Nobody Has Counted the Rats: Surveillance, Inference, and the Culling Paradox in Urban Rodent Management

1. Introduction: a number everyone repeats and nobody has

Ask how many rats live in a given city and you will get an answer. The answer will usually be a ratio to the human population, delivered with confidence, and it will be traceable to nothing at all.

The honest position is the one taken by researchers who have spent more than a decade studying this exact question. Asked whether there were more rats in Vancouver's Downtown Eastside than elsewhere in the city, Kaylee Byers, who has worked on the problem since 2013, acknowledged plainly that nobody knows.²

The Vancouver Rat Project itself was launched partly in response to public concern about an exploding rat population, a characterisation that was, as one account puts it, exploding anecdotally, because no one had counted it.¹

1.1 What this paper argues

Three claims. First, that the measurement problem is real and severe rather than a matter of insufficient effort. Second, that where careful work has been done it has produced findings that contradict standard practice, most importantly on culling. Third, that the correct response is not to keep trying to produce a citywide population number but to change what we measure, because abundance was never the variable that mattered most.

2. The Canadian evidence base before 2010

It is worth establishing how thin the foundation was, because it explains how the figures in circulation came to be untethered.

The Vancouver Rat Project was started in 2010 by Dr Chelsea Himsworth, a veterinary pathologist at the University of British Columbia School of Population and Public Health, as a way to learn more about the rat population of Vancouver's Downtown Eastside, including the diseases they carry.² Before its launch, there had never been a comprehensive study of rats in Canada.²

The scale of the prior gap is best conveyed by a single sentence from an account of the project's origins: the sum total knowledge of Canada's wild rats could be boiled down to a single study of 43 rats living in a landfill in nearby Richmond in 1984.¹

Forty-three animals, in one landfill, in one year, four decades ago. That was the national evidence base for a species present in every Canadian city, implicated in structural damage, food contamination and zoonotic disease, and the subject of continuous municipal expenditure.

3. Why rats resist counting

The gap is not simply neglect. Several properties of the animal make abundance estimation unusually hard.

They live underground. Norway rats are particularly hard to study because they live underground, in burrow networks.² The population is not observable without trapping, and trapping samples only the fraction willing to enter a trap.

They are neophobic. As discussed elsewhere in this journal, R. norvegicus avoids novel objects in familiar environments, sometimes for a week or more. Any trapping-based estimate is therefore biased toward bold individuals, and the bias is not constant across populations or over time.

Effort does not scale. The Vancouver work set ten Tomahawk live traps per city block, covering nine city blocks at a time, with traps set in the early evening.² That is intensive fieldwork for a small area. Extrapolating the same intensity to a whole city is not a funding problem so much as a category error.

Traditional methods are slow and narrow. Byers and Himsworth note directly that traditional methodologies for describing rodent movement, such as mark-recapture, are hindered by their time-consuming nature and limited geographic scope, and that as a result our understanding of how rodents interact with urban environments remains limited.³

4. What the Vancouver Rat Project actually did

Rather than attempting a citywide census, the project pursued questions that were answerable. The methodology is worth describing because it sets a realistic standard.

Rats were captured in live traps, photographed, checked for fleas and diseases, ear-tagged and released.¹ The team collected urine, blood and faecal samples.⁴ Over fifteen years the project has combined a range of tools to study rat ecology, the epidemiology of zoonotic pathogens, the mental and physical health threats rats pose to people, and municipal rat management policy and programming.⁵

The disease work alone is substantial: Leptospira interrogans ecology in an inner-city neighbourhood,¹ methicillin-resistant Staphylococcus aureus in urban Norway rat populations,¹ multiple human enteropathogens in rats from an under-resourced Vancouver neighbourhood,¹ and a systematic review of rat-associated zoonoses in urban centres.¹

4.1 The methodological shift

The project's most useful contribution to method may be the argument that population genetics can substitute for the mark-recapture work that does not scale. Byers and Himsworth argue that population genetic principles and tools have the capacity to greatly increase understanding of rodent population dynamics, ecological relationships and movements across space, while noting that the field is often unapproachable to non-scientist pest management professionals.³

Their commentary is explicitly aimed at promoting collaborative and integrative rodent pest management by introducing these principles to practitioners.³ We would endorse that aim, and this paper is in part an attempt to carry it further.

5. The block as the ecological unit

The genetics produced a finding that should reorganise how operators think about rodent territory.

Using population genetics, the project found that rats within a block were in general closely related, with 99 per cent of rats trapped in the same block as a close relative, meaning a parent, offspring, or full or half sibling.¹

Complementary work on movement examined genetic material from 600 related rats and found similar DNA in 1,200 pairs of rats within 30 metres.²

5.1 What this means operationally

The city block is not an administrative convenience. It is approximately the biological unit. Rat colonies are spatially discrete, genetically coherent family groups, and movement between blocks is limited enough that relatedness drops off sharply across them.

Three consequences follow. Treating a single property while the colony occupies the block is the rodent equivalent of the unit-scope error in multi-family bed bug management discussed elsewhere in this journal. Reinvasion after successful control is more likely to come from the same block than from a distant source. And an operator working a single address is intervening in a population whose boundaries do not correspond to the property line, the client relationship, or the invoice.

6. Genomics across cities

The Vancouver work sits within a larger comparative effort. Combs and colleagues, with Byers and Himsworth among the authors, examined brown rats to test hypotheses about the repeatability of neutral evolution across four cities: Salvador in Brazil, New Orleans, Vancouver and New York City.⁶

At least 150 rats were sampled from each city and genotyped for a minimum of 15,000 genome-wide single nucleotide polymorphisms.⁶ The framing of the study is that urbanisation often substantially influences animal movement and gene flow, but few studies have examined gene flow of the same species across multiple cities.⁶

Related work has examined urban population genetics of rats in Salvador's slums,¹ and the relationship between sewer system characteristics and rat presence in Seattle.¹

6.1 Why comparative genomics matters for practice

If gene flow patterns are broadly repeatable across cities, then findings from Vancouver carry some weight in Winnipeg without requiring the same fieldwork to be repeated locally. If they are not repeatable, that is equally important to know, because it means local data are indispensable. The multi-city design exists precisely to answer which of these is true.

7. The culling paradox

This section concerns the single most consequential result to come out of this body of work, and the one that is least reflected in practice.

7.1 The finding

The project examined the effects of culling on Leptospira interrogans carriage by rats.¹ The expectation was straightforward: remove rats, reduce the reservoir, reduce the pathogen.

That is not what happened. While the proportion of individuals carrying the bacteria did not decrease in city blocks where rats were removed, cases increased everywhere else.²

7.2 Why this is not an argument against rodent control

It is important to state the scope of the finding precisely, because it is easy to over-read.

The outcome measured was pathogen carriage, not rat abundance or the structural and food safety harms that motivate most commercial rodent work. A programme protecting a food premises from contamination has a different objective and this result does not speak directly to it.

What the finding does establish is that population reduction and health risk reduction are not the same variable, and that an intervention can succeed on the first while failing or backfiring on the second. Byers characterises the culling effect as more of a preventative measure.²

8. What culling does to a social animal

The mechanism deserves attention because it generalises beyond Leptospira.

Rats within a block are close kin.¹ That relatedness reflects stable social structure: established colonies with territories, dominance relationships and familiar contacts. Removing a substantial fraction of such a group does not leave a smaller version of the same structure. It leaves a disrupted one.

Disruption has predictable consequences. Vacated territory attracts immigrants from adjacent blocks. Surviving animals shift ranges. Encounters occur between individuals that would not previously have met, and those encounters include the aggressive interactions through which several rat-borne pathogens transmit. Work from this group has associated wounding with Leptospira carriage, comparing ecologically distinct urban environments.³

8.1 The generalisable principle

Any control method that partially reduces a socially structured population while leaving habitat and resources intact risks increasing movement and contact. This is a known phenomenon in wildlife disease management and the Vancouver results place urban rats within that pattern.

The corollary is that interventions removing the resource base, meaning harbourage and food, rather than removing animals, do not carry the same risk. They reduce carrying capacity instead of creating a vacuum inside it.

9. Complaint data as accidental surveillance

In the absence of abundance data, municipalities rely on service requests and complaints. It is worth being explicit about what that instrument measures.

A complaint requires that a rat be seen, that the observer identify it correctly, that the observer know a reporting mechanism exists, that they expect reporting to produce a response, and that they be willing to be identified with a rat problem. Each of those conditions varies systematically across a city.

Reporting propensity tracks tenure and trust. Owners and renters differ. Neighbourhoods with strong municipal relationships differ from those without.

Stigma suppresses reporting in exactly the settings where infestation is most likely, a pattern that parallels the disclosure suppression documented for bed bugs in rental housing.

Visibility varies with built form. Alleys, dumpster arrangements and lighting change the probability of a sighting independently of abundance.

Media coverage generates artefacts. A local rat story reliably produces a complaint spike that reflects attention rather than ecology.

The result is that complaint density is a composite of abundance, visibility, civic trust and news cycle. It is not useless, because it does carry signal, but treating it as a population index will systematically under-represent exactly the under-resourced neighbourhoods where the health burden is concentrated, which is precisely the population the Vancouver work has focused on.¹

10. Environment as the measurable variable

If abundance is hard to measure and complaints are biased, what can actually be measured?

Himsworth and colleagues addressed this directly. Noting that rat abundance is largely determined by features of the environment, but that the specific urban environmental factors influencing density had yet to be clearly identified, and that there were no well described tools for evaluating the relationship, they developed a systematic environmental observation tool using methods borrowed from the field of systematic social observation.⁷

The tool combined quantitative and qualitative methodologies and was used to identify environmental factors associated with the relative abundance of Norway rats in an inner-city Vancouver neighbourhood.⁷

10.1 Why this is the right move

Environmental features are observable from the street without trapping, without genetics and without a research budget. They are stable enough to be re-measured. And because abundance is largely determined by environment, they are causally upstream of the thing we cannot count.

The related Seattle work on sewer system characteristics and rat presence follows the same logic.¹ Measure the habitat, not the animal.

11. What a defensible programme would measure

Combining the evidence above, a municipal or commercial rodent programme could reasonably be built on the following, none of which requires a population estimate.

Environmental condition indices, assessed systematically and repeatedly along the lines of the observation tool,⁷ giving a comparable score across areas and over time.

Block-level rather than property-level scoping, because the block approximates the colony.¹

Activity indices rather than abundance estimates, using consistent methods such as bait take, burrow counts or camera detections, accepting these as relative indices rather than attempting to convert them into population numbers.

Explicit separation of objectives. Structural protection, food safety and zoonotic risk reduction are different goals with different success criteria, and the culling result demonstrates they can move in opposite directions.²

Complaint data used as a service-demand signal, which is what it actually is, rather than as an abundance proxy.

12. Implications for Winnipeg

Manitoba occupies an unusual position in this literature, and it is worth setting out carefully.

A correction is needed first, because Manitoba is routinely confused with Alberta on this point. Alberta has maintained essentially rat-free status since 1950 by way of a control zone along its eastern border.⁸ Manitoba has no such programme. Norway rats entered Saskatchewan in the 1920s and spread northwest from there,⁸ having already passed through Manitoba, and they are established in this province. Winnipeg is not Vancouver's Downtown Eastside and the extrapolation should not be made carelessly, but neither is it a rat-free jurisdiction.

But several of the findings transfer on mechanistic rather than epidemiological grounds. Block level colony structure follows from rat social biology, not from Vancouver's climate.¹ The social disruption mechanism behind the culling result follows from the same biology.² And the measurement problem is universal: nobody has counted Winnipeg's rats either.

12.1 The specific local opportunity

What Manitoba does have is an established urban rat population in a cold-winter prairie city, which is a setting almost entirely absent from the published literature. The Vancouver work was done in a temperate coastal climate; the multi-city genomics compared Vancouver, New York, New Orleans and Salvador.⁶ A continental climate with sustained sub-zero winters is a genuinely different environment for a species that cannot overwinter outside human structures.

Pairing the VKORC1 genotyping proposed elsewhere in this journal with block-scale relatedness analysis would produce, in a single sampling effort, both a resistance map and a movement map. The samples required are the carcasses operators already handle. The analytical methods are established and published. What is missing is the collaboration, not the technique.

13. Limitations and open questions

One city, one neighbourhood. The core findings come from Vancouver's Downtown Eastside, an under-resourced inner-city neighbourhood with particular building stock and waste management conditions.¹ The multi-city genomics work provides some basis for generalisation⁶ but block-scale results should be treated as demonstrated there and plausible elsewhere.

The culling result is about carriage. It concerns Leptospira interrogans prevalence, not abundance and not other harms.² We have tried to state this scope precisely and readers should resist broader readings of it, in either direction.

Our account of the mechanism is partly inference. The social disruption chain in §8 is consistent with the published association between wounding and carriage³ and with general wildlife disease theory, but the specific causal pathway is our reconstruction rather than a measured result.

Complaint data biases are argued, not quantified here. Section 9 sets out mechanisms rather than measured effect sizes. We are not aware of a published quantification of reporting bias for rodent complaints in a Canadian city, and that is itself a research opportunity.

Chart values are illustrative. The evidence-base chart compares study sizes drawn from different designs and purposes, and is intended to convey the scale of the pre-2010 gap rather than to compare methods.

14. Conclusion

The number of rats in a Canadian city is not known, and the confident figures in circulation are not measurements. Until 2010 the entire national evidence base for wild rats amounted to a study of 43 animals in a landfill.¹ The researchers who have since done the work say plainly that they do not know whether one neighbourhood has more rats than another.²

What is known is more useful than a population figure would be. Rats within a block are close kin, with 99 per cent trapped alongside a parent, offspring or sibling,¹ which makes the block rather than the property the operational unit. Abundance is largely determined by environment, and environment can be measured systematically where animals cannot.⁷ And culling a block did not reduce pathogen carriage there while carriage rose elsewhere,² which means removal and risk reduction are not interchangeable.

The instinct when facing an unmeasured problem is to demand better counting. We would argue the better response is to stop treating abundance as the target variable. It is difficult to measure, weakly connected to several of the harms that matter, and, on the evidence of the culling work, not reliably improved by the intervention most often deployed against it.

For Winnipeg the immediate implication is modest and achievable. The province has a rodent population under unusually well-documented long-term management, carcasses pass through operators' hands every week, and the analytical methods for extracting resistance and relatedness data from those carcasses are published and established. The gap is a collaboration that nobody has yet convened.

References

1. Himsworth, C.G. et al. From Data to Knowledge to Wisdom: Lessons Learned From the Vancouver Rat Project. Integrative Zoology. Source for block-level relatedness with 99% of rats trapped alongside a close relative, and for the project's associated literature including Lee et al. (2018) on effects of culling on Leptospira interrogans carriage, Emerging Infectious Diseases 24:356-360; Lee et al. (2019) on MRSA; Himsworth et al. (2013) PLoS Neglected Tropical Diseases 7:e2270; Himsworth et al. (2023) PLoS NTD 17:e0011669; Himsworth et al. (2012) Vector-Borne and Zoonotic Diseases 13:349-359; Guo et al. (2022) on Seattle sewers, Urban Ecosystems 25:1699-1709; and Kajdacsi et al. (2013), Molecular Ecology 22:5056-5070. Also the origin account noting the single 43-rat Richmond landfill study of 1984. https://onlinelibrary.wiley.com/doi/full/10.1111/1749-4877.70125
2. Vancouver Rat Project field reporting. Source for the project's 2010 launch by Dr Chelsea Himsworth, Kaylee Byers joining in 2013, the trapping protocol of ten Tomahawk live traps per block across nine blocks set in early evening, the 600-rat genetic analysis finding similar DNA in 1,200 pairs within 30 metres, the statement that nobody knows whether the Downtown Eastside has more rats than elsewhere, the difficulty of studying subterranean Norway rats, and the culling result in which carriage did not fall in treated blocks while cases increased elsewhere. https://www.vancouverisawesome.com/local-news/vancouver-weather-rat-study-dtes-2023-7484209
3. Byers, K.A. & Himsworth, C.G. Commentary on population genetic approaches for rodent pest management. Source for the limitations of mark-recapture, the case for population genetics as an alternative, and the aim of making these methods approachable to pest management professionals. Includes reference to Minter, Himsworth, Byers and Costa on age and wounding associated with Leptospira interrogans carriage in ecologically distinct urban environments. https://www.researchgate.net/publication/405427145_From_Data_to_Knowledge_to_Wisdom_Lessons_Learned_From_the_Vancouver_Rat_Project
4. Vancouver Rat Project sampling methodology, including capture, ear tagging, and collection of urine, blood and faecal samples. https://www.vancouverisawesome.com/animal-stories/rats-vancouver-city-birth-control-infestations-rodent-experts-9183306
5. Vancouver Rat Project, Who We Are. Canadian Wildlife Health Cooperative, British Columbia. Source for the project's scope across rat ecology, zoonotic pathogen epidemiology, health threats to people, and municipal rat management policy and programming. https://vancouverratproject.ca/whoweare
6. Combs, M., Byers, K.A., Ghersi, B.M., Blum, M.J., Caccone, A., Costa, F., Himsworth, C.G., Richardson, J.L. & Munshi-South, J. (2018). Urban rat races: spatial population genomics of brown rats (Rattus norvegicus) compared across multiple cities. Proceedings of the Royal Society B, 285(1880), 20180245. doi:10.1098/rspb.2018.0245 https://royalsocietypublishing.org/rspb/article/285/1880/20180245/102240/Urban-rat-races-spatial-population-genomics-of
7. Himsworth, C.G., Parsons, K.L., Feng, A.Y.T., Kerr, T., Jardine, C.M. & Patrick, D.M. (2014). A Mixed Methods Approach to Exploring the Relationship between Norway Rat (Rattus norvegicus) Abundance and Features of the Urban Environment in an Inner-City Neighborhood of Vancouver, Canada. PLOS ONE, 9(5), e97766. Source for the systematic environmental observation tool and the finding that abundance is largely environment-determined while the specific factors had not been clearly identified. https://journals.plos.org/plosone/article?id=10.1371%2Fjournal.pone.0097776
8. Government of Alberta. History of rat control in Alberta. Source for the Rat Control Zone established along the Alberta-Saskatchewan border in the early 1950s, the arrival of Norway rats in Saskatchewan in the 1920s and their northwestward spread, and Alberta's essentially rat-free status. Manitoba has no equivalent programme. https://www.alberta.ca/history-of-rat-control-in-alberta