Rabbits

References and Resources for this tutorial

Suckow, Mark A., Douglas, Fred, The Laboratory Rabbit, a volume in the Laboratory Animal Pocket Reference Series (Boca Raton: CRC Press, 1997)

Suckow, Mark A., Brammer, David W., Rush, Howard G, Chrisp, Clarence E., "Biology and Diseases of Rabbits," Chapter 9 in Laboratory Animal Medicine, 2nd Edition, James G. Fox, et. al., editors (New York: Academic Press, 2002)

"Lagomorpha" in The UFAW Handbook on the Care and Management of Laboratory Animals, 7th Edition (Malden: Blackwell Science, 1999)

Overview

In general, rabbits are timid and non-aggressive, sociable with each other, although the males will fight each other after maturity. They require cooler temperatures than most other laboratory mammals; between 61-72 degrees F is considered optimal. They eat continuously throughout the day and food is provided ad lib, usually pellet form feed. They consume up to 120 ml/kg a day of water; excreting up to 50-75 ml/kg daily. They can play with both food and water supplies/equipment so need to be monitored to be sure basic needs are being met. Due to high urine output, catch pans need to be cleaned often (changing twice weekly is standard) and room ventilation maintained at 10-15 air changes per hour. They are generally quite docile to work with but handlers will need special training; they usually thrust vigorously with their hind legs when taken from the cage or restrained and if not properly held, will break their back.

The Animal Welfare Institute, and the Animal Welfare Information Center have cross posted an overview of the laboratory rabbit, including commentary on their housing and behavior, including photographs, entitled "Comfortable Quarters for Rabbits in Research Institutions." Interestingly enough, the standard of care for the majority of rabbits used in the United States is single housing in contrast to the more widespread group housing practiced in Europe and described in this article.

The rabbit and science

Rabbits have been domesticated since the sixth century, when they were kept for food and fur. They have also been selectively bred over the years for varieties in the fur and are also a popular pet. After mice and rats, they are the most common laboratory animal. As many as 76 different breeds of rabbit are recognized by the British Rabbit Council but the New Zealand White (NZW), bred in the 1920s has become the one most commonly used in research. Historically, they have been most used for antibody development and as testing sentinels for a wide array of products. 

It was actually this use of rabbits for toxicity testing that initiated one of the first large scale public protests against the use of animals in research. Since the eyes of the rabbit are large and do not have tear ducts, eye irritancy tests (the Draize test) for various household and cosmetic products have been conducted since 1944.  Due to large scale protests in the 1970s and 1980s use of the Draize test has declined; the cosmetic industry has been instrumental in seeking alternatives. Here is an example of how society has had a direct impact on scientific practices--research into alternatives such as cell lines or the use of bovine eyes collected after slaughter are two examples. Since safety tests of some sort are legally required by the United States’ Food and Drug Administration, research into alternatives and guidelines when live animals are used have been established. Oklahoma State University has posted NFPA Guidelines for toxicity testing.

Since rabbits can be easily restrained in stocks and are generally docile and cheap to maintain, they have been used for a wide range of toxicity testing, especially on their skin. e.g. see “Effects of Military Fuels on the Eyes and Skin,” from a publication by the National Academies Press. Both rabbits and pigs have been utilized extensively in burn research: this raises tremendous ethical issues. If one chooses the rabbit species, as opposed to the dog (social protest due to emotional attitudes towards dogs) or the pig (high intelligence and expense to maintain) for burn research, then the basic moral issue of inflicting pain and distress on an animal for human benefit must still be addressed. For some burn research, rats (low social concerns and ease of maintenance and handling) have been used instead of rabbits. In an online article, “Experimental Burn Models” researchers in Spain discuss various procedures—we note this article as one for thought and discussion on perimeters in the use of research animals as tools.

Biology

Rabbits have incisors that grow continuously; they rely on normal occlusion and use to wear them down. They use their premolars and molars to chew food with a sideways motion. They need to be watched for malocclusion (mandibular prognathism); when this occurs, the teeth need to be trimmed every 2-3 weeks, otherwise they are unable to obtain and chew their food and can die of starvation. This genetic problem is usually solved by getting rabbits from vendors who have selected out this recessive trait.

Rabbits have only striated muscle in their esophagus and down including the stomach cardia; they are unable to vomit. Their stomachs are never entirely empty: they can at times have difficulty with ingested hair from grooming. The reasons for hairball obstruction are complex: signs can be anorexia or hunched posture but often the amount of hair in the stomach is only revealed at the time of autopsy. There is some anecdotal evidence that mineral oil is helpful but sometimes surgical intervention is necessary. The cecum is very large with a capacity 10X that of stomach. The colon is divided into proximal and distal portions: “cecotrophs” also known as night feces, or soft pellets are eaten by rabbit directly from anus to increase utilization of B vitamins and proteins. It is important to be sure to provide for the normal ingestion of cecotrophs. Hard pellets are 2/3 of fecal output.

Nasal breathing is rapid, 20-120 times per minute. Since the diaphragm initiates breathing, artificial respiration is performed with an up and down motion of head 30-45 times per minute. Lung volume increases with age. The pharynx is long and narrow, and the rabbit has a large tongue. Intubation is difficult with laryngeal spasms occurring easily.

The cardiovascular system has some anatomical structural aspects that have given rise to the use of the rabbit in cardiovascular research. For example, the tricuspid valve of heart has only 2 cusps (vs 3 in most mammals) as well as a small group of pacemaker cells. Blood supply to brain is mainly via the internal carotid artery .

The rabbit urogenital system produces urine that is different from other animals: excretion is usually from 50-75 ml/kg daily with the urine varying in color from yellow to reddish to brown—it is clear in young rabbits but can be very cloudy in adults. The residue on pans needs to be scraped, and/or soaked in acid before cleaning; high volume often necessitates minimum twice weekly maintenance.

Their body temperature ranges from 38.5-39.5 C. Their ears have large surface area and are highly vascular; due to this the ears are a common site for blood collection. The ears also have thermoregulatory function.

Nutritional requirements

Rabbits are strict herbivores, needing diets high in fiber, high in protein and low in carbohydrates. They prefer pellets to meal type feed; if they are fed supplements such as greens, with high water content, they will drink less water. They are very sensitive to imbalances of vitamin A (reproductive dysfunction, e.g.); vitamin E deficiencies will result in reproductive problems as well. Long-term feeding of high calcium diets can result in renal problems: “Calcium Metabolism in Rabbits,” by Leah Postman, DvM discusses calcium complexities and husbandry needs in rabbits.

Most commonly, rabbits are fed high quality pelleted food obtained from vendors; the storage area should be out of direct sunlight and monitored frequently for signs of insect or fungal infestation. Rabbits are usually fed from hoppers mounted on the cage; they can have a tendency to play with the food and empty the hopper; something else to keep an eye on.

Water

Since their body is 58% water they need at least 120 mg/kg daily. If water deprived, rabbits will decrease eating; after three days without water they will basically stop eating. If food deprived they will increase water intake. This is why frequent monitoring is so important for rabbits--a daily schedule including weekends and holidays is standard. As an example, a group of rabbits can be fine on Friday morning, happily playing with their water bottles. If they are not monitored again till midday on a Monday it is entirely possible they will have emptied the bottles midday Friday and thus been without water until checked midday on Monday.        

The water system can be either an automatic watering system (piping into room with individual valves leading from pipe to each cage). The benefit is that water is always available with the caveat that the valves need to be checked daily to be sure they are working. Leaking can cause flooding. With automatic watering it is not possible to monitor how much the rabbit is drinking. Individual water bottles with sipper tubes have the advantage of allowing for evaluation of drinking. The valves can be open or have inner ball valves. Here you need to check that the rabbit has not emptied the bottle by playing with the valves..

Major illnesses

Many of the more common diseases can be avoided by careful husbandry; rabbits are easily stressed, especially at weaning time, and when stressed, and/or eating poorly, their weakened immune system might not fight off an illness. Or, if an illness has remained sub-clinical, stressors might trigger a full blown sickness. The University of Missouri Veterinary School has posted information on Rabbit Diseases Click on teaching and training, and then on teaching resources and then Diseases of Research Animals to get to the rabbit webpage. Three of the more commonly seen diseases are Pasteurellosis, Tyzzer's Disease and Enterotoxemia.

Pasteurellosis often presents as a rhinitis (thus the common name for this illness, “snuffles”); aside from the nasal discharge, the front paws can be wet due to the rabbit wiping the exudate from the nose. There can also be sneezing, coughing and eye involvement (conjunctivitis, hair loss around the eyes, swollen eyelids, etc.) This is a bacterial disease caused by Pasteurella multocida, that can present with different types of systemic involvement, e.g. pneumonia, sinusitis, otitis media, genital infection and spread through a colony. The disease can progress rapidly and rabbits that become septic can die quickly. Sometimes anorexia and depression are the only clinical signs before the disease progresses. A colony can become infected via direct contact, oral, respiratory or aerosol: the infection can remain sub-clinical, only to surface during situations of stress. Pneumonia can be sub-clinical. One of the difficulties of detecting it is due to the low activity level of caged rabbits; there may not be enough respiratory challenge to indicate any distress.  Rabbits affected with the otitis form, can develop a head tilt (“torticollis”): sometimes circling in the direction of the affected ear is noted. When the rabbits become stressed for some reason, a low level pasteurella infection, one sub-clinical, may increase to noticeable levels. In the chapter, “Biology and Diseases of Rabbits,” the authors note that, “the most common research complication associated with pasteurellosis is infection of injection sites of rabbits immunized for production of polyclonal antisera.” (p.340) and for this reason, most facilities use only pasteurella free rabbits provided by vendors to avoid problems.

Gastrointestinal diseases

There are a number of different gastrointestinal diseases—actually GI disease in the rabbit is a complex family of diseases and obtaining rabbits from reputable vendors who screen for these is the usual practice. We present two specific illnesses here as examples.

Tyzzer’s disease

This bacterial disease, caused by Clostridium piliforme, also affects other lab animals, particularly rodents. It occurs most often in weanling animals, presenting with watery diarrhea, anorexia, dehydration, and lethargy. It can have a short course, with death after a day or two, or go on to become a chronic state, with weight loss. The route is oral, via spores. A high level of sanitation is critical to help prevent this illness; preventing stress is important as well. Again, be sure your source for the rabbits is able to assure disease-free animals. Since the spores can infect different species, sanitation measures are critical throughout the entire animal facility.

Enterotoxemia

Quite often, with this illness, there are no obvious clinical signs. There may be watery diarrhea and/or staining around the perineal region. Symptoms may include anorexia, dehydration, polydipsia, depression, fever, bloat or grinding of the teeth. You might see the rabbit hunched near the water bottle or food hopper. The cecum can be bloated with gas, with mucosal hemorrhaging and ulceration. It is important to avoid situations that may change the normal gut flora such as antibiotic administration, abrupt diet changes, or husbandry stress, e.g. lack of water or food for periods of time.  Weaning is a particularly vulnerable time: the young should not be overfed, given sufficient fiber and antibiotics should be used carefully.

There are two online postings about laboratory rabbits that are useful for overview. One is the University of Iowa’s “Biomethodology of the Rabbit,” which covers biology and husbandry. A paper by by Dr. Jo Lynn Wilber, of the Armed Forces Institute of Pathology in Washington, D.C.,“Pathology of the Rabbit,” is an online overview focusing on diseases both infectious and metabolic with a review of basic rabbit biology.

Husbandry and welfare considerations

The amount of time laboratory animals spend undergoing a procedure of some sort is usually minor compared to their time in their quarters. Husbandry is thus of central importance to them. In “The Ill-Effects of Uncomfortable Quarters, William M. S. Russell talks of the necessity for healthy, unstressed animals in research. 

It is important to monitor the environment the rabbits are in as well as the rabbits themselves.  Animals have keen senses of hearing and smell and what we would not notice might be quite noxious to them. The noise of cage banging, loud music, noisy animals (primates, dogs)—all this is especially stressful to rabbits since they are able to hear sounds at high frequency. There are some computers that emit a hum below our hearing range but quite audible to a rabbit; their housing location thus should not be near machinery (e.g. cage wash areas or machine rooms) or noisy animals.  It is important to check the room for appropriate temperature, humidity and ventilation. (Data from Barnett, op.cit.)

Room temperature	61-72 degrees Fahrenheit
Humidity	55%-/+ 10%
Ventilation	10-15 air exchanges per hour

Traditionally, rabbits have been singly housed, in stainless steel cages, with space requirements as set out in “the Guide:” (See Table 2.2)

Weight (kg)	Floor area/animal (ft2)	Height (in)
Less than 2	1.5	14
Up to 4	3.0	14
Up to 5.4	4.0	14
Greater than 5.4	5.0	14

Catch pans below the cage bottom collect urine and fecal pellets; some sort of absorbent material is usually placed in them to reduce the ammonia in the room and protect the catch pan from the scale buildup than can occur with rabbit urine.

One of the welfare issues for rabbits is their single housing; this is a sociable species and there is a growing body of information about group housing of rabbits. In 1995, Environmental Enrichment Information Resources for Laboratory Animals (1965-1995): Birds, Cats, Dogs, Farm Animals, Ferrets, Rabbits, and Rodents was published jointly by the U.S. Department of Agriculture and the Universities Federation for Animal Welfare. The chapter on Rabbits is available electronically. This chapter includes an extensive references list, as well as an annotated bibliography on rabbits. For those interested in group housing male rabbits previously singly housed, in “Group Housing for Male New Zealand White Rabbits,” authors Rage and Stewart report their findings.

All laboratory animals, as well as their environment should be monitored on a daily basis: the University of Minnesota, Research Animal Resources, has produced an excellent set of guidelines for this entitled, Housing and Husbandry Guidelines for Laboratory Animals, which is available electronically.

Handling and Procedures

As a preventative measure, be sure that the surfaces for any handling or procedure are not slippery: rubber mats work well to give them a surface to grip onto to help avoid slipping; they can easily panic. Although they are generally tractable when handled, they often struggle when restrained, particularly if they are not being correctly handled; this can result in a traumatic vertebral fracture (broken back). They will attempt to spring with their forceful hind legs: when carrying a rabbit, as well as when removing and returning them to the cage they need to be carefully lifted and supported both around the chest and under the hindquarters. When removing them from their cage it is critical to be sure they are supported fully before removing; when returning, do so back first. Without trained and careful handling, back injury can occur easily. The University of Minnesota’s Research Animals Resources has a page on handling of specific species: scroll down to the rabbit for a diagram and text.         

The University of Minnesota’s Research Animal Resources “Experimental Guidelines for the Prevention, Assessment and Relief of Pain and Distress in Laboratory Animals” gives a listing of those specific procedures or situations that are sources of pain, as well as a listing of criteria to use when assessing pain/distress 

Monitoring

The paper “Group-housing Rabbits” by Dr. James A. Love and Kim Hammond, can serve as a good overview of husbandry details about managing rabbits in research: the authors cover many topics other than housing, such as disease, behavior, acclimatizing new rabbits, and treatment data.

As previously noted, frequent monitoring is essential. An experienced technician or researcher will have an intuitive sense of something “not quite right;” familiarity with what is normal and expected is a key skill to gain. It is useful to look into the room before entering as some animals will instinctively become active in your presence, but when not feeling observed, their behavior might be more revealing of problems. Things to assess quickly are posture (hunching indicates discomfort or pain) and grooming (poor or absent grooming is often a sign of a sick animal.) Over-grooming is also something to watch for since this can lead to excessive hairball build up in the stomach. Another useful check is to see if all the water bottles are at reasonably similar levels: this can indicate a water bottle leak or a rabbit that is not drinking up to par. By the same token, a quick inspection of catch pans can reveal too much or too little urine or feces in a particular animal. Once inside the room, you can work through the colony, checking them quickly and thoroughly following a systematic pattern. If the rabbits are on wire cages instead of solid floors, their feet need to be examined regularly for sore hocks (ulcerative pododermititis), a condition that develops from pressure sores of the feet. Their nails need to be regularly trimmed as well so inspect them when you check their feet.

Monitoring Checklist:

It is best to decide on a formal repeatable examination pattern. For example:

Overall: coat should be clean, dry, shiny, well groomed, urine and feces of amount and consistency and color as usual and similar to other rabbits, no obvious weight gain or loss, normal activity level, expression BAR (bright, alert, reactive).

Breathing: normal is rapid, 30-60 bpm

Nose: pink, dry

Mouth: check that teeth occlude correctly, gums should be pink, moist, not overly wet or dry.

Eyes bright, eyelids clean, not swollen

Ears: (pinna) fur clean, present and full on outside, veins and surface on inside clean, dry, smooth, not reddened. Inside of ear free of crust (possible mite Psorptes cuniculi infection). (It is important to monitor for ear mites.)

Tail/Anus: clean, no evidence of mucous or feces

Genitalia: clean, no exudates (if female in estrus the vulva will be reddened and inflamed)

Legs: no stiffness or damage or swelling, check claws for breaks or trimming needs, check hocks to be sure full furred and not sore. For front legs be alert for any sign of matting, wetness that can indicate nasal discharge.

Current research using rabbits

Three of the ways that rabbits are used in research are: 1) to produce serum antibodies; 2) as models for disease or biological systems; and 3) as sentinels or “test animals” for products.

Polyclonal antibody production

The rabbit has been the breed of choice for polyclonal antibody production for some time, due to the size and blood volume and the accessibility of the vascular system via the ear veins. There is also a large database of information about rabbit immunoglobulins. This continues to be one of the major uses of rabbits in research today.  The Canadian Council on Animal Care (CCAC) in their Guidelines on Antibody Production, notes,

Biological model

Rabbits are used as models for cancers mainly in the following six categories:

1. VX-2 tumor

2. Spontaneous endometrial adenocarcinoma

3. Monoclonal gammopathies

4. Nephroblastoma

5. Lymphoblastic leukemia

6. Malignant fibroma

The systemic diseases studied in the rabbit are generally in the following areas:

The infectious diseases most studied using the rabbit are:

1. Campylobacter enteritis

2. Chagas’ disease

3. Cryptococcal meningitis

4. Herpes simplex encephalitis

5. Staphylococcal blepharitis

Study/discussion question:

In the United States single caging of rabbits is the standard while in Canada and Europe, group housing is more common. Review Comfortable Quarters for Rabbits in Research Institutions, from the Animal Welfare Institute, a United States private organization, and Examples of Environmental Enrichment, Module 7 from The Canadian Council on Animal Care. What reasons would you cite for the differences in standard housing between our country and some others? If you had to decide, which type of housing would you choose and why? Which sort of housing makes the most sense scientifically? Ethically? What does your answer say about the inherent value of rabbits?