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The Rat




Diseases of Rats


The laboratory rat, Rattus norvegicus, belongs to the order Rodentia and family Muridae. Rats were first used for experimental purposes in the mid 1800s. Strains were developed to study neuroanatomy, nutrition, endocrinology, genetics and behavior.

The rat has short hair, a long naked tail, rounded erect ears, protruding eyes, a pointed snout with long whisker (vibrissae) and five toes on each foot. Albino rats have poor eyesight and depend on facial vibrissae and olfaction for sensory input. Rats have no gall bladder. Rattus norvegicus (brown rat) has a short tail compared to Rattus rattus (black rat), which has a much longer tail.

Rats have a pair of incisors and three pairs upper and lower of molars. Molars are permanently rooted while the incisors have an open root and grow continuously. Due to this continuous growth of the incisors rats can have problems with incisor overgrowth when the upper and lower incisors do not meet properly (malocclusion). Malocclusion can be hereditary or follow trauma, disease or inappropriate diet and/or soft food. There is no permanent cure for overgrown teeth. The only treatment is to trim the teeth every 2-3 weeks, if malocclusion persists.

Rats have a large horseshoe-shaped Harderian gland deep within the orbit. Secretions from the gland contain varying amount a reddish-brown porphyrin pigment depending on the physiologic state, age, strain and sex of the rat. The amount of secretions increases during stress and appears as 'red crusts' around the eyes and nostrils.

Normative values for rats

Lifespan2.5-3.5 years
Adult weightMales 300-500g, Females 250-300g
Birth weight5-6g
Heart rate330-480 beats per minute
Respiratory rate85 breaths per minute
Body temperature35.9-37.5ºC
Blood volume50-70 ml/kg
Urine volume3.3 ml/100g bwt/day
AllergensDander, urinary protein

Rats are communal animals. Young are raised communally with shared nursing responsibilities. Males can be co housed as fighting rarely occurs among adults. Rats are burrowers and take advantage of that whenever the opportunity presented. They are nocturnal animals but adapt to their environments. Rats can inflict severe bite wounds and must be handled gently and with care to avoid biting.

Rats have two distinct cervices and uterine bodies. There are separate urethral and vaginal openings. There is a vaginal closure membrane, which is lost at puberty. The inguinal canal remains patent throughout life. Rats have an os penis or os clitoridis associated with external genitalia. Mammary tissue is widely distributed and may extend to the lateral and dorsal areas of the abdomen.

Rats have a four to five-day estrous cycle, divided into characteristic phases: proestrus, estrus, metestrus and diestrus. The stage of the estrous cycle can be determined by vaginal cytology. Ovulation occurs at the end of metestrus. Receptive females exhibit lordosis when a downward pressure is applied to the pelvis. Such females will adopt a rigid posture with the hindquarters raised. Mating leads to formation of a vaginal plug. Plugs persist for 16-24 hours and may last as long as 48 hours.

Rat vaginal smear
Samples for making a vaginal smear can be collected by inserted a cotton tipped swab moistened with phosphate buffered saline into the vaginal cavity of a rat. The swab should be applied gently against the vaginal wall and rolled slightly before withdrawing. The moist swab is then rolled onto a clean glass microscope slide. The specimen is spray fixed using 95% ethanol. Air drying or dipping the slides in 100% alcohol does not produce but the very good cytologic preparation of rat vaginal smears. Samples for making a vaginal smear can also be collected by introducing 1.0 ml of saline into the vaginal cavity using a blunt tipped disposable pipette, and recovering .25 ml of vaginal fluid a short time later. This material is then applied onto a microscope slide and processed as above. Fixed specimens are then stained using Papanicolaou stain for best results and examined using a light microscope under low power. Giemsa or Diff-Quick® can also be used to stain the cells. Estrus lasts up to 12 h and is indicated by the presence of large cornified cells in the vaginal smear. Metestrus lasts 21 h and usually has many neutrophils in the smear and scattered squamous epithelial cells. Diestrus lasts up to 57 hours and there are abundant neutrophils and a few nucleated non-cornified epithelial cells. Proestrus lasts 3-12 hours and has abundant nucleated non-cornified epithelial cells.

Rat young are born incompletely developed

The young are born incompletely developed (altricius). They are born hairless and their eyes open after 10-12 days. Young are weaned after 21 days. Puberty is attained at 7-9 weeks. Breeding onset is after 9 weeks and breeding life is 9-12 months. It may be preferable to replace breeders when they are 6-9 months old.

Pregnancy lasts 21-23 days. Females will build a nest prior to parturition if opportunity is provided. Birth usually occurs at night with 10-12 pups being born. Stretching and hindleg extension are usually signs of impeding birth. Babies are born either head or tail first (breech). The female usually eats the placenta. Delivery lasts 1-4 hours, if labor persists call a veterinarian (5-3713). There is a fertile postpartum estrus. Maternal antibody is transferred to the fetus in utero and to the newborn via colostrum.

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Rats are generally fed a diet containing low fiber (5%), protein (20%) and fat (5-10%). Feed may be pelleted or powdered. The pelleted feed is supplied as regular, breeder, certified, irradiated or autoclavable. Rats are usually supplied feed free choice and they eat 10-30 g a day (5 g/100 g body weight/day). Water is supplied free choice and they usually drink 20-50 ml a day (10 ml/100 g body weight/day). Water may be supplied using a bottle or automatic waterers, and may be further treated by reverse osmosis, ozone, ultraviolet radiation, hyperchlorination or acidification.
Rat rooms are usually maintained at 30-70% relative humidity and a temperature of 18-26ºC with at least 10 room air changes per hour. Rats are usually housed in standard shoebox cages with or without filter tops. Filter tops prevent cross contamination of rats limiting the spread of disease and keep facilities clean. Cages with filter tops may have a slightly higher temperature, relative humidity, carbon dioxide and ammonia than the room air. Microisolator tops provide even a higher level of protection than bonnet type filter tops, since they seal better. Static cages as described above are usually changed one to two times a week depending on cage density and housing style. In ventilated cages air is forced into the cage at up to 60 air changes per hour. This keeps the cage dry and reduces build up of ammonia and carbon dioxide. In such situations cages are changed once every 1-2 weeks. Ventilated cages may be kept positive or negative to room air depending on the study being performed.

Rats are usually provided with bedding in the shoebox cages. Bedding can be paper, wood shaving, wood chips or corncob. In very rare instances rats are housed on wire floors. Housing on wire floors must be justified and approved by the animal care and use committee.

IdentificationEar punch marks to identify rats
Rats should always be clearly identified on cage cards indicating protocol number, strain (using standard nomenclature), sex, age, supplier, investigator and contact person. A combination of marks made by an ear punch can be used to identify rodents as indicated in the figure.
Procedures performed on the animal should be clearly indicated. Individual rats can be identified using ear punches, ear tags, tattoos, fur dyes, indelible mark on tail or microchips.

Sex is determined using the anogenital distance. Males have a greater (1.5-2 times) anogenital distance than females as well as a larger genital papilla. In neonatal males the testis may be visible through the abdominal wall. Conspicuous bilateral rows of nipples are visible in females at about 9 days of age. Absence of testicles is not a useful criterion for sexing since the testis is retractable throughout life into the open inguinal canal.

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Rats should be acclimatized to handling (gentling) to reduce stress. Always talk quietly, move hands slowly and handle them frequently. Rats should be handled at the base of the tail using your fingers. Pick up rats by placing the hand firmly over the back and the rib cage and restraining the head with thumb and forefinger immediately behind the mandibles. Holding the rat upside down keeps it distracted and reduces the chances of biting. A variety of restraint devices are available to assist in handling rats.

Handle rats at the base of the tailRestrain head with thumb and forefingerHolding upside down reduces chances of bitingPicking up a rat

Blood collection
An adult rat has a circulating blood volume of about 15-35 ml (5-7% of the body weight), however in older and obese animals this may be lower. Up to 10% of the circulating blood volume may be taken on a single occasion from a normal healthy animal on an adequate plane of nutrition with minimal adverse effect. Always make sure the animal has recovered safely from the procedure and give warm isotonic fluids. This volume may be repeated after 3-4 weeks. For repeat bleeds at shorter intervals, a maximum of 1% of an animal's circulating blood volume can be removed every 24 hours.

Blood can be collected from several sites in the rat including tail vein, saphenous vein, retro-orbital sinus, brachial vessels, vena cava or cardiac puncture. Always ensure complete hemostasis before returning the rat to its home cage.

It may be necessary to warm the tail by exposing it briefly to a heat lamp or placing it in a bowl of warm water. The rat should be restrained in a device for the collection. Blood can be collected from the tail vein (and artery) by making a snip in terminal =5 mm of the tail with a scalpel or sharp scissors. Stroke the tail gently with thumb and finger to enhance flow of blood into the collection vial. Because of the thermoregulatory function of the tail no more than the distal 3 mm should be taken at a time. At the end of the collection apply pressure to the cut end with a gauze bandage and ensure that blood has completely stopped flowing before returning the rat to the cage. A small nick can also be made at side of the tail 0.5 -2cm from the tail base to collect blood. A fine gauge needle introduced through the skin at a shallow angle can be used to withdraw blood from the tail vein. Apply a tourniquet around the base of the tail to aid in the collection. A butterfly catheter with only about 5 mm of tubing attached to it (rest cut off) may be used instead of a needle and syringe.

Blood collection equipmentCollecting blood from a ratRestrain and extend the hind leg

Saphenous veinWipe shaved area with alcohol or sterile gel
Restrain and extend the hind leg applying gentle downward pressure above the knee joint. This stretches the skin making it easier to remove the hair and immobilizes the saphenous vein. Wipe the shaved area with alcohol or sterile lubricating gel and use a 25-gauge needle to puncture the vein (the vein is next to the dark highlight in the picture below). If done correctly a drop of blood forms immediately at the puncture site and can be collected in a microhematocrit tube. Gentle pressure over the puncture site or relaxation of the restrainer’s grip is usually sufficient to stop the blood flow. The scab at the puncture site can be rubbed off at a later date to allow additional blood collection.

Use a 25-gauge needleLocation of veinDrawing blood

Retrorbital sinus
The retrorbital sinus is a system of dilated venous channels at the back of the orbit. Blood can be collected form this area in anesthetized rats using a microhematocrit tube. There should be no movement of the head during the procedure. Pressure down with the thumb and forefinger just behind the eye and pull back on the skin to allow the eyeball to protrude. Position a microhematocrit tube along the inner corner of the eye (medial canthus) beside the eyeball. Insert the tube gently but firmly through the conjunctiva towards the back of the eye along the orbit. Rotate the tube gently as you proceed. Blood should flow freely, if, the tube is properly inserted. Tilt the head slightly downward to improve flow. After collecting the blood withdraw the tube and apply pressure on the closed eyelids to stop any bleeding. Remove excess blood with gauze. Complications include damage to the eye and surrounding tissues.

Brachial vessels
Blood can be collected from the brachial plexus as a terminal procedure in deeply anesthetized rats. Make a cut through the skin at the side of the thorax into the angle of the forelimb (axilla) to expose the axillary vessels. Transect the vessels and allow blood to pool into the pocket created by tenting the skin. Aspirate the mixed venous arterial blood is into an appropriate receptacle.

Vena cava and abdominal aorta
Blood can be obtained from the posterior vena cava or abdominal aorta in a deeply anesthetized rat following laparotomy. Approach the vessel at a shallow angle using a fine gauge needle attached to a small syringe. This is a terminal procedure.

Cardiac Puncture
Up to 10 ml of blood can be obtained from the heart of a deeply anesthetized rat in a terminal procedure. The most common approach is to lay the rat on its back and insert a 25 to 30 gauge needle attached to a 3-5 ml syringe just behind the xiphoid cartilage and slightly left of the middle. The needle should be introduced at 10-30 degrees from the horizontal axis of the sternum in order to enter the heart. Alternatively approach the heart laterally immediately behind the elbow at the point of maximum heartbeat.

Cardiac puncture

Cardiac puncture

Terminal blood collection by cardiac puncture in a deeply anesthetized rat.

Intramuscular injectionAdministration of substances
Materials to be administered to rats can be given orally e.g. in water or feed or injected systemically through a variety of routes. The average daily consumption of feed and water for an adult rat is 15-25 g and 30-50 ml respectively. The following volumes can be injected into rats safely: 2-5 ml subcutaneously, 0.1-0.2 ml intramuscularly (0.1 ml per site), 1.5-2.5 ml intravenously, 3-5 ml into the stomach and 3-5 ml intraperitonealy. Intramuscular injections are usually not recommended in rats because of the small muscle mass. A fine gauge needle should be used to make injections in the anterior thigh muscle. It is good practice to use a new needle each time you perform an injection.
Intramuscular injection
Intramuscular injection in a rat

Oral gavage is performed using a ball ended feeding needle. Estimate the distance that the needle needs to be inserted into the rat (usually from the nose to the first rib) and mark it on the needle. Restrain the rat with the rat extended in a straight line to facilitate introduction of the gavage needle. Introduce the needle in the space between the left incisors and molars, and gently direct it caudally toward the right ramus of the mandible. The rat usually swallows as the feeding tube approaches the pharynx, facilitating entry into the esophagus. If the animal struggles or appears to be in respiratory difficulty withdraw the tube and begin all over again. Once the desired position is attained, inject the material and withdraw the syringe. Monitor the animal after the procedure to ensure that there are no adverse effects.

Gavage needles
Gavage needles

Estimating the length of needle
Estimating the length of needle to be inserted into the animal

Inserting a gavage needle
Inserting a gavage needle into a rat

Subcutaneous injections
Subcutaneous injections are usually made into the loose skin over the neck or flank using a fine gauge needle. Insert the needle 5-10 mm through the skin before making the injection. Lack of resistance to the injection is indicative that you are in the right location. Check for leak back especially if a larger volume is injected.

Intraperitoneal injections
Intraperitoneal injections are usually made in the lower right quadrant of the abdomen. The rat is restrained with its head tilted lower than the body to avoid injury to internal organs or major blood vessels. After swabbing the lower right quadrant with alcohol, a fine gauge needle is introduced slowly through the skin, subcutaneous tissue and abdominal wall. Withdraw the syringe plunger to ensure that you are not in the bladder or intestines. If nothing is withdrawn inject the material slowly. If you accidentally enter the bladder or intestines withdraw and discard the needle and syringe.

Intraperitoneal injection

Intraperitoneal injection

Picture demonstrating intraperitoneal injection in a rat

Intravenous injections
Intravenous injections are usually made into the dorsal tail vein. Warm the tail by immersing it in warm water or placing the animal under a heat lamp. The tail vein is easier to see in non-pigmented rats. A fine gauge needle should be used for this procedure.

Warming the tail

Making the injection

Tail vein injection demonstrating warming the tail to dilate the vessels and making the injections

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Signs of pain in rats

Acute pain

Chronic pain

Anorexia (no fecal pellets)Decreased body weight
Decrease in appetite (few fecal pellets)Reluctance to move
Rubbing or scratching surgical siteChange in behavior
Biting or shaking affected body partPoor grooming
VocalizationChange in bowel or urinary activity
RestlessnessRough hair coat
Porphyrin discharge (red-brown pigment around eyes and nostrils)
Increased respiration

Pain relief
Generally opioids e.g. buprenorphine or non-steroidal anti-inflammatory agents e.g. acetaminophen, ketoprofen, caprofen, ibuprofen are used to relieve pain. Drugs can be administered in water, in jello, as oral drops or injected. Drugs administered in water may be broken down in water, or insufficient quantities may be taken due to poor solubility in water or palatability problems.

In general inhalant anesthetics are safer than injectable anesthetics. Halothane and isoflurane are the safest ones to use. Methoxyflurane is no longer available. Use of ether at Johns Hopkins University is subject to restrictions due to safety concerns. Ketamine and xylazine is a common and relatively common injectable anesthetic combination. Sodium pentobarbital can be used, but it has a narrow safety margin and is associated with a prolonged recovery period.

Euthanasia in rats is most often performed by carbon dioxide asphyxiation or overdose of an anesthetic agent. Use of cervical dislocation or decapitation in absence of deep anesthesia must be scientifically justified. All individuals performing euthanasia must be properly trained. Individuals must also ensure that animals are dead before the carcass is disposed. Exsanguination or opening the thoracic cavity will ensure death. AVMA Panel on euthanasia report

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Diseases of rats

Diseases of rats are usually handled as a herd (colony) health problem rather than on an individual animal basis. In most cases the goal is to prevent introduction of a disease into a colony than to treat individual animals. Disease prevent is practiced by institution of a health monitoring (sentinel) program based on serological and microscopic diagnosis of problems in a representative sample of animals. Due to the widespread movement of animals all over the world with advent of genetic manipulation of animals, the possibility of introducing disease agents in a colony has markedly increased. The expanded use of genetically modified and immunocompromised animals greatly exacerbates the problem. Furthermore the practice of transplanting tumor material into animals provides a portal where these agents can be introduced into animal, especially if the tumors are not screened for adventitious infectious agents. Some important rat diseases are discussed below to draw attention to the need to adhere to practices recommended by the veterinary staff to avoid these diseases. For additional information see Implications of infections agents on research results and Opportunistic Infections in Laboratory Rodents.

Pinworms (Syphacia and Aspicularis) inhabit the intestine (cecum, rectum, colon) and have a direct lifecycle. The eggs are particularly resistant and survive for a long time in the environment. The disease is usually subclinical being marked in weanlings and immunocompromised animals. Symptoms include poor body condition, rough hair coat, reduced growth rate and rectal prolapse. Infection with pinworms has a negative impact on gastrointestinal, growth, behavioral and immunology studies.

Mites affect the skin of rats and up to 100% of the animals may be affected. Affected animals are scruffy, pruritic (itchy), loose hair and have scratch wounds, which can become infected with bacteria. There are changes in the immune responses of affected animals.

Sialodacryoadenitis virus
Sialodacryoadenitis virus (rat corona virus) affects the salivary and lacrimal glands and the respiratory tract of rats. The virus spreads extremely rapidly in a susceptible population resulting in conjunctivis, nasal and ocular discharge, porphyrin (red) staining around the eyes and nostrils, corneal ulceration, swelling of the neck and protrusion of the eyes (exopthalmus) resulting in drying of the cornea. There are changes in the respiratory passages (squamous metaplasia) during recovery from the disease. The disease may persist for at least 6 months in immunocompromised animals. The disease interferes with growth and reproduction in rats, and adversely impacts on studies of respiratory, ocular, olfactory and immune systems. It adversely affects studies involving infectious agent especially in the respiratory system as well as tumor studies.

This is a fungal disease affecting a wide range of laboratory animals and humans. The organs are primarily localized in the lungs but may also involve other organs including the eyes, skin etc. It causes a slowly progressive chronic pneumonia with weight loss and eventually death in a large number of immunocompromised animals. The disease has a severe negative impact in research involving immunocompromised animals, pulmonary function and immunology.

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