Tumors in Rabbits
Both male and female rabbits can develop many forms of cancer, and any organ can be a primary site – or a secondary site as a result of metastasis. Cancer becomes a greater concern as a rabbit ages, though it can also occur in young bunnies. Other tumors (benign) develop in rabbits as well.
Since this article discusses malignant and benign tumors, it should be understood that the word “cancer” technically refers only to a malignant tumor. Benign tumors are not considered cancer. When referring to tumors in general, the words “neoplasm, “neoplasia” and “neoplastic” may also be used. The words refer to an abnormal new growth of tissue, either malignant or benign.
The two most common cancers occur in unspayed female rabbits, and both cancers involve the reproductive system. Though House Rabbit Society and other rabbit rescue groups promote spay/neuter of our domestic bunnies, the very nature of rescue work means that organizations and private individuals often receive unneutered rabbits, and some of them will have tumors. Thus, this article focuses on the reality of reproductive cancers in rabbits as well as other cancers that do not occur as frequently.
The three main topics in this article are:
- Uterine Cancer (Uterine Adenocarcinoma)
- Cancer of Mammary Glands (Mammary Adenocarcinoma)
- Other Tumors and Growths
UTERINE CANCER (UTERINE ADENOCARCINOMA)
Unaltered female rabbits are at risk for uterine cancer by two years of age, and the risk greatly increases beyond that age. The cancer is invasive, spreading to surrounding tissues and through the circulatory system to other sites, including the brain, lungs, and bones. Spaying (removal of the uterus and the ovaries) is the primary preventive measure and also reduces the risk of mammary adenocarcinoma (breast cancer), discussed below.
Veterinarians generally prefer to spay a rabbit between four and six months of age, when they are near sexual maturity and have a well-developed reproductive tract but before the risk of uterine cancer increases. Certainly, rabbits older than six months can be spayed as well although earlier (before two years of age) is considered better for both cancer prevention and behavior modification.
Signs of Uterine Cancer
A rabbit may not exhibit noticeable signs of uterine cancer in the early stages. Dr. Susan Brown, who has been an exotic animal veterinarian for over thirty years, with a special interest in rabbits, offers the following explanation:
One sign that may be seen with a uterine tumor is a spot or pool of blood in the middle of where the rabbit has urinated. This may not be noticed in the rabbit’s bedding, but it is clearly seen on a flat surface such as the floor or a newspaper. The blood is not coming from the urine so it is not mixed evenly in the urine. Rather, the blood is coming from the uterus and, thus, it appears as a separate spot or pool within the urine pool. This is one sure sign of a reproductive problem, but not all affected rabbits will exhibit it. Any sign of blood at the time of urination is cause to take your rabbit to a veterinarian immediately.
More often the neoplasm is discovered during a routine veterinary examination, during palpation of the abdomen. An abnormally large or lumpy uterus may be felt. Prior to surgery to remove the uterus and ovaries, a radiograph would be recommended to take a look at the uterus and also other organs such as bones and lungs to make sure there are no problems there.
At the time of surgery, abnormal tissues should be sent to a pathologist to confirm if the abnormality is cancer or some other benign condition. The lab results will determine the course of treatment.
If uterine cancer becomes advanced and spreads through the body, signs that may be seen are weight loss, reduced appetite, difficulty breathing, and generalized weakness. Uterine cancer tends to spread slowly, and it may take one to two years for it to be terminal.
Diagnosis, Treatment, and Prognosis for Recovery
Dr. Peg Frank, who regularly treats rabbits for the Minnesota Companion Rabbit Society, advises that the only “cure” for uterine cancer is to spay the rabbit, though consideration is given to metastatic spread (transmission from the original site). As the cancer progresses, other abdominal organs are commonly affected; the cancer can also be spread through the blood to other areas of the body. It must be noted that clinical signs may not show up until the cancer is advanced.
Dr. Frank comments further:
I’m always hopeful that the rabbit is in early stages, with the cancer contained within the uterine walls. However, invasion to other areas of the body may not be grossly visible, so there is always the chance that the cancer had started to spread before removal of the reproductive organs. If the histopathology confirms the diagnosis of uterine cancer, then I want to see the rabbit every six months and take a full-body radiograph each time as well.
When early-stage surgery removes the cancer while still contained, the prognosis is excellent and most rabbits live a full life. If the rabbit is not treated in the early stages, surgery is not effective because of metastasis. In these cases, the rabbit is severely compromised and the prognosis is very poor. If the rabbit is suffering and we cannot control the pain, I assist my clients through the difficult decision of euthanasia.
Dr. Stewart Colby, who treats many rabbits at his hospital, including bunnies from the Georgia House Rabbit Society, adds from his experience:
I feel that uterine cancer is so common and pervasive that I can think of very few scenarios that would prevent me from spaying a stable female rabbit. Surgery would be postponed, however, if the rabbit suffered from severe systemic infection, an immediate life-threatening trauma, or something of that nature. And of course, if the cancer had metastasized, for example to the lungs, then I would address the prognosis with the rabbit’s caregiver.
Once overt [observable] clinical signs are present, my long-term prognosis is guarded at best. In my experience, if one can remove the cancerous uterus in its entirety and no other organs are affected, then the long-term prognosis is good to very good.
Home Care of a Rabbit with Uterine Cancer
Depending on how involved the surgery is, aftercare is generally the same as for a standard spay, and pain medication is prescribed to control post-surgical pain. Oftentimes, the bunny feels so much better after the tumor is removed that she is up and around within hours.
CANCER OF MAMMARY GLANDS (MAMMARY ADENOCARCINOMA)
A female rabbit has eight to twelve mammary glands and is less likely to experience this type of cancer if she is spayed before two years of age, advises Dr. Frank.
There is a higher mortality risk with this cancer, as there is only a small window of opportunity to treat it. In addition, this multi-centric cancer can show up in a gland at one end of the body, then in another gland at the other end. Because it travels through the lymphatic system, the cancer spreads throughout the body, including the brain and nervous system.
Dr. Colby provides additional information:
Diagnosis and treatment of mammary cancer in rabbits is very similar to how it is diagnosed and treated in canines. Only when it is caught early and surgically corrected is there a fair long-term prognosis.
It is not unusual for a rabbit with this cancer to also have uterine cancer. There is some association between the incidence of uterine cancer and cyst formation in mammary glands, which often leads to mammary adenocarcinoma.
Signs of Cancer of Mammary Glands
Because a rabbit does not exhibit signs of this cancer during the early stages, finding a lump is often incidental. It may be found during playtime, while grooming or – as is Dr. Frank’s experience – during a veterinary exam.
The smallest neoplasm I’ve felt was about the size of a BB, but of course the area may be swollen with a much larger growth. A slightly discolored (blood-tinged) discharge from the teat [nipple] may be visible.
Since the rabbit may initially seem completely normal, she will have to be closely watched to determine when pain medication is needed. Depending on how quickly and to which organs the cancer metastasizes, other signs will become evident. The rabbit may stop grooming, playing, exercising, and interacting normally; these are all definitive signs that she needs pain control. Weight loss, weakness and equilibrium problems, and anorexia may also occur.
Diagnosis, Treatment, and Prognosis for Recovery
Fine needle aspiration, which uses a needle to suction out cells from the lesion, can give some initial information as to whether a tumor is a cancer, a granuloma, or an infection. If the cytology indicates that the neoplasm is suspicious, Dr. Frank removes the mass with excisional biopsy (lumpectomy) and sends it to the lab for pathology analysis and definitive diagnosis.
The lumpectomy is scheduled as quickly as possible to prevent any potential spread of cancer. I try to get wide clear margins around the mass to try to remove all the abnormal cells. Mammary adenocarcinoma is an invasive cancer that generally recurs, even after surgery. I may do several lumpectomies on a single rabbit.
I would also recommend to the caregiver that the rabbit be spayed, regardless of the age, to decrease the number of potential tumors and hopefully to increase the longevity and quality of the bunny’s life. However, the spay would not necessarily be done at the time of the lumpectomy because it could result in too much surgery time as well as tissue loss. This is something that I determine on a case-by-case basis.
Dr. Colby shares from his experience:
In most cases the rabbit will be acting and behaving normally when the lump is noticed. Given the rapidity of spread, I generally recommend immediate excision over aspiration.
In my experience it appears that mammary cancer often moves up the mammary chain, rather than crossing from a gland on one side of the rabbit’s body to a gland on the other side. I usually remove the mammary gland in front of the affected one as well. Sending more tissue to the pathologist allows for a better idea of active spreading or movement of neoplastic cells.
Following surgery, I closely monitor the rabbit, checking her each week. If I’m satisfied with what I’m seeing, after three weeks I recommend bimonthly exams. However, if any lumps are noticed, then I look for metastasis through radiographs of the lungs and abdomen. If there appears to be no sign of spread to other organs, I then recommend removal of the entire mammary chain [on the one side of the body] as soon as possible.
This is a very invasive procedure, and I always discuss with the caregiver the surgery as well as the aftercare needed for the rabbit. When the mammary gland cancer appears to be contained to one side of the mammary chain, then the long-term prognosis for the rabbit changes from fair to good. In this case, the caregiver may elect the surgery and commit to the extensive aftercare. Not only will the rabbit require pain medication, but her movement will be hampered, affecting eating and drinking. Extensive supportive care and watching for signs of GI stasis [gastrointestinal shutdown] are important, as is maintaining the rabbit in as stress-free environment as possible.
If, however, it appears that both mammary chains are affected, then the long-term prognosis is poor. The invasiveness of removing all mammary glands is severe in any animal, and especially for one as sensitive as the rabbit. It’s a very difficult decision at this point.
Home Care of a Rabbit with Cancer of Mammary Glands
Depending on how and where the cancer metastasizes, the rabbit will likely exhibit other signs, requiring the bunny’s caregiver and veterinarian to devise a maintenance plan for daily care, which may include pain medication. If surgery is performed, your veterinarian will work out a home-care plan with you.
Prevention is the best option: have a qualified veterinarian spay the rabbit before two years of age or as soon as she is adopted (assuming good health). In addition, be vigilant for any abnormal lumps or bumps during handling sessions (e.g., playtime and grooming).
OTHER TUMORS AND GROWTHS
It has already been noted that cancer may metastasize from the original site. For example, cancer may spread locally (e.g., from the inside to outside of the uterus) or, depending on the type of cancer, through the circulatory or lymph systems. As previously stated, any site may be primary or secondary.
According to Dr. Colby, after reproductive cancers, lymphoma is the most common cancer affecting female rabbits. Arising in the lymph nodes or in some other lymphoid tissue, the cancer spreads through the rabbit’s system via the lymphatic system. Tumors are usually malignant. Dr. Colby adds this:
Lymphoma is a cancer of lymphocytes, which constitute approximately twenty-five percent of white blood cells in a healthy body. Lymphocytes are critical to the proper functioning of the immune system, so when cancer invades, the rabbit’s condition is very serious.
Lymphoma can affect any part of the body. I might find an intra-abdominal mass or maybe the cancer has infiltrated the liver, spleen, and gastrointestinal tract, making those organs fail. Treatment is not well understood as this cancer is usually well advanced when it is detected. In a case like this, I would discuss euthanasia with the rabbit’s caregiver because advanced-stage lymphoma is very debilitating and painful.
An unneutered male rabbit may get testicular cancer, and the risk increases with age. As with other cancers, the condition may not be immediately noticed by a caregiver. Signs include enlarged testicles and weight loss as the cancer progresses. To help ensure health, neutering male rabbits at the appropriate age is recommended.
Osteosarcoma (bone cancer) most often occurs in the jaw, and it may recur even after surgical removal of the initial tumor. For skin tumors, a biopsy is required to determine the type of growth, and current treatment is generally based on the protocol used for cats and dogs.
A caregiver may also notice papillomas, which are cancer-like growths but not true cancers. The overgrowth of cells in the epithelial tissue commonly occurs around the anal area and around the mouth and on the tongue. Dr. Brown advises:
Papillomas are proliferative growths that are caused by a virus. They appear to be benign and do not metastasize; they may resolve on their own. However, if they interfere with defecation, the papillomas may need to be surgically removed. Oral and tongue papillomas can be transmitted from rabbit to rabbit, but they don’t tend to spread to other areas of the body. Growths that interfere with eating may need to be surgically removed.
Caregivers may want to consider supplementing their rabbit’s care with alternative therapies, some of which are addressed in separate articles posted on this website. There are times when combining the expertise of both standard and alternative treatments offers the best supportive care.
When researching complementary treatments, be aware of training and qualification requirements and give careful consideration to the health, nature, and needs of your rabbit. Be clear and realistic about your expectations and goals for treatment, which should prioritize your rabbit’s comfort and quality of life.
by Marie Mead with Drs. Susan Brown, Stewart Colby, and Peg Frank
© Copyright 2013 by Marie Mead. Used by permission. All rights reserved. Portions of this article appeared in “Spay/Neuter: More Than Just Population Control,” Bunny Mad (UK), Issue 8, Spring/Summer 2009.
I wish to extend my sincere gratitude to Drs. Susan Brown, Stewart Colby, and Peg Frank for sharing their expertise in this article and additionally to Dr. Brown for her overall review of the article. Warm thanks also to Cheryl Abbott, Sandi Ackerman, Heidi Anderson, Dr. Stephanie Crispin, and Gary McConville for their suggestions. – Marie Mead
Marie Mead has been involved in various capacities with animal rescue, advocacy, and education for over twenty years. She has made a home with special-needs rabbits and other animals, all of them rescues. Author (with collaborator Nancy LaRoche) of Rabbits: Gentle Hearts, Valiant Spirits – Inspirational Stories of Rescue, Triumph, and Joy, Marie has also written rabbit-related stories and articles for other publications. Additional writings have covered topics such as aging and the environment.
Susan Brown, DVM, is the founder and former owner of Midwest Bird and Exotic Animal Hospital (originally in Westchester, Illinois) and the current owner of Rosehaven Exotic Animal Veterinary Services and The Behavior Connection (North Aurora, Illinois). She is coauthor of Self-Assessment Color Review of Small Mammals and author of numerous lay and professional writings on rabbit medicine and care; she has also lectured extensively in the United States and Europe. She is involved in exotic animal care at rescue organizations and shelters. Utilizing the principles of behavior and training, she is teaching ways for people to live in harmony with their companion animals.
Stewart Colby, DVM, obtained his veterinary degree from the University of Georgia and is the founder and veterinarian at Windward Animal Hospital in Alpharetta, GA. He specializes in small animal medicine and surgery and has extensive experience with avian and exotic species. He works with the Georgia House Rabbit Society to help rehabilitate abused and abandoned animals, and he also assists in the care of exotic species at the Chestatee Wildlife Preserve and the North Georgia Zoo.
Peg Frank, DVM, obtained her veterinary degree from the University of Minnesota and completed an internship at the University of Prince Edward Island-Atlantic Veterinary College in Canada. Dr. Frank practices at Cottage GroveAnimalHospital in Cottage Grove, Minnesota, and works closely with the Minnesota Companion Rabbit Society in the treatment of their rabbits.
My sincere thanks to the veterinarians named in this article for sharing their expertise during personal interviews and in subsequent feedback. In addition to House Rabbit Handbook: How to Live with an Urban Rabbit, the following list of publications, although by no means comprehensive, may assist those who desire additional research.
- BSAVA Manual of Rabbit Medicine and Surgery. Anna Meredith and Paul Flecknell (Eds.)
- Clinical Radiology of Exotic Companion Mammals. Vittorio Capello and Angela Lennox
- Color Atlas of Small Animal Anatomy: The Essentials. Thomas O. McCracken and Robert A. Kainer with David Carlson
- Exotic DVM (journal)
- Exotic Pet Behavior: Birds, Reptiles, and Small Mammals. Teresa Bradley Bays, Teresa Lightfoot, and Jorg Mayer
- Ferrets, Rabbits, and Rodents: Clinical Medicine and Surgery. Katherine E. Quesenberry and James W. Carpenter
- The 5-Minute Veterinary Consult: Ferret and Rabbit. Barbara L. Oglesbee
- Journal of Exotic Pet Medicine
- Notes on Rabbit Internal Medicine. Richard A. Saunders and Ron Rees Davies
- Rabbit and Rodent Dentistry Handbook. Vittorio Capello with Margherita Gracis
- Rabbit Medicine & Surgery. Emma Keeble and Anna Meredith
- Textbook of Rabbit Medicine. Frances Harcourt-Brown