As my pups come up to six months of age, I thought it timely to reproduce an article reseached a few years ago which concisely summarises the pros and cons of spay and neuter for our dogs, as evidenced by peer-reviewed publications. I hope you find it helpful.
Long-Term
Health Risks and Benefits
Associated with Spay / Neuter in Dogs
Laura J. Sanborn, M.S.
May
14, 2007
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
RutgersUniversity
New
Brunswick,
NJ08901
INTRODUCTION
Dog owners in America are frequently advised to spay/neuter their dogs
for health reasons. A number of 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.
Other studies found a rate of total complications from spaying of 17.7%
and
23%
A study of
Canadian
veterinary private
practitioners found complication rates of 22% and 19% for spaying
female dogs and
neutering male dogs,
respectively
4.
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.4The
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 Europe
and
the other
involved a dog population in America.
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”
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.”
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 studies,
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%.
However, the prognosis for treating testicular tumors is very good
owing to a
low rate of metastasis
,
so
testicular cancer is an
uncommon cause of death in intact dogs. For example, in a Purdue
University
breed health survey of Golden Retrievers,
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 dogs.
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 testicles
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
dogs.
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 dogs>.
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 study
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.
The risk of
osteosarcoma increases with increasing breed size and especially height.
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 Retrievers and
is even
more common in larger breeds.
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 Norway
15 where
spaying is much less common than in the USA.
50-60% of
mammary tumors are
malignant, for which there is a significant risk of metastasis
16.
Mammary
tumors in
dogs have been found
to have estrogen receptors
,
and the published research
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”
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 yrs.
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
tumors.
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 age.
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 coefficients.
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 dogs
.
Spaying
will remove the risk
of ovarian tumors, but the risk is only 0.5%.
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 dogs.
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 risk./span>
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 Dogs.
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 females.
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.
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. .
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.
The
lifetime risk of hypothyroidism in breed health
surveys was found to be 1 in 4 in Golden Retrievers,
1 in 3 in
Akitas,
and 1 in
13 in Great Danes.
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 females.
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 dogs.
A survey
study of veterinary
practices in the UK
found that
21% of dogs were
obese.30
Being
obese
and/or overweight is
associated with a host of health problems in dogs. Overweight dogs are
more
likely to be diagnosed with hyperadrenocorticism, ruptured cruciate
ligament,
hypothyroidism, lower urinary tract disease, and oral disease.
Obese
dogs are more likely to
be diagnosed with hypothyroidism,
diabetes
mellitus, pancreatitis,
ruptured cruciate ligament, and neoplasia (tumors)
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 neutering.
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
males.
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.
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%
for
spayed
females
compared
to
only 0.3% in intact
females.
Urinary
incontinence is so
strongly linked to spaying that it is
commonly
called “spay
incontinence” and is caused by urethral sphincter incompetence,
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.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 females.
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.
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.
The
risk is higher still
for female dogs spayed before puberty.
Pyometra
(Infection of the Uterus)
Pet
insurance data in Sweden
(where
spaying is very
uncommon) found that 23% of all female dogs
developed
pyometra before 10
years of age.
Bernese
Mountain
dogs,
Rottweilers, rough-haired
Collies,
Cavalier
King Charles Spaniels
and Golden Retrievers were found to be high risk breeds.
Female
dogs
that have
not whelped puppies
are at elevated risk for pyometra.
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 death.
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 dogs.
German
Shepherd Dogs and Irish
Setters are more likely to develop perianal fistulas than are other
breeds.
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 years
.
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.
BPH is
linked to other problems
of the prostate gland, including infections, abscesses, and cysts,
which can sometimes
have serious consequences.
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),
suggesting an increased risk of hip dysplasia with spaying.
Spaying
was
also found to cause
a net loss of bone mass in the spine 50.
Since the
growth plates in various bones close
at different times, spay/neuter that is done after
some
growth
plates have closed
but before other growth plates have closed might result in a dog with
unnatural
proportions, possibly
impacting performance and long term durability of the joints.
Perhaps
this is associated with the increased
risk of obesity.
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 disease.
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 dogs,
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.
Compared
to
intact dogs,
spayed/neutered dogs were found to have a 3.1 fold higher risk of
patellar
luxation.
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 dogs.
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 cycle.
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”
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 the odds of overall good health or degrades them
probably
depends on the age of the dog and the relative risk of 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. 
