[Annual Rodent Control Issue] On the Trail of Troublesome Rodents

[Annual Rodent Control Issue] On the Trail of Troublesome Rodents

A team of researchers has been diligently roaming NYC streets in search of rodents that are not only elusive, but that may carry a flea species capable of transmitting plague pathogens.

September 2, 2015

At 5:30 a.m., on a Sunday, Harlem’s 125th street is asleep and strangely quiet. A man dressed in dirty jeans and work boots steps off the train and makes his way to a dark street corner. Leather gloves, forceps, a flashlight, flea comb, and miscellaneous supplies fill his backpack. A van approaches from the west and slows to a stop. The door slides opens and sleepy eyes meet his own. Another day of rodent research has begun.

The van makes its way downtown through abandoned streets in this city that never sleeps. The first stop is a building where cage traps were set the night before to live-capture the city’s most notorious pest: Norway rats. For one full week traps were pre-baited with pieces of chicken, cucumber and apple slices, and rodents in the building’s crawlspace have acclimated to the new objects and are now more likely to be caught.

The Task at Hand.

The team of researchers is investigating the pathogen and ectoparasite burden of New York City rats — a preliminary step in understanding the risk of human contact with rodent-borne disease. And although rats abound in Manhattan, live-trapping the diabolically clever animals requires patience and skill, especially an ability to identify and interpret rodent evidence such as droppings, sebum trails, gnaw marks, footprints and burrows (see related story on the right). Pathway analysis and placement of rodent management devices in commonly used runways, whether live traps, snap traps or bait stations, will greatly improve their efficacy.

Know the Signs

Being able to identify and interpret rodent evidence such as droppings, sebum trails, gnaw marks, footprints and burrows is essential to controlling these urban pests.

Droppings. A single Norway rat may deposit 40 to 50 ¾-inch droppings per day. According to noted rodentologist and PCT technical editor Dr. Bobby Corrigan, droppings are the roadmap to rodent activity, and their presence can help identify high-activity areas. Droppings can tell you about the structure of the population (small/large mixed can indicate adults and juveniles), or how recent activity might be (fresh vs. old).

Sebum. Mammals protect their hair and skin by secreting an oily or waxy material called sebum. Because rodents tend to travel the same pathways, this material gets deposited on surfaces and leaves a trail or grease mark. Combined with urine, these trails contain pheromones that orient rodents within their environment.

Gnaw Marks/Entry Points. Rodents gnaw to obtain actual or perceived resources. Specifically, rodents may create or expand an entry point, or chew on wires that mimic twigs and vines. Gaps larger than ¼ inch tall, and openings 3/8 inch wide permit entry by rats.

Burrows. Rats burrow in soil, with a typical nest of eight rodents having three 3.5- to 4-inch burrow openings. In most cases, one opening is smooth, while two are escape holes camouflaged by leaves, trash and other debris.

Footprints. Whether in dust, dirt-floor crawlspaces or in talc powder, rat pathways may be identified by their ¾- to 1¼-inch footprints and tail drag.

Emerging from a dirt floor crawlspace with hardhats, headlamps, respirators and full protective equipment, the scientists remove eight traps containing rats from the building’s underbelly. Not a bad haul. It’s a short ride from here to the main office of Assured Environments, a pest management firm in New York that donated laboratory space for the group to conduct its study. Water bottles are affixed to cages, which are then covered with dark cloth pillowcases to keep the rats calm for their journey.

Before most New Yorkers take their first sip of coffee, the team has euthanized, investigated the pelage, and dissected three rats. While tissue samples will be analyzed at a later date for the presence of pathogens, a flea comb with round teeth and short spaces between tines is used to meticulously inspects the rat’s fur for crawling arthropods. The first critters to be found are not surprising. Since early human history, Norway rats have stowed away on vessels and have been transported by humans across the globe. Where the rats went, so too did their ectoparasites, including those found on New York City rats: the tropical rat mite, the spiny rat mite and the spined rat louse. Although these organisms can bite humans in the absence of their preferred host (rats), the good news is that rodent ectoparasites will not infest humans, and are not known to transmit human disease.

The ‘Unusual’ Suspects.

Among the 133 rats collected in the study, however, lurks a foe of infamous repute, and one that has not been reported in New York since the 1920s. Known for its role in transmitting murine typhus and plague, the Oriental rat flea was collected on 30 percent of New York City rats. What’s more, rats hosted an average of four fleas each, with several rats having more than 25 fleas in their fur. The formula for disaster is not hard to imagine: reservoir host (rats) + vector (fleas) + disease organism (plague/typhus) could spell trouble. Fortunately, no evidence of plague or typhus was detected in the fleas or the rats themselves, but fleas were infected with Bartonella, a bacteria that can cause febrile illness in humans and one that is likely underreported.

The authors found that neither the rats nor their ectoparasites were evenly distributed within New York City, a phenomenon well known among pest professionals. Some experts consider these rat “hot spots” to serve as reservoirs for the surrounding area. Indeed, 94.1% of all Oriental rat fleas collected in this study were recovered from a single site that was heavily infested with rodents.

When tissue samples from the rats were analyzed, a variety of pathogens were detected that can cause acute or mild gastroenteritis in people, including atypical enteropathogenic Escherichia coli, Clostridium difficile and Salmonella enterica. Furthermore, infectious agents that cause fever-like illnesses were also found: Bartonella spp., Streptobacillus moniliformis, Leptospira interrogans and Seoul hantavirus. Interestingly, a number of novel viruses — new to science that is — were detected in rats, while the presence of known viruses such as hepatitis C may help scientists to better understand human infection by this pathogen.

The two studies, funded by the Center for Infection and Immunity at Columbia University, highlight a pressing need. Although the number of rats in New York and other cities remain elusive (and likely impossible to estimate), detection of new viruses and medically important vectors in a sample of only 133 rats is telling. The authors suggest that more work is needed to fully characterize the pathogen and ectoparasite profile of rats in NYC, and to understand how these relate to the incidence of human disease.

For PMPs that engage in rodent management the message is clear: additional PPE may be needed when entering areas with a large rodent infestation, especially humid dirt floor crawlspaces with lots of droppings. While mites and lice are crawling arthropods that likely spend all of their time on the host, flea larvae develop off the host near rodent nests and can jump onto approaching organisms for a blood meal — including humans.

The research also provides an opportunity. How can PMPs help reduce human interactions with rodents and their ectoparasites? It isn’t necessarily through trapping or poisoning, which are critical to rodent management, but can cause ectoparasites to leave the host body in search of a blood meal. Instead, pest professionals can offer exclusion services to keep rodents out of structures where people live, work, learn and play. In fact, as an industry, PMPs are uniquely qualified for this task, armed with the ability to identify and interpret rodent evidence and pathways. We can learn from the guide Pest Prevention by Design,* a document that provides specifications for the type of materials needed to keep pests out, including the thickness of woven hardware cloth, galvanized sheet metal, and concrete. Importantly, this document helps to differentiate weatherization (making a structure efficient in terms of heating and cooling) and pest exclusion (keeping pests out). A distinction that matters because materials such as expanding foam may insulate a space, but will not prevent a rat from entering.

For the homeowner, PMPs can recommend good sanitation to keep areas free of the food, water and shelter that sustain pests. The combined efforts of population reduction, pest exclusion and sanitation lead to effective pest management.

As the Sun Goes Down.

At 8:00 p.m., the sun has set and darkness returns to the city. Contrary to the early morning quiet, the streets are alive with restaurant-goers and sightseers. Among them, lurking in the shadows, are rats — rats carrying pathogens and ectoparasites. They are in the streets, subways, offices, residences and parks. But their cryptic nature cannot protect their secrets forever. Scientists in New York, Baltimore, Los Angeles, New Orleans, Vancouver and around the globe peer into their mysterious lives, searching for ways to reduce human-rodent interactions. Meanwhile, in Harlem, a weary scientist boards a train as colleagues drive north to deliver samples for analysis. The day is long, but before he drifts off to sleep he thinks, “some day this will make for a great story.”


About the author: Matt Frye is Community IPM Extension Educator at Cornell University New York State IPM Program, and can be contacted at mfrye@gie.net.

*Geiger, C., and Cox, C. “Pest Prevention by Design, Authoritative Guidelines for Designing Pests Out of Structures.” SF Environment Brochure (November 28, 2012)