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Deer Ticks (Ixodes scapularis), also called blacklegged ticks, are one of four major species that commonly bite people in Canada.

Disease-carrying pests and toxic weeds are on the rise because of climate change and urbanization. As Wency Leung reports, it's time to get to know them

Rodents. Blood suckers. Toxic plants.

You don't need to venture far this summer to encounter potentially harmful fauna and flora. They'll come to you.

Thanks to changes in the environment, such as urbanization and climate change, pests and invasive plants are multiplying and encroaching into new territory, bringing with them a variety of health risks.

The proliferation of blacklegged ticks, a relatively new species in Canada, for instance, has contributed to a surge in cases of Lyme disease, which, if left untreated, can lead to persistent symptoms, including severe headaches, neurological and cardiac problems.

According to federal government data, 987 cases of Lyme disease were reported last year, up from 144 cases in 2009.

Dr. Jianghong Wu at York University has been mapping the expansion of suitable habitats for blacklegged ticks in Canada and he notes these areas are expanding northward. Much of Ontario, Quebec, Nova Scotia and parts of Manitoba are favourable habitats, says Wu, the Canada Research Chair in Industrial and Applied Mathematics.

In Ontario, "it's not a matter of spreading now," he says. Rather, blacklegged ticks seem to be establishing themselves in certain areas, including Toronto, meaning they are thriving and their populations are increasing in density, thus raising your risk of encountering and getting bitten by one.

Meanwhile, local authorities in British Columbia, Ontario and Quebec warn of the potential dangers of giant hogweed, an invasive species that can lead to severe skin burns. While the noxious plant is mostly found in Ontario and in B.C.'s Lower Mainland, it appears to be spreading in Quebec and there have been a few isolated reports of it in the Maritimes, says Dr. François Tardif, a professor of plant agriculture at the University of Guelph.

You don't even have to leave home to have an unwanted encounter. Media reports suggest rat populations in Vancouver and Toronto are exploding. Earlier this year, Ontario's Ministry of Health and Long-Term Care reported three human cases of Seoul virus infection, a type of hantavirus, linked to prolonged contact with rats. Although no serious health problems were reported, the ministry noted that in rare cases, infection can lead to a type of severe kidney disease. It urged people to educate themselves about rat-borne diseases and to take precautions if they come in contact with rats and contaminated materials.

Here, we zero in on four of the noxious encroachers you're most likely to meet this summer and how best to defend yourself against them:


Rats can carry a wide variety of zoonotic diseases, or diseases that can be transmitted from animals to humans, including viruses, bacteria and parasites.

Why they're a threat: Where there are people, there are rats. So with increased urbanization and cities becoming denser, rats are becoming a growing concern, says Dr. Chelsea Himsworth, assistant professor of the school of population and public health at the University of British Columbia. "When there's more people, there's more resources, and then there's more rats."

In Canada, the most prevalent species are Norway rats and black rats, which are invasive species that actually originate from Asia, Himsworth says. (Norway rat is a misnomer.) But today, rats are found all over the world. Norway rats are a particularly robust and aggressive species, so they tend to displace black rats in territories where the two compete.

Rats can carry a wide variety of zoonotic diseases, or diseases that can be transmitted from animals to humans, including viruses, bacteria and parasites. Leptospirosis, for instance, is a known rat-borne bacterial disease that affects people who come in contact with rat urine.

But a recent finding from Himsworth's research shows rats can also carry human pathogens. So not only do they have their own rat-specific microbes, she says, "they act as this incredible sponge and they'll actually soak up the human-origin microbes … and potentially propagate them and transmit them back."

She and her team found that rats in Vancouver carried human strains of the antibiotic-resistant staph bacteria MRSA and the diarrhea-causing Clostridium difficile.

In addition, the mental toll of experiencing a rat infestation is often neglected, Himsworth says. Living with rats can lead to anxiety and feelings of helplessness. People living in impoverished inner-city communities are particularly vulnerable as they're often forced to live with infestations, she says.

How they spread disease: There are a number of ways in which rats transmit diseases to humans. The most obvious is by getting bitten by a rat. (The aptly named rat-bite fever, characterized by fever, vomiting and joint-and-muscle pain, is transmitted in this manner.)

"But what most people don't know is that you don't have to have direct contact at all," Himsworth says. "In fact, you don't even have to have seen a rat to potentially be exposed."

The majority of rat-borne diseases, she says, are transmitted through rat urine, feces and fleas.

Seoul virus, for instance, is spread not only by rat bites, but through exposure to urine, feces, saliva and contaminated bedding, according to the Ontario Ministry of Health and Long-Term Care.

The majority of rat-borne diseases, Dr. Chelsea Himsworth says, are transmitted through rat urine, feces and fleas.

Why they're tricky to tackle: Since rats may infect people with human diseases, it's likely impossible to know whether an infected person has caught his or her illness from a rat or another human, Himsworth says.

Moreover, physicians may misdiagnose patients with rat-borne illnesses if they're not aware of them or lack the tests to identify them.

Further complicating matters, the diseases that rats carry differ according to their geographical area and they can vary not only by city, but by city block, Himsworth says. That's because rats form tightly-knit family groups called colonies, which act almost as self-quarantining units. They have a small home range, usually less than a city block. And there's little to no contact between colonies, she says.

"That's interesting from a public health point of view because it suggests counter-intuitively that your risk of getting disease from a rat is not proportional to the number of rats you're exposed to," she says.

In other words, you could stand in the middle of a colony of thousands of rats and not get sick. Conversely, you could happen to come into contact with urine from a single infected rat and become horribly ill.

Why you should leave it to the experts: If you have rats in your home, your instinct may be to just kill the critters. But hear Himsworth out before you start laying out traps.

Diseases are transmitted amongst rats through specific social interactions, she says, adding that rat groups are hierarchical. If you eliminate individual rats within a family group, it disturbs its family dynamics.

"Now they have to reorganize themselves," Himsworth says, noting that a frenzied scramble occurs. Rats that wouldn't normally interact start making contact with each other as they try to regroup. "And every one of those contacts could be a disease-transmission event."

By the time the colony reorganizes itself a few weeks later, more of its members are likely infected. Plus, since rats are notoriously prolific, producing six to 10 pups a litter several times a year, that colony will be back to its original size in no time.

What you can do to protect yourself: One of the few things individuals can do is prevent infestations in their own living spaces, Himsworth says. That means ensuring your personal environment doesn't attract rats by making sure food sources such as garbage, bird seed and compost are well managed, and sealing off entryways by repairing cracks and crumbling foundations.

If you do have an infestation, call a licensed pest-control professional who can assess your home to determine why you have an infestation and tackle your rat problem accordingly, she says.

The trouble is this tactic only covers a small amount of space in which you come in contact on a daily basis, Himsworth says.

To truly reduce the harm these pests can create, she says, cities need to have organized, systematic and scientifically based rat-control programs. These include measures such as collecting data on rat infestations, which could help identify problem areas and lead to ways of improving sanitation, as well as implementing rat-specific bylaws, such as requiring property owners to deal with infestation.


Culex mosquitoes, pictured, are the principal vectors of West Nile virus, which is arguably the most well-known mosquito-transmitted disease in the country.

Why they're a threat: The majority of the roughly 82 species of mosquito found in Canada are harmless to humans. Not all species bite humans and only female adults feed on blood so they can develop eggs. In fact, many mosquitoes are important pollinators since they feed on plant nectar.

But, of course, mosquitoes are not only a nuisance when they do bite us, a variety of species can also transmit diseases, says Dr. Manisha Kulkarni, assistant professor in the school of epidemiology and public health at the University of Ottawa.

"Mosquito-borne diseases are on the rise globally. And with the changing climate, different species may be arriving further north, so it's good practice to take personal precautions and use repellents and try to avoid mosquito bites as much as possible," she says.

Fewer than 10 species in Canada pose the greatest risks. These include members of the genus Culex and Aedes, as well as certain species of genus Culiseta and Anopheles, Kulkarni says.

Culex mosquitoes are the principal vectors of West Nile virus, which is arguably the most well-known mosquito-transmitted disease in the country. In Eastern Canada, these are the species Culex pipiens and Culex restuans, and in Western Canada, Culex tarsalis.

The first human case of West Nile virus in Canada was reported in Ontario in 2002. Since then, the number of cases in the country has fluctuated from year to year, up to 2,215 cases in 2007 to a low of five cases in 2010. While 104 cases were reported last year, according to federal government records, none have yet been reported this year.

Most people infected with West Nile virus are asymptomatic, so they won't even know they have it, Kulkarni says. Around 20 per cent of them will experience febrile illness, in other words, a mild fever and other symptoms, such as body aches, that eventually go away. But fewer than 1 per cent of infected individuals can go on to experience severe neurological disease, such as encephalitis, or inflammation of the brain and meningitis, which is inflammation of the protective membranes of the brain and spinal cord. There's no specific treatment for West Nile virus.

Since the types of mosquitoes that carry West Nile virus tend to thrive in urban areas, that's where human cases of the virus often show up. Some of the other mosquito-born diseases are transmitted between mosquitoes and animals in rural or wooded areas, where humans are unlikely to get involved in those transmission cycles, Kulkarni says. "Only rarely would they spill over to people," she says.

Other mosquito-borne viruses in Canada that can make people ill include eastern equine encephalitis, western equine encephalitis, California serogroup viruses, which include Jamestown Canyon and snowshoe hare viruses, St. Louis encephalitis and Cache Valley virus.

Although the mosquito-borne Zika virus has captured headlines in recent years, the Canadian government notes the mosquitoes that transmit the virus, Aedes aegypti and Aedes albopictus, are not established in Canada because of its colder climate. However, University of Notre Dame researchers reported finding Aedes aegypti mosquitoes as far north as Washington, in a study published in 2016.

According to the U.S. Centers for Disease Control (CDC), areas where there's a risk of Zika include Mexico, Central America, the Caribbean, most of South America, much of Africa, South Asia, Southeast Asia and the Pacific Islands.

Cases of Zika have also been reported in Florida – though the CDC no longer has recommendations to avoid travelling there – as well as one mosquito-borne case reported in Texas late last year.

A warmer winter has allowed more adult female Culex mosquitoes to survive and a rainy spring has created standing water for them to lay eggs, says Jessica Fang, an aquatic biologist for the Toronto and Region Conservation Authority.

Where you'll find them: Because of climate change, the areas that mosquitoes inhabit will likely expand, Kulkarni says. But their populations fluctuate dynamically from season to season. Warmer temperatures earlier in the year, for example, increase the likelihood of more mosquitoes.

And across the Toronto area, a mild winter and wet spring have created prime conditions for mosquitoes. Jessica Fang, an aquatic biologist for the Toronto and Region Conservation Authority, says she and her team have collected significantly more mosquitoes through their West Nile virus mosquito-monitoring program so far this year than in previous years.

A warmer winter has allowed more adult female Culex mosquitoes to survive and a rainy spring has created standing water for them to lay eggs, Fang says. Moreover, she says, certain species of Aedes mosquitoes lay eggs, which can be viable for up to 15 years, in moist soil, so they need floodwater to hatch. Eggs that did not have enough moisture to hatch last year have the opportunity to hatch this year, Fang says.

"Just so many factors compounded to [create] a really bad mosquito season this year," she says.

The chances of infection, including the amount of time it takes for a virus to develop within a mosquito and the biting rates of mosquitos, are highly dependent on seasonal temperatures, Kulkarni says.

"So many aspects of mosquito biology are driven by temperature," she says.

In a single year, there are usually a few generations of Culex mosquitoes. Culex mosquitoes diapause, or become dormant, over the winter as adults and lay eggs after the snow melts. They develop into larva and pupae stages in the spring and the first generation of adults are typically seen in June and July. During summer and into the fall, depending on how warm it stays, this cycle repeats itself two or three times.

Thus, their populations increase over the summer as the weather gets warmer and the burden of virus within the population also builds up, Kulkarni says.

"That's why we see the most infected mosquitoes late in the summer, and that's when we see more human cases as well," she says.

Environments in which mosquitoes thrive is specific to each species. But Culex mosquitoes favour stagnant water, which is why they're often found in urban areas, in eavestroughs and storm drains, Kulkarni says.

How to protect yourself: Mosquitoes tend to appear in greater numbers in the late afternoon and early evening, so if you can hide out indoors during that time of day, you're less likely to be bitten.

And the more you cover up with clothing, the better off you are, Kulkarni says, though she acknowledges, "It's not always realistic in the Canadian summer, when people don't want to be covering up."

So your best defence against mosquitoes if you're planning to enjoy your summer evenings outdoors, without wearing fall and winter gear? Use repellent with DEET or picaridin, both of which are recommended and proven effective, Kulkarni says. (Plus, they also protect you from ticks. See below.)


Blacklegged ticks, also called deer ticks, are getting plenty of attention these days, since they transmit the Borrelia burgdorferi bacterium that causes the much-dreaded Lyme disease.

Why they're a threat: There are 40 species of ticks in Canada, but most are associated with their preferred hosts. Groundhog ticks are usually found on groundhogs. Rabbit ticks are most likely found on rabbits.

"They're not all attracted to humans," says Dr. Kateryn Rochon, assistant professor of entomology at the University of Manitoba.

The ones that do feed on humans tend to be "generalists," attracted to a variety of hosts and, in Canada, there are only four major species that commonly bite people, Rochon says: the blacklegged tick, also called deer ticks ( Ixodes scapularis); the American dog tick, also called wood ticks; the Rocky Mountain wood tick, which is found in Saskatchewan and Western Canada; and in the Pacific, the western blacklegged tick (Ixodes pacificus).

Blacklegged ticks are getting plenty of attention these days, since they transmit the Borrelia burgdorferi bacterium that causes the much-dreaded Lyme disease. (Western blacklegged ticks can transmit Lyme disease, too, but they don't get infected much, Rochon says.)

In Manitoba, the first reported blacklegged tick was recorded in 1989, according to Rochon. They're believed to have been introduced to Canada by migrating birds. Millions of blacklegged ticks are believed to be dropped into the country every year by birds, Rochon says, but the reasons why they've become established here are multiple and complicated.

One of the best hypotheses, she says, is that reforestation efforts over the past several decades have created habitats for deer, mice and other animals that are hosts for ticks. Climate change may also mean hosts are moving into new areas. As well, she says, milder and shorter winters may allow ticks to survive once they've been dropped by migratory birds, whereas in previous decades, the climate may have been too cold for them to survive.

How they spread disease: In addition to Lyme disease-causing Borrelia burgdorferi, blacklegged ticks are also able to transmit Anaplasma phagocytophilum, a bacterium that causes granulocytic anaplasmosis, which can be fatal in rare cases; the blood parasite Babesia microti; the bacterium Borrelia miyamotoi and Powassan virus. Individuals who are infected with these pathogens can experience a range of symptoms, from no symptoms at all, to flu-like symptoms, to long-term neurological problems.

Rocky Mountain wood ticks and American dog ticks do not transmit these illnesses, but both are able to transmit Rocky Mountain spotted fever, typically characterized by a sudden fever and headache and, rarely, tularemia.

While Borrelia burgdorferi, Anaplasma phagocytophilum, Babesia microti and Borrelia miyamotoi are the four pathogens you're more likely get from tick bites, Rochon says, some of these are prevalent in only 1 per cent to 5 per cent of ticks. "It's not like all these ticks are infested and you'll be sick for sure," she says.

Ticks can attach to any part of the human body but are often found in hard-to-see areas such as the groin, armpits and scalp.

When a tick ingests the blood of an infected host, the pathogen passes through the gut membrane and travels through the tick's body to the salivary glands. It then crosses the barrier of the salivary glands into the tick's saliva, Rochon says.

To keep their hosts' blood flowing when they feed, ticks and other blood-sucking arthropods inject their hosts with saliva, which contains molecules that prevent the blood from coagulating and prevent the host from noticing them.

"It's part of why they're so good at transmitting pathogens because their saliva modifies the local immune response. So when the bacteria goes in, it kind of has a nice little bed of modified immune response" that allows them to thrive, she says.

Ticks are able to transmit pathogens such as Anaplasma and Babesia within an hour. But when it comes to Lyme disease, the Borrelia burgdorferi bacterium in an infected tick's gut doesn't pass into its body right away. It needs blood to "wake it up," before it makes its way into the tick's body, which only happens after the tick starts feeding, Rochon says. Once in the body, the bacterium then enters the salivary glands and gets into the saliva. All of this takes time. Which is why, if you remove a tick within 24 hours of it attaching itself to you, your chances of getting Lyme disease are extremely low, she says.

When they're active: Female blacklegged ticks lay eggs in late spring. The larvae that hatch are usually pathogen-free and they typically feed on small animals such as mice, not humans. The larvae then molt into nymphs, which is generally regarded as the most important stage from a public health standpoint, Rochon says. Nymphs tend to be most active during summer.

If they've fed on an infected animal during the larval stage, nymphs can transmit pathogens, Rochon says. They're also extremely small, about the size of a poppy seed, so they may go undetected and people who develop symptoms may not recall getting a tick bite.

It can take up to four years for blacklegged ticks to reach the adult stage. Adults tend to be active until there's snow on the ground. In the spring, they're back "questing," or waiting to attach themselves to hosts. Rochon advises people to keep an eye out for ticks from early in the year when the snow melts until there's snow on the ground again.

How to protect yourself: Use repellent containing DEET or picaridin. If your repellent has a lower concentration than 30 per cent DEET, it will still work, but you'll have to reapply more often, Rochon says. Picaridin is a good choice for children, she adds. "It doesn't stink like DEET does." (But don't bother with citronella oil, she says. It won't keep the ticks away.)

Ticks climb up long grass and low shrubs to seek hosts and they seek shelter in lower vegetation and leaf litter. If you're heading into an area where you know there are ticks, wear long pants and tuck the legs into your socks and tuck your shirt into your waistband, Rochon says.

"It's not sexy, but health is sexy," she says, explaining these measures, along with wearing light-coloured clothing, will allow you to see and remove ticks as they crawl up your body in search of a place to attach themselves.

Giant hogweed

Brushing up against the massive leaves of a giant hogweed plant won’t necessarily cause you harm. But if the plant is broken in any way, beware.

Why it's a threat: Giant hogweed is a particularly impressive and nasty member of the carrot family, known for its enormous size and ability to cause severe skin burns. Since giant hogweed plants produce thousands of seeds that spread easily, this prolific species is an environmental menace. It out-competes native plants, dominating areas where it grows.

Giant hogweed originates from the Caucasus Mountains, around Eastern Turkey. But in the 19th century, it was introduced to Western Europe as an ornamental plant, says Tardif, professor of plant agriculture at the University of Guelph. In Canada, over the past 15 to 20 years, giant hogweed has become more prevalent as an invasive species, Tardif says, though it's unclear why. Greater public awareness may, in part, be driving increased reports of the presence of this noxious plant.

Brushing up against the massive leaves won't necessarily cause you harm. But if the plant is broken in any way, beware. It's the sap that makes giant hogweed a public-health concern, Tardif says. The sap contains chemical compounds called furanocoumarins, which penetrate the skin if you come in contact with it, Tardif says. These compounds make your skin cells sensitive to UV rays. Thus, subsequent exposure to sunlight results in a chemical burn that could require medical attention.

"It's not immediate burning," Tardif says. Burns occur "between 24 and 48 hours if you're exposed to sun after [contact]."

Although he has never experienced this himself, Tardif says others have described it as a painful burning sensation, unlike the itching that comes from contact with poison ivy. (A warning to the squeamish: a Google search of "giant hogweed" will turn up truly ghastly images of raw and blistered limbs.)

The skin discoloration that follows can take a long time to disappear, Tardif says. Dark brown scars can last for years. Although giant hogweed is believed to cause blindness, Tardif says he isn't aware of any documented cases of blindness because of hogweed in medical literature. It may be possible that one's tears would wash away any sap from the eyes before it causes real damage, he says. But then, of course, no one wants to try.

With white flower-clusters, hogweed resembles other members of the carrot family, such as angelica or Queen Anne’s lace. But its size sets it apart. Darren Calabrese/THE CANADIAN PRESS

Where it grows: It's uncertain when giant hogweed was introduced to Canada, although Tardif says early records show it was here by the mid-20th century and he notes it was likely sold here as a garden plant.

"It's a very nice plant, very tall, very spectacular," he says.

These days, "you can find it pretty much everywhere," he says, noting that although it is widespread, "that doesn't mean it's abundant."

Giant hogweed thrives in wet areas, mostly along riverbanks, creeks and streams. It grows well in both shaded and sunny areas. Once established, a colony can creep up riverbanks and into fields, but for the most part, it is found near water.

How to spot it: With white flower-clusters, hogweed resembles other members of the carrot family, such as angelica or Queen Anne's lace. But its size sets it apart. Mature plants are generally three- to four-metres tall (sometimes as high as five metres) and they grow leaves as long as a metre wide. Young plants, which are smaller, may be harder to identify and thus, can be more dangerous when they grow in June and July, Tardif says.

Size aside, giant hogweed also has distinct hairy, bristly stems, with red blotches and red dots at the base of these hairs and on the stem.

How to protect yourself: Identifying giant hogweed is the first step, Tardif says. If you spot a large colony on your property, it's best to call professionals for help. But if you're trying to get rid of only a few plants, be sure to cover up in trousers, long sleeves, rubber gloves and rubber boots before you start weeding, Tardif says. Be careful not to let the sap splatter; this is not a plant you want to tackle with a weed whacker.

He suggests cutting the plant at the base and letting it rot in plastic bags before discarding it. Then, get a shovel to dig up the roots, which can grow more than half a metre deep.

If you get the sap on your skin, wipe it off as quickly as you can. Tardif says his team of researchers always carries moist towelettes and baby wipes when working with giant hogweed. And, he says, make sure you cover up the area with long sleeves and trousers, and avoid sunlight for at least a few days afterward.

A previous version of this article incorrectly identified aquatic biologist/taxonomist Jessica Fang as Jennifer.