November 24, 2014

Rabbit GI Physiology and Nutrition

by Susan A. Brown, DVM

Rabbits are strict herbivores. Their relatively small body size makes it difficult to store large volumes of coarse fiber as might be done in the cow or horse. The gastrointestinal tract (GIT) attempts to eliminate fiber as quickly as possible. Large fiber particles stimulate motility of the GIT. The rabbit then utilizes the non-fiber portion of the food to produce the nutrients that are needed for life, some of which are absorbed directly from the GIT and some of which are reingested in the form of cecotropes. This particular system allows for a large volume of food intake, with a rapid digestive transit time thus increasing the total amount of energy stored and minimizing the need to store fiber.

Ingested food first enters the stomach, where the pH is approximately 1-2 creating very acidic environment. Preweaning rabbits have a stomach pH much closer to neutral (5-6.5) which allows bacterial flora to be initiated into the GIT. After weaning the pH drops dramatically. It is not known what the pH of the stomach is in all states of disease or anorexia and even in health the pH may not be constant. However, published research tends to support that the stomach environment is normally quite acidic.

Ingesta in the stomach is essentially sterilized, massaged a bit and broken down into smaller particles. It then moves into and through the small intestine where nutrients are extracted, and more water is added resulting in a fluid content. It may take several days for the stomach to completely empty, so fasting a rabbit to empty its GIT for diagnostic testing as in the case of radiography to detect a gastric foreign body, generally does not work. In addition, the fasting itself will slow down gut motility because the fiber which “drives” the system is not being taken in. It is not recommended to fast a rabbit prior to a surgical procedure. Rabbits do not have ability to vomit.

At the end of the small intestine is the ileocecocolonic junction. The cecum is a large blind sac in which resides a specific population of bacteria that break down the non-fiber portion of the gut content. Through bacterial fermentation proteins, fatty acids and certain vitamins are produced. Some of these items are directly absorbed thorough the wall of the cecum, but most are returned to the rabbit when it eats the cecotropes which are formed nutrient rich “feces” that come directly from the cecum. Hard waste feces (what is found on the floor of the cage) which have a high fiber content, are produced for approximately the first four hours after the rabbit eats its food and the cecotropes are produced during the next four hours (therefore not only at night). The cecotropes are ingested directly from the anus, have a mucous coating, are soft, moist have a stronger odor and are brighter green color that the dry waste feces. This mucous coating helps to protect the microflora through the acid pH of the stomach. The dominant bacteria in the cecum of the healthy adult rabbit is Bacteriodes with small amounts of Clostridium sp. and E. coli. Note that Lactobacillus species are not common or normal inhabitants of the adult rabbit GIT, therefore, in my opinion it makes no sense to feed products that contain Lactobacillus to our pets. In addition, unprotected live bacterial products fed to a rabbit will be destroyed in the very acid pH of the stomach. If we truly want to repopulate to GIT with healthy bacteria, we should be using the species that are normally present in health. Some practitioners have advocated giving the fresh cecotropes of healthy rabbits to ill patients. This is probably not a bad idea, but the problem is that it may be difficult to collect the cecotropes due to the fact that rabbits eat them directly and the healthy individual doesn’t usually “drop” them in the cage unless they have a collar placed on them, which is stressful. If cecotropes are used, they must be retained in their “whole” form to protect their mucous coating (i.e. not ground up). Probably only 2 or 3 cecotropes are needed.

When the liquid small intestinal contents get to the area of the junction of the large intestine and cecum it enters sac-like areas in the wall of the cecum and colon called haustra which move food along by muscular contractions. When the liquid small intestinal contents reach this junction the long fibers are separated from the digestible portion of the food and moved into the center of the colon where they become the hard, dry waste feces and are passed out of the body. The digestible portion of the intestinal contents are moved into the cecum to undergo fermentation. The haustra move the liquid ingesta back and forth in the cecum and in the colon continually separating fiber from digestible particles. In fact, in the colon, water is actually excreted into the lumen to aid in the separation of fiber and non-fiber portions. The large fibers are moved towards the center of the lumen and the digestible particles accumulate along the wall in the haustra of the large intestine. These digestible particles are then are moved in a retrograde fashion back into the cecum.

There is a constant flux back and forth in the cecum and upper third of the colon to mix and separate food particles. Because it is so important to have a liquid consistency to the ingesta in this area to allow sorting of materials, it could be detrimental to administer such materials as psyillium (Metamucil) to rabbits, because these products tend to absorb moisture to create bulk and the end result may actually be constipation.

So back to diet of the HOUSE RABBIT….I want to stress that we are speaking here of the nonproduction, nonreproductive house pet specifically. We recommend feeding the house rabbit a diet that is high in fiber and relatively low in calories (especially fats and starches). Unfortunately, over the years, we have seen pelleted diets become a problem in the maintenance of the house rabbit. Pelleted diets were originally formulated for the rapid growth and ease of care of the meat or fur production rabbit, and for laboratory rabbits. Most of these rabbits were not meant to live out their full life span. The pellets perform an excellent function in these situations, as they produce rapid growth, good weight gain, are efficient, economical and easy to feed. The problem comes when we have a house rabbit that is usually neutered, is expected to live out its full life potential, and unfortunately may not get all the exercise it needs. Pelleted diets are typically made up of chopped, compressed alfalfa hay, various grains and other added nutrients. Grains can be quite high in calories (starches and fats) and usually lower in fiber than just hay. The alfalfa hay in pellets is chopped and compressed and heated and may lose some of its fiber quality.

The problems that we and other practitioners have seen over the years when pets are fed an unlimited primary pelleted diet are obesity, chronic soft stools (mixed with normal stools) and periodic bouts of anorexia (commonly known as “hairballs”, but what I feel is more likely a GIT motility problem.) We have also seen less frequently, calcification of blood vessels (some pellets are quite high in calcium), and bladder and kidney stones. I am not going to say that all of these problems are entirely caused by the diet, but my observation is that diet plays a very big role. If we correct the diet, then we can attend to other factors that may be still be present. Some manufacturers of pellets have been sensitive to the needs of the house rabbit and are producing higher fiber and lower calorie pellets. Unfortunately other manufacturers have gone the opposite direction and have decided to add all kinds of dangerous things to the pellet mix such as seeds, nuts and additional grains in the name of marketing without sufficient knowledge of what the consequences can be. Regardless, I think that those of us who deal with house rabbits should not depend on pellets as the total food source for our house rabbits.

The diet that we recommend for the ADULT, NONREPRODUCTIVE HOUSE RABBIT (and we did not “originate” this, there are plenty of practitioners around who have done this for years) is no more than 1/8 cup/4 lbs of body weight of a high fiber maintenance type pellet (18% or higher fiber) per day. (Some adult animals are given no pellets at all if they have trouble losing weight or have chronic GIT problems). In young growing animals the pellets may be given free choice until they are about 6-8 months of age, then cut back to the maintenance amount. Fresh hay should be offered FREE CHOICE throughout the pet’s life. THIS IS THE MOST IMPORTANT PART OF THIS DIET AND MUST BE AVAILABLE ALL THE TIME. Young bunnies should be exposed to hay as soon as they can eat on their own. Mixed grass hay or timothy hay is the preferred type because it is lower in calcium and calories than alfalfa hay. Try contacting a horse barn or feed store for your source. If you have three or more bunnies, just buy a bale and store it in a cool dry place, because you will use it up quickly! If you cannot get the grass hay, then use alfalfa, but be cautioned that it is much higher in calories, and calcium. I prefer that rabbits on 100% alfalfa hay not get pellets at all, because it is somewhat redundant.

We also like for our bunnies to get greens and lots of them. We pick the tough fibrous greens which are rich in a variety of nutrients. We suggest feeding a minimum of 3 types daily in a total MINIMUM total amount (of all types of greens together) of 1 heaping cup/4 lbs body weight. Note, that this is a minimum, because as the bunny adjusts to it more can be fed. By feeding several types of greens daily, you will provide a variety of nutrients as well as not creating a finicky rabbit. Some of the excellent greens are kale, collards, beet tops, carrot tops, parsley, dandelion greens, mustard greens, romaine lettuce, broccoli leaves, Brussels sprouts, outer cabbage leaves, raspberry leaves, peppermint leaves, escarole, endive, raddich io, wheat grass, alfalfa sprouts, etc. Don’t feed light colored greens (i.e. iceberg and bibb lettuce) or the mixed gourmet greens in a bag as the only source. Other vegetables such as carrots, pea pods (not the peas), green pepper, squash, can be fed. Stay away from starchy foods such as legumes (beans and peas) and corn and other grains. Fruit can also be fed with some restrictions. Stay with high fiber fruits such as apples, pears, peaches, papaya, pineapple, and strawberries, but stay away from sugary fruits such as bananas and grapes. The fruit and vegetables we feed in the amounts of 1 -2 Tablespoon/4 lbs body weight daily.

Do not feed grains such as oats, corn, wheat, crackers, Cheerios, bread, crackers, pasta, etc. There is research to suggest that high starch and low fiber diets may be two of the contributing factors to often fatal cases of enterotoxemia. Enterotoxemia can be caused by changes in cecal pH resulting in the overgrowth of certain bacteria that produce dangerous iota toxins that when absorbed into the body ultimately lead to death. I know the bunnies love this stuff and in small amounts, and in adult rabbits it wouldn’t normally be a problem, but often clients overdo and it may result in serious GIT disease. We have seen rabbits that continued to have periodic soft stools when all else was corrected about the diet, yet they still got two crackers a day. When the crackers were removed, the stools returned to normal.

For obese rabbits and those that have that chronic intermittent soft stool mixed with normal stool, I take them entirely off of pellets and feed only hay free choice for two weeks. Then I will add back in some greens and then eventually try them on small amounts of pellets. Obviously, you must make sure that the rabbit is eating hay before embarking on this diet, or else it might starve. In addition the bunny should have a thorough physical examination and diagnostic tests, if appropriate, to rule out other disorders prior to starting this diet. Removing all the pellets from the diet sounds drastic, but it works well and the bunnies seem happier and more lively as the GIT starts to work more normally again. I have had clients tell me about complete personality changes (for the better usually) when we got the weight off their pets or got rid of those soft stools that stick all over the fur and make the rabbits and the owner miserable. Some rabbits can never go back on pellets again, because the soft stools may return or the weight goes back up. In addition, rabbits that have renal or bladder stones will also be taken off pellets and alfalfa hay for life to help reduce the calcium intake.

I feel that it is a mistake to “fast” rabbits for long periods each day to reduce weight, as in the cases where rabbits may be given pellets for only a certain amount of time a day. This leaves the pet with nothing to do physiologically and mentally for long hours. In an animal that was designed to eat large amounts of food frequently it can be frustrating and stressful. In addition, I fear that it may lead to a sluggish GIT due to lack of stimulation. These pets will frequently start eating paper, wood and anything else they can get their teeth on the stave off their cravings. How often have you seen the pet that has stopped eating pellets, but eats all the newspaper in the cage? These pets are usually not on unlimited (or usually any) hay or greens and are craving fiber.

Practitioners worry that if we take the rabbits off the pellets, they will not get all the nutrients, vitamins and minerals that they are supposed to get. Remember, that the rabbit manufactures its own rich supply of nutrients in the cecum in the form of the cecotropes, because they were designed to be able to live off of a “poor” quality diet in the wild. I have not yet able to detect nutritional deficiencies on the diet we recommend and we have been recommending it for at least 5 years. In addition I rarely see a case of “hairball” on this diet. The cases of “hairball” that we see in the practice are on a primary pellet diet with little or no hay or greens. In my opinion, “hairballs” are an accumulation of ingesta an hair that takes place over time due to low GIT motility, until it reaches such a size that the rabbit stops eating. Treatment for this problem is aimed primarily at correcting the underlying dietary problems.

As far as other supplements, there has been a lot of talk about using enzymes, and bacteria, etc. I think that these things do no harm, but are not necessary when the pet in put on a more “natural” diet. I used to recommend some of these items myself, no longer because I do not see the need to do so. I would like to see those people who are using these products first make the diet changes as suggested in this article and then be able to quantitatively document that the addition of the other “supplements” made any difference in the appearance or behavior of their pet. I certainly have been proven wrong before, but I feel more scientific research needs to be done on these various supplements to really determine if they are making a difference.

I will stress that there are a wide variety of diseases that can affect the rabbit and certainly they are not all going to be cured by a diet change. There must be a thorough physical examination and appropriate diagnostic testing performed prior to any drastic life style change for the pet.

Let’s feed our pets the way they were designed to eat..lots of food with high fiber content. When they can “fill up” on hay and greens, many of them lose interest in chewing up paper and furniture (although they never lose interest in electrical cords). Let them out to exercise also, to get the weight off, keep it off and keep all the body’s systems in good working order.

References:

Cheeke P. Rabbit Feeding and Nutrition. Orlando, FL. 1989.

Jenkins JR, Brown SA. A Practitioners Guide to Rabbits and Ferrets. American Animal Hospital Association, 1993. (Part of the Professional Library Series)