Lyme disease (borreliosis)

Borrelia (Lyme disease) is a bacterial infection transmitted by ticks of the Ixodes genus, caused by the Borrelia burgdorferi spirochete. The bacterium infects humans when a tick remains attached to the skin for a sufficient time. It is estimated that around 20–25 % of ticks carry Borrelia. In Finland, Lyme disease occurs throughout the country, with the highest incidence on Åland and along the southwest coast.

Several subspecies of Borrelia burgdorferi exist; North America has only one, while Europe hosts multiple. Finnish studies have shown that both the geographical distribution and population of ticks have expanded in recent years—likely due to climate change—possibly increasing Lyme disease cases in the future.

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Symptoms and Treatment

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The incubation period typically ranges from 3 to 30 days. A common early sign is a slowly expanding skin rash called erythema migrans, which can appear in a ring shape or as a uniformly colored area. In children, a soft bluish-red bump—known as lymphocytoma—may appear, for example, on the earlobe. Sometimes, no skin reaction occurs, and symptoms are limited to mild, flu-like signs.

If the infection is not treated early with antibiotics, approximately 50 % of cases can progress to late-stage Lyme disease—manifesting in various symptoms affecting the joints, nervous system, skin, muscles, or heart. The clinical picture varies depending on the subspecies of Borrelia burgdorferi present in different regions.

In North America, joint symptoms are more common, while neurological symptoms are more prevalent in Europe. Late-stage Lyme disease can develop years after the initial tick bite. Although late-stage disease is almost always treatable with antibiotics, symptoms can persist long after successful treatment.

There is no natural immunity against Lyme disease—meaning repeat infections are possible.

Currently, there is no vaccine for Lyme disease, though research is underway. The existing "tick vaccine" protects against tick-borne encephalitis, a viral disease, but does not protect against Lyme disease nor prevent tick attachment.

Text updated: February 15, 2024