MICE

mouse-lgMice are small RODENTS, and like all rodents they possess gnawing teeth. The word mouse, however, has no distinct meaning beyond that in scientific classification systems.

It has been applied loosely to many rodents simply because of their size and superficial similarities, such as a somewhat pointed snout, a slender and sparsely haired tail, relatively conspicuous ears, and an elongate body. (The word rat is used in an equally loose way for larger rodents of similar appearance.)

In addition, many small rodents with distinctly different characteristics have also come to be known as mice. Among them are the short-tailed field mice, or field voles, Microtus; the pocket mice, family Heteromyidae; the jumping mice, family Zapodidae; and the squirrellike dormice and related forms, families Gliridae, Platacanthomyidae, and Seleviniidae. Many of these animals carry a variety of viral, bacterial, and parasitic diseases . Mice associated with humans also have considerable economic impact through crop damage, destruction of trees, and food contamination.

Garlough and Spencer (1944) noted the word “mouse” can be traced to the Sanskrit word musha which is derived from a word “to steal.” Mice were well known to the ancient Greeks and Romans and were featured prominently in their art and literature. This cosmopolitan rodent is believed to have come originally from Central Asia. It is now found throughout the world from the tropics to the Arctic and Antarctic.

Linnaeus originally named the house mouse Mus musculus. Later, taxonomists cosidered that two subspecies of house mice had been introduced from Europe into the New World: Mus musculus domesticus and M. m brevirostris. Especially in the Midwest these two forms came into contact and interbred. Taxonomic “splitters” came to recognize several hundred verieties.

In recent years opinions have been revised again, and you are likely to see Mus domesticus cited in technical papers. Some specialists now want to reserve M. musculus for those mice in Eastern Europe. However, continuing to use our present technology (i.e., M. musculus) seems to me the appropriate and least confusing course of action.

Life History Of The House Mouse. House mice are extremely prolific breeders. At 35 days of age they mature and carry embryos for 18 to 21 days. If a female aborts, she can become pregnant again within 48 hours. The average litter size is about six. Variations in genetics, food supply and temperatures affect these numbers. At low temperatures, mice produce fewer offspring, and they are smaller in size. A female suckles her young for about four weeks. The female may become pregnant following parturition or while still lactating. In such cases, the gestation period will be lengthened. Therefore, if all goes well, a female can have litters approximately every 40 to 50 days. Under optimum conditions (often found indoors) mice can breed throughout the year. Outdoors, mice are more seasonal breeders, peaking in the spring and fall.

Newborn mice are extremely small, blind, pink and naked, except for short vibrissae. They weigh between 0.02 and 0.03 ounces. After two weeks, the eyes and ears open, and the young mouse is fully covered with hair, makes short trips from the nest and begins feeding on solid food. At four months of age an adult mouse weighs about 25 grams, slightly less than 1 ounce. Eaton and Cabell (1949), on studying laboratory mice, state: “Young mice may be moved from their dam at three weeks of age and the dam rebred. A female is not usually productive after 15 months, but may live much longer. Male mice have been known to live as long as 2-1/2 to three years.”

Garlough and Spencer (1944) note mice usually live for 15 to 18 months, and some have lived up to six years. However, considering their natural enemies and the diseases to which they are susceptible, it is generally agreed their life expectancy is less than a year.

Storer (1931), Orr (1944) and others have observed “community nests of mice” wherein several females may share the same nest with their accumulated brood. Orr observed as many as three females to use the same nest. Storer found one nest to harbor 36 young. “Of these one brood was newly born, another of a size ready to leave the nest, and remainder of intermediate development, with eyes still unopened.” Under such crowded conditions, survival may be relatively low.

Feeding Patterns. Garlough and Spencer (1944) noted house mice “eat about the same kinds of food as do human beings, including meats, grains, cereals, seeds, fruits and vegetables. They prefer sweet liquids to pure water for drinking.” According to Mills (1947), house mice feed 15 to 20 times a day, consuming 100 to 200 milligrams of food at each feeding. Ives (1948), speaking of the feeding habits of mice, says: “They do not sit down and eat a large amount of food at one sitting like a rat does. Instead, they duck in and out and dart around picking up a morsel of food here and there.” The same author notes that four seeds suitably treated with a poison will kill a mouse, and a mouse will eat that many at one time.

While mice are nibblers and feed many times in many places, they have two main feeding periods. They eat at dusk and just before dawn, interspersed with many other feeding bursts approximately 3/4 to 1-1/4 hours apart. They have to consume about 10 to 15 percent their body weight every 24 hours. When water is available, they drink.

Southern and Laurie (1946), who studied the house mouse in grain stacks in the field, make the following notes about the water requirements of house mice: “Unlike rats, mice do not feed to forage outside a rick for water; in captivity they live in good health on a diet of wheat, which itself contains about 15 percent of water, though they will take a small amount more if it is provided. This is generally less than 1 ml per day, and this quantity would be easily obtained on the outside in the form of dew or rain. This probably accounts for most mice in a rick visiting outside, and for the readiness with which they take poison bait mixed with water.”

Social Behavior. Women’s liberation has a long way to go in the world of house mice. Each male mouse stakes out a territory and guards it. Within the territory can be several females and lower ranking males. While the dominant male is busy defending his territory, the female mice may be “getting acquainted” with lower ranking males. Female mice will often mate with more than one male. A mouse’s territory depends upon a number of factors, including number of mice in the entire structure and arrangement of materials within the structure. The more mice present, the less territory each has. Some mice can remain in a desk or pallet for an entire lifetime, but it is important to keep in mind that the mouse is climbing up and down within the materials stored on the pallet or in the desk. Mice entering an already occupied territory are not welcome and are driven off. When mouse populations swell, the mice will seek out rodent bait stations, exposed window ledges and any other area where they can hide from other more aggressive mice.

Mice are cannibalistic and will feed on each other when hungry. Mice caught on glue boards may be partially eaten by other mice. Mice in multiple live traps will often be eaten by other mice caught in the same trap.

Because mice scurry from place to place and deposit fecal droppings wherever they please, the easiest way to determine if mice are present is to locate their droppings. Other signs include gnaw marks, small holes in walls and doors, and the pungent odor of their urine. The easiest way to discern active infestations is to sweep up the droppings and see if new ones appear the next day.

Emlen (1950) made a study of the diseases mice travel by capturing 1,572 mice, marking them and then releasing them at the point where they were caught. He found the average distance was 12 feet and that 90 percent moved less than 30 feet and 70 percent less than 10 feet. From this he concludes mice are “stay-at-homes” and do not move around unless disturbed. As a result of his studies, Emlen recommends the distribution of a large number of poison baits sparced no more than 20 feet apart. Southern and Laurie (1946) note the average range of mice in cellars is not more than 48 square feet in 24 hours.

RATS

Rats are extremely important pests on the farm. Hamilton (1947) states, “On the farm, rats eat incredible
quantities of foodstuffs, destroy poultry, lay waste the stored fruits and vegetables, and riddle buildings
with their sharp teeth. Rats tear down growing corn, eat melons, pumkins and tomatoes on the vines, and
even take an appreciable toll of cherries, climbing the tree in search of the fruit.” Rats also are a major
problem to the poultryman. They destroy feed, carry disease and kill chicks. Various estimates suggest That rats destroy or contaminate at least 20 percent of the world’s food supply.

Hamilton (1947), speaking of the depredations of rats on wildlife, notes: “In America, game keepers and
conservationists might well look to the rat as one of the chief predators of game. Rats victimize the nests
of robins and ground-nesting birds, insular colonies of terns have been completely destroyed by rats, and
rat destruction of quail chicks in the South is well known. On game farms, the abundant supply of feed
attracts horders of rats, which in turn kill the young pheasants and other birds. Rats ate the legs off 40
young black ducks on a Long Island game farm. A single rat destroyed, in three nights, 120 pheasant chicks.”

In rural areas, the young of pigs and lambs are not immune to their depredations, and it has been reported
they will on occasion gnaw holes in the bellies of swine, tear the nipples from a farrowing sow and
otherwise attack animals. A circus owner in Germany was forced to kill three elephants because the rats
gnawed the feet of these animals, and a pest control operator observed rats to gnaw on the hides of living
alligators in a Los Angeles alligator farm.

House mice similarly infest our structures, destroy and contaminate our food supply, and even gnaw into
the backs of densely penned poultry. Mice, unlike Norway rats, occur readily apart from man in fields and
second-growth vegetated areas.

Commensal rodents are responsible for damage in the home and warehouse. They gnaw upholstery, bolts
of silk, papers, books and like items for nesting material. Accounts of the rat’s ability to start fires by
carrying matches to their hiding places, as well as by causing short circuits through the gnawing of
electrical wires, are commonplace.

Diseases Carried By RATS:

Disease organisms may be transmitted directly through the rodent’s bite, carried from the rodent (i.e.,vectored) by a flea, tick or mite which bites man and transfers the pathogen, or by contamination of food or water with feces or urine.

HANTAVIRUS: By now, most avid readers of newspaper, pest control trade magazines, and a variety of other publications have probably heard about hantavirus. The disease has various strains that may induce symptoms as minor as high blood pressure or as deadly as pulmonary collapse due to fluid buildup and the overwhelming of the body defenses. The hantavirus strain know as hantavirus pulmonary syndrome (HPS; also sometimes referred to as Muerto Canyon hantavirus or Four Corners hantavirus) was first identified in 1993 in New Mexico and is one of the deadliest known strains of the disease. It produces pneumonia-like symptoms that can quickly become severe; 40% of the more than 100 cases of HPS that has been documented in the U.S. since 1993 have been fatal. The various strains of hantavirus occur in different rodents, primarily mice. The Four Corners strain is known to occur in field mice or deer mice and is transmitted to humans through dried fecal material. This mouse is commonly found in rural areas and occasionally suburban areas, but rarely in urban settings. Most urban cases of hantavirus are likely due to the victim visiting a rural area and contracting the disease there, then returning home. It should be noted that none of the three most common urban rodent pests –the house mouse, the norway rat, or the roof rat — have been found to carry hantavirus. When in a suburban setting, where dried mouse feces are present, it is best to err on the side of safety and assume the worst.

SALMONELLA: Associated with exposure to rotting food, salmonellosis may occur after contact with infested droppings of either rats or mice. The salmonella bacteria can enter victims via the mouth, by inhalation, or through direct contact with open cuts or sores. The disease is also commonly associated with bird infestations. (More information about this disease at the bottom of this page under “Salmonellosis”.)

HISTOPLASMOSIS: Histoplasmosis is a fungal disease associated with dust generated from pigeon and rodent droppings. The disease affects many humans without requiring hospitalization; it usually causes no more serious symptoms than a drawnout cold or cough, although it can become more severe and can affect the lungs, liver, spleen, and central nervous system. Adequate respiratory protection is required to prevent inhalation of the spores of any fungal disease. Many other fungal diseases spores are found in bird droppings, so similar precautions should be taken when performing bird control services.

PLAGUE:. The great plague of London that killed more than half of the city’s inhabitants, and the “black death” that devastated Europe for more than 50 years in the 14th century, killing some 25,000,000 individuals, were in part due to the abundance of rats. The plague-infected rats carry plague-infected fleas which in turn infect man. Fortunally, such epidemics no longer devaste Europe, yet it is estimated that from 1898 to 1923 11 million lives were lost from the plague in India, China, Mongolia and other parts of Asia. The thousands of miles of water separating us from the Asiatic and European shores were not sufficient to spare us from this dread scourge, for plague outbreaks occurred in San Francisco in 1900, in Oakland and San Francisco in 1907 and 1908, in New Orleans in 1914, in Galveston in 1920, in Los Angeles in 1924 and in other cities since then. Plague is a bacterial disease of the circulatory and respiratory systems. The germ Yersinia pestis, which invades the body, was discovered independently in 1894 by the Japanese investigator Kitsato and by the French investigator Yersin. At the time it was established that rat plague and human plague were identical. In man, plague may manifest itself in four ways:

( 1 ) Bubonic plague. Here the blood is infested and the bacilli are arrested in the lymph glands, particulary in those of the groin and under the armpits, resulting in inflamed glands or buboes which suppurate. This is the most common form of plague and results from the bite of a flea. The mortality may range from 40 to 70 percent. It should be noted bubonic plague also can be contracted by contact of the abraded skin with infected dust or body fluids.

( 2 ) Septicemic plague. In most serious cases the lymph glands fail to arrest the bacilli, which appear in large numbers in the blood. Numerous hemorrhages occur under the skin, which turn black, accounting for the name “Black Death”. This form of plague also is spread by the bite of an infected flea, but since the disease in this case is very virulent, death nearly always results.

( 3 ) Pneumonic plague. Here, where the bacilli are in the lungs, we have the most dangerous
form of plague from a public health standpoint, since it is spread so readily through contact and coughing, as well as by the consumption of contaminated food. This form of plague nearly always results in mortality above 90 percent. It is now thought that the initial 14th century European plague outbreak was of this form, since the spread was so rapid.

( 4 ) Sylvatic plague. This form of plague wherein the virulence is greatly diminished. Groundsquirrels presumably contracted the plague in San Francisco in 1900, so now it is enzootic or established in wildlife in this country. This form of plague was first discovered in 1908 and gradually spread throughout the West. It is now found in ground squirrels, wood rats, deer mice and woodchucks. Silver (1927) states: “The sylvatic form of the plague is apparently not highly contagious to man, as an average of only about one human case each year has been reported. The menace, however, remains a most disturbing one because of the ever-present possibilities that house rats may become reinfested in the population centers and that human cases of bubonic plague contracted from native rodents may develop the secondary, or pneumonic form, which is highly contagious directly from person to person.” Elsewhere in the world, plague remains active. Major endemic (and enzootic) foci occur in Africa, Asia and South America. Murinetyphusfever. USPHS (1948) states there are two kinds of typhus fever, “epidemic or European, and endemic or murine. The epidemic form is transmitted from person to person by body lice, while murine typhus is contracted from domestic rodents, probably both rats and mice, rats being the more active in spreading infection. If louse-infested individuals contract murine typhus, the infection may then be transmitted by the patient’s lice to other people.”

Andrews and Link (1947) note the Oriental rat flea is an important agent of transmission of the disease: “It must be emphasized, however, that rickettsiae have been found in the excrement of rat fleas and in the urine of rats. Thus, the possibilities of transfer to man by inhalation of dried flea feces in dust, or by the consumption of food or drink contaminated by flea feces or rat urine must be considered as well.”

Leptospirosis, Infectious Jaundice or Weil’s disease. Although this is a common disease in the Orient, what is not so well known is its prevalence in the United States. The disease is caused by the spirochete, Leptospira icterohaemorrhagiae, which is found in the blood and urine of the rat. (Other Leptospira species are characteristic of other mammals and also are transmissable to man.) Human beings may become infected “by handling or eating things contaminated with rat urine. It is also contracted by swimming and wading in contaminated water.” USPHS (1948) states the disease “does not usually cause death, but is very debilitating, confining the patient to his home for a week or longer. ‘Yellow jaundice’ may be caused by a number of conditions, but rats are probably responsible for many undiagnosed cases.” Storer (1948) notes the disease has caused epidemics among city dogs in California.

Rat-bite fever. As was previously noted, Richter (1946) showed seven of 65 cases treated for rat bites in The Johns Hopkins Hospital in Baltimore developed rat-bite fever. The symptons of this disease may develop after the wound has healed. The infected individuals may have a relapsing type of fever for weeks or months. Larson (1941) showed that rat-bite fever is due to two bacterial organisms, Spirillum minus and Streptobacillus moniliformis.

Jellison et al. (1949) have the following to say in discussing a case in Montana where a girl was bitten by mice:

“Rat-bite fever is most frequently communicated to man by the bite of rats, Rattus spp., occasionally by the bite of other rodents, and rarely by the bite of dogs, cats or ferrets which presumably have become contaminated by eating infected rodents.” The disease is particularly dangerous to babies and small children, since they are the ones most frequently bitten by rats. Seldom, however, is laboratory confirmation attempted.

Trichinosis. This disease of man is caused by a minute roundworm, Trichinella spiralis. Rats and mice are principal agents in the dissemination and the perpetuation of the disease. Large numbers of Trichinella in the adult or sexual state are most commonly present in the intestine of man, pigs and rats. The worms may be found encysted in the muscles of mammals and birds. It has been estimated the flesh of an infected human being contained 100,000,000 encysted worms. While encysted, the worms suspend animation and undergo no further development. Further development of the encysted and sexless worms will only take place if the infected flesh is eaten by another animal in which the worm is capable of living (e.g.,man, pig or rat). Once this is done, the cysts are dissolved by the digestive juices, the worms escape, become sexually mature, mate and migrate, producing the disease again. Rats become infected by feeding upon excrement or meat infected with these worms. Pigs eat the rats and mice, or food fouled by excrement of the rodents. Man eats the trichinous pork and becomes infected when the meat is not properly cooked or has not been frozen for an extended period.

Food Poisoning (Salmonellosis). USPHS (1948) notes both “rats and mice suffer from intestinal infections that are communicated to man, who is infected from eating foods contaminated by the excreta of infected rodents. Acute food poisoning of this type is probably much more common than generally realized, and many involve a large number of persons at one time.” This same source states it is possible for man to contract amoebic dysentery by eating food contaminated with rat excreta. It is these enteric diseases and this contamination problem that Pest Control Operators and householders should be most concerned about