Assessing the Risk of Lyme Disease

Lyme disease is on the rise in Ontario. Pharmacists may be experiencing an increase in the number of questions regarding ticks, and many may not feel comfortable assessing a patient’s risk for Lyme disease. The following is meant to provide some guidance when trying to identify this risk.

The probability of acquiring Lyme disease depends on the type of tick, the geographic location where the tick was acquired, and the duration of attachment. In Ontario, the tick of concern is the blacklegged deer tick, Ixodes scapularis. Nymphs (immature ticks) and adult ticks can transmit the bacterium, B. burgdorferi. Nymphs are about the size of a poppy seed, and adults are closer in size to a sesame seed. In Canada, the highest risk of acquiring Lyme disease will be in areas where ticks carrying the bacterium have been identified; endemic areas are present now in six provinces. See this updated 2019 map to identify the specific areas in Ontario where the risk is greatest. Risk areas for the rest of Canada can be found here.

The most difficult part about assessing the risk for Lyme disease may surround the timing of tick attachment. A tick should be removed quickly. Tick attachment under 24 hours will usually not cause Lyme disease, as a tick must be attached for at least 36 hours before the bacterium can be transmitted. However, even with these time periods, the bacterium is still not usually transmitted within the first 48 – 72 hours of being attached. Thus, there is an increased risk if a tick has been attached for at least 72 hours. How much the tick is engorged can be used to estimate how long the tick may have been attached. Images can be seen in the Infectious Diseases Society of America (IDSA) guideline.

When to Refer

After a tick bite, a small red bump similar to a mosquito bite is common, but this is not a sign of Lyme disease and will usually disappear within 1-2 days. The difficulty surrounds symptoms of Lyme disease, which can vary depending on the stage of infection (Table 1). If fever, facial paralysis, rash, or arthritis are present and occur after a tick bite, or after a patient has been to an area where Lyme disease has been identified, the patient should seek medical attention. Illness within several weeks after removing a tick bite should also be assessed.

It is important to be aware that 1) an erythema migrans (EM) rash may not occur in all patients with Lyme disease, and 2) if an EM does occur, it may not necessarily have the typical “bull’s-eye” appearance. Examples of how EM may present in Lyme disease can be seen here.

Table 1. Signs and symptoms of different stages of Lyme disease.*

Time Period Signs & Symptoms Comments
Early localized Lyme disease (< 30 days)
  • Headache
  • Fever
  • Myalgia
  • Arthralgia
  • Single, local skin lesion called erythema migrans (EM)


  • If the bacterium was transmitted, most patients present with EM within 7 days of a tick bite, although incubation can vary from 3-30 days.
  • Appearance of EM: greater than 5cm, slowly increases size and is not usually itchy or painful.
  • Sometimes the EM will be clear in the centre (creating a bull’s-eye), or may look like a bruise in dark-skinned patients.
  • An EM suggestive of Lyme disease can be: blistering, solid, crusted, blue-purple, or a bulls-eye appearance.
Early disseminated Lyme disease

(< 3 months)

  • Fatigue, general weakness
  • Skin reactions (e.g. multiple EMs)
  • Cardiac symptoms
  • Neurological symptoms
  • Rare symptoms (uveitis, keratitis, conjunctivitis, mild hepatitis, splenomegaly)
  • Untreated, the bacteria can travel to other sites of the body and damage tissue.


Late Lyme disease

(> 3 months)

  • Musculoskeletal (e.g. Baker’s cyst, chronic arthritis)
  • Neurological (e.g. memory, concentration issues)


  • Untreated, late Lyme disease can last months-years.
Post-treatment symptoms


  • Even when treated, some patients may continue to have symptoms months-years after.

*Information compiled from: Government of Canada. For health professionals: Lyme disease. 30 May 2017. Accessed 27 June 2019 from

General prevention includes using an insect repellant with 10-20% picaridin for those patients > 6 months of age for up to seven hours of protection; or 10% DEET for ages 6 months – 12 years for up to four hours of protection; or 20-30% DEET for patients > 12 years old for four to eight hours of protection. Patients should also wear a long-sleeved shirt, pants, socks and boots all tucked in when in risk areas. Skin should also be checked after possible exposure and patients should shower within two hours. For information on tick removal, see these CDC instructions.

Overall, if Lyme disease is suspected, timely treatment with antibiotics in the early stages can enable quick recovery. Therefore, after a tick bite, patients should be monitored for symptoms of Lyme disease up to 30 days after being bitten.

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