TECHNICAL | PPC105 NOVEMBER 2021
Did 18 months of lockdowns cause an increase in rodent populations and sightings? Dr Mark Lambert, Defra, writes for PPC about what the available data shows, from here to Tokyo.
In 2020, shortly after the outbreak of the SARS-CoV-2 (Covid-19) virus in the UK, BPCA ran a survey to find out how the outbreak had impacted pest control professionals in the UK.
One of the most striking findings was the report of substantial increases in rodent activity. Over 900 people responded to the survey, and nearly 50% of them reported an increase in rat and mouse activity.
Six months later, 708 people responded to a follow-up survey and 78% reported an increase in rat activity, while 63% reported an increase in mouse activity.
Inevitably these results generated considerable interest in the UK media. Reporting the results of the BPCA surveys, the BBC’s Science Focus asked “Has the pandemic unleashed a plague of rats on our cities?”
Increases in rodent sightings were also reported by the media in some other countries, and these increases were linked to lockdown measures.
Interestingly however, in some countries, the number of rat sightings stayed the same or declined during lockdown.
The BBC article reported that in Manhattan, New York, the number of rat sightings following lockdown decreased by 30% compared to a normal year.
This decline was attributed to a reduction in rodent numbers caused by the closing of restaurants, and it was suggested that selection pressures led to changes in the behaviour of the remaining rodents who exploited alternative feeding sources.
Reopening of the restaurants meant more opportunities to find food, and reports of rat sightings in New York went on to exceed pre-lockdown levels.
An increase or decrease in rodent sightings does not necessarily equate to a change in the number of rats or mice of course, and changes in the number of sightings could be a result of changes in rodent distribution.
In Sydney, Australia, a rapid increase, and then decline, in the number of rodents trapped during lockdown was attributed to the redistribution of rodents in response to changing patterns of food resources.
The number of rodent-related complaints received by the City of Sydney Council was not affected by lockdown, although the spatial distribution of rodent-related residents’ complaints did change between lockdown and post-lockdown, providing further evidence that the distribution – rather than the number of rodents – had changed.
Professional vs public sightings
Another recent study highlighted a mismatch between changes in the number of sightings reported and changes in the number of rats or mice trapped during the lockdown.
In Tokyo, Japan, the number of public service calls regarding rat sightings increased after the implementation of social distancing measures, however the majority (60-70%) of pest management professionals said they had not experienced an increase in their activities.
It is difficult to be certain whether these reports are evidence for changes in rodent numbers, changes in rodent distribution, or changes in our daily activities (such as working from home) that mean we are more likely to see, or report, rodents.
However, an increase in sightings or reports of rodent activity is obviously a cause for concern, particularly as during lockdown many people were spending more time at home, and many of the increases in sightings or increases in reports of rodent activity are likely to have been in and around domestic properties.
The BPCA surveys recorded reports of rodent activity by pest control professionals, rather than public sightings. This suggests that, unlike in Tokyo, the changes in the UK were not due to an increased likelihood of reporting by the public.
Is this an indication of increased rodent activity in and around our homes, and could these increases really be a direct result of the lockdown measures?
It turns out we have quite a good idea about the factors that influence the likelihood of rodent prevalence in domestic properties, and increased levels of resources is one of them.
In a 15-year survey of domestic properties in England published in the nature journal Scientific Reports in 2017, data from the English House Condition Survey (EHCS) and English Housing Survey (EHS) indicated that rats and mice were more likely to be found in homes with pets or livestock (and hence animal feed) in the garden for example.
Like in Sydney and Manhattan, rodent distribution could have shifted in response to lockdown measures.
With fewer feeding opportunities from restaurants and other food outlets that closed during the lockdown, it is likely that some rodents will seek out feeding opportunities elsewhere, which could include domestic properties.
This might not be the only reason for the increased rodent activity reported by the BPCA surveys however.
We know from extensive surveys of farm buildings carried out by staff of the then Ministry of Agriculture, Fisheries and Food (MAFF) in the late 1960s, that the prevalence of Norway rats in farm buildings fluctuates seasonally, with peaks in spring and autumn.
Dr Mark Lambert, Defra
Lockdown measures in the UK were first introduced in March 2020, and the first BPCA survey was carried out shortly after this.
The second BPCA survey was six months later, in the autumn.
We know from extensive surveys of farm buildings carried out by staff of the then Ministry of Agriculture, Fisheries and Food (MAFF) in the late 1960s, that the prevalence of Norway rats in farm buildings fluctuates seasonally, with peaks in spring and autumn.
These seasonal changes seem to be related to changes in the distribution of food resources on farms, which drives the movement of rats out into the fields during summer, and back again during autumn when crops are harvested and weather conditions deteriorate.
Seasonal peaks in breeding activity, which again could be resource-dependent, are also likely. However, our analysis of the EHCS and EHS data and other studies have found no seasonal association with urban rat prevalence.
We did find a seasonal effect for mice, with lower prevalence in domestic dwellings during summer, although this was a relatively subtle effect.
It is likely that under normal circumstances, food resources do not dramatically change throughout the year in urban environments, and hence rodent populations tend to be relatively stable.
It is therefore unlikely that seasonal effects explain all of the increases in rodent activity reported in the BPCA surveys, although they could explain some of the increases in rural areas.
We also know that some rodent populations change dramatically from year to year. This is particularly apparent in Scandinavia and elsewhere in Northern Europe, where vole populations undergo regular cycles with peaks every three to five years.
The causes of these population cycles are not fully understood, although climate, and a mechanism whereby oscillations in vole populations and their predators are interdependent, have been suggested as possible reasons.
Regular population cycles in rodents become less obvious at lower latitudes, however in our analysis of the EHCS and EHS data we found two peaks in the prevalence of mice inside, and rats around, domestic dwellings in England, one in 2002 and the other in 2008.
Extrapolating this trend, we suggested that another peak would be expected in 2014, and another in 2020.
To establish whether reports of increased sightings or increased rodent activity are a result of lockdown measures it is important to make comparisons with previous years.
We are fortunate in the UK to have a relatively long-term data set that shows that the increases we have seen here could be, at least in part, due to an underlying cycle of rodent prevalence.
While it is tempting therefore to link an increase in rodent sightings with a high-profile event such as lockdown, this could just be a coincidence.
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Source: PPC105