Rodents on the Rise

Climate and human factors seen as common causes.

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It’s being seen:

…from California: “California is experiencing a massive spike in its rodent population that is both measured by available data sets and observed by field personnel.” (Reform California)

…and Oregon:“There’s been a 20% climb in the rodent population in just the last three years, a local (Eugene, Ore.) pest elimination expert said.” (

…to New York:New York has always been forced to coexist with the four-legged vermin, but the infestation has expanded exponentially in recent years, spreading to just about every corner of the city. ... We’ve seen rats the size of Cleveland.” (New York Times)

…and Philadelphia:“In January, Philadelphia made national headlines as having the most rat sightings of any U.S. city. ... Philadelphia, like most major cities, especially on the East Coast, has a rat problem that’s not going away.” (Philly Voice)

Numerous media reports have cited 2019 as having brought a rise in rodent populations in cities across the U.S. — and according to pest management industry experts, it’s more than media hype. As National Pest Management Association (NPMA) Chief Entomologist and Vice President of Technical and Regulatory Affairs Jim Fredericks said, “NPMA members are seeing extensive rodent activity across the United States.” Some urban areas are experiencing increased activity, especially where there are sanitation concerns or homeless encampments with abundant food and harborage, and suburban towns and subdivisions are experiencing rats, likely due to rat migration from high density urban centers into these new locations as the suburban infrastructures mature, he said.

RMC Consulting President Bobby Corrigan, a renowned rodent expert, has seen rodents universally increasing for the past 10 years, not just across the U.S. Rather, he said, “It is global.”

WHY ARE THERE MORE RODENTS? Corrigan sees a number of possible causes for the increase of rodents, including:

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  • Consecutive mild winters in most areas. “Rodents traditionally shut down their reproduction during the cold months and resume in earnest in the spring, summer, and fall.” But, Corrigan said, “A mild winter enables a maximum window for perhaps one more litter before shut down. That ‘one more litter’ adds up.”
  • Increases in human population into urban areas (many of which have food operations such as manufacturing, foodservice, food refuse systems, etc.) and more humans in general — which equates to more food and harborage for rodents.
  • Incomplete control programs due to cost. “The marketplace is often not willing to bear the cost of 100% eradication of a rodent infestation on a premise,” Corrigan said.
  • Increasing behavioral resistance to trap devices (particularly snap traps and glue traps for mice) especially in areas where these have been in use for a decade or more.

Additionally, he said, “When taken across the entire scale of most cities, and the pervasiveness of mice and rats in these cities, suburbs, rural areas, partial rodent control programs keep rodents on a slow burn for long periods of time with periodical flare ups.”

Other potential causes cited by experts include:

  • The unprecedented heat of the summer — with July being the hottest month on record for the planet, according to NOAA — could have caused rodents, especially rats, to get closer to buildings and be reported more than in the previous years, said Zia Siddiqi Consulting Owner Zia Siddiqi. “In my professional opinion, this would be true for rats or other exterior rodents trying to enter buildings, but not for house mouse since these are mostly established indoors and may hitchhike on incoming shipments.”
  • “We often see increases in rodents associated with construction and environmental extremes,” said Pest Management and Food Safety Professional Kim Kelly-Tunis. “For example, in areas that are experiencing a lot of new construction, rodents are often displaced and try to shelter in the closest structure.”

Overall, the warming weather was most frequently cited as a likely cause of the increase in rodents, with Cornell Community IPM Extension Area Educator Matt Frye providing the following explanation:

Think of the rodent population as a steadily increasing line; in a typical year, the cold winter months stop growth and cause the line to drop dramatically. It then takes time for the population to rebound, start growing at the same rate, and eventually reach the point where it previously dropped off.

But with winters being milder, that drop in rodent populations is less dramatic, and populations are able to rebound and start growing again much faster, he said. This means it takes less time for the population to reach — and potentially surpass — the previous peak.

Rodents’ habit of defecating and urinating on items and surfaces as they move about Is one of the leading causes for the contamination of products, equipment, and food prep surfaces in food facilities.
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PUBLIC HEALTH SIGNIFICANCE. “In terms of the public health significance in food facilities, rodents are reservoirs of pathogens that affect people and animals,” Frye said.

“Recent research has demonstrated that both mice (house mice) and rats in New York City are carrying a number of pathogens that affect people,” he said, adding that cases of both leptospirosis and hantavirus pulmonary syndrome have been reported from metropolitan areas.

Echoing such citations, Corrigan said, “Multiple independent studies from different parts of the world have formally documented the foodborne pathogens associated with rats and mice.” Citing 2016 studies from Columbia University, he said that wild house mice have been shown to commonly carry Salmonella, E. coli, shigella, and clostridium difficile. Rats have been strongly associated with these same pathogens as well as campylobacter.

As such, Fredericks said, “One of the most important public health concerns with regard to rodents in food plants is their ability to transmit Salmonella through their feces and mechanically on their feet and fur.”

It is exactly that — the rodents’ habit of defecating and urinating on items and surfaces as they move within their home range — that, Kelly-Tunis said, “Is one of the leading causes for the contamination of products, equipment, and food prep surfaces within food facilities.”

TODAY’S TECHNOLOGIES. As in so many aspects of food processing plant management today, new technologies are bringing a greater range of options for dealing with rodents — even with the increase. One of those is remote digital monitoring technology, which, Corrigan said, “Is finally making its appearance in real-world pest management for the food industry.”

Digital monitoring systems can provide electronic analysis of rodent activity around and inside a food facility illustrating the causative reasons for rodents.
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As described in “What’s the Buzz?” in the September/October 2018 issue of QA, (, it is a digital system of rodent detection, data collection, and notification with sensors installed in rodents traps throughout a facility or in specified locations. The traps are set to automatically monitor and report rodent entry and alert the service provider (and/or food facility) of a capture. And, regardless of the specific developer, provider, or design, “electronic monitoring is becoming more and more common in food facilities and can be an attractive addition to traditional pest management services,” Fredericks said.

Among the benefits and challenges of the electronic rodent monitors, as identified by the experts, are:

  • Rodent capture can be monitored on a 24/7/365 basis. This is particularly beneficial in hard-to-reach and out-of-sight areas (such as false ceilings) that breeding rodents love but are very time-consuming to access and inspect.
  • The technology reduces response time: When a rodent enters a trap, trained technicians can be alerted immediately, setting into motion the appropriate steps to remove the pest and prevent reinfestation.
  • The systems can provide electronic analysis of rodent activity around and inside a food facility illustrating the causative reasons for rodents (e.g., a door that is not properly rodent proofed; an improperly managed dumpster, a cluttered portion of the warehouse, etc.)
  • The technology can help to increase technician effectiveness, allowing pest management professionals the opportunity to spend less time checking traps and more time identifying opportunities to prevent infestations in the future, shifting the focus from pest control to pest prevention.
  • Digital monitors enable a facility to have a truly preventive rodent pest management program in line with FSMA directives, as early or immediate detection of rodents allows for immediate intervention to prevent rodents from increasing in number before the next “scheduled” pest management service visit. This can dramatically reduce the the false negative potential of a person surveying a food facility only once or twice a month for only minutes per location.
  • The associated software allows for dashboard data organization, which is very valuable in pest prevention and record keeping.
  • The use of electronic rodent monitors can help to improve the quality of pest management service being provided in the food facility, and they not only increase detection in the area of the facility that is being monitored for activity, they also provide the real-time collection of data documenting rodent behaviors.

But because of the newness of digital monitoring technology and its continuing evolution, there also are some challenges with its implementation:

  • While it does offer the benefits of identifying if a rodent has been trapped and trigger follow ups, from the pest management provider’s perspective, it may trigger unscheduled follow up every time there is an alert, depending on the expectation of the food facility, which also could impact the service cost.
  • It is often marketed that the technology eliminates the need to check all the traps if there is no alert. But from a food safety perspective, as required by FSMA’s preventive controls and GFSI, each and every area of the food facility must be inspected.
  • Any remote monitoring system that is implemented should be made a part of the operational software system being used by the pest management provider; otherwise two systems would be run with different reports required.
  • The systems require various interfacing tools (such as a WiFi router, repeater, modem, etc.), which may increase the initial cost and maintenance fees. However, some systems are, instead, incorporated into the traps without use of exterior technologies, enabling the data and reports to be available through one operating system.
  • A major question, in some cases, is who owns and stores the monitoring data? While most data are stored in the remote monitoring company’s server, the food facility managers may feel that because the data belong to them, they would want full control over it.

Other significant new technologies include the use of wildlife cameras (“cams”) to detect elusive rats and mice in facilities. “What used to take hours and hours of human inspection time can now often be solved in one night of camming — pending good location spotting of the cams,” Corrigan said.

He noted that there also is additional research in the early stages, such as rodent-sniffing dogs (similar to bed bug sniffing dogs) to locate mice and rats hiding in product pallets, walls, etc., and the use of drones in large food distribution centers as ongoing sentries for spotting rodents on floors and pallet tops.

While experts agree that rodent pressure is increasing across the U.S., and even around the world, they also are seeing significant innovation in technologies to help fight the escalation and protect against the pathogenic contamination potential of rodent pests.

The author is Editor of QA magazine. She can be reached at

September October 2019
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