Lyme disease, or Lyme borreliosis, is an infectious disease transmitted to man through the bite of a tick infected with the bacterium Borrelia burgdorferi.

How is Lyme disease diagnosed?

Erythema migrans is the only symptom that allows to diagnose Lyme disease without doing additional analyses. It is a red circular rash on the skin which expands gradually on the site of the bite.

If there is no erythema migrans, other later symptoms (facial paralysis, inflammation of the joints, cardiac problems) are suggestive for Lyme disease, which will be verified by a blood test.

The blood test detects antibodies against the Borrelia bacteria. If they are present they indicate a present or a past conact with the bacteria. The blood test is only of use 6 weeks after the bite because:

  • early antibodies (IgM immunoglobulins) appear between 2 and 4 weeks after the infected tick bite and peak between the 6th and 8th week
  • delayed antibodies (IgG immunoglobulins) appear between 6 and 8 weeks after the infected tick bite. They decrease very slowly or persist for years.

If the results of the blood test are positive, a second confirmation test is carried out (Western Blot) to check for “false positives” results.

If the result of the second analysis is positive in the presence of symptoms, antibiotic treatment must be started.

Challenges in making the diagnosis

It can be difficult to diagnose Lyme disease because:

  • some people who develop the disease do not recall a tick bite
  • the localised reaction which may appear immediately after the tick bite can be confused with an erythema migrans
  • the symptoms may be very different and vary from one person to another
  • the blood test should not be done too early because at the beginning of the infection the antibodies are not yet detectable (risk of “false negatives”)
  • the immune system produces antibodies for months, even years, after the infection (including after antibiotic treatment) and a test could therefore be positive although the bacterium is no longer present and the disease is cured.

4 reasons for not being alarmed

  • Not every bite is infectious (in Belgium only about 10% of ticks are infected).
  • An infected tick does not necessarily transmit the bacteria.
  • An infected person does not necessarily develop the disease.
  • Lyme disease can be treated effectively with antibiotics if it is diagnosed in time.


Sciensano ensures epidemiological surveillance of Lyme disease through a monitoring network of laboratories and general practitionersand a National Reference Centre  for Borrelia burgdorferi. 

QR code

QR code for this page URL