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Below is an article I've written about uterine cancer that should help sort out the scientific information available and referred to on the internet. The article was written for the breeder and as a short introduction to the subject but pet owners can also benefit. Some additional information is presented after the article, just some additional book references and statements from people that should also help your understanding of the subject. As I get additional information about UC in rabbits it will be presented on the site.
I would bet many of you reading this have never experienced uterine cancer in your herd. Therefore it is understandable that many breeders doubt the information passed out by many rabbit rescue groups, especially when they throw figures out such as 85% of all unspayed does will get uterine cancer. The veterinarian community not working for "rescue" groups is also mute on the subject, some cite book references but do not share their experiences with us. Those who benefit from a relationship with rabbit "rescue" just regurgitate what "rescue" tells them to. I had my doubts about the high incidence of uterine cancer and set out to find the facts about it. I wish to share what I have learned with all of you in order to better the knowledge of the breeding community.
Uterine cancer is the most studied form and most common of cancers found in rabbits. Many studies have found incidence rates of 1.3%-2.6%, The Biology of Laboratory Rabbits report studies finding numbers such as 16 rabbits with cancer out of 599, 4 in 150, 2 from 400 (1). It appears there is a low incidence of uterine cancer in these studies however the rabbits in these studies were juveniles and not considered old enough to be "cancer-prone". TBLR reports that most rabbits used in research colonies are between 4 and 24 months of age. Cancer occurrence in younger populations is normally a low amount.
Harry S. Greene spent years researching cancer in rabbits and his findings are the source of the infamous 80% figures for uterine cancer incidence. He studied a colony for 30 years and his findings reveal a significant incidence of uterine cancer in older does. "Greene reported that 16.7% of 849 female rabbits (dying of various causes) were found to have uterine adenocarcinoma (Greene, 1958a)." (1). When one examines uterine cancer in the age groups we find the incidence increases with age. Greene reported an incidence of 4.2% in does 2-3 years of age and 79.1% in those 5-6 years old. Other researchers have also found similar results in aged rabbits. "Notwithstanding heredity as a factor, the incidence of uterine carcinoma has been noted to exceed 50% in certain colonies of random-bred females kept past age 5- 6 years." (1).
There is a problem with the Green studies however, it appears the number of does in the 5-6 year range from which the was an 79% incidence of UC, was a small sample size. I have not gotten a copy of the Greene study which spells out the exact numbers of rabbits in each age group however someone posted the information to a pet rabbit list. The information was posted by Marieke Tieman on Nov 29, 1996. I have not found anyone who contradicts this so I'm assuming it was posted correctly. The person posted that the number of rabbits in the various age groups were as follows:
491 rabbits aged 2-3 yrs old
259 rabbits 3-4
71 rabbits 4-5
24 rabbits 5-6
4 rabbits 6-7
Now remember the "rescue" groups are saying does have an 80% or more risk of getting UC and they are basing it on the Greene studies. What the Greene studies tell us is the risk of UC increases with age. It does not tell us 80% or more of all does in the general population will get UC. Once again "rescue" is using scientific studies and twisting it to try and back-up their agenda of speutering ALL rabbits. Same was done with studies involving pine and cedar shavings. See my article The Truth About Pine Shavings
Dr Barbara Deeb DVM in Washington state reported that in her practice among spay surgeries she performed during 1994-1996, she found 9 out of 16 does over 3 years of age with adenocarcinoma (56.3%). In does 1-3 years old it was 6 out of 37 (16.2%), however 9 (24.3%) in that group had endometrial hyperplasia (precursor to uterine cancer). Does under 1 year of age had no occurrence of uterine cancer, but 4 out of 77 had endometrial hyperplasia. (2).
A pet owner on the Petbunny mailing list has been tracking incidence of uterine cancer among house rabbits. She has a survey pet owners fill out and she is keeping records based on it. The last update appeared in September of 1997, she had records on a total of 209 rabbits with the average age of 2.9 years. The total incidence is 14%, but does over 3 years old have an incidence of 40% (24 out of 60). Does 1-3 years old have an incidence of 3.2%. There were a total of 18 does over 6 years of age and 8 had uterine cancer (44.4%). This information suggests the occurrence of uterine cancer is much greater than many of us have experienced.
Greene also found that reproductive problems occur in the does prior to tumor detection (3). The reproductive disturbances he reports include: diminished fertility, reduced litter sizes and many dead young, retention of litters, abortion, or resorption. In one fourth of the uterine cancer cases cystic breast changes were also observed. He also found that the incidence vary in relation to age, breed, and other constitutional factors (3). "No instance of the tumor occurred in the Belgian or Rex breeds, and the arrangement of breeds in order of increasing incidence stands as follows: Polish, Himalayan, Sable, Beveren, Chinchilla, English, Marten, Dutch, Havana, French Silver, and Tan." (3). Another interesting note from Greene is that crossbred animals had a total of 21.1% incidence while purebreds had 14.2%. The crossbreds were kept because they showed or transmitted "constitutional variations" while the purebreds were considered "normal".
Greene also found that there was a link between pregnancy toxemia and eventual development of uterine cancer(3). Apparently tissue changes and blood chemical alterations were the same in fatal cases of pregnancy toxemia as "mild" cases. All animals experiencing toxemia later developed uterine cancer. This is the link between endometrial hyperplasia and uterine cancer, it always preceded the development of a tumor in the studies. Greene's paper goes on to suggest that liver function is affected by pregnancy toxemia and can last up to a year. During this time the inability of the liver to suppress estrogen could ultimately result in tumors.
Also of interest is that carcinoma of the cervix in rabbits is apparently non-existent. Greene's laboratory searched for it during autopsies of almost 4,000 does over 2 years of age but didn't find a single case (3). Greene says there is an anatomical basis for this.
Although no one has reported "the" incidence of cancer among the general population of rabbits, pet or breeder, we should pay attention to what the studies are showing us. Adams (1962) made the observation that the incidence of uterine cancer in breeder rabbits and aged virgin rabbits was the same. Many breeders do not keep their does past breeding age and often will not keep does around who have reproduction difficulties, so this may explain why many have not experienced the incidence.
I believe there is enough evidence to support the position that there is a significant risk of uterine cancer in older female rabbits. I do not believe that the supposed risk or "stat" from "rescue" groups is accurate. I further believe they say these things to scare pet owners into spaying and neutering everything. After many years of hysterics on uterine cancer "rescue" groups began saying bucks are at high risk of testicular cancer despite no scientific evidence. I believe it is our responsibility to inform pet owners of uterine cancer and the accurate information about it and to promote spay and neutering of pet rabbits. In addition to removing any risk of uterine cancer it also provides a behavioral benefit. Some people who abandon a pet rabbit do so because of hormone driven behavior such as spraying, aggression, and mounting, however spay/neuter can often prevent or lessen these problems. And the facts are lessen, it isn't a cure all for behavioral problems
The complication here is that veterinarians charge far too much to spay rabbits and few are qualified to do so. It is sad that "rescue" groups spend so much time and energy attacking breeders while doing nothing to lower the cost to spay rabbits and educate more veterinarians. For example the House Rabbit Society has up an "activist" page with all sorts of links about ending breeding, use as food, etc yet what are they doing about getting the price of a rabbit spay lowered? In my own state I have heard a range of spay prices from $200-$500. Many pet owners simply can't afford to have the rabbit spayed. And if the price is right there aren't that many qualified veterinarians.
So if you're a pet owner wondering what to do I'd suggest if you can find a qualified rabbit vet and the price is affordable you may want to consider having your doe spayed. If you cannot find a vet or it costs too much I would take solace in that the 80% and higher "stats" thrown around by those in "rescue" does not seem to be supported. You may wish to watch your does carefully as they age for signs such as blood in the urine.
On e final factor is, what are the downsides to spay/neuter in rabbits? This area has not been researched as far as I am aware. One would also have to question if it is safe to spay/neuter young rabbits, or would it be better to wait until they have reached maturity. Studies on dogs have found there to be some drawbacks to spay/neuter. It would be naive to think that there are no health concerns in spay/neuter in rabbits.
Domestic Rabbits May/June 1991 pub. by ARBA, "Medical and Surgical Care of the Pet Rabbit" by Robert C Clipsham
The article mentioned that uterine cancer was one of the most common forms of cancer in rabbits. Ovahysterectomy (spaying) was mentioned as preventative care for does not destined for breeding. As a benefit it also helps to lower what the author referred to as "the very high rate of endometritis and endometriosis documented". It was also mentioned that these reproductive disorders account for a lower expected lifespan for rabbits (6 yrs vs a potential of 15 yrs.)
From e-mail conversations with an experienced rabbit vet, I found out he sees an estimated 20% incidence of uterine cancer in his practice and he does most spays at 5 months of age. He also said that he has read several articles that cite incidence rates of between 14%-35%. He also told me that the mortality rate for spay surgeries should be less than 1% and that he has never lost a doe so far. I have read other sources they state a mortality rate should be less than 1%.
The UC survey Suzy Shaker is conducting can be found at : http://www.earthlink.net/~suzys/survey.html
As of 10/8/98 Suzy Shaker's survey is reporting the following information:
Rabbits diagnosed with cancer 13% 42 out of 320-this is all ages
Rabbits over age 2 diagnosed with cancer: 31% 37 out of 120
over age 4 43% 22 out of 51
over age 6 50% 11 out of 22