[Cover Story] House Mouse Menace

As pest management professionals, many of us share a common bond with our customers: parenthood. What parent who sees mice scurrying across their floor is not worried about more than just an impending scream from their children?

Mouse allergens, in the form of mouse urine or dander, are widely distributed in both progressive urban centers and inner cities, and may be a significant contributing factor to the childhood asthma epidemic in these urban areas, according to two studies by Johns Hopkins University researchers published in the Journal of Allergy and Clinical Immunology.

Prior to these studies, mice were not recognized as an allergen in structures. Now we know that houses are full of it, and we were surprised that mice turned out to be even more important in inner-city asthma than cats, dogs or dust mites. While cockroaches are the more important inner-city allergen, mice are second in line. Physicians need to take mouse allergens into account when evaluating children with asthma.

A HUMAN PERSPECTIVE. Throughout  the centuries, house mice have both spread disease and been used by scientists to help cure disease. The albino lab mouse, the icon of scientific lab research, was bred from the house mouse and has been used in everything from cancer studies to stem cell research.

At one point, cooked mouse was a folk remedy for flu-like symptoms, and house mice were considered a helper for conditions like baldness and constipation. They also carry a variety of bacteria and viruses that are dangerous to humans, including tularemia, bubonic plague, spotted fever, typhus and Salmonella.

In the Western world, infectious diseases spread by mice are overshadowed by autoimmune diseases of allergies and asthma induced by house mouse bodily excreta. In the Johns Hopkins study of eight cities, scientists discovered that 95 percent of all homes in the study had mouse allergen in at least one room; Baltimore was the worst with 100 percent. Eighteen percent of the children were allergic to mice and those children tended to have more severe asthma. Finally, researchers found that the more a person was exposed to mice, the greater the chances that future encounters would cause a reaction.

AN UNKNOWN ALLERGEN. For many years, researchers have known that cats, dogs, dust mites and cockroaches can cause allergies that trigger the wheezing and constricted airways of asthma. But while doctors have treated people who work with mice in laboratories for allergies, until now, not much was known about mouse allergy in the general population.

To fill this knowledge gap, scientists turned to data from the National Cooperative Inner-City Asthma Study, (NCICAS), a multi-center study of more than 1,500 children. First, the researchers analyzed dust samples from the homes of 608 children. Ninety-five percent of these homes had detectable mouse allergen in at least one room, with the highest levels found in the kitchen, followed by the bedroom and living room. Eighty-seven percent of the samples from each room in the study had detectable mouse urine or dander.

Researchers then set out to examine the effects of this common allergen. Of the 608 children, 499 had undergone puncture skin tests for several types of allergies, including cockroaches, mice, grasses and cats. Children came from homes with adequate dust samples.

The remaining homes did not have large enough samples because of superior cleaning skills, difficulty getting dust from non-carpeted floors, or because dust had been used up during other tests. Researchers found 18 percent of these children had mouse allergy, and there was a connection between the allergy and asthma severity.

A CHANGE IN APPROACH. Researchers recommend that doctors begin evaluating asthmatic children for mouse allergy. Currently, it is not routine to test asthmatic children for allergy to house mice. Doctors need to change their approach to inner-city asthma. Medical professionals need to test for this allergy, ask families about mouse infestation, and recommend aggressive pest management of mice. Researchers will continue to define house mouse allergen as significant.

The author is a consulting physician, medical entomologist and environmental health specialist with Springer Pest Solutions, Des Moines, Iowa.

A Widespread Disease

Asthma affects approximately 15 million people in the United States, 5 million of which are children. Nationally, it is estimated that more than seven percent of children now have the disease. Researchers estimate that asthma is twice as common in the inner city in comparison to other areas.

The NCICAS study consisted of 1,528 children, ages four to nine years, from eight major inner-city areas: Baltimore, the Bronx borough in New York City, Chicago, Cleveland, Detroit, East Harlem, St. Louis and Washington, D.C. The children were diagnosed with asthma, and lived in neighborhoods where 30 percent or more of the households had incomes below the 1990 poverty level.

Mice at a Glance

Pest management professionals are well versed when it comes to the biology and behavior of the house mouse. Here’s another look at this incredibly adaptable creature.

Taxonomy: Mus musculus Linnaeus, 1758

Additional common names: English: Mouse; French: Souris domestique; German: Hausmaus; Spanish: Ratón común.

Physical characteristics: Body length 5 to 9 inches; tail 2.3 to 3.9 inches; and weight 0.6 – 0.8 ounce. A typical mouse has grayish brown hair on top, relatively large ears, and a dusky-scaly, nearly hairless tail. The hair on its bottom is only slightly lighter than the hair on its top, and it has non-grooved incisors.

Distribution: Mice spread to Europe from Asia and appeared in the 16th century in the New World as immigrants on the ships of explorers. In the 17th century, they appeared on the North American continent, and have proliferated since due to their high reproductive capacity. House mouse subspecies can be found across the world.

Habitat: Underground burrows, which some subspecies equip with storage rooms. Each mouse will make its own nest, but will share burrows with other individuals in the colony. Many house mice are co-habitants with humans, living beneath large appliances or inside the walls of homes. Some house mice live temporarily as migrants in grain fields, where they breed, feed and leave when the field is plowed. Between 1926 and 1928, house mice nearly took over the fields of California’s Central Valley, living within the fields at density of 202,000 mice per 2.5 acres. Sometimes, they eat animal and plant pests in fields, but they inevitably wind up in barns and silos where they contaminate food. In general, the house mouse does not stray far from cover, with the best habitats offering copious amounts of food, water and places to hide. Their home ranges vary, from 10 feet for some indoor mice, to more than two miles for some outsiders.

Behavior: A social species, the house mouse lives in groups with others of its kind, and aggressive males have hierarchical ranks and tend to dominate colonies. Each group lives in a territory bound by scent markers, and animals within the colony have their own nests. They will groom each other and display both aggressive and submissive postures, common within the species.

Feeding ecology: House mice eat up to 10 percent of their body weight on a daily basis, and feed up to 20 times each day. They consume grains, fruits, vegetables, meat, insects, and have been known to eat glue, paste, and even soap. If they eat moist food or a seed diet of 12 percent protein, they can live without water. The house mouse has been known to feed on caterpillars, larvae, flightless moths and earthworms.

Reproductive biology: They reproduce copiously (polygynous), and a biological contraceptive keeps their populations in check, females’ ovaries become inoperative and the animals become infertile. They breed throughout the year, with females giving birth to litters of 3 to 12 offspring about 5 to 10 times annually. Gestation is about three weeks. When the young arrive, they are hairless and their eyes are closed. Young are weaned by 21 days, they begin to reproduce in their second month of life, and they can live, depending on predation, to be six years old. Most wild mice live about a year, while those in captivity generally live about two years.