Rat plague following COVID-19?

By Dr. Liji Thomas, MDJul 9 2020

The COVID-19 pandemic led to the widespread closure of many business establishments, including eateries and restaurants. This kind of mass change in human behavior has historically led to changes in animal behavior as well. This is especially observed with respect to urban rat behavior.

A new study published on the preprint server medRxiv* in July 2020 describes the increased rat sightings following the imposition of social distancing and business closures at the start of 2020. Soon after this, local governments and public health authorities were alerted to an increasing number of rat sightings, with increasingly aggressive behavior and a loss of fear of people. Some have reported cannibalism among rats.

Study: Rats and the COVID-19 pandemic: Early data on the global emergence of rats in response to social distancing. Image Credit: anatolypareev / Shutterstock

This news article was a review of a preliminary scientific report that had not undergone peer-review at the time of publication. Since its initial publication, the scientific report has now been peer reviewed and accepted for publication in a Scientific Journal. Links to the preliminary and peer-reviewed reports are available in the Sources section at the bottom of this article. View Sources

Of course, the calls related to rats could be more than just a reflection of actual increases in rat sightings; they could be because of greater health consciousness following the universal jitteriness about COVID-19 transmission, and a feeling that these animals are linked to unsanitary conditions and disease. Again, when one report is cited in the media, especially social media, more people are likely to report the same if it occurs near them.

However, if rats are indeed encroaching upon human territory, the health consequences are significant. Rats can carry a multitude of germs and eat food, worth billions of dollars every year. One common saying is that rats have caused more human deaths than all the wars since the world began. Moreover, since rats are typically disliked and feared, the report of rats is enough to create panic or emotional stress, especially among poor and densely populated areas.

Issues with Tracking Urban Rat Populations

However, there are no validated quantification methods for rat movements, nor established rodent-transmitted pathogen surveillance programs. Rat sightings are thus the only way at present in which cities can decide where the risk of rodent-borne disease is greatest so as to focus on reducing the risk.

Rat sightings are more prominent in warmer seasons in the colder climates, both because rats retreat underground and humans go outside less. The current paper aims to help understand how human reporting behavior and the wild environment were respectively contributing to this phenomenon of increased rat sighting.

The researchers looked at observable shifts in reports to public services like the non-emergency phone system 311, without reference to seasonal changes. They also looked at calls to pest control services, which, unlike the other, are not free. Food availability in the form of restaurants and other food services is also explored.  

The study took place in Tokyo and New York City, using calls to public and private pest control services. Poland and Canada were also included, but only pest control associations exist in these countries, through which surveys were distributed.

The researchers found that rat movements may indeed have changed, contributing to the pattern of increased sightings, rather than changes in human reporting behavior only. First of all, rodent sightings are surprisingly high near food service counters and restaurants. Secondly, the hotspots of rat sightings have changed during the pandemic.

The changes in requests for pest control interventions were localized and specific to certain locations, near regions with a high density of restaurants in both cities. Still, no changes were observed in Warsaw, where restaurants are not packed within densely populated areas.

Contradictory Trends in Different Areas

In New York City, the 12 month study period witnessed lower daily rat sightings as based on the number of calls to pest control institutions. The businesses themselves reported that they felt the number of sightings had gone up. The number of calls related to food service establishments went up after lockdown. The hotspots were consistently around the clusters of food service establishments throughout the study.

In Tokyo, there was a 23% increase in calls relative to the last five years, but the survey results showed that people were not aware of this shift.

In North America, 13% of businesses said that all their rat-extermination jobs were from new clients, and an equal number said none of these jobs were from new clients. Overall, a third of these companies said that half or all of their work post-lockdown, related to rats, was from new clients. This is a pattern seen after calamities and reflects actual changes in the rat population.

In Tokyo, the proportion of jobs from new clients was low for 60%, while in both Poland and North America, more than a third gave this response.

No new clients asked for rat-related jobs in a fifth of businesses in Tokyo. However, about 90% of those in North America who said their business had increased said they had more clients compared to last year.

Explanations for Geographical Differences

In New York City, most sightings are near subways and public parks or leisure spaces, but these were much less used, since most people in April 2020 were at home or had left the city. The sudden reduction in food supply for rats resulted in a decrease in the number of rats.

Also, the venue of sighting makes a difference in the number of calls and the number that is called. One rat in a restaurant could mean many calls to 311, while multiple rats in one home could mean one call to a private pest exterminator to get immediate action.

The Picture in Tokyo

In Tokyo, the reason that the increased calls to the pest control operator Tokyo Pest Control Association (TPCA) did not correlate with increased calls to private pest control businesses could be due to different impacts of the lockdown on the two different species of rats. Roof rats were mentioned in about 80% of the complaints, but are smaller and more elusive, and nest in roofs or ceilings.

Moreover, roof rats are more likely to be found in smaller buildings because larger buildings must have pest control measures by law. Moreover, since smaller restaurants and bars remained open for take-aways, roof rats had access to food supplies. In short, this rat population would not have contributed to a significant proportion of the calls.

In eastern Tokyo, on the other hand, the downtown area with a heavy concentration of restaurants saw a large amount of garbage placed from midnight to morning for kerbside pickup. This provided the other rat species, Norway rats, abundant access to food, and this increased the number of calls to TPCA, while not creating enough of a menace to call in a private exterminator. Moreover, most businesses in pest control have yearly contracts with eateries, and this would not be affected by the lockdown.

In Poland, not only is a public pest extermination request system non-existent, but mandatory pest control ensures strict control of rats at all times, which would not be affected by the lockdown. Moreover, restaurants are sparse in areas with a heavy population.

Migration of rats under stressed conditions could lead to more interbreeding, and the genetic profile of urban rats in the future might well change, as a more significant number of genotypic variations enter the genetic pool.

Human Health Hazards Caused by Rats

Do rats get COVID-19? Experts think it unlikely, as only two species of rodents have been found to be infected with this virus. However, theoretically, rodents under stress or who are wounded are also less likely to groom themselves, and dirty fur can transmit more infections even when the rat itself is not a carrier. Secondly, if the rat carries pathogens that cause immunosuppression, such as the ticks that cause Lyme disease, the infected individual might be at risk for other diseases, including COVID-19. Thus, rat populations and the shifts in rat movements need to be monitored more diligently to allow better management.

This news article was a review of a preliminary scientific report that had not undergone peer-review at the time of publication. Since its initial publication, the scientific report has now been peer reviewed and accepted for publication in a Scientific Journal. Links to the preliminary and peer-reviewed reports are available in the Sources section at the bottom of this article. View Sources