OTTAWA -- Lyme disease has settled so deeply into parts of Canada many public health units now just assume if you get bitten by a tick, you should be treated for Lyme disease.

In Ottawa, where more than two-thirds of the ticks tested in some neighbourhoods carry the bacteria that causes Lyme disease, the public-health unit no longer bothers to test ticks at all.

Dr. Vera Etches, the unit's top doctor, said that in 2016 and 2017 more than one-fifth of black-legged ticks tested in Ottawa came back positive for Lyme.

"That's a threshold," she said. "Once you know that more than 20 per cent of the ticks in your area carry Lyme disease bacteria then we don't need to check in on that. That is what we now call an 'at-risk area.'"

That means if a tick is found on a person, and is believed to have been there for more than 24 hours, then the patient should get antibiotics to prevent Lyme infection, even without any testing of the tick. It takes 24 hours before bacteria in the tick's gut move to its salivary glands and are transferred to a person.

After three days, the preventive treatment won't work so patients then wait for symptoms or enough time for antibodies to evolve to show up on a test. It can take more than a month before symptoms appear. They're mostly similar to the effects of influenza, including fever and aches, as well as -- usually but not always -- a rash. It typically takes just about as long for the immune system's antibodies to show up on a lab test.

If left untreated, Lyme disease can cause serious illnesses such as meningitis, but Etches is quick to point out that because it is caused by a bacteria, it's treatable with drugs.

"It's a good-news story, actually, that there is antibiotics that work to treat Lyme disease," she said.

Most public-health offices in Canada used to test ticks submitted by the public, as well as conducting their own surveillance by actively seeking out tick populations and testing them. Some, including Ottawa's, have decided now that Lyme is endemic, they should shift to public education and prevention as well as treatment.

Lyme disease was named after the town of Lyme, Conn., where the first case was diagnosed in 1975. It is caused by bacteria that are traded back and forth among black-legged ticks and migratory birds and small mammals like mice and chipmunks. Ticks bite birds and small mammals infected with the bacteria and get infected and then spread the disease when they bite their next victims.

Before 10 years ago, most of the cases diagnosed in Canada were in people bitten by ticks while travelling in the United States. But climate change has led to southern Canada seeing milder winters, which means the ticks that migrate to Canada on the backs of migratory birds are now surviving the winter in larger numbers, spreading the Lyme-causing bacteria more rapidly.

Canada started keeping track of Lyme disease cases in 2009, when 144 cases were confirmed or considered probable. Only 79 of those cases were believed to have been contracted in Canada.

In 2017, more than 1,400 cases were confirmed or probable across the country, more than two-thirds of them in Ontario and most of them believed to have been contracted locally.

National statistics for 2018 are not yet available but in Ontario, the number actually fell significantly, from 967 in 2017 to 612 in 2018. Etches said that was because 2018 was hotter and drier than 2017, and ticks thrive in wet, cool weather.

A 2014 study by the National Collaborating Centre for Infectious Diseases at the University of Manitoba suggested the Lyme-carrying ticks are expanding their territory by about 46 km a year, an expectation being borne out in health units' mapping

In 2017 and 2018, Point Pelee National Park near Windsor, Ont., was considered to be an at-risk region but the rest of Windsor-Essex County in Ontario's southernmost tip was not. In 2019, almost all of the county has been added as an at-risk area.

In 2017 all of Nova Scotia was declared to be at risk for Lyme Disease.

In New Brunswick, six of 15 counties were declared at-risk as of 2018.

There are also at-risk areas for Lyme in southern Manitoba, northwestern Ontario, British Columbia and Quebec. While some cases of Lyme have been found in the other four provinces, the numbers are very low and mostly contracted elsewhere.