Public Health Importance of Urban Rodents

The purpose of this article is to provide an important research update addressing the significance of city mice and rats and everyday public health. As PMPs likely already know, public health updates are important for the pest management industry. They not only help us better serve our residential and commercial clients (e.g., schools, restaurants, office buildings), but even for our own lives, homes and families.

VIRUS HUNTERS. In New York City, the well-respected Center for Infection and Immunity is located at Columbia University’s School of Public Health. The scientists at this center have been studying the microbial pathogens (e.g., viruses, bacteria) associated with various insect and animal vectors (e.g., mosquitoes, rodents, ticks, etc.) for years.

This update also provides important tips for safely working with rodents on your everyday accounts. Moreover, these research findings should serve to emphasize why the integrated portion of Integrated Pest Management (IPM) must continue to be strongly emphasized in urban rodent control.

One group of these scientists is led by Dr. Ian Lipkin, the virus hunter internationally recognized for his work with West Nile virus and SARS. Dr. Lipkin served as the science consultant for the film Contagion — a film that was acclaimed for its scientific accuracy.

Beginning in 2012, Dr. Lipkin’s teams began live-capturing and analyzing everyday city rats and mice from New York. The goal was to study which viruses and bacteria these rodents were carrying, and whether or not they might pose public health threats. Columbia published their important research findings of city rats (Rattus norvegicus) in 2014. (See references on page 46.)

The more recent rodent research update (and addressed here) discusses Columbia’s second rodent research project, which was conducted similarly to the rat study, except it focused only on wild house mice (Mus domesticus). The research findings were published in two separate journal papers earlier this year and received quite a bit of attention from the global press (which no doubt, many of our clients read online and may have had follow-up questions for you out in the field). The following is an overview of the research findings.

CITY MICE & GERMS. The city mouse project was led by Simon Williams, a research scientist with Ian Lipkin’s lab at Columbia. Simon was joined by a team of virologists, microbiologists, epidemiologists and rodentologists.

The mouse team live-trapped 416 wild house mice from various residential and multi-functional buildings throughout greater New York. By employing a range of sophisticated laboratory techniques, the droppings, urine and the mice themselves were analyzed, allowing Williams’ team to isolate and identify the bacteria and viruses found on and within the mice.

DANGEROUS FINDINGS. Importantly, the collected house mice were found to be carrying several of the bacteria responsible for human gastroenteritis (inflammation of the stomach and intestines, typically resulting from bacterial toxins or viral infection).

Five bacterial pathogens and one protozoan were discovered in the wild house mice with significant frequency:

1. Salmonella (different strains)
2. Escherichia coli
3. Clostridium difficile
4. Shigella
5. Leptospiria spp.
6. Toxoplasma gondii (protozoan)

Depending on the acquired infection level and the specific bacterial pathogen, should a person ingest (directly or indirectly via contact or inhalation), the resultant sicknesses ranges from mild to life-threatening. Salmonella bacteria, for example, is a leading cause of bacterial food poisoning in the United States with 1.4 million reported cases annually along with 15,000 hospitalizations and 400 deaths. And if we consider the impact of all foodborne pathogens, the Centers for Disease Control and Prevention in 2010 reported a staggering 76 million cases of foodborne illness and between 3,000-5,000 associated deaths occurring every year in the United States.

Among the millions of less severe cases of foodborne illness infections (and if you are reading this, no doubt you have “been there, done that”), most result in diarrhea, fever and stomach cramps with accompanying and sometimes severe vomiting one or two days after contact or somehow ingesting the bacteria or viruses from fecal or other contact from rodents or other pests (flies, cockroaches) or animals (see Figures 2 and 3).

Toxoplasma gondii is a protozoan parasite that causes toxoplasmosis, of which the cat is the definitive host. Cats become infected by killing and eating house mice infected with T. gondii. When the parasite gains a foothold around mouse-infested homes and apartment complexes (via cat feces accumulating in basements, crawlspaces, unkempt apartments, etc.), the house mouse serves as an intermediate host contributing to the parasite’s persistence and propagation. If pregnant women contact infected cat feces, they can become infected, resulting in spontaneous abortions or various fetal abnormalities, such as lifelong retinal damage causing partial or complete blindness in one or both eyes.

The leptospira bacteria found in the NYC mice (and rats) are probably the most widespread and most prevalent of all zoonotic diseases (i.e., transferable from lower animals to man). The leptospirosis bacteria can be transmitted to people via the urine from rodent pests around buildings. This happens via skin contact with leptospirosis-contaminated water, moist soil and vegetation, or sometimes via the direct ingestion of food contaminated by infected rodents.

Symptoms of leptospirosis often closely mimic the common flu: fevers, headaches, diarrhea, chills and vomiting. As such, many cases are too casually dismissed by both the patients and their physicians as “the flu.” In severe cases, however, this bacteria can be deadly as it can cause severe kidney damage, jaundice and hemorrhaging. Just last year (2017) in New York City, rodent-transmitted leptospirosis killed one and hospitalized others in the Bronx. Because leptospirosis is prevalent among rodents in American cities containing high populations of rats and mice, perhaps health departments should undertake more preventive programs with appropriate serology and analyses of local rat populations. (See the comments of Lieberman.)

It should be noted that the discovery of these particular germs with the NYC mice isn’t revolutionary. During the past several decades, other researchers have recorded rodents carrying various microbial pathogens. What is significant is this was the first time a random sample of just a tiny portion of the wild house mice of America’s most highly dense and populated city has been profiled — which begs the question: “What might be the profile of any of the other millions of mice not sampled?”

There was an additional interesting and important finding in the study. Columbia also discovered within the NYC mice the evidence of genes widely distributed that can bring about antimicrobial resistance (AMR) to several of our most common antibiotics including the fluoroquinolones and -lactam compounds. Notably, these particular antibiotics are, in fact, among the most commonly used drugs to combat the bacterial gastrointestinal infections caused by the pathogens discovered in the wild mice of this study.

For example, one of the fluoroquinolone antibiotics is ciprofloxacin (trade name: Cipro). This drug is used worldwide for the treatment of gastroenteritis. And, the -lactam drugs include the universally known penicillin derivatives. For many years, more than half of all commercially available antibiotics in use were of the -lactam compounds.

With the Columbia study then, it is yet another example of the axiom “the deeper we look, the more we find.” Finding the AMR association in everyday house mice certainly opens up the door for additional questions regarding one of our most pressing worries — the threat of continual increase of antibiotic resistance.

NOVEL VIRUSES. Columbia’s second study on house mice (also published in the journal Molecular Biology), investigated the viruses present in mouse droppings. Interestingly, 36 viruses, including six new viruses, were identified. None of the viruses were found to be varieties that cause human illness; this was different from what was found in the bacteria study.

This is not to say the wild mouse viruses were cleared of all guilt. The study did identify genetic sequences matching important viruses that infect insects, dogs, chickens and pigs, findings which will certainly be of interest to veterinarians and livestock producers everywhere (considering how prevalent mouse infestations are around livestock, animal hospitals, zoological parks and so on).

Nevertheless, the lack of human-pathogenic viruses — at least in the sample of mice collected in this study — is a bit of a relief compared to what we’ve been alerted to with mouse viruses during the past 30 years (e.g., hantavirus and deer mice, LCMV, etc.).

NO GUARANTEED TRANSMISSION. It is important to keep in mind, not only for this study, but for any research addressing pest-species vectors, that just because animals are found to be harboring germs, it doesn’t guarantee those germs will be transmitted to other animals around them. Such is the case with this house mouse study as well. More research is needed to measure, if possible, the likelihood of transmission of microbial pathogens found on mice and rats, or even cockroaches and flies, for that matter, under typical everyday conditions.

Still, it’s sobering that in only the space of a period at the end of a sentence several million viruses can fit. And, only one mouse in a restaurant can produce upwards of 125 fecal pellets and upwards of thousands of microdroplets of urine in 24 hours (see Figure 3).

What’s more, we know mice constantly dart about in thousands of “spots” throughout the rooms where they are active day-in and day-out and that they tend to investigate many of the everyday objects they encounter during those forays. So, it seems an obvious likelihood that sooner or later an infected dropping or micro-droplet of urine will find its way into “our world.”

(Photo: Bobby Corrigan)

One example of this would be the all-too-common mouse infestations that occur in the ceilings above the kitchens in our commercial accounts (e.g., restaurants, bakeries, delis, grocery convenience stores, etc.). For these infestations, there is a decent possibility of pieces and parts of feces, hair, urine fragments and so on routinely falling down (partly by the daily vibrations that occur from all the moving of objects and activity below busy commercial ceilings) onto foods, food boxes, preparation areas, utensils, plates, pots and pans and the like. Similarly, droppings and urine can be deposited within food containers stored within the mouse-attracting dark, quiet drawers and closets of common household pantries of residences everywhere (see Figure 2).

The bottom line: transmission of bacteria and viruses from interior mice can happen at any time and in any number of ways. This also includes when we are actually trying to clean up the presence of germs and dirt via sweeping and vacuuming — which can then render microbes airborne and inhaled by those doing the cleaning.

WHAT’S IN YOUR WALLET? Perhaps you are thinking, “Well, the rats and mice collected in these studies were from New York City, a very crowded, old seaport city with more than 900,000 buildings and lots and lots of garbage, litter and the like. It doesn’t mean the rodents in my city, town or area are carrying the same pathogens.” Well, maybe. But until a city rodent/pathogen profile is conducted in your city, the question truly is, “What’s in your wallet?” Maybe it’s less? But maybe it’s more.

This then leads us back to the Will Mayo quote at the beginning of this article about being preventive (in part, via effective, well-designed and well-implemented pest management programs at the ground level). Because in all cities around the entire globe, the fact of the matter is millions of cases of unexplained febrile illnesses year after year are not actually diagnosed.

When it comes to being preventive around any city where mice and rat infestations occur, Jay M. Lieberman, M.D., an infectious diseases specialist and former professor of clinical pediatrics at the University of California, Irvine, suggested in an important 2009 publication salient advice for all: namely, that when any ill person is brought in for medical attention with an unexplained fever-related (i.e., febrile) illness or infectious disease, clinicians should routinely ask about any potential exposures to animal pests such as mice and rats that have been active around the domicile or workplace.

TAKE-AWAY TIPS. Rodent control is among our industry’s highest revenue-generating services, as well as one of our primary identities as to why we truly do help protect the health and lives of humans. So how can we use this important research update in what we do and what we offer in our services, not to mention in our own daily work?

Here are some tips:

• The Columbia research further substantiates the importance of the basic tenets of exclusion and sanitation within the urban IPM model. If rodents can’t get into the buildings in which we eat, sleep and work in the first place, they can’t deposit fecal matter of any sort containing pathogens and/or the genes for facilitating anti-biotic resistance that we may later contact, ingest or inhale.

• Ensuring rodent-proof doors, sealed holes and detailed/cleaned floors in shadowy, hard-to-reach spots (i.e., the favorites of house mice) is not, as the saying goes, rocket science. Updates in the public health risks of rodent pests can help our clients be aware of the importance of the essential partnership in any IPM program between them and us. Company fact sheets, website blogs and simple handouts with highlighted points recapping such research can go a long way in your role in promoting public health to schools, homes, apartment and office buildings, restaurants and so on.
• Any client with a current rodent infestation and/or who has called upon you to provide the service to eliminate such rodent infestations should be reminded as to the importance of personal hygiene (e.g., frequent washing of hands) — especially until the rodent infestation is corrected.
• Services to address rodent infestations must be sold with an attitude of achieving high control on an ASAP basis. Inexpensive services can hardly input the up-front labor necessary to get the population eliminated fast. (Consider all the fecal material that can be laid down by a few mice in just a few days.)
• Rodents in our commercial accounts tend to favor nesting and reproducing and being most active in those structural areas that are often hard-to-reach (and thus often prone to being skipped over during service visits). Employing any of the electronic remote sensors now available on the market serve as important food safety and public health monitors in this regard.
• Cleaning up any excrement and associated rodent filth is the client’s responsibility. Clients should clean and disinfect all areas in which rodents have been traveling and/or have left fecal material (advise clients to refer to the www.cdc.gov for proper methods of disinfecting small and large amounts of animal excrement).
• All on-the-job pest professionals must always wear issued PPE (gloves, respirators when inside enclosed spaces) while performing inspections, and/or laying down indoor rodent control equipment within infested areas.
• Always wash any coveralls or discard any Tyvek-style suits in the proper donning and doffing procedures after using them in crawlspaces, attics or other areas/surfaces where rodent infestations occurred. (Check the cdc.gov site for donning and doffing in these scenarios.)
• Even after wearing your disposable gloves during rodent control work, wash your hands after removing the gloves each and every time.
• Never, ever, clean out rodent-visited exterior bait stations or traps by sweeping the station’s contents (feces, dirt, hairs, etc.) onto any client’s property.
• To dispose of dead rodents found on the job (including contaminated glue traps) always double-bag and remove the contents from the premises and discard in an appropriate trash receptacle.

SUMMARY. Perhaps the most succinct way to summarize this important Columbia University research would be to consider two things. First, what if the mice collected from this study targeted only the mice of restaurants, or schools, or office buildings? Would less, or more pathogens have been found?

(Photo: Bobby Corrigan)

And second, I’d like to repeat the answer to a question I had posed to the famous food microbiologist Dr. Michael Doyle, of the University of Georgia, at a food safety symposium awhile back: “Would you, Dr. Doyle, eat in a restaurant if you knew it had only one mouse positive for Salmonella enteriditus hiding somewhere in the kitchen?

”Dr. Doyle’s answer: “I would not. Would you?”

The author is an urban rodentologist with RMC Pest Management Consulting, Briarcliff Manor, N.Y.

References (Free via open access [OA])

Firth C., Bhat M., Firth M.A., Williams S.H., Frye M.J., Simmonds P., Conte J.M., Ng J., Garcia J., Bhuva N.P., Lee B., Che X., Quan P-L, Lipkin WI. 2014. Detection of zoonotic pathogens and characterization of novel viruses carried by commensal Rattus norvegicus in New York City. mBio 5(5):e01933-14. doi:10.1128/mBio.01933-14.

Simon H. Williams, Xiaoyu Che, Ashley Pawulick, Cheng Guo, Bohyun Lee, Dorothy Muller, Anne-Catrin Uhlemann, Franklin D. Lowy, Robert M. Corrigan, W. Ian Lipkin. New York City house mice (Mus musculus) as potential reservoirs for pathogenic bacteria and antimicrobial resistance determinants. mBio, 2018; 9 (2): e00624-18 DOI: 10.1128/mBio.00624-18

Simon H. Williams, Xiaoyu Che, Joel A. Garcia, John D. Klena, Bohyun Lee, Dorothy Muller, Werner Ulrich, Robert M. Corrigan, Stuart T. Nichol, Kormal Jain, W. Ian Lipkin. 2018. Viral diversity of house mice in New York City. mBio 9:e01354-17doi: 10.1128/mBio.01354-1717 April 2018 mBio vol. 9 no. 2 e01354-17

Lieberman, J.M. 2009. North American Zoonoses. Pediatr Ann. 2009 Apr:38(4):193-8