soon spread to the lymph, blood, heart, bone and other organs. Signs of infection generally are absent from naturally infected wildlife, and most wildlife seem to tolerate the infection, though some mice display neurological disturbances. In general, there is no impact of B. bergdorferi on wildlife populations. In contrast, skin rash, arthritis and cardiac disease are prevalent in humans following infection.
The key to protecting humans from Lyme disease is personal protection. Hikers should seek to avoid tick infested habitats, wear light clothing to enable detection of ticks, wear long pants and sleeves to minimize skin exposure, tuck pants into boots to keep ticks away from skin, use insect repellents, and search for and remove ticks regularly. Tick control is of limited value. Habitat modifi cation is useful if it decreases contact of humans with wildlife, and the ticks borne by wildlife.
