Hard-bodied ticks of the genus Ixodes are the primary vectors of Lyme disease. The majority of infections are caused by ticks in the nymph stage, since adult ticks are more easily detected and removed as a consequence of their relatively large size. Transmission is relatively rare, with only about 1% of recognized tick bites resulting in Lyme disease: this may be due to the fact that an infected tick has to be attached for at least a day for transmission to occur.
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 people who become infected with Lyme disease by the deer tick can remember having been bitten,[34] 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 United States as far west as Texas, is unlikely to transmit the Lyme disease spirochete Borrelia burgdorferi,[35] 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, 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.
It has also been heard that the original carrier of the disease is the white-footed deer mouse. Ticks are thought to receive it in early stages of life, when the tick feeds on mice.[citation needed]
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.