Reaching almost five metres into the air, the fading corn-stalks still sprout black, rotting, husk-covered stubs that, less than a month ago, were plump ears of corn. Lovingly tended by an immigrant Latin American group, the corn was planted alongside squash and beans in an ancient Mesoamerican pattern called las tres hermanas, or “the three sisters”.
It was the first year that the Latin community had tried growing traditional foods at the Portland Hotel Society’s Hastings Urban Farm, a community garden at 58 West Hastings Street, across the street from the Army and Navy store. The group was especially looking forward to sharing the fruits of its labour with other gardening volunteers at the upcoming annual September feast.
Then the rats came.
“They came one day before harvest,” says Sarah Common, manager of Hastings Urban Farm. “They knew exactly what they were doing. They ate the kernels; they took everything. They finished them in two days. It was crushing.”
Garden became rats' personal pantry
Common has managed the Hastings Street garden for one year. Created three years ago to provide Downtown Eastside (DTES) residents with skills training and nutritious food as well as a serene, green respite from the grim Hastings landscape, the garden was treated by the rats as if it was their personal pantry. They had discriminating tastes, seeking out sweet crops like peppers and tomatoes as well as the corn, Common says.
Common—a graduate of UBC’s global resource systems program, specializing in sustainable communities and food security—tried to outwit the rats, planting choice crops in the middle of the garden and creating a moat of pollinator-friendly flowers and alliums like onions and garlic, which rats snub the way Jamie Oliver shuns fast food.
But the encroachment continued, Common says, pointing to a freshly dug rat hole under a raised vegetable bed. Ideally, she says, the beds would have heavy metal mesh underneath to prevent such tunnelling. “I would like to mesh this whole acreage in. But that’s really expensive,” says Common, who won’t set out rat bait for fear of accidentally poisoning the volunteers or their children and pets.
Rats chased kids out of playground
Controlling the rat population hasn’t been on Vancouverites’ radar until this past summer. Rats were seen scurrying around places like the Commercial-Broadway SkyTrain station, sparking a raft of complaints. Rodents were so prolific near a daycare located under the Burrard Bridge that the children weren’t allowed onto the adjoining playground. Gardeners complained of devastated tomato, fig, and pepper harvests, while rats—normally nocturnal—were seen scampering about in the middle of the day.
Other than the yuck factor, is this apparent escalation in rat numbers cause for worry? Aren’t rats—admittedly unattractive—part of Vancouver’s Beatrix Potter–like charm that includes skunks, raccoons, gulls, rabbits, and squirrels?
Vancouver’s Chelsea Himsworth, a veterinarian with a PhD in zoonotics—the study of disease transmission between animals and humans—says the rodents are in a class of their own. Rats, interlopers from Asia who came here on ships, are worrisome for several reasons, Himsworth says. They carry pathogens, reproduce exponentially (a rat dam can birth up to a dozen pups four to six times a year), and are so well adapted to humans they are dependent upon us, a phenomenon called commensalism.
Will city composting rules just feed rats?
We unwittingly nurture this, allowing garbage bins to overflow, leaving fruit on trees, and creating urban gardens and compost heaps that serve as both larder and lodgings. Come January 1, Vancouver’s citywide composting edict takes effect, forcing condo and apartment dwellers to separate organic matter from garbage to place it in special green bins. It also means, Himsworth says, that rats, which can “chew through lead pipes to get at grain stores”, may have even greater access to tasty human detritus. “Rats are thriving,” Himsworth says, noting that the city rodent population is a mystery because no one tracks their numbers.
Himsworth has headed the Vancouver Rat Project for the past three years, overseeing the trapping of 725 rats—brown Norway rats (Rattus norvegicus, also known as common rat or wharf rat) and black rats (Rattus rattus)—in a 42-block area of the DTES where they proliferate. (It wasn’t their cute button noses but their ubiquity that made rats the official mascot of the DTES’s Poverty Olympics in 2010.) Himsworth’s team of researchers from UBC’s school of population and public health carried out disease testing on the nabbed rats.
They also sequenced the DNA of the bacteria carried by the rats. One of the youngest team members, biologist Kaylee Byers, a UBC PhD candidate, is currently testing the fleas found on the rats to determine what diseases the parasitic hitchhikers carry. Byers will also be undertaking rat-DNA sequencing to establish family trees, which helps researchers understand how rats’ social structures influence the spread of disease.
Rats and their fleas: partners in medical crimes
Historically, fleas and rats are partners in crime: it was the Yersinia pestis bacterium carried by the common rat flea (Xenopsylla cheopis) that was responsible for three globally devastating bubonic plagues. According to the United States Centers for Disease Control and Prevention, the first pandemic, or Plague of Justinian, was a 200-year scourge that began in AD 541 and killed up to 100 million people worldwide. The 14th-century Black Death dispatched up to 60 percent of Europe’s population. The Third Pandemic, or modern plague, appeared in China in the 19th century and spread to port cities around the world via rats until 1959.
Unfortunately, according to Himsworth, the bubonic plague refuses to be relegated to the annals of medical history. There were 13 reported outbreaks of rat-associated plague between 2009 and 2013 in seven countries including Tanzania, Uganda, Peru, and China. Madagascar has been especially hard hit, with that country’s health ministry reporting 300 to 600 cases of bubonic plague (brought in by rats) in the past several years. All it would take, Himsworth says, would be for a few flea-ridden stowaway rodents to hop on a merchant ship headed for Vancouver to “introduce the plague and other diseases to the city”.
Rats carry one in four human diseases
Discussion of an ancient disease that tends to sprout up only in the world’s poorer places may come across as fear-mongering. It isn’t. Rats are the doyens of zoonotic disease. There are 352 generic human infections in the world today; rodents carry 85 of them that can cross the species barrier, according to Dr. Steve Berger, cofounder of California-based GIDEON, a global infectious-diseases database for health professionals.
Canada is no stranger to the bubonic plague, Berger says. Although there is no recorded case of bubonic plague in a person, wild rodents like squirrels, gophers, and bushy-tailed wood rats are known to be carriers. In the United States, prairie dogs carry bubonic plague.
Over at McMaster University in Hamilton, Ontario, Hendrik Poinar researches a slightly more esoteric database. An evolutionary geneticist and biological anthropologist, Poinar studies the genome of the bubonic plague, dating back to medieval times. Smaller outbreaks, or endemics (as opposed to global pandemics), provide an opportunity to understand the bacterium’s evolving genome, Poinar says. Scientists are seeking to identify the mutations that may have been responsible for the massive mortalities of past pandemics. These will help identify potential pandemic-level strains currently residing in some rodent populations, “which is probably not what people want to hear,” Poinar says.
Once rats deliver disease, we do the rest
Chillingly, once the Yersinia pestis bacterium enters the human population via a rat, it no longer needs the animal to do its dirty work. Poinar says that the bubonic plague, which infects the lymphatic system, can become pneumonic: a severe lung infection that spreads via airborne droplets.
Ebola hemorrhagic fever is an example of a zoonotic disease that popped up occasionally in isolated clusters in Africa starting in the 1970s, then became a runaway train earlier this year, killing more than 4,500 people since February. “What’s so scary about these zoonotic diseases is that we understand them so poorly,” Himsworth says. “We’ve invested so little research in them that there is a high probability of them surprising us.”
Himsworth’s four-year study, which is poised to enter its next phase, has spawned 15 peer-reviewed papers so far. One, published earlier this year in the journal PLOS ONE, “Carriage of Methicillin-Resistant Staphylococcus aureus by Wild Urban Norway Rats (Rattus norvegicus)”, is an analysis of this bacterium, which is found in both rodents and humans and is better known simply as MRSA. The malady it causes brings skin and soft-tissue infection. It is also considered a “superbug” because of its resistance to antibiotics.
Drug-resistant human bacteria passed to rats
MRSA is present in many of Vancouver’s intravenous-drug users, who shoot up in filthy surroundings in the rat-infested areas of the DTES. Himsworth and her team swabbed the nose and throat of the rats they caught and found that MRSA had “colonized” about four percent of the rodents, although some individual rat communities had infection rates of 60 percent. In a medical twist, after undertaking genome sequencing of the MRSA in the rats, Himsworth discovered it was identical to the strain found in humans. The rats had caught MRSA from people, a zoonotic reversal called anthroponosis.
“That’s why I think rats are a threat,” Himsworth says. “MRSA is living and replicating in them. We are giving the rats this particular bug, so why, then, couldn’t it go in the opposite direction?”
Although Himsworth refuses to call the DTES ground zero for a future disease outbreak, she considers the city’s homeless problem a serious health issue. Sleeping on cement and in back alleys brings Vancouver’s homeless directly into contact with rat feces and urine, which are full of bacteria and viruses. These include bacteria such as the Leptospira types, which can cause an infection that may result in kidney failure, and Clostridium difficile, which is responsible for a growing scourge that has infiltrated hospitals, attacking the weak and causing death. Rats also carry Salmonella, which causes severe food poisoning. Other rat-borne, disease-causing bacteria species that Himsworth found included Rickettsia typhi, Leptospira interrogans, and Bartonella henselae.
Health official: nothing to see here, move along
Not everyone thinks that Himsworth’s findings are cause for worry. Dr. Réka Gustafson, Vancouver Coastal Health’s medical health officer and medical director of communicable-disease control, dismisses Himsworth’s research as “an interesting academic exercise. It has very few implications for public health in British Columbia.”
Pathogens such as Clostridium difficile, Leptospira, and MRSA are ubiquitous in the environment and are not making the leap from rodent to human, thanks to basic hygiene practices and integrated pest management, Gustafson says. “The existence of the organism is not enough to cause human disease; you need a pathway,” she says. “From my perspective, there is no worry at all.”
Vancouver Coastal Health takes a hands-on approach to pest management, especially in restaurants, according to Richard Taki, regional director of health protection. Because rats can fit through a hole the size of a quarter, keeping restaurants rodent-free requires ongoing monitoring, Taki says. VCH inspectors make surprise visits to about 6,000 eateries one to three times a year, looking for evidence of feces, urine stains, and access points.
Drug-resistant pathogen has many pathways to people
Vigilance at restaurants is crucial. MRSA, for example, can produce a poison called Staphylococcus aureus toxin that can lead to organ failure and persist even in cooked and baked food, says veterinarian David Waltner-Toews, professor emeritus in the department of population medicine at the University of Guelph. Waltner-Toews, author of The Chickens Fight Back: Pandemic Panics and Deadly Diseases That Jump From Animals to Humans, also warns that the disease pathway between rodents and humans usually isn’t as linear as rat to food to human. Seagulls, which eat garbage alongside rats, can be infected by rodent MRSA. The pathogen could then be transferred to humans via the simple act of cleaning gull poop off a car window, Waltner-Toews says. “There are lots of different kinds of pathways for an organism to start spreading around once it gets into the environment.”
Still, the probability of contracting a disease from a rat is very low, at least for those living outside the DTES, Waltner-Toews says. Nonetheless, it is important to control rat numbers to ensure that some disease-carrying whiskery escapee from a merchant ship doesn’t have a vast population of pals waiting to be infected. “You don’t want to be at the point where you say, ‘Oh, my God, we should have done something about rats 10 years ago,’ ” Waltner-Toews says.
Rats teach each other to avoid poison
To date, the main tactic for controlling rats is baiting them with poison. However, this strategy is doomed to backfire, Himsworth warns. Killing off a colony causes surviving rats to migrate. The effects are twofold: infected rats travel to uninfected colonies, spreading disease. Meanwhile, reestablishing the social hierarchy through fighting increases the number of diseased animals within a group. (Waltner-Toews adds that street-smart rats teach each other to avoid poison.)
Rats’ astonishing fecundity is an important piece of the control puzzle. SenesTech Inc., a biotech company based in Arizona, has developed a system of rat sterilization that prevents the problem of colony collapse and migration. This month, SenesTech began a second round of field experimentation in the rat-infested subway of New York City, according to marketing director Ali Applin. Rat numbers in New York are estimated to be 32 million, and decades of trapping and baiting have proven a failure. Rats thrive because of the amount of food that is thrown away in the subway, as well as its warmth, allowing them to tunnel out their own cozy, underground metropolis.
The only thing lacking underground is water, and SenesTech created a liquid cocktail of two chemicals that sterilizes both male and female rats. Applin would not disclose the chemicals, as the mixture is patent pending.
SenesTech’s first field trial, held last year in New York’s subway and funded by the National Institute of Health, was considered a success, as the rats lapped up the cocktail. (There are no data yet on whether rat numbers declined.) Nonetheless, other field trials throughout the U.S. are under way where rats are proliferating. Applin says that this method is superior to rat poisons, usually anticoagulants that cause a slow and painful death and kill any animal that eats the hapless rodent.
Rats on the rise worldwide: time to act?
Vancouver isn’t alone in its rat population explosion. Last year, Rome declared a rat emergency, shutting down several schools as hordes of rodents commandeered the streets. Chicago declared war on rats this spring. Rats have even besmirched the memory of Winnie-the-Pooh, decimating bird and small-mammal populations in Ashdown Forest in England’s East Sussex, the inspiration for the fictional Hundred Acre Wood.
On a global scale, increasing rat numbers are a harbinger of climate change and environmental degradation. As the wild spaces once inhabited by megafauna like lions, elephants, giraffes, and polar bears disappear due to human encroachment, rats move in. Stanford University biologist Rodolfo Dirzo presented this dystopian future in a paper published recently in the journal Science. Dirzo wrote that we are on the brink of a sixth mass extinction—a planetary “anthropocene defaunation”, or biodiversity loss. Rats, as with Mary Shelley’s Frankenstein, seem to be a monster created in our own image.
It looks like rats and their diseases are here to stay. The question is: what are we going to do about it?