Lyme disease is caused by Gram-negative spirochetal bacteria from the genus ''Borrelia''. At least 11 ''Borrelia'' species have been discovered, 3 of which are known to be Lyme-related. The ''Borrelia'' species that cause Lyme disease are collectively known as ''Borrelia burgdorferi sensu lato'', and show a great deal of genetic diversity.
The group ''Borrelia burgdorferi sensu lato'' is made up of three closely related species that are probably responsible for the large majority of cases: ''B. burgdorferi sensu stricto'' (predominant in North America, but also present in Europe), ''B. afzelii'', and ''B. garinii'' (both predominant in Eurasia). Some studies have also proposed that ''B. bissettii'' and ''B. valaisiana'' may sometimes infect humans, but these species do not seem to be important causes of disease.
Transmission
Lyme disease is classified as a zoonosis, as it is transmitted to humans from a natural reservoir among rodents by ticks that feed on both sets of hosts. Hard-bodied ticks of the genus ''Ixodes'' are the main vectors of Lyme disease. Most infections are caused by ticks in the nymphal stage, as they are very small and may feed for long periods of time undetected. Larval ticks are very rarely infected. Tick bites often go unnoticed because of the small size of the tick in its nymphal stage, as well as tick secretions that prevent the host from feeling any itch or pain from the bite. However, transmission is quite rare, with only about 1% of recognized tick bites resulting in Lyme disease; this may be due to the fact that an infected tick must be attached for at least a day for transmission to occur.In Europe the vector is ''Ixodes ricinus'', which is also called the sheep tick or castor bean tick. In China ''Ixodes persulcatus'' (the taiga tick) is probably the most important vector. In North America, the black-legged tick or deer tick (''Ixodes scapularis'') is the main vector on the east coast. The lone star tick (''Amblyomma americanum''), which is found throughout the Southeastern United States as far west as Texas, is unlikely to transmit the Lyme disease spirochete ''Borrelia burgdorferi'', though it may be implicated in a related syndrome called southern tick-associated rash illness, which resembles a mild form of Lyme disease. On the West Coast of the United States, the main vector is the western black-legged tick (''Ixodes pacificus''). The tendency of this tick species to feed predominantly on host species such as lizards that are resistant to ''Borrelia'' infection appears to diminish transmission of Lyme disease in the West.
While Lyme spirochetes have been found in insects as well as ticks, reports of actual infectious transmission appear to be rare. Lyme spirochetes have been found in semen and breast milk, however transmission of the spirochete by these routes is not known to occur. Congenital transmission of Lyme disease can occur from an infected mother to fetus through the placenta during pregnancy. The risk for fetal harm is much higher in the first three months of pregnancy than later. Prompt antibiotic treatment almost always prevents fetal harm. Pregnant Lyme-disease patients cannot be treated with the first-choice antibiotic, doxycycline (see below), as it is potentially harmful for the fetus. Instead, erythromycin is usually given; it is less effective against the disease but harmless for the fetus.
Tick-borne co-infections
Ticks that transmit ''B. burgdorferi'' to humans can also carry and transmit several other parasites such as ''Theileria microti'' and ''Anaplasma phagocytophilum'', which cause the diseases babesiosis and human granulocytic anaplasmosis (HGA), respectively. Among early Lyme disease patients, depending on their location, 2–12% will also have HGA and 2–40% will have babesiosis. Ticks in certain regions, including the landscapes along the Eastern Baltic Sea, also transmit ''tick-borne encephalitis''.Co-infections complicate Lyme symptoms, especially diagnosis and treatment. It is possible for a tick to carry and transmit one of the co-infections and not ''Borrelia'', making diagnosis difficult and often elusive. The Centers for Disease Control studied 100 ticks in rural New Jersey and found that 55% of the ticks were infected with at least one of the pathogens.
Adapted from the Wikipedia article Lyme disease, under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki
