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Posted on August 2nd, by admin in Vectors. No Comments

The rats and mice considered pests are often referred to as commensal rodents. The word commensal means “sharing one’s table.” This is appropriate as these rodents have shared man’s table for centuries.  The term “rodent” means “to gnaw.” Like all rodents, rats and mice have a single pair of chisel-like incisor or front teeth that grow throughout their lifetime. Their teeth are kept sharp and are sized due to their guarding against one another and by the pests’ continuous gnawing on a variety of objects.

The three most important commensal rodents are

  • House Mouse
  • Norway Rat
  • Roof Rat


Rodents have been a major cause of death and disease for many years. Today, the threat is not as significant as it once was because of sanitation and rodent control programs. However, the threat is still evident, and the potential of disease must always be kept in mind.

Some rodent-transmitted diseases include:

  • Plague — This was caused by a bacteria that is spread from rats to people by the oriental rat flea. The Black Death that killed 25 million people in Europe during the 14th century was the result of Plague. Plague does not exist in commensal rodents in the U.S., but it is still found in some native rodents, such as brown squirrels, in the western United States and other parts of the world.
  • Salmonellosis (acute food poisoning) — Salmonella bacteria can thrive on decaying food, meat, and poultry, and in sewers, livestock facilities, septic tanks, accumulated garbage, and other unsanitary environments. Rodents visit contaminated areas and then visit homes and food facilities, causing the spread of the salmonella. The bacteria grows in the intestinal tract of rodents and spreads through food contaminated with rodent feces.  There are about two million cases of salmonellosis in the United States each year. Current research shows that rats and mice do not play a major role in the spread of salmonellosis to humans but can significantly spread the disease to animals.
  • Rat-bite fever — Rat bite fever is caused by a bacteria that lives in the saliva glands of both rats and mice. It causes flulike symptoms that last for several days and in severe cases can be deadly. Although it is not common in the United States, it is possible many cases are unreported, as the symptoms can be mistaken for the flu. Conservative estimates place the number of people bitten by rats and mice between 14,000 and 20,000 annually.
  • Typhoid and dysentery — Rats have been shown to carry the organisms of these diseases.
  • Trichinosis — Rats serve as vectors by carrying the trichina worm, which is responsible for trichinosis. This disease occurs when the worm is ingested when eating undercooked pork.
  • Hantavirus — There were several outbreaks of this virus in the southwestern United States from 1993-1995. Hantavirus has been associated with deer mice, not commensal rodents.


Rats and mice cannot see well beyond 3 or 4 feet, but they are sensitive to motion up to 30-50 feet away. They are color-blind, but are sensitive to and may avoid light-colored and reflective objects. Therefore, rodents are most active during the night hours when light levels are low and they can rely primarily on their other senses, especially the sense of smell, touch and hearing.

Rodents have a very strong sense of taste, which can lead to rejection of bait if they are contaminated with odors or other chemicals. To ensure acceptance of bait, use fresh, food-quality grain ingredients.

Smell is one of their most important senses. Rodents mark objects and pathways with urine or other secretions, then use their sense of smell to recognize the odors for guidance, to distinguish members of their own colony, and to tell if a stranger is threatening.

Rodents are able to hear at high frequencies.  They use hearing to locate objects within a close range.

If rodents cannot go around an object, if possible they will tunnel through it. They can gnaw through many materials, including cinderblock, aluminum siding, glass, and improperly cured concrete. They can squeeze through very small openings — as small as 1/2” for a rat and 1/4” for a mouse.


House Mouse (image coming soon)

Distinguishing characteristics — The house mouse has a small and slender body 2- 3 1/2” long.  The tail is very slender and as long as the head and body. It has large ears, pointed nose, small eyes, light gray or brown fur.  Its droppings are rod-shaped, pointed, and about 1/4’ long.

Focus locations — The house mouse may be found throughout a structure; however, it is most often associated with a food source. Frequented areas include the food pantry, under the false bottom of kitchen cabinets, and attics. Commercial areas include grocery store shelving and food storage pallets. They are usually found within 10 to 30 feet of the nest site.

 Norway Rat (image coming soon)

Distinguishing characteristics — The Norway rat has a large robust body 7-9 1/2” in length.  The tail is thick and shorter than its body length. It has a blunt nose, small ears covered with small hairs, brown to dark gray fur, and small eyes. They are good swimmers.  Droppings are up to 3/4” long and 1/4” in diameter with rounded ends.

Focus locations —  Indoors, nests are found around the lower floors of buildings. They may be in wall voids, in crawl spaces and among stored pallets and supplies. Territories may extend 50 to 150 feet from the nest.  Outdoors, look for burrows along foundation walls, in embankments and within areas of high vegetation.

Roof Rat (image coming soon)

Distinguishing characteristics — The Roof Rat has a slender, sleek body 6-8” long with tail longer than the length of body. It has a pointed nose, large and nearly hairless ears, light gray to light brown fur, and large prominent eyes. Very agile. Good climbers and jumpers. Droppings are spindle-shaped and 1/2” long.

Focus locations — Indoors they are found in upper levels in aerial nests. They may climb along utility lines and fences moving from one structure to the other. Look for oily rub marks on runways, usually above floor level.  Outdoors, they may nest in vines on the sides of buildings or fences, as well as in trees.

–Contributed by Stephen Gates

Sources and more information:

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