In Europe, the sheep tick, castor bean tick, or European castor bean tick (Ixodes ricinus) is the transmitter.
In North America, the black-legged tick or deer tick (Ixodes scapularis) has been identified as the key to the disease’s spread on the east coast. Only about 20% of persons infected with Lyme disease by the deer tick are aware of having had any tick bite, making early detection difficult in the absence of a rash. 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. The lone star tick (Amblyomma americanum), which is found throughout the southeastern U.S. as far west as Texas, has been ruled out as a vector of the Lyme disease spirochete Borrelia burgdorferi, though it may be implicated with a clinical syndrome southern tick associated rash illness (STARI), which resembles the skin lesions of Lyme disease.
On the west coast, the primary vector is the western black-legged tick (Ixodes pacificus). The tendency of this tick species to feed predominantly on host species that are resistant to Borrelia infection appears to diminish transmission of Lyme disease in the West.
While Lyme spirochetes have been found in insects other than ticks, reports of actual infectious transmission appear to be rare. Sexual transmission has been anecdotally reported; 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, however prompt antibiotic treatment appears to prevent fetal harm.
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