Quebec’s public health institute says the number of cases of Lyme disease is on the rise, with more cities each year considered to be in an endemic zone.

Data from the Institut national de santé publique du Québec shows that 103 cases were recorded in the province as of June 22.

About half of those were found in the Eastern Townships region, which is by far the most affected. That region also declared 322 of the province’s 652 cases last year.

Public health doctor Geneviève Baron says the region’s high case load can be partially explained by its proximity to the United States.

She said the region didn’t record any Lyme cases until around 2011, but that some people caught the disease when travelling south of the border, where ticks could survive the winter.

She said the region is also known for high numbers of deer, which are carriers of the ticks that cause Lyme disease.

“With the milder climate, ticks have slowly established themselves in our region,” said Baron, who works at the health authority in the Eastern Townships. “There are areas that are perhaps more affected because ticks have been established there for longer, but which are also areas that are more suitable as habitat.”

The number of Quebec municipalities deemed to be in an endemic zone has exploded, going from 117 in 2023 to 461 this year. However, the public health institute says this rise can be attributed in part to changing criteria.

The institute has a map on its website that details all the towns where black-legged ticks, which carry the disease, are present and active, as well as the endemic zones for Lyme disease.

To be considered an endemic zone, a municipality has to have declared at least three locally acquired human Lyme disease cases in the past five years, or had 23 black-legged ticks found on humans who submitted them for analysis.

If all three stages of the tick (larva, nymph, adult) are spotted or at least six specimens of the same stage are collected in one year, the area is also classified as an endemic zone. A new criterion has been added, that of being 20 kilometres or less from a municipality that meets one of the criteria to be considered endemic.

“In 2024, the definition of an endemic zone for Lyme disease was enlarged to better represent the epidemiological situation of this disease in Quebec,” the institute said in an email.

Under the new definition, the Quebec City and Chaudière-Appalaches regions have been added to the list of endemic zones.

Baron believes it won’t be long before all of southern Quebec will be affected by the disease. “Slowly, with birds — because birds can carry ticks — with deer and other small animals, it’s slowly extending towards the north,” she said.

The first symptoms of Lyme disease, which often appear within a month of a tick bite, include redness that expands with time, fever and muscular and joint pain.

Baron says people can avoid tick bites by wearing insect repellent containing DEET, wearing long pants and shirts, and avoiding walking through tall vegetation.