David A. Khan, Charles S. Barnes, J. David Miller, Jonathan A. Bernstein, Christopher Randolph, John Oppenheimer, David M. Lang, Stephen A. Tilles, Richard A. Nicklas, Diane E. Schuller, David I. Bernstein, P. Brock Williams, Joann Blessing-Moore, Linda Cox, Dana V. Wallace, James L. Sublett, Jay M. Portnoy, Carl Grimes, Elizabeth C. Matsui, Sheldon L. Spector, James M. Seltzer, Kevin Kennedy, and Wanda Phipatanakul
Rodents (Rodentia) are the largest order of mammals in the world, with estimates ranging from 1,500 to 2,000 different documented species. The order includes commonly known mammals, such as rats, mice, guinea pigs, hamsters, and gerbils. The order Rodentia also includes beavers, muskrats, porcupines, woodchucks, chipmunks, squirrels, prairie dogs, marmots, chinchillas, voles, lemmings, and many others, although significant human exposure to these rodents is uncommon. The presence of 2 sets of incisors (top and bottom), used for chewing, is the common characteristic of rodent species. Incidentally, rabbits, hares, and a few other species make up the Lagomorpha order and therefore are not classified as rodents. Figure 1 shows an abbreviated taxonomy of the order Rodentia.2 Figure 1 Taxonomy of the order Rodentia. Adapted from Taxonomy of Common Rodent and Rodent-like pets.2 The common house mouse (Mus musculus) is a small, primarily nocturnal mammal. Although usually considered to be pests, mice also are popular pets. Mice are known to invade and establish residence in homes and other buildings, including schools, to obtain food and can at times be harmful, causing structural damage. Certain rodents, such as the deer mouse, can also spread diseases such as hantavirus through their feces and possibly also through their urine and saliva.3,4 Mice also are commonly used in research laboratories, where laboratory animal handlers come into contact with their allergens. Mice tend to have litter sizes of 4 to 8 pups and can have 6 to 8 litters per year, depending on the availability of food. Their life span in the wild is less than 1 year because of predation, although under laboratory conditions they can live as long as 2 years.5 Mice have the ability to jump 12 inches up and down 8 feet to the floor. They can run up almost any vertical surface, including wood, brick, metal pipes, wire mesh, and cables, and can run along suspended electric wires and ropes. They can squeeze through a 1/4-inchdiameter hole, travel upside down, cling from 1/4-inch hardware mesh, swim well, and survive at 24 °F for many generations.6 Rats are long-tailed mammals also of the order Rodentia. The most important rats from a human perspective are the black or roof rat, Rattus rattus, and the brown or Norway rat, Rattus norvegicus. The Norway rat is found in every state of the United States, whereas the roof rat tends to be found in coastal states. Rats usually can be distinguished from mice by their larger size. Rats also can serve as vectors for certain pathogens, such as Lassa fever. They are popular as pets but also are commonly used in research laboratories, where laboratory animal handlers come into contact with their allergens.6 Rats tend to be nocturnal and search for food and water between dusk and dawn; however, they will come out in daylight if their habitat is overcrowded or food is lacking. They require daily water; prefer traveling along edges, pipes, and rafters; and can even travel along overhead utility lines. They prefer to not cross open spaces to avoid predation, and although they have poor visual acuity, they have acute senses of smell, taste, hearing, and touch using their whiskers. Rats tend to have litter sizes of 5 to 12, and they can have up to 9 litters per year, depending on food supplies. Their life span generally is less than 1 year. Rats can pass through openings as small as 3/4 inch, theycan climb up vertical surfaces and even inside vertical pipes, and they can crawl horizontally on any type of pipe or conduit. They can jump vertically 24 inches and fall more than 50 feet and survive. They are able to swim under water for up to 30 seconds, tread water for up to 3 days, and swim up to ½ mile in open water. This permits them to enter buildings through drains and toilets.7 Although guinea pigs (Cavia porcellus), hamsters (Mesocritecus auratus), and gerbils (Gerbillus jerboa) also are rodents, they usually are kept as pets in a confined area and usually are not considered to be pests. Patients who keep such pets clearly can develop allergic symptoms after sensitization; however, other than in the case of laboratory animal workers, exposure to such rodents can usually be contained, and complete abatement, should it become necessary, is easily accomplished with removal of the offending animal.