How to Control Rodents as Disease Vectors

in survival settings, it’s been said that rats will do a better job of surviving than humans. Rats, mice, and other rodents are well-known causes of “zoonotic” infections.  A zoonotic disease is one that can be transmitted from animals to humans.  The animal in question may not have symptoms of the disease itself, but may serve as a “vector”; that is, it carries the disease to a human target.

Rats and mice belong to the order Rodentia, from the latin word rodere (“to gnaw”).  This order contains various families, including beavers, porcupines, squirrels, and gophers.  As you are unlikely to have an infestation of beavers in your home, we’ll concentrate on rats and mice. Ai pair of rats could produce 1,500 offspring in one year if they all reproduced. Most rats and mice that cause issues for humans come from the “Old World”.  These include:

Brown rats (rattus norvegicus): Also called Norway rats, although they didn’t originate there (Norway has no more rat issues than other countries). Brown rats may reach 16 inches (including the tail) and are good swimmers; the term “sewer rat” was coined for them.

Black rats (rattus rattus): Thought to have introduced the Plague to Europe through their fleas. The black rat, also called the “roof rat”, is slightly smaller than its brown cousin and is an excellent climber.

House mice (Mus musculus): Used to living in close quarters with humans, mice are “nibblers” and can contaminate an entire pantry by taking a few bites out of multiple food items. Mice and other rodents can also chew through electrical wiring, thereby constituting a fire hazard.

Rats and mice are some of the world’s most invasive species. Every year, a percentage of the world’s food supply is contaminated by their droppings, urine, and hair. These items, known as “fomites”, may contain disease-carrying organisms and, as such, render food unfit for human consumption.

Before I go further, let me tell our readers who have rats and mice as pets that they (the pets, not necessarily the owners) are generally clean, intelligent creatures.  I have had the privilege of working with them in university laboratories as a student.  Despite this, it is indisputable that the diseases they may carry are cause for concern.

MEDICAL ISSUES CAUSED BY RODENTS

From a medical perspective, what diseases might one contract from a rodent or its droppings?  These include:

Plague:  The Plague is caused by a bacterium known as Yersinia Pestis. It is carried by fleas. The black rat’s arrival in Europe in the Middle Ages (and with it, its fleas) caused pandemics of the disease that wiped out a third of the population. Even today, Plague exists in developing countries and, there have been hundreds of cases in the U.S. over the past three decades.

Hantavirus: Hantavirus, transmitted by mice in urine, droppings or saliva, causes a serious lung disease that may become fatal without the availability of intensive care.

Leptospirosis: Caused by consuming food contaminated by rat urine, Leptospirosis causes a flu-like syndrome that progresses to kidney and liver failure if untreated.  This disease can also be carried by certain livestock.

Lymphocytic Chorio-Meningitis Virus (LCMV): LCMV may be contracted from mice urine or droppings or from pets in contact with mice, such as hamsters.   It causes a flu-like syndrome that occasionally causes complications in the nervous system, especially in people with weakened immune systems or pregnant women.  LCMV may cause miscarriage or birth defects.

Salmonellosis: Infection with the bacteria Salmonella may occur as a result of handling of pet rats or mice, especially if they have had diarrhea.  It causes severe diarrheal disease in humans, and is one good reason for owners of rats and mice to wash their hands after handling.

Rat Bite Fever: Infection with the bacterium Strebtobacillus occurs from rat bites and scratches or from ingesting food or water contaminated with rat droppings. Abrupt onset of fevers, rashes, vomiting, and headaches are noted at first, with general deterioration afterwards. If untreated, there is a 10% death rate.

RODENT-PROOFING A RETREAT

It’s simply common sense to take measures to prevent rodent infestation in the home and to eliminate those already there. Once an infestation has occurred, much more effort is required to dislodge these unwanted guests. Rodent-proofing a home requires careful evaluation for points of entry from the level of the foundation to the roofline.  This includes sewer lines, bathroom vents, pipes and gutters, doors and windows, and vegetation near concrete slabs.

Some rodent-proofing techniques for homes include:

• Sealing cracks in building foundations, walls, siding, and roof joints with, for example, mesh hardware cloth or concrete patching. Rodents only need ¼ inch of opening to gnaw their way into your home. Metal mesh scouring pads or galvanized window screening (not steel wool, which quickly deteriorates) may be stuffed into crevices as a temporary solution.
• Installing vent guards in bathroom or washer/dryer vents.
• Placing barriers to prevent climbing rodents from going up pipes or gutters.
• Trimming trees so that branches don’t come close to the roof.
• Contacting the utility company for strategies to prevent rats from traveling along power lines to your house.
• Preventing rodents, especially rats, from tunneling under the foundation by placing flat concrete pavers or gravel for the first 3 feet from the base of the house.

Rodent control also involves careful attention to both indoor and outdoor sanitation.  Here are some suggestions for the wise homeowner:

• Never leave food or water out overnight. Keep your countertops clean and disinfected.
• Breadboxes may seem old-fashioned, but they are there for a reason: To keep the bread away from rats and mice.
• Never leave pet food outside, clean all bowls daily, whether they are used inside or out. Rodents love to eat dog and cat food.
• Clean under kitchen appliances. Even a few crumbs will make a meal for a mouse or rat.
• Keep garbage disposals and sinks clean with a cup of bleach once a month.
• Never flush grease down the sink drain.
• Keep toilet lids down until needed.
• Store dry foods, even pet foods, in sealed containers at least 18 inches off the floor.
• Construct barriers around birdhouses and bird-feeders to prevent seed from being accessible to rodents.
• Remove any fruits or vegetables from your garden that you won’t use.
• Keep garbage can lids tightly closed.
• Keep the side and back yards free of debris that might serve as shelters.
• Deny access to water by fixing leaky faucets.
• Avoid putting animal products in your compost bin.

IDENTIFYING INFESTATIONS

If you’re not sure that your home is currently rodent-free, you might consider:

• Looking for any partially eaten food, gnawed containers, or nesting material.
• Inspecting your home’s interior at night with a flashlight; look especially closely at the bases of walls, as rats and mice prefer to travel along them. Little used areas of the home should be especially targeted.
• Looking for rodent droppings. Mice and rat defecate 50 times a day; if they are in your home, you should be able to find their feces along floorboards, in attic crawl spaces, and in basements.
• Setting out a thin layer of flour or talcum powder by areas through which rats and mice might enter your home. Place some, as well, along floorboards; rodents prefer to travel along walls. The rodents will leave tracks which will prove their presence.
• Having cats and dogs as “mousers”. They may or may not be efficient, but they usually will alert you when a rodent is near.
• Listening for squeaking and scrabbling noises inside walls at night.
• Check for unusual smells. If there are a lot of rats in your home, you may notice an odor from their urine.

ELIMINATING THE PROBLEM

Once you have made the determination that you have rats or mice in your home, it’s time to reduce the population.  It should be noted that long-term control will be difficult if you haven’t followed my earlier suggestions for indoor and outdoor sanitation.

There are myriad mouse and rat-traps on the market and a number of poisons available to kill rodent invaders. It makes more sense to use traps, in my opinion, as poisons may leave you with a bunch of dead, rotting animals inside your walls. The stench may last a month or more, and sometimes deodorizer is needed to be inserted through a hole drilled in the wall.

If you have a lot of rats in your yard, you shouldn’t use poisons, as they may be ingested by neighborhood pets or even children. You should, however, consider trapping boxes. These can be snap traps, electronic “zappers”, glue traps or even catch and release versions. Both rats and mice will readily go for a small amount of fresh peanut butter as bait. Advice to the soft-hearted: Brown rats, black rats, and house mice are not native wildlife; besides other damage, some will cause casualties among endangered songbird eggs and young if released.

Glue traps are popular but controversial.  They are better weapons against mice than rats. Unfortunately, they usually leave you with a live animal to kill.  If you must use them, euthanize the rodent by throwing the trap and animal into a bucket of water or by striking it with a stick several times just behind the head. Another disadvantage of the glue trap is that it loses effectiveness in dusty areas or in extreme temperatures.

Snap traps should always be placed in perpendicular fashion, with the bait side against the wall.  Never use just one trap: Place a number of them several feet apart in the rodent’s usual path. Traps can be fastened to pipes with wire or thick rubber bands.

When cleaning out a building that has been infested with rats or mice, specific safety precautions should be followed to avoid infection. First and foremost, remember that you should never handle a wild rodent, alive or dead, without disposable gloves. Masks should be worn when cleaning. Other steps to follow:

• Open windows and doors before cleaning to allow it to air out, then leave for an hour.
• Avoid raising dust if at all possible.
• Steam-clean all carpeting and upholstery.
• Clean all surfaces with a diluted bleach solution or other household disinfectant, soaking areas that held dead animals, nests, or droppings.
• Wash all bedding linens, pillows, etc. and use the high heat setting on your dryer.
• Eliminate any insulation material contaminated by rodent urine, feces, or nesting material
• As ultraviolet light can kill viruses, place contaminated items that cannot be thrown away (such as important documents), outside in the sun for several hours. If this isn’t possible, “quarantine” the items for a week in a rodent-free area.  This should give enough time for viruses to be inactived.
• Dispose of any contaminated items or dead rodents in a plastic bag, and then place them in an exterior garbage can.
• Thoroughly wash hands after cleaning. Consider showering with soap and hot water.

We share our world with many other creatures. Some of these creatures invade our homes and can damage our possessions and, more importantly, our health. With careful attention to sanitation and the occasional surgical strike, we can eliminate unwanted guests and make our homes safe environments for our families.

Joe Alton, MD

Learn more about animal-borne diseases and 150 other medical topics in the Third Edition of the Survival Medicine Handbook: The Essential Guide for When Medical Help is Not on the Way, now available at Amazon.com.