The Norway rat is native to Japan and Eastern Asia and is mainly a burrowing species. It lived along stream banks in Asia and spread rapidly into the canals and rice fields although in Malaysia it was strictly confined to seaports. This species was not known in Europe until about 1553 and it probably came into western Europe by ships instead of by overland caravans. It arrived in North America about 1775. The Norway or Norwegian rat is more often associated with populated regions but where a mean annual temperature above 11 C (52 F) occurs and free living populations exist. They are extremely adaptable and inquisitive but they tend to avoid new objects in their natural environment which may account for much of their craftiness. The Norwegian rat in the wild will eat practically anything it can cut with its teeth. A group of experimental animals given a free choice of food over a five year period selected different foods in such proportions so that they maintained good health. The diet consisted of a wild variety of plant and animal foods such as seeds, nuts, grains, vegetables, fruits, meats, fish and invertebrates. In the wild these rodents destroy far more than they eat. They also gnaw insulation from wire and occasionally cut through lead pipes and concrete dams. On a world basis the direct damage caused each year amounts to billions of dollars. The Norwegian rat harbors and carries such diseases as Bubonic plague, typhus, Salmonella (food poisoning), Rabies, Tularemia, and Trichinosis. The plague, which is transmitted to human beings by rat fleas, reduced the population of Europe by 25% or more at various times. Rat-born typhus has altered human destiny more than the influence of any person in history. Rat-born diseases are believed to have taken more lives in the last ten centuries than all the wars and revolutions ever fought. The common laboratory rat, an albinistic strain of Norwegian rat is used in many phases of biological research, including the study of diets, diseases, and genetics in man. On the whole, the Norwegian rat and the house mouse have probably affected man's existence both for better or worse more than any laboratory animal. The rat gastrointestinal system has two unique characteristics in that the rat does not possess a gall bladder and is incapable of vomiting. The latter characteristic makes the rat a good model for toxicity testing and is also taken into consideration in the selection of rodenticides for wild rat population control.
Handling is easily accomplished if the operator approaches the animal in a calm and slow approach. The rat's natural inquisitiveness should be taken into consideration and the animal should be given the opportunity to smell your hand as you approach it. It is recommended that gloves not be used, in that the glove masks the natural human odor of the operator and prevents the animal from becoming acquainted with the operator and recognizing him over a period of time when handling may be necessary. The rat should always be handled by grasping the body utilizing the thumb and little finger under the pectoral girdles and using the forefinger to gently restrain the head. The hand should be reclined to rest the body of the rat in the palm of the hand. In this position the rat can easily be injected IP in the lower quadrants of the abdomen and also identified with the various techniques discussed in the paper on identification techniques in rodents. Handling the rat by the tail can lead to serious problems for both the rat and the operator. The rat dislikes being handled by the tail and is much more inclined to be aggressive. This can lead to rather severe bites and scratches which can easily be eliminated by correct handling. Handling the rat by the tail if not properly done can also lead to slippage of the skin and underlying tissue from the bone and cartilage of the tail which can lead to necrosis of this tissue with subsequent sloughing and loss of that aspect of the tail. This can be very serious in that one of the selected bleeding and injection sites for the rat is the tail vein. An injury to the tail would preclude the utilization of these valuable bleeding and injection sites. If the tail is used, it should be grasped at the base of the tail, the animal lifted up and placed immediately on a safe surface. Any prolong time in the air will cause the animal to go into a spin. Injection sites on the rat include the IP injection in the lower quadrants of the abdomen, the tail vein which runs on the dorsal aspect of the tail and, in some cases, a cut down on the femoral vein or artery can be accomplished with minimal problems and closed with skin staples or sutures subsequent to the injection. Small samples of blood may be obtained from the medial canthus of the eye utilizing a capillary tube and introduction of the tip of the tube into the medial canthus, rotating the tube gently until the desired amount of blood is obtained. This is best accomplished under light anesthesia. Repeated blood sampling utilizing this method can be accomplished if a suitable period of two to three days between drawings is allowed. Subcutaneous injections can be accomplished utilizing the skin fold at the back of the neck.
The laboratory rat is second only to the mouse in the utilization as a research animal. The rat has been utilized for a wide range of studies involving practically all fields of research as well as other nutritional, genetic, and environmental studies. There are various strains of inbred rat that are available with spontaneous tumor production and other physiologic and anatomical differences and other unique aspects that should be considered before and any research study is undertaken utilizing the rat as a model. Various random bred strains are used in toxicology studies and other nutritional and biochemical work. The rat is widely used because of its suitability for a wide variety of experimental designs, its docility and ease of handling and care, its short gestation period, wide dietary preferences, intelligence and availability. When selecting a particular inbred or random bred strain of animals for utilization, the availability and source should be closely considered. The laboratory rat is commercially available through many sources, some of which are much more preferable than others. The investigator is encouraged to investigate fully a suggested commercial source before utilizing the animals from these sources as the quality of the animal received and supplied may vary greatly from one supplier to another. Once a particular strain is selected, a suitable and practical breeder should be selected and maintained through the entire study. Strain differences between individual breeders may be considerable and it is therefore recommended that changing from one supplier to another be minimized.
Rats should be kept in rooms with the temperature set at about 70F and humidity at 50%. Lights should not be too bright since white rats are albinos and too much light damages their retina. They are diurnal which means they need about 12 hours of light and 12 hours of darkness each day. Their bedding should not be wood shaving since some wood emits hepatotoxic fumes. They should have fresh mouse or rat food and water available at all times. Their bedding should be changed 2 or 3 times a week to prevent the buildup of urea.
Signs of Disease
Signs of disease in the rat can be broken down into several groups:
Cutaneous Signs - Excessive hair loss may be one o the first signs of a skin abnormality. Rough hair coat can indicate several situations from old age to disease. Rats housed together have a tendency to tail bite. Care should be taken for an swelling or mass in or under the skin. It could be an abscess or a tumor. Any abnormal temperatures or humidities, especially rapid changes, can induce injuries. Uncomplicated Sendai virus infection can cause deaths in pregnant or aged rats.
Gastrointestinal Signs - Diarrhea is seldom seen in rats. Slobbering in rats is associated with suffocation, heat stress, and malocclusion.
Respiratory Signs - Sniffling and a nasal discharge are associated with respiratory Mycoplasmosis commonly. A red ocular and nasal discharge indicates many debilitating diseases and stress conditions.
Neuromuscular Signs - Otitis interna, pituitary adenoma, and encephalitis may cause a head tilt in rats. Trauma, encephalitis, pituitary neoplasia, the rat virus, and otitis interna can lead to incoordination in rats. Brain lesions, trauma of the spinal cord, malnutrition, and arthritis can produce limb weakness or paralysis. Abscesses at the head of the tail or in the perineum often extend into the spinal canal. Corneal opacities can be related to irritation from food, bedding dusts, to injury or infection.