A pet rat climbs out of a box at a home in Los Angeles. ((Damian Dovarganes/Associated Press))

To some people, rats are affectionate, intelligent, clean animals well suited as pets. But to most, they are seen as dirty rodents more often associated with disease and infestations.

Rats are mainly distinguished from mice by their size. The larger rodent can grow to 12 centimetres long or more and can weigh half a kilogram. The Rattus genus includes 56 types, but most people know only two of them: the Norway rat and the house rat.

These two thrive in urban centres around the world. The house rat is typically found in warmer climes, while the Norway rat, also known as the brown rat, dominates more temperate locations.


In Alberta, the provincial government goes to extraordinary measures to keep rats out.

For the past 60 years, it has been illegal for anyone except research institutions and zoos to import or possess live rats. Pest control inspectors man the border. And signs draw attention to the province's rat-free status. All suspected infestations are investigated by field staff.

Ironically, one of the biggest problems is most Albertans can't actually identify rats and the signs of rats because they're so unfamiliar with them.

Rats were first reported on a farm along the eastern border in 1950 and by autumn of 1951, there were 30 confirmed infestations along the border. Over the next decade, officials successfully struggled to get the problem under control.

A buffer control zone was set up along the Saskatchewan border. Natural barriers like the Rocky Mountains in the west and boreal forest to the north have helped stop rat migration into the province.

From time to time, people unwittingly bring rats into the province as pets.

In 2002, Alberta declared its first year with no rat infestations. Between 2002 and 2007, there were only two known rat infestations.

Source: Government of Alberta

What do they eat?

Rats will eat almost everything in urban centres. The Norway rat, though omnivorous, prefers a carnivorous diet, including shrimp, snails, insects, bird eggs, amphibians, poultry and rabbits. The Norway rat can swim and will also enter lakes, streams and sewers to hunt for fish and other food.

The two common rats thrive in urban centres where human and pet food is plentiful. The animals will also eat stored grains and kill poultry. On some oceanic islands, infestations of rats have been blamed for depleting or causing the extinction of small mammals, birds and reptiles, according to the Encyclopaedia Britannica.

Do they spread diseases?

The Norway and house rats are believed to be contributors to the spread of 40 diseases among humans, including most famously the bubonic plague. Food poisoning, schistosomiasis, murine typhus, tularemia and leptospirosis are other diseases rats can help spread.

In June 2010, an Australian microbiologist warned that rat bite fever, a potentially life-threatening disease associated with squalour, could be on the rise in suburban homes because of people keeping the rodents as pets. Since first reported in the U.S. in 1839, there have been more than 200 cases worldwide, mostly in North America.

Two cases were reported in Ontario in the early 2000s. Both the 29-year-old man and the nine-year-old girl who contracted the disease got it from a pet rat.

The Norway rat, however, is also frequently used in laboratory experiments, which help improve human health through research.

What other damage do they cause?

According to the Alberta government, where rats are forbidden, "Norway rats are one of the most destructive creatures known to man." Not only can they destroy crops, they can also weaken foundations of buildings, streets, sewers and water lines with their tunnels. They can also gnaw holes in floors, walls, insulation and supporting structures.

Where do they live?

Rats typically dwell on the ground, but some are excellent climbers. The house rat, also known as the ship rat, black rat or roof rat, is a good climber and can run along narrow branches and wires.

Some live in burrows or build nests under such things as boulders and rotting tree trunks. They can even live in rock crevices or caves or large city buildings. They can't survive the winter outdoors in Canada.

Tips for preventing an infestation

  • Store garbage and compost bins with tight-fitting lids.
  • Reduce clutter to prevent hiding spots.
  • Eliminate sources of food, like fallen food on the floor, pet food outside, grass seed, etc.
  • Check for possible entry points, any opening larger than 1½ centimetres in diameter. Cover with heavy-gauge wire mesh for vents and metal sheeting for holes.

For more on figuring out if you have an infestation and tips on getting rid of rats, see the  Toronto Public Health's rodent control page.

Where did they come from?

It's believed the Norway, or brown, rat originated in northern or northeastern China and reached Europe in the mid-1500s. According to the Encyclopaedia Britannica, the house rat, meanwhile, most likely originated in India.

Rats came ashore on the east coast of North America in 1775 and spread westward. It's estimated they reached Saskatchewan in 1919 and moved northwest at about 24 kilometres a year.

How often do they reproduce?

The brown rat's reproduction is among the most studied. It reaches sexual maturity at three months and can produce up to 12 litters per year. Each litter can range from two to 22 young, though eight or nine is more typical. The gestation period is from 21 to 26 days. Peak times are spring and autumn.