Vancouver rats carry disease that can infect humans

Jessica Barrett, Vancouver Sun 



A groundbreaking study shows some rats in the Downtown Eastside are carrying a bacterial disease that can infect humans and, in severe cases, lead to kidney failure, bleeding in the lungs and meningitis.

The first results from the study, published Thursday in the journal PLoS ONE, found rats trapped over the course of one year tested positive for leptospirosis, a disease that mostly causes flu-like symptoms in humans and is commonly found in developing countries.

The results are the first definitive findings to come out of the Vancouver Rat Project, a three-year study helmed by Abbotsford-based researcher Dr. Chelsea Himsworth that aims to fill a knowledge gap about the characteristics of local rats, the pathogens they carry and the potential crossover to humans.

“There’s never been a comprehensive study of rats on any subject in Canada. Ever,” explains Himsworth, a veterinarian completing her PhD through the University of B.C.’s school of population and public health.

With a specialty in neglected zoonotic risks — diseases that spread from animals to humans — in urban centres, Himsworth embarked on the Vancouver Rat Project in 2011 to fill what she sees as a blind spot in the scientific research community.

“It really fascinates me that when you actually look into the scientific literature, we know a lot more about elephants and whales and grizzly bears than we do about rats,” she says.

Himsworth attributes that fact to the failure of the parasitic rodents to capture our collective imagination, in spite, or perhaps because of, their proximity to us.

Indeed, in the hierarchy of wildlife nobility, rats often rank near the bottom. While we celebrate in science and art animals that remind us of our more becoming attributes — loyal horses, strong elephants, mysterious, intelligent whales — rats are more a lurking presence, half-hidden in the shadows.

As Himsworth puts it, they are our closest, strangest bedfellows, yet, from a scientific standpoint, we really don’t know who we’ve been sleeping with.

That dearth of knowledge could leave us vulnerable to new health challenges posed by rats in a rapidly changing, globalized world.

With half the world’s population now living in cities and a corresponding increase in zoonotic diseases, Himsworth became consumed by what she calls “the rat question.” She wondered whether the same critters that spread pandemics such as the bubonic plague in the Middle Ages could potentially become the source for other zoonotic diseases in an era that has seen SARS, bird flu and swine flu outbreaks in the last decade.

The leptospirosis findings are particularly timely, Himsworth says, given the disease is beginning to present in humans living in cities in the developed world. Rat-borne instances of leptospirosis have been discovered in people living in Baltimore and cities in France. The U.S. Centre for Disease Control also states the disease, which has historically been found in rural communities and the developing world, is increasingly showing up in inner-city children in the U.S.

Himsworth’s next phase of study will determine whether there’s been any crossover to humans in the Downtown Eastside, where the disease could spread to humans through exposure to rat urine or contaminated water. Her team is spending this year collecting blood samples of 200 residents in the neighbourhood.

People living in poverty with compromised immune systems can be particularly susceptible to rat-borne pathogens, Himsworth says, adding human cases likely wouldn’t be picked up in regular health screening, simply because doctors aren’t expecting to see them surface in cities like Vancouver.

“For most of these diseases, you’d never make the diagnosis unless you were predicting a risk in the first place. So it’s kind of a circular reasoning,” she says. “If you’re not expecting those diseases to be present, you’re not testing for them.”

The findings also revealed some vital information that could be used to concoct better pest-control procedures. Contrary to the common belief that more rats equal more disease, Himsworth found the prevalence of leptospirosis was unrelated to the overall number of rodents on a given city block.

“So we would have blocks with hundreds of rats and none of them had lepto, and then we would have blocks with very few rats and almost all of them were diseased.”

The importance of this finding, she said, is that it suggests rats tend to stay in “self-quarantining” family groups, with a home territory of just a few metres. However pest control measures such as trapping and poisoning destabilize the family unit, causing infected rats to migrate to other colonies and spread disease.

“So there’s kind of this dogma that more rats must mean more disease, but actually, for lepto anyway, that was kind of the opposite of the truth.”

The leptospirosis study is just the tip of Himsworth’s research iceberg, which saw her screen more than 700 rats for diseases including Seoul Hantavirus, the parasite toxoplasmosis and the “superbug” MRSA. She also employed a state-of-the art research technique called metagenomics that can potentially uncover previously unknown diseases.

“It’s basically a technology that screens all of the DNA in the rat,” she explains. “Then you use computing technology to kind of subtract the rat and piece together all of the remaining DNA fragments.”

Himsworth said her findings could eventually call into question existing public health policy, which has long held that Canadian rats don’t pose a threat to human health.

“Really, it makes you wonder how health officials gauge rat-associated health threats without that knowledge base,” she said.

But Dr. Reka Gustafson, medical health officer for Vancouver Coastal Health is adamant Himsworth’s findings do not show any increased risk to human health. The leptospirosis study is merely of academic interest, she said.

“The public has absolutely no reason to be concerned at all.”

Vancouver Coastal Health does not sample rat populations for pathogens because it is more effective to survey the human population for evidence of zoonotic diseases. If someone presents with symptoms that are not shown to be caused by typical infections, advanced testing would reveal unusual diseases such as leptospirosis, she said. So far the only confirmed cases in B.C. have been found in travellers.

“In Vancouver ... we know that because we aren’t getting human disease where the vector is known to be rats, we know they are a nuisance, but they are not a public health concern in our area.”

Dr. David Patrick, an infectious disease expert with the B.C. Centre for Disease Control and UBC — and who is involved in the Vancouver Rat Project — said it would be “a step beyond logic” to conclude a widespread public health threat from Himsworth’s findings to date. However, he didn’t rule out the possibility her further research could uncover cases of leptospirosis in humans.

“One thing we know now, is we’re not having people dying with leptospirosis, going to the emergency room with leptospirosis,” he said.

Rather, Patrick said the Vancouver Rat Project’s real value is to lay the scientific roadmap of rat ecology that can be an important resource in the event of an outbreak similar to SARS, which was eventually traced back to a virus carried in bats.

And for Himsworth, that task has become much more than a PhD project — but a life calling to uncover more about our most ubiquitous urban pest.

“So that when you call me 10 years from now, I can say, are things better, are they worse, are they different? So we can have a better-informed approach to rat-associated health risks.”



