|
Long-Term Health Risks and Benefits Associated with Spay
/ Neuter in Dogs
Laura J.
Sanborn, M.S.
Precis
At
some point, most of us with an interest in dogs will have to consider
whether or not to spay / neuter our pet. Tradition holds that the
benefits of doing so at an early age outweigh the risks. Often,
tradition holds sway in the decision-making process even after
countervailing evidence has accumulated.
Ms
Sanborn has reviewed the veterinary medical literature in an exhaustive
and scholarly treatise, attempting to unravel the complexities of
the subject. More than 50 peer-reviewed papers were examined to
assess the health impacts of spay / neuter in female and male dogs,
respectively. One cannot ignore the findings of increased risk from
osteosarcoma, hemangiosarcoma, hypothyroidism, and other less frequently
occurring diseases associated with neutering male dogs. It would
be irresponsible of the veterinary profession and the pet owning
community to fail to weigh the relative costs and benefits of neutering
on the animal’s health and well-being. The decision for females
may be more complex, further emphasizing the need for individualized
veterinary medical decisions, not standard operating procedures
for all patients.
No
sweeping generalizations are implied in this review. Rather, the
author asks us to consider all the health and disease information
available as individual animals are evaluated. Then, the best decisions
should be made accounting for gender, age, breed, and even the specific
conditions under which the long-term care, housing and training
of the animal will occur.
This
important review will help veterinary medical care providers as
well as pet owners make informed decisions. Who could ask for more?
Larry
S. Katz, PhD
Associate
Professor and Chair
Animal
Sciences
INTRODUCTION
Dog owners
in
health benefits
are cited, yet evidence is usually not cited to support the alleged
health benefits. When discussing the health impacts of spay/neuter,
health risks are often not mentioned. At times, some risks
are mentioned, but the most severe risks usually are not.
This article
is an attempt to summarize the long-term health risks and benefits
associated with spay/neuter in dogs that can be found in the veterinary
medical literature. This article will not discuss the impact of
spay/neuter on population control, or the impact of spay/neuter
on behavior. Nearly all of the health
risks and benefits summarized in this article are findings from
retrospective epidemiological research studies of dogs, which examine
potential associations by looking backwards in time. A few are from
prospective research studies, which examine potential associations
by looking forward in time.
SUMMARY
An objective
reading of the veterinary medical literature reveals a complex situation
with respect to the longterm health risks and benefits associated
with spay/neuter in dogs. The evidence shows that spay/neuter correlates
with both positive AND adverse health effects in dogs. It also suggests
how much we really do not yet understand about this subject.
On balance,
it appears that no compelling case can be made for neutering most
male dogs, especially immature male dogs, in order to prevent future
health problems. The number of health problems associated with neutering
may exceed the associated health benefits in most cases.
On the positive
side, neutering male dogs
·
eliminates
the small risk (probably <1%) of dying from testicular cancer
·
reduces
the risk of non-cancerous prostate disorders
·
reduces
the risk of perianal fistulas
·
may
possibly reduce the risk of diabetes (data inconclusive)
On the negative
side, neutering male dogs
·
if
done before 1 year of age, significantly increases the risk of osteosarcoma
(bone cancer); this is a
common cancer
in medium/large and larger breeds with a poor prognosis.
·
increases
the risk of cardiac hemangiosarcoma by a factor of 1.6
·
triples
the risk of hypothyroidism
·
increases
the risk of progressive geriatric cognitive impairment
·
triples
the risk of obesity, a common health problem in dogs with many associated
health problems
·
quadruples
the small risk (<0.6%) of prostate cancer
·
doubles
the small risk (<1%) of urinary tract cancers
·
increases
the risk of orthopedic disorders
·
increases
the risk of adverse reactions to vaccinations
For female
dogs, the situation is more complex. The number of health benefits
associated with spaying may exceed the associated health problems
in some (not all) cases. On balance, whether spaying improves the
odds of overall good health or degrades them probably depends on
the age of the female dog and the relative risk of various diseases
in the different breeds.
On the positive
side, spaying female dogs
·
if
done before 2.5 years of age, greatly reduces the risk of mammary
tumors, the most common malignant tumors in female dogs
·
nearly
eliminates the risk of pyometra, which otherwise would affect about
23% of intact female dogs; pyometra kills about 1% of intact female
dogs
·
reduces
the risk of perianal fistulas
·
removes
the very small risk (_0.5%)
from uterine, cervical, and ovarian tumors
On the negative
side, spaying female dogs
·
if
done before 1 year of age, significantly increases the risk of osteosarcoma
(bone cancer); this is a common cancer in larger breeds with a poor
prognosis
·
increases
the risk of splenic hemangiosarcoma by a factor of 2.2 and cardiac
hemangiosarcoma by a factor of >5; this is a common cancer and major
cause of death in some breeds
·
triples
the risk of hypothyroidism
·
increases
the risk of obesity by a factor of 1.6-2, a common health problem
in dogs with many associated health problems
·
causes
urinary “spay incontinence” in 4-20% of female dogs
·
increases
the risk of persistent or recurring urinary tract infections by
a factor of 3-4
·
increases
the risk of recessed vulva, vaginal dermatitis, and vaginitis, especially
for female dogs spayed before puberty
·
doubles
the small risk (<1%) of urinary tract tumors · increases the risk of orthopedic disorders
·
increases
the risk of adverse reactions to vaccinations
One thing
is clear – much of the spay/neuter information that is available
to the public is unbalanced and contains claims that are exaggerated
or unsupported by evidence. Rather than helping to educate pet owners,
much of it has contributed to common misunderstandings about the
health risks and benefits associated of spay/neuter in dogs.
The traditional
spay/neuter age of six months as well as the modern practice of
pediatric spay/neuter appear to predispose dogs to health risks
that could otherwise be avoided by waiting until the dog is physically
mature, or perhaps in the case of many male dogs, foregoing it altogether
unless medically necessary.
The balance
of long-term health risks and benefits of spay/neuter will vary
from one dog to the next. Breed, age, and gender are variables that
must be taken into consideration in conjunction with non-medical
factors for each individual dog. Across-the-board recommendations
for all pet dogs do not appear to be supportable from findings in
the veterinary medical literature.
FINDINGS
FROM STUDIES
This section
summarizes the diseases or conditions that have been studied with
respect to spay/neuter in dogs.
Complications
from Spay/Neuter Surgery
All surgery
incurs some risk of complications, including adverse reactions to
anesthesia, hemorrhage, inflammation, infection, etc. Complications
include only immediate and near term impacts that are clearly linked
to the surgery, not to longer term impacts that can only be assessed
by research studies. At one veterinary teaching hospital where
complications were tracked, the rates of intraoperative, postoperative
and total complications were 6.3%, 14.1% and 20.6%, respectively
as a result of spaying female dogs1.
Other studies found a rate of total complications from spaying of
17.7%2
and
23%3.
A study of Canadian veterinary private practitioners found complication
rates of 22% and 19% for spaying female dogs and neutering
male dogs, respectively4.
Serious complications
such as infections, abscesses, rupture of the surgical wound, and
chewed out sutures were reported at a 1- 4% frequency, with spay
and castration surgeries accounting for 90% and 10% of these complications,
respectively.4
The
death rate due to complications from spay/neuter is low, at around
0.1%2.
Prostate
Cancer
Much of the
spay/neuter information available to the public asserts that neutering
will reduce or eliminate the risk that male dogs develop prostate
cancer. This would not be an unreasonable assumption, given that
prostate cancer in humans is linked to testosterone. But the evidence
in dogs does not support this claim. In fact, the strongest evidence
suggests just the opposite.
There have
been several conflicting epidemiological studies over the years
that found either an increased risk or a decreased risk of prostate
cancer in neutered dogs. These studies did not utilize control populations,
rendering these results at best difficult to interpret. This may
partially explain the conflicting results.
More recently,
two retrospective studies were conducted that did utilize control
populations. One of these studies involved a dog population in Europe5
and
the other involved a dog population in America6.
Both studies found that neutered male dogs have a four times
higher
risk of prostate
cancer than intact dogs. Based on their results, the researchers
suggest a cause-and-effect relationship: “this suggests that
castration does not initiate the development of prostatic carcinoma
in the dog, but does favor tumor progression”5
and also
“Our study found that most canine prostate cancers are of ductal/urothelial
origin….The relatively low incidence of prostate cancer in intact
dogs may suggest that testicular hormones are in fact protective
against ductal/urothelial prostatic carcinoma, or may have indirect
effects on cancer development by changing the environment in the
prostate.”6
This needs
to be put in perspective. Unlike the situation in humans, prostate
cancer is uncommon in dogs. Given an incidence of prostate cancer
in dogs of less than 0.6% from necropsy studies7,
it is difficult to see that the risk of prostate cancer should factor
heavily into most neutering decisions. There is evidence for an
increased risk of prostate cancer in at least one breed (Bouviers)5,
though very little data so far to guide us in regards to other breeds.
Testicular
Cancer
Since the
testicles are removed with neutering, castration removes any risk
of testicular cancer (assuming the castration is done before
cancer develops). This needs to be compared to the risk of testicular
cancer in intact dogs. Testicular tumors are not uncommon in older
intact dogs, with a reported incidence of 7%8.
However, the prognosis for treating testicular tumors is very good
owing to a low rate of metastasis9,
so testicular cancer is an uncommon cause of death in intact dogs.
For example, in a Purdue University breed health survey of Golden
Retrievers10,
deaths due to testicular cancer were sufficiently infrequent that
they did not appear on list of significant causes of "Years of Potential
Life Lost for Veterinary Confirmed Cause of Death” even though 40%
of GR males were intact. Furthermore, the GRs who were treated for
testicular tumors had a 90.9% cure rate. This agrees well with other
work that found 6-14% rates of metastasis for testicular tumors
in dogs11.
The high cure rate of testicular tumors combined with their frequency
suggests that fewer than 1% of intact male dogs will die of testicular
cancer.
In summary,
though it may be the most common reason why many advocate neutering
young male dogs, the risk from life threatening testicular
cancer is sufficiently low that neutering most male dogs to prevent
it is difficult to justify.
An exception
might be bilateral or unilateral cryptorchids, as testicles that
are retained in the abdomen are 13.6 times more likely to
develop tumors than descended testicles12
and
it is also more difficult to detect tumors in undescended testicles
by routine physical examination.
Osteosarcoma
(Bone Cancer)
A multi-breed
case-control study of the risk factors for osteosarcoma found that
spay/neutered dogs (males or females) had twice the risk of developing
osteosarcoma as did intact dogs13.
This risk
was further studied in Rottweilers, a breed with a relatively high
risk of osteosarcoma. This retrospective cohort study broke the
risk down by age at spay/neuter, and found that the elevated risk
of osteosarcoma is associated with spay/neuter of young dogs14.
Rottweilers spayed/neutered before one year of age were 3.8 (males)
or 3.1 (females) times more likely to develop osteosarcoma than
intact dogs. Indeed, the combination of breed risk and early spay/neuter
meant that Rottweilers spayed/neutered before one year of age had
a 28.4% (males) and 25.1% (females) risk of developing osteosarcoma.
These results
are consistent with the earlier multi-breed study13
but
have an advantage of assessing risk as a function of age at neuter.
A logical conclusion derived from combining the findings of these
two studies is that spay/neuter of dogs before 1 year of age is
associated with a significantly increased risk of osteosarcoma.
The researchers
suggest a cause-and-effect relationship, as sex hormones are known
to influence the maintenance of skeletal structure and mass, and
also because their findings showed an inverse relationship between
time of exposure to sex hormones and risk of osteosarcoma.14
The risk
of osteosarcoma increases with increasing breed size and especially
height13.
It is a common cause of death in medium/large, large, and giant
breeds. Osteosarcoma is the third most common cause of death
in Golden Retrievers10
and
is even more common in larger breeds13.
Given the
poor prognosis of osteosarcoma and its frequency in many breeds,
spay/neuter of immature dogs in the medium/large, large, and giant
breeds is apparently associated with a significant and elevated
risk of death due to osteosarcoma.
Mammary
Cancer (Breast Cancer)
Mammary tumors
are by far the most common tumors in intact female dogs, constituting
some 53% of all malignant tumors in female dogs in a study
of dogs in Norway15
where
spaying is much less common than in the
50-60% of
mammary tumors are malignant, for which there is a significant risk
of metastasis16.
Mammary tumors in dogs have been found to have estrogen receptors17,
and the published research18
shows
that the relative risk (odds ratio) that a female will develop mammary
cancer compared to the risk in intact females is dependent on how
many estrus cycles she experiences:
# of estrus
cycles before spay Odds Ratio
None 0.005
1 0.08
2 or more
0.26
Intact 1.00
The same data when categorized differently showed that the relative risk (odds ratio) that females will develop mammary cancer compared to the risk in intact females indicated that:
Age at Spaying
Odds Ratio
_
29
months 0.06
_
30
months 0.40 (not statistically significant at the P<0.05 level)
Intact 1.00
Please note
that these are RELATIVE risks. This study has been referenced elsewhere
many times but the results have often been misrepresented as absolute
risks.
A similar
reduction in breast cancer risk was found for women under the age
of 40 who lost their estrogen production due to “artificial
menopause”19
and
breast cancer in humans is known to be estrogen activated.
Mammary cancer
was found to be the 10th
most
common cause of years of lost life in Golden Retrievers, even
though 86% of female GRs were spayed, at a median age of 3.4 yrs10.
Considering that the female subset accounts for almost all
mammary cancer cases, it probably would rank at about the 5th
most
common cause of years of lost life in female GRs. It would rank
higher still if more female GRs had been kept intact up to 30 months
of age.
Boxers, cocker
spaniels, English Springer spaniels, and dachshunds are breeds at
high risk of mammary tumors15.
A population of mostly intact female Boxers was found to have a
40% chance of developing mammary cancer between the ages of
6-12 years of age15.
There are some indications that purebred dogs may be at higher risk
than mixed breed dogs, and purebred dogs with high inbreeding coefficients
may be at higher risk than those with low inbreeding coefficients20.
More investigation is required to determine if these are significant.
In summary,
spaying female dogs significantly reduces the risk of mammary cancer
(a common cancer), and the fewer estrus cycles experienced
at least up to 30 months of age, the lower the risk will be.
Female
Reproductive Tract Cancer (Uterine, Cervical, and Ovarian Cancers)
Uterine/cervical
tumors are rare in dogs, constituting just 0.3% of tumors in dogs21.
Spaying will remove the risk of ovarian tumors, but the risk is
only 0.5%22.
While spaying will remove the risk of reproductive tract tumors,
it is unlikely that surgery can be justified to prevent the risks
of uterine, cervical, and ovarian cancers as the risks are so low.
Urinary
Tract Cancer (Bladder and Urethra Cancers)
An age-matched
retrospective study found that spay/neuter dogs were two times more
likely to develop lower urinary tract tumors (bladder or urethra)
compared to intact dogs23.
These tumors are nearly always malignant, but are infrequent,
accounting for less than 1% of canine tumors. So this risk is unlikely
to weigh heavily on spay/neuter decisions.
Airedales,
Beagles, and Scottish Terriers are at elevated risk for urinary
tract cancer while German Shepherds have a lower than average risk23.
Hemangiosarcoma
Hemangiosarcoma
is a common cancer in dogs. It is a major cause of death in some
breeds, such as Salukis, French Bulldogs, Irish Water Spaniels,
Flat Coated Retrievers, Golden Retrievers, Boxers, Afghan Hounds,
English Setters, Scottish Terriesr, Boston Terriers, Bulldogs, and
German Shepherd Dogs24.
In an aged-matched
case controlled study, spayed females were found to have a 2.2 times
higher risk of splenic hemangiosarcoma compared to intact
females24.
A retrospective
study of cardiac hemangiosarcoma risk factors found a >5 times greater
risk in spayed female dogs compared to intact female dogs and a
1.6 times higher risk in neutered male dogs compared to intact male
dogs.25
The
authors suggest a protective effect of sex hormones against hemangiosarcoma,
especially in females.
In breeds
where hermangiosarcoma is an important cause of death, the increased
risk associated with spay/neuter is likely one that should factor
into decisions on whether or when to sterilize a dog.
Hypothyroidism
Spay/neuter
in dogs was found to be correlated with a three fold increased risk
of hypothyroidism compared to intact dogs.
26.
The researchers
suggest a cause-and-effect relationship: They wrote: “More important
[than the mild directimpact on thyroid function] in the association
between [spaying and] neutering and hypothyroidism may be the effect
of sex hormones on the immune system. Castration increases the severity
of autoimmune thyroiditis in mice” which may explain the link between
spay/neuter and hypothyroidism in dogs.
Hypothyroidism
in dogs causes obesity, lethargy, hair loss, and reproductive abnormalities.27
The lifetime
risk of hypothyroidism in breed health surveys was found to be 1
in 4 in Golden Retrievers10,
1 in 3 in Akitas28,
and 1 in 13 in Great Danes29.
Obesity
Owing to
changes in metabolism, spay/neuter dogs are more likely to be overweight
or obese than intact dogs. One study found a two fold increased
risk of obesity in spayed females compared to intact females30.
Another study found that spay/neuter dogs were 1.6 (females) or
3.0 (males) times more likely to be obese than intact dogs, and
1.2 (females) or 1.5 (males) times more likely to be overweight
than intact dogs31.
A survey
study of veterinary practices in the
Diabetes
Some data
indicate that neutering doubles the risk of diabetes in male dogs,
but other data showed no significant change in diabetes risk with
neutering33.
In the same studies, no association was found between spaying and
the risk of diabetes.
Adverse
Vaccine Reactions
A retrospective
cohort study of adverse vaccine reactions in dogs was conducted,
which included allergic reactions, hives, anaphylaxis, cardiac arrest,
cardiovascular shock, and sudden death. Adverse reactions were 30%
more likely in spayed females than intact females, and 27% more
likely in neutered males than intact males34.
The investigators
discuss possible cause-and-effect mechanisms for this finding, including
the roles that sex hormones play in body’s ability to mount an immune
response to vaccination.34
Toy
breeds and smaller breeds are at elevated risk of adverse vaccine
reactions, as are Boxers, English Bulldogs, Lhasa Apsos, Weimaraners,
American Eskimo Dogs, Golden Retrievers, Basset Hounds, Welsh Corgis,
Siberian Huskies, Great Danes, Labrador Retrievers, Doberman Pinschers,
American Pit Bull Terriers, and Akitas.34
Mixed
breed dogs were found to be at lower risk, and the authors suggest
genetic hetereogeneity (hybrid vigor) as the cause.
Urogenital
Disorders
Urinary incontinence
is common in spayed female dogs, which can occur soon after spay
surgery or after a delay of up to several years. The incidence rate
in various studies is 4-20%
35,36,37
for
spayed females compared to only 0.3% in intact females38.
Urinary incontinence is so strongly linked to spaying that it is
commonly
called “spay incontinence” and is caused by urethral sphincter incompetence39,
though the biological mechanism is unknown. Most (but not all) cases
of urinary incontinence respond to medical treatment, and in many
cases this treatment needs to be continued for the duration of the
dog’s life.40
A retrospective
study found that persistent or recurring urinary tract (bladder)
infections (UTIs) were 3-4 times more likely in spayed females dogs
than in intact females41.
Another retrospective study found that female dogs spayed before
5 ½ months of age were 2.76 times more likely to develop UTIs compared
to those spayed after 5 ½ months of age.42
Depending
on the age of surgery, spaying causes abnormal development of the
external genitalia. Spayed females were found to have an increased
risk of recessed vulva, vaginal dermatitis, vaginitis, and UTIs.43
The
risk is higher still for female dogs spayed before puberty.43
Pyometra
(Infection of the Uterus)
Pet insurance
data in
that have
not whelped puppies are at elevated risk for pyometra45.
Rarely, spayed female dogs can develop “stump pyometra” related
to incomplete removal of the uterus.
Pyometra
can usually be treated surgically or medically, but 4% of pyometra
cases led to death44.
Combined with the incidence of pyometra, this suggests that about
1% of intact female dogs will die from pyometra.
Perianal
Fistulas Male dogs are twice as likely to develop perianal fistulas as females, and spay/neutered dogs have a decreased risk compared to intact dogs46. German Shepherd Dogs and Irish Setters are more likely to d evelop perianal fistulas than are other breeds.46
Non-cancerous
Disorders of the Prostate Gland
The incidence
of benign prostatic hypertrophy (BPH, enlarged prostate) increases
with age in intact male dogs, and occurs in more than 80% of intact
male dogs older than the age of 5 years47.
Most cases of BPH cause no problems, but in some cases the dog will
have difficulty defecating or urinating. Neutering will prevent
BPH. If neutering is done after the prostate has become enlarged,
the enlarged prostate will shrink relatively quickly.
Orthopedic
Disorders
In a study
of beagles, surgical removal of the ovaries (as happens in spaying)
caused an increase in the rate of remodeling of the ilium (pelvic
bone)48,
suggesting an increased risk of hip dysplasia with spaying. Spaying
was also found to cause a net loss of bone mass in the spine
49.
Spay/neuter
is associated with a two fold increased risk of cranial cruciate
ligament rupture51.
Perhaps thisis associated with the increased risk of obesity30.
Spay/neuter
before 5 ½ months of age is associated with a 70% increased aged-adjusted
risk of hip dysplasia compared to dogs spayed/neutered after 5 ½
months of age, though there were some indications that the former
may have had a lower severity manifestation of the disease42.
The researchers suggest “it is possible that the increase in bone
length that results from early-age gonadectomy results in changes
in joint conformation, which could lead
to a diagnosis of hip dysplasia.”
In a breed
health survey study of Airedales, spay/neuter dogs were significantly
more likely to suffer hip dysplasia as well as “any musculoskeletal
disorder”, compared to intact dogs52,
however possible confounding factors were not controlled for, such
as the possibility that some dogs might have been
spayed/neutered
because they had hip dysplasia or other musculoskeletal disorders.
Compared to intact dogs, another study found that dogs neutered
six months prior to a diagnosis of hip dysplasia were 1.5 times
as likely to develop clinical hip dysplasia.53
Geriatric
Cognitive Impairment
Neutered
male dogs and spayed female dogs are at increased risk of progressing
from mild to severe geriatric cognitive impairment compared to intact
male dogs55.
There weren’t enough intact geriatric females available for the
study to determine their risk.
Geriatric
cognitive impairment includes disorientation in the house or outdoors,
changes in social interactions with human family members, loss of
house training, and changes in the sleep-wake cycle55.
The investigators state “This finding is in line with current research
on the neuro-protective roles of testosterone and estrogen at the
cellular level and the role of estrogen in preventing Alzheimer’s
disease in human females. One would predict that estrogens would
have a similar protective role in the sexually intact female dogs;
unfortunately too few sexually intact female dogs were available
for inclusion in the present study to test the hypothesis”55
CONCLUSIONS
An objective
reading of the veterinary medical literature reveals a complex situation
with respect to the longterm health risks and benefits associated
with spay/neuter in dogs. The evidence shows that spay/neuter correlates
with both positive AND adverse health effects in dogs. It also suggests
how much we really do not yet understand about this subject.
On balance,
it appears that no compelling case can be made for neutering most
male dogs to prevent future health problems, especially immature
male dogs. The number of health problems associated with neutering
may exceed the associated health benefits in most cases.
For female
dogs, the situation is more complex. The number of health benefits
associated with spaying may exceed the associated health problems
in many (not all) cases. On balance, whether spaying improves theodds
of overall good health or degrades them probably depends on the
age of the dog and the relative riskof various diseases in the different
breeds.
The traditional
spay/neuter age of six months as well as the modern practice of
pediatric spay/neuter appear to predispose dogs to health risks
that could otherwise be avoided by waiting until the dog is physically
mature, or perhaps in the case of many male dogs, foregoing it altogether
unless medically necessary.
The balance
of long-term health risks and benefits of spay/neuter will vary
from one dog to the next. Breed, age, and gender are variables that
must be taken into consideration in conjunction with non-medical
factors for each individual dog. Across-the-board recommendations
for all dogs do not appear to be supportable from findings in the
veterinary medical literature.
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32
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• Vol. 4, No. 2, 2006.
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347-350.
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and Neutering: Effects on the
Urogenital System. Proceedings of the Third International Symposium
on Non-Surgical
Contraceptive Methods for Pet Population Control
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