The Significance Of Commensal Rodents

Rodents impact our food, our shelter, our health, and our comfort. In some parts of the world, they are the No. 1 pest, more important than even the most troublesome insects. This article presents an overview of the significance of rodents as pests in the urban environment.

No doubt, commensal rodents are among the most significant of all pests on earth. In our cities and towns, rats and mice take advantage of our food and shelters, multiply into populations of millions, and attempt to co-exist with us in our homes, offices, and food preparation and storage facilities. Rodents are also especially well designed for carrying and transmitting diseases, and they frequently damage and destroy our buildings, utility systems, and electronic communications systems through their gnawing and burrowing activities.

The cost of controlling rodents on a global scale is estimated to be in the billions of dollars. Paradoxically, however, rodents are also among the most beneficial of all mammals to humans, as they are invaluable in health and pharmaceutical research. In non-urban environments, they are an integral component for maintaining balance within most ecosystems.

Rodents impact our food, our shelter, our health, and our comfort. Commensal rodents are among the top five most important pests in urban and industrial communities. In some parts of the world, rats and mice are the most important pests insects included. The purpose of this article is to present an overview of the significance of rodents as pests in the urban environment. By understanding how rats and mice impact people and other animals in everyday life, the importance of professional rodent control programs quickly becomes apparent. It is not an exaggeration to say that the earth and its inhabitants would be dramatically affected for the worse without rodent control.

RODENTS IN CITIES. Much is said and speculated about the numbers of rats and mice in our cities and towns. Statements such as "there is one rat for every person in New York City" and "there are more rats in the world than people" are still prevalent among urbanites. Often pest professionals are asked, "How many rats live in my area?" or, "How many mice do you think we have in our warehouse?"

Interesting questions. But even rodentologists cannot provide accurate answers to these questions. In general, not many cities in the world contain rodent populations that equal the human population although there are some where the rodents outnumber the people.

For most developed areas of the world, however, the rodent populations are "clumped," with large populations occurring in some areas and smaller numbers in others. The rodent populations are usually distributed according to the specific environmental conditions of an area. For example, rodent populations are typically larger in urban areas where general sanitation is poor (i.e., abundant garbage and rodent harborage) and rodent control programs are lacking. In those parts of the city where sanitation and control programs are in place, however, only small populations of rats will be able to eke out a living.

It is safe to say that the total population of commensal rats and mice in the larger cities of the world is in the millions. We also know, from body counts of rodent "cleanouts" of badly infested buildings, that thousands of rodents can exist in one building. Documented cases also exist of occasional mouse "plagues" where hundreds of thousands of mice are observed overrunning towns and farms. One Australian farmer recorded 28,000 dead mice on his porch after one night's poisoning. Another 70,000 were killed in his crop field in one afternoon. But such plagues are uncommon, and are cyclic population explosions that result from various coinciding environmental conditions. They are not likely to ever occur in urban areas.

So, we cannot say with any accuracy that "15 million" rats live in New York City or Cleveland or Chicago. For the pest management professional, the more realistic concern relative to estimating rodent population sizes regards a specific population inhabiting a particular building and its immediate environs. When designing control programs, the mistake of underestimating the size of a serious rodent infestation in a building can become costly, due to repetitive callbacks from dissatisfied clients.

DAMAGE TO FOOD. On a global scale, it is difficult to measure the total amount of food consumed or contaminated by rodents, but it is significant. Other books and papers address this subject in detail and provide various statistics regarding the impact of rodents on agricultural production. In 1982, for example, it was reported by the Food and Agricultural Organization of the United Nations that rats destroyed more than 42 million tons of food at a cost of $30 billion.

It has been repeatedly estimated that between one-fifth to one-third of the world's food supply never reaches the table due to losses from rodents. In some parts of the world, the entire crop grown for human consumption has been lost due to rodent outbreaks. In general, the monetary resources lost to combatting rodents could be used to feed, clothe, and shelter all the impoverished peoples of the world. Surely, humans truly compete with rats and mice for the earth's food and space.

For most urban and industrial areas, the concern about rodents and our food is really more of rodent contamination, spoilage, and damage to food rather than consumption. With the exception of food storage facilities, rodents are often mere scavengers of garbage scraps or discarded food residues left lying about from improper cleaning. Rodents living in zoos, parks, and livestock and farm facilities often feed on the food that is spilled or discarded by other animals or people.

In cases of severe rat infestations, however, food consumption and loss can be significant. A rat can consume up to 1.05 ounces (30 grams) of food each evening.

Consider that a realistic population of 50 rats living in a grain storage facility can consume as much as 3 pounds (1.4 kilograms) of grain each week.

Nevertheless, the contamination of food by rodents is a more serious wide-scale problem than the amount of food they consume. One mouse can excrete between 40 and 100 droppings per day, and can deposit hundreds of small droplets of urine during its travels. A rat typically produces between 20 and 50 droppings and excretes 0.5 ounces (14 milliliters) of urine daily.

Obviously, the concern of food industry and health professionals regarding food contamination is justified. In processed foods, minimum amounts of rodent contamination are permitted in the food before the food is considered contaminated and subject to seizure by the U.S. Food and Drug Administration (FDA). This amount is called the Defect Action Level (DAL). The various foods have different action levels, but generally, the levels are very low. Wheat, for example, has a defect action level of 9.0 milligrams of rodent excreta or dropping fragments per kilogram of wheat.

Regardless of the action levels established by the FDA, the reality for pest management professionals is that food and health inspectors will not tolerate the presence of any rodents in areas where they might contaminate food or food production surfaces. In addition, consumers certainly are not tolerant of discovering a rodent dropping in their breakfast cereal.

Even one rodent in a food handling establishment, therefore, is a big deal, with potentially serious consequences for the food processor who fails to recognize the importance of an effective rodent control program. Pest management professionals must also acknowledge this and implement programs designed with long-term control and prevention objectives in mind.

DISEASE TRANSMISSION. Historically, commensal rodents have been responsible for some of the most devastating disease outbreaks of all time. In the past century alone, more than 10 million people have died from rodent-borne diseases. Plague and typhus outbreaks are two well-noted examples that have contributed to this astonishing figure.

Rats and mice have been implicated in about 55 different diseases, representing a diverse range of pathogens from viruses to parasitic worms, as listed below:

But from a practical aspect, are the rats and mice living in and around our buildings a serious health threat? This question does not have an easy answer, and perhaps is best answered in terms of potential and actual risks.

In general, rodents, like other wild animals, insects, and people, are capable of carrying diseases directly into buildings. And they can spread or accelerate the spread of established pathogens from contaminated areas to uncontaminated areas via their droppings, feet, fur, urine, saliva, or blood. Thus, the potential for a rat in a house, or a mouse in the local restaurant to transmit salmonella food poisoning, leptospirosis, or some other disease to people is real providing these pathogens already exist in the environment or the structure, or the rodents immigrated or were imported from an infected area. But the mere presence of rodents themselves does not necessarily indicate an imminent disease threat.

In the United States and other developed countries that enjoy organized sanitation programs, modern pharmaceuticals and professional pest management programs, the actual risk of disease transmission from rodents to humans with any degree of frequency is low. This is borne out when you consider the sheer numbers of mice and rats that have co-existed with humans over the past 50 years. In fact, the spread of disease by rodents is no longer the primary reason for rodent control programs in developed nations. In underdeveloped countries that lack the benefits of modern technology and funding, the risks from zoonoses (diseases that can be transmitted to people by vertebrate animals) of all types are significantly greater.

Nevertheless, when we consider that we are surrounded by and truly co-exist with rodents, and that they are especially "well-designed vehicles" for disease transmission (see story on page 40), we must remain vigilant and concerned about the potential for disease transmission. Epidemiologists are keeping an watchful eye on "emerging infections" and readily acknowledge the potential role of rodents in transmitting infections. This came to light with the 1993 hantavirus/deer mouse outbreak. Thus, despite the low risk associated with rodent-borne diseases, we cannot afford to be complacent in our efforts to control rodents in populated areas.

Some of the more common rodent-borne diseases of concern to people in urban environments diseases such as relapsing fever, Lyme disease, plague, food poisoning, leptospirosis, hantavirus, and others occur throughout the world. In the United States, plague is endemic (that is, constantly present) in ground squirrels and a few other noncommensal rodents throughout much of the western section of the United States. Rodents can also be health threats in livestock operations and in zoological facilities, where expensive animals are vulnerable to various rodent-transmitted pathogens.

We should not be alarmists about the occasional rodent around the home, yard, or restaurant, but the potential of rodents to transmit serious diseases cannot be easily dismissed. It is this potential threat that emphasizes the importance of the pest management professional in protecting public health.

For a more complete discussion of rodent-borne diseases, the reader should refer to any of the excellent references which provide greater details on this subject especially the publications of Norman Gratz.

RODENT BITES. Rodent attacks especially by rats on people is a topic that generates concern (and fear) among the public. Both rats and mice bite people regularly, especially in areas where rodent populations are high and they live in close proximity to people (multi-family housing, homes in older neighborhoods, residential areas abutting commercial areas or industrial sites, etc). Most rat bites occur in lower socio-economic areas of urban areas. Unfortunately, it is babies in cribs, the confined elderly, and the indigent homeless that are most vulnerable to foraging rats and mice. Statistics that provide an accurate number of rodent bites per year worldwide are difficult to obtain because most bites go unreported.

It is safe to state that in the large, highly populated cities of the world — especially those without formal rodent control programs — the number of rodent attacks on people is significant. In the United States, it is estimated that as many as 50,000 people are bitten each year by rats, the majority of them children. Neglected children and homeless people in infested areas may suffer serious wounds from repeated attacks by one or more rats. In may cases, foraging rodents are attracted to the food residues on the hands, fingers or faces of babies or sleeping adults. The rodent may bite when attempting to consume the food residues completely from the flesh of the person, or the rodent may also consider the flesh itself as food. Rodents also bite children when they attempt to pet or pick up the furry animal that has joined them in bed.

RODENTS AND RABIES. Most people are fearful that rodents transmit rabies when they bite. Although rats and mice can carry the rabies virus, commensal rodents have not been shown to be infected with the rabies virus in wild populations, and thus are not of concern in rabies transmission to people. Consequently, the U.S. Public Health Service does not recommend rabies treatments in the cases of rat or mouse bites.

Robert M. Corrigan is a rodent specialist with the Purdue University Center for Urban and Industrial Pest Management. This article is excerpted from Corrigvan's forthcoming book, Rodent Control: A Practical Guide For Pest Management Professionals, which will be published by PCT in 1996. Additional discussion regarding the significance of the rodents as pests will be covered in the book.