Causes of Lyme disease

Learn about what causes Lyme disease, how it is spread and where it is found.

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What causes Lyme disease?

Lyme disease is caused by the bacteria called Borrelia burgdorferi.


This figure shows the size and colour of female blacklegged ticks in various stages of feeding.

How is Lyme disease spread?

Lyme disease is an infectious disease spread through the bite of infected ticks. There are two types of ticks that can spread Lyme disease in Canada:

  1. blacklegged tick (or deer tick, Ixodes scapularis) in southeastern and south-central regions of Canada
  2. western blacklegged tick (Ixodes pacificus) in British Columbia

Ticks need blood to survive, so they attach on animals and humans to feed. Ticks become infected with Lyme disease bacteria by feeding on infected wild animals, such as:

  • birds
  • rodents

Once infected, ticks can spread the bacteria to humans and pets, particularly dogs. In most cases, the infected tick must attach and feed for at least 24 hours before the bacteria can be transmitted.

Most humans are infected through the bite of immature ticks that are approximately the size of a poppy seed, called nymphs. Adult ticks, approximately the size of a sesame seed, can also spread Lyme disease. Ticks are very small and their bites are usually painless, so you may not know that you have been bitten.

Blacklegged ticks are most often found in wooded or forested areas, in leaf litter or on shrubs and tall grass in some parts of Canada. Blacklegged ticks in at risk areas of southeastern and south-central Canada are more commonly infected.

Can Lyme disease spread through human contact?

There is no evidence that Lyme disease can spread from person-to-person. A person cannot get infected from touching, kissing, having sex or other forms of casual human contact.

Can pets spread Lyme disease to humans?

Pets can carry unattached and potentially infected ticks into your home and your yard which could increase your chance of being bitten by a tick. You cannot be infected directly by your pet.

Can Lyme disease spread by blood transfusion?

There are no cases of Lyme disease linked to blood transfusion. Ask your local blood donor clinic about other possible restrictions to donating blood if you have been infected with tick-associated infections including Lyme disease.

Can Lyme disease spread to my unborn or breastfeeding child?

If you are pregnant and have been bitten by a tick, contact your doctor immediately. Untreated Lyme disease during pregnancy may cause complications. There are no reports of Lyme disease transmission from breast milk.

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