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. 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.
Uterine Cancer In Rabbits
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 is also mute on the
subject, some cite book references but do not share their experiences with us. 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).
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. As responsible breeders we should be sure to warn any pet owners
who buy a rabbit from us that has had a history of problems, they would be at great risk of developing uterine cancer.
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 believe it is our responsibility to inform pet owners of it and to promote spay and neutering of all pet
rabbits. In addition to removing any risk of uterine cancer it also provides a behavioral benefit. Many 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.
References:
1) The Biology of Laboratory Rabbits 2nd Ed 1994, Manning, Ringler, Newcomer
2) March 1997 Veterinary Conference in CA Rabbit Medicine and Procedures for Practitioners Program and Abstracts
"Neoplasia in Rabbits" pg. 171.
3) Adenocarcinoma of the Uterine Fundus In the Rabbit by Harry S. N. Greene pg. 535-542, Annals New York
Academy of Sciences.
ADDITIONAL NOTES:
Domestic Rabbits May/June 1991 pub. by ARBA, "Medical and Surgical Care of the Pet Rabbit" by Robert C Clipsham
DVM.
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 I recommend all
pet owners fill it out because that will be the best way we can come up with an estimate of the incidence of uterine cancer in
does.
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
I urge all owners of does to fill out this survey, it is likely that this survey will be the most comprehensive look at uterine cancer in rabbits and can be another important key to determining whether or not you spay your pet rabbit.
The decision to spay or not is up to you but it is important you research all the factors and talk to your veterinarian. I hope
you have found the information on my site helpful in your decision. Personally I recommend spaying for pet rabbits because
it will eliminate the possibility of your doe having cancer and there is a behavioral benefit. Just be sure you go to an
experienced rabbit vet who has a good (or non-existent) mortality rate for the surgery.
