There is no evidence to suggest that leeches transmit any disease. In some cases, there may be irritation and itching after a bite, and rarely, the wound may get infected. There is absolutely no cause for you to worry following a leech bite. And the scar that you have mentioned will disappear in a short time. In fact, even till late 19th century, using leeches to treat illness was a very common practice in medicine (though without any physiologic basis). Today, leeches are used by plastic surgeons to control venous congestion at operative sites and the anticoagulant found in them is used to treat black eyes and ear inflammation.