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Veterinary Medicine: Research and Reports 2015:6 171–180
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REVIEW
open access to scientific and medical research
Open Access Full Text Article
http://dx.doi.org/10.2147/VMRR.S53264
Current perspectives on the optimal age
to spay/castrate dogs and cats
Lisa M Howe
Department of Small Animal Clinical
Sciences, College of Veterinary
Medicine and Biomedical Sciences,
Texas A&M University, College Station,
TX, USA
Correspondence: Lisa M Howe
Department of Small Animal Clinical
Sciences, College of Veterinary
Medicine and Biomedical Sciences,
Texas A&M University, College Station,
TX 77843-4474, USA
Tel +1 979 845 2351
Fax +1 979 845 6978
Email lhowe@cvm.tamu.edu
Abstract: Spaying and castrating of dogs and cats has been considered for decades to be a routine
standard of practice in veterinary medicine in the US for the prevention of numerous undesirable
behaviors, medical conditions, and diseases. Additionally, the procedures have been promoted as a
method of curbing the severe pet-overpopulation problem in the US. Recently, however, this routine
practice has come under scrutiny and become a very controversial topic. The general wisdom and
safety of the procedures have been questioned by those who are concerned that the procedures
may have some unintended consequences that are only recently being recognized. The purpose
of this paper is to critically examine the scientific literature regarding elective spay/castration
procedures and present both risks and benefits of elective gonadectomy. After the literature is
examined, it becomes clear that there may not be a single absolute optimal age to spay or castrate
all dogs and cats, but that the optimal age may be dependent upon several factors, including spe-
cies, breed, body size, and breed-specific diseases, among others. Determining the optimal age
to perform elective gonadectomy is much clearer in cats, and the literature demonstrates that the
procedures can typically be safely performed at any age after 6–8 weeks of age. The optimal age
to spay or castrate dogs of certain breeds (rottweiler, golden retriever, Labrador retriever, and
vizsla) is becoming less clear as studies are being conducted as to the health benefits and risks
in those breeds. This review will examine these controversies and make recommendations as to
the optimal age to spay/castrate dogs based upon the scientific literature.
Keywords: gonadectomy (neuter), ovariohysterectomy (spay), castration, neoplasia, longevity,
orthopedic disorders
Introduction
In veterinary practice, surgical sterilization of cats and dogs is one of the most common
surgical procedures performed. Routine spaying/castrating is often performed because
of its value in preventing reproductive tract disease, including pyometra and mammary
neoplasia in female cats and dogs, and benign prostatic hyperplasia and testicular
neoplasia in male dogs.1–8 Elective gonadectomy is also routinely performed in shelter
situations in animals as young as 6–8 weeks of age as a method of contraception to
help with the pet-overpopulation problem in the US.9–12 However, recently the routine
practice of sterilizing all nonbreeding animals has come under scrutiny.
Veterinarians attempting to determine best practice for spaying/castrating of cats
and dogs are often confronted with conflicting findings from various studies, as well as
differences of opinion as to the optimal age to perform these procedures. The purpose
of this review is to assimilate the literature, identify the risks and benefits of elective
spaying/castrating in dogs and cats, and summarize and discuss these findings, as well
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as attempt to answer the question: when is the optimal age
to spay or castrate dogs and cats?
Considerations for spaying/
castrating
In the current literature, there have been recent studies exam-
ining the effects of gonadectomy on dogs of certain breeds,
and results suggest that hormones may have an influence
on the risk of some types of neoplasia (both positively and
negatively) and certain orthopedic disorders. Additionally,
longevity appears to be influenced by neuter status in the
general population of dogs. There are also many older
“landmark” studies that are still referenced and relevant
to discussions of the risk of neoplasia, as well as the more
general risks and benefits of spaying/castrating in male and
female dogs and cats. The following subsections will exam-
ine some of the more relevant, and sometimes controversial,
issues facing the veterinarian making recommendations to
clients regarding elective gonadectomy in the dog and cat.
Reproductive neoplasia
Mammary neoplasia is among the most common types of can-
cer seen in intact female dogs, and approximately 50% of these
neoplasms are malignant.13,14 There may be differences, how-
ever, in the risk of malignancy among different sizes of dogs.
With small-breed dogs, the risk of histologically malignant
mammary tumors is 25%, while the risk is 58% in large-breed
dogs.15 It is believed that if a female dog is spayed after the sec-
ond estrus cycle or after reaching 2.5 years of age, the relative
risk of developing mammary neoplasia is 26%.1 In the boxer
breed, there is a 45% risk of developing mammary neoplasia,
and in the beagle breed the lifetime risk for the development
of mammary neoplasia is 63%, and the risk for developing a
malignant tumor is 23%.16–18 It has also been shown that ovari-
ohysterectomy at the same time as mastectomy in dogs that have
already developed mammary neoplasia may improve the odds
of preventing new mammary tumor formation.19 However, one
recent systematic review of the literature based on Cochrane
guidelines found that the association between the age at spay-
ing and the risk of mammary neoplasia was weak.20 Depending
upon the timing of gonadectomy, spaying female dogs prevents
this potentially deadly neoplasm (Table 1).
Mammary tumors are also common in intact female
cats, but the risk of mammary tumors is approximately half
that of dogs.2,21 Unfortunately, between 85% and 93% of
these tumors are malignant, and more than 80% of cats with
malignant mammary tumors also have evidence of metasta-
sis (Table 1). Cats spayed before a year of age are reported
to have an 86% reduction in risk of developing mammary
neoplasia compared to intact cats.3,21 As with dogs, appro-
priately timed gonadectomy in cats is extremely beneficial
in preventing mammary neoplasia.3,21
In the male dog, castration prevents testicular neoplasia,
although it has been estimated that less than 1% of intact male
dogs will die of testicular cancer.22,23 This low death rate is
due to a high cure rate coupled with the fairly low risk. Cats
very rarely develop testicular neoplasia.24
Confusion sometimes exists regarding the effect of cas-
tration on prostate cancer in dogs. Castrated dogs are at an
increased risk of developing prostate neoplasia.25 Fortunately,
the risk of prostatic neoplasia is extremely low (0.6%), negat-
ing undue concern about this risk.25
Nonneoplastic reproductive disorders
Pyometra is a life-threatening disease seen in unspayed dogs
and cats, but the disease is much more frequent in dogs than
in cats.4,5,26,27 The risk of pyometra in unspayed dogs has been
reported to range from 25% to 66% in dogs over 9–10 years
old, and some breeds (collies, golden and Labrador retrievers,
rottweilers, and German shepherds) appear to have a strong
Table 1 Benets and risks of disease process with and without gonadectomy
Benet Risk with gonadectomy Risk without gonadectomy
Longevity in dogs Males: 13.8% increase in life span
Females: 26.3% increase in lifespan
–
Pyometra 0 Dogs: 25%–66% in those over 9–10 years old
Cats: risk unknown, but less than in dogs
Mammary tumors Dogs: 0.5% if prior to the rst estrus
Cats: 86% decreased risk if before 1 year
Dogs: 23%–63%, with 50% being malignant
Cats: half the risk of dogs, with 85%–93% being malignant
and 80% already metastasized when diagnosed
Benign prostatic hyperplasia 0 75%–80% of intact dogs by 6 years: predisposes to other
life-threatening prostate diseases
95%–100% of intact dogs by 9 years: predisposes to
other life-threatening prostate diseases
Testicular neoplasia 0 Risk unknown, but ,1% will die from tumor
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Optimal age to spay/castrate dogs and cats
predilection for pyometra (Table 1).28–30 Complete removal
of the ovaries and uterus of female dogs prevents pyometra
from occurring.
In male dogs, there are several nonneoplastic diseases
of the prostate that may occur with some frequency in dogs.
Benign prostatic hyperplasia is the most common, and is
reported to be evident in 75%–80% of intact dogs by 6 years
of age and in 95%–100% of intact dogs by 9 years of age
(Table 1). 25,31–33 Dogs with benign prostatic hyperplasia are
predisposed to prostatitis, which can result in life-threatening
prostatic abscesses and infected prostatic cysts.6,7,34,35 Pros-
tatic cysts and paraprostatic cysts can also result from benign
prostatic hyperplasia.25,36–38 Castration is an effective treat-
ment for benign prostatic hyperplasia.36,39,40 Additionally,
castration is an effective preventive.38 Intact male cats only
rarely are affected by prostatic disorders.41
Urinary tract disorders
Female dogs that are spayed are at greater risk for urethral
sphincter-mechanism incompetence, which can result in uri-
nary incontinence, than are intact female dogs.42–47 Concerns
exist regarding early age gonadectomy in female dogs,
because of a possible increased risk of urinary incontinence.
Several studies have looked at this issue, and have had some-
what similar findings in that the risk of urinary incontinence
increases after ovariohysterectomy, although the studies dif-
fered as to the timing of gonadectomy and its effect on incon-
tinence.43,47 One such study demonstrated that the greatest risk
of urinary incontinence was for pups spayed before 3 months
of age.48 The authors of that study recommend avoiding gon-
adectomy until 3–4 months of age, if possible, recognizing
that in sheltering situations this may not always be possible.
A recent study evaluating the prevalence of urinary incon-
tinence in 566 spayed dogs found that the risk of acquired
urinary incontinence to be 5.12%.49 No significant difference
in the age at the time of ovariohysterectomy between conti-
nent and incontinent dogs was identified; however, the incon-
tinence rate of small dogs was 1.37% and 9.12% for dogs
$15 kg (medium and large dogs), respectively. Since all dogs
of the study were older than 12 weeks of age when spayed,
the authors were unable to make any statements regarding
the risk of urinary incontinence of dogs spayed younger than
3 months of age. Urinary incontinence does not appear to be
a problem in female cats following ovariohysterectomy.44
Lymphoma
Lymphoma is a tumor that originates most commonly in
lymphoid tissue, but may arise in any tissue in the body.50–52
Concerns have been raised about an increased risk of
lymphoma in spayed/castrated dogs, and recent studies have
examined the issue. One study examined the effect of sex and
hormones on lymphoma using 14,573 case-control pairs of
dogs identified based on breed and age from the Veterinary
Medical DataBases (database of over 7 million records
submitted by 26 universities).53 The study found that intact
males were at the greatest risk for development of lymphoma
(odds ratio 1.32) compared to intact females, castrated males,
or spayed females. Age at the time of gonadectomy was not
discussed.
The results of a study of vizsla-breed dogs were conflict-
ing compared to the first study.54 Using an online survey of
vizsla breeders to evaluate risk of several types of cancer, the
study found that the odds of gonadectomized dogs having
lymphoma or lymphosarcoma were higher (4.3 times) than
those of sexually intact dogs. When examining the raw data
from the study, however, the findings of the study result
from the fact that the risk of lymphoma in dogs spayed after
12 months of age was higher (1.1%) than the other spayed
group (spayed ,12 months of age – 0.3%) or the intact group
(0.1%), which had similar risks (Table 2).
Another study of 759 golden retrievers revealed findings
that were different than the other studies.55 In this study, the
rate of lymphosarcoma in intact male dogs was less than that
of dogs castrated when less than 12 months of age. In dogs
castrated 12 months of age, the risk of lymphoma was 0
versus 3.5% in intact dogs, and 9.6% in males castrated at
or before a year of age. There were no differences relative
to neuter status in female dogs (Table 2). The results of this
study would suggest that male golden retriever dogs should
be castrated just after a year of age to reduce the risk of
lymphoma. A recent study of 1,500 Labrador retriever dogs
found that the risk of lymphosarcoma was not higher at any
neuter time period compared with intact animals for either
males or females (Table 2).56
These studies illustrate that the findings of one breed-
specific study cannot be extrapolated to the population as a
whole, or to other breeds, since certain types of cancer (or
other diseases/conditions) naturally occur more frequently in
certain breeds than in others. Therefore, studies examining
the effects of gonadectomy in a specific breed are only appli-
cable to other dogs of the same breed. Studies that examine
the population as a whole can typically be applied to the
entire population of animals, recognizing that some breeds
may respond differently than the population as a whole. In the
cat, there are no studies suggesting any association between
spaying/castrating and lymphoma.
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Hemangiosarcoma
One of the major causes of death in certain breeds of dogs,
including boxers, Labrador and golden retrievers, and
German shepherds, is splenic hemangiosarcoma, which
accounts for approximately 2% of all canine tumors.57–60
Three recent breed-specific studies have reported on neuter
status and the risk of hemangiosarcoma (Table 2). In vizslas,
the risk of hemangiosarcoma was greatest in females spayed
after 12 months of age (1.1%) and lowest in the intact dogs
(0.1%).54 Females spayed before 12 months of age had a
risk of 0.5%. In male vizslas, the risk was greatest in those
castrated after 12 months of age (0.6%) and lowest in those
castrated before 12 months of age (0), while the risk was
0.4% in the intact males. The study of golden retriever
dogs found similar results in the female dogs.55 In that
study, females spayed after 12 months of age had a risk
of 7.4%, which was markedly higher than in dogs spayed
prior to a year of age (1.8%) or intact females (1.6%). In
males, differences were not seen between neuter status
groups and the risk of hemangiosarcoma. In an older, age-
matched, case-controlled study of multiple breeds (92 dogs
with hemangiosarcoma compared to 1,369 group-matched
controls), gonadectomized females were found to be at a
2.2-time-greater risk compared to intact females for splenic
hemangiosarcoma, although age at the time of gonadectomy
was not discussed.61 In the Labrador retriever study, the
risk of hemangiosarcoma was not found to be higher at
any neuter period compared with those remaining intact
for either male or female dogs.56
While the overall risk of hemangiosarcoma is low in the
general population, the results of the golden retriever study
should be kept in mind when discussing timing of spay-
ing, and efforts should be made to spay these dogs before
1 year of age to potentially decrease the risk of splenic
hemangiosarcoma. Splenic hemangiosarcoma occurs infre-
quently in cats, and no known association with gonadectomy
could be identified in the literature.
Osteosarcoma
The most common bone tumor in dogs is osteosarcoma, and
it is a common cause of death in medium-size and large-
and giant-breed dogs.62–65 The prognosis for osteosarcoma
remains poor, with median survival rates of 5–18 months.64,65
Concerns have been expressed that breeds that are predis-
posed to osteosarcoma may be at increased risk if they have
been spayed/castrated.
One study examining this issue involved a mail survey of
owners of 683 rottweiler dogs.63 The study found that dogs
that underwent a spay or castration before a year of age were
3.8 and 3.1 times, respectively, more likely to develop osteo-
sarcoma compared to intact dogs. Sexually intact dogs of that
study had an 8% risk of developing osteosarcoma compared
with a 28% risk in males and a 2 5% risk in females gonadec-
tomized before 1 year of age (Table 2). However, the age at
Table 2 Breed-specic lifetime risk of disease processes based on neuter status and timing of gonadectomy
Breed Sex Risk Gonadectomy #12 months Gonadectomy 12 months Intact
Rottweiler M
F
OSA 28%
25%
–
–
8%
8%
Golden retriever M Hip dysplasia 10.2% 3.1% 5.1%
Golden retriever M
F
CCL rupture 5.1%
7.7%
1.4%
0
0
0
Golden retriever M LSA 9.6% 0 3.5%
Golden retriever F HSA 1.8% 7.4% 1.6%
Golden retriever F MCT 2.3% 5.7% 0
Labrador retriever M CCL rupture ,6 m – 7.6%
6–11 m – 2.8%
12–23 m – 1.9%
2–8 y – 0
2.3%
Labrador retriever MElbow dysplasia ,6 m – 4.2%
6–11 m – 0
12–23 m – 0
2–8 y – 2.2%
0.6%
Labrador retriever F Hip dysplasia ,6 m – 5.4%
6–11 m – 5.1%
12–23 m – 4.3%
2–8 y – 0
1.7%
Vizsla M
F
LSA 0.3%
0.3%
0.4%
1.1%
0.2%
0.1%
Vizsla M
F
HSA 0
0.5%
0.6%
1.1%
0.4%
0.1%
Vizsla M
F
MCT 0.5%
1.1%
1.3%
1.9%
0.5%
0.6%
Abbreviations: M, male; F, female; OSA, osteosarcoma; CCL, cranial cruciate ligament; LSA, lymphoma/lymphosarcoma; HSA, hemangiosarcoma; MCT, mast-cell tumor;
m, months of age; y, years of age.
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Optimal age to spay/castrate dogs and cats
the time of death was actually greater (despite the increased
risk of death from osteosarcoma) for the gonadectomized
dogs compared to the intact dogs among the dogs that were
deceased at the time of the study. The higher overall cancer
risk seen in the early gonadectomized dogs may be due to
longer life spans.
A study of the Veterinary Medical DataBases exam-
ined the role of host risk factors, including breed and sex
and neuter status, on the development of osteosarcoma.62
The case-controlled study (but not age- or sex-controlled)
involved 3,062 purebred dogs with confirmed diagnosis of
osteosarcoma compared to 3,959 purebred controls found
that dogs that had undergone gonadectomy had twice the
risk of developing osteosarcoma as intact dogs. There were
no sex-related differences in risk. Increasing height was
associated with an increased risk of osteosarcoma, as was
increasing body weight when controlling for breed. No
attempt was made to correlate increased body weight with
neuter status (gonadectomized dogs tend to gain weight
following gonadectomy). The authors speculated that there
could be confounding factors that were not measured, such
as diet, since owners of gonadectomized animals might be
more health-conscious and more likely to supplement their
dogs with minerals and vitamins, which could promote bone
growth and increase osteosarcoma risk.
Findings of these studies are of concern, particularly in
the rottweiler breed, but it remains to be seen whether these
findings are confirmed in larger well-controlled prospective
studies or in differing breeds. Recent studies in the golden and
Labrador retriever and vizsla breeds reported no increased
risk of osteosarcoma in gonadectomized dogs compared to
intact dogs, which illustrates the difference between dog
breeds and the danger of extrapolating findings from breed-
specific studies to other breeds or the population as a whole.
Osteosarcoma is an uncommon tumor in cats, and an associa-
tion with gonadectomy in cats has not been identified.64
Mast-cell tumor
Mast-cell tumors are a very commonly seen type of skin
tumor in dogs (and cats), with boxers, Boston terriers, golden
retrievers, Labrador retrievers, and others demonstrating a
breed disposition.65–68 While mast-cell tumors may account
for a large portion of skin tumors seen in dogs, the actual
risk of these tumors is estimated to be around 0.06%–0.13%
in the general population.69,70 In the golden retriever breed,
mast-cell tumors were seen in 2.4% of the dogs in one
study.55 While no differences were seen in the risk of mast-
cell tumors in male dogs relative to neuter status, in female
dogs the tumors were seen more frequently in dogs spayed
after 1 year of age (5.7%) compared to intact dogs (0). Dogs
spayed before a year of age had a risk of 2.3% (Table 2).
Labrador retrievers that have been spayed or castrated
have no greater risk of developing mast-cell tumors, regard-
less of the age at neutering, compared to intact Labrador
retriever dogs.56 In the vizsla breed, gonadectomized male
and female dogs had a higher risk of mast-cell tumors.54
Male dogs castrated after 1 year of age had the highest risk
of mast-cell tumors (1.3%), followed by dogs castrated before
1 year of age (0.5%) or remaining intact (0.5%). Similarly,
females spayed after 1 year of age had a higher risk of mast-
cell tumors (1.9%), whereas the risk in those spayed before
1 year of age was 1.1% and 0.6% in dogs remaining intact
(Table 2).
Whether the findings of these studies apply to other
breeds is unknown, but these studies would suggest that when
gonadectomizing dogs of the vizsla and golden retriever
breeds, it would be best to perform the procedure prior to a
year of age if there is concern about an increased risk of mast-
cell tumors. There is no identifiable association with mast-cell
tumors and spaying/castrating status in the literature in cats.
Orthopedic disorders
Questions have also been raised as to whether gonadectomy
in dogs may result in an increased risk of hip disease. It is
well known that hip dysplasia is a multifactorial disease that
is influenced by environment, genetics, diet, and many other
factors.71–76 In one study of 759 golden retriever dogs, the risk
of hip dysplasia in females was not influenced by ovariohys-
terectomy or the timing of the procedure.55 However, males
castrated before 1 year of age had a higher risk (10.2%) of
hip dysplasia than intact dogs (5.1%) or those castrated after
1 year of age (3.1%).
A study of 1,500 Labrador retrievers identified no dif-
ference in the risk of hip dysplasia in males at any neuter
period compared to intact dogs.56 In females, however,
the risk of developing hip dysplasia was higher for those
spayed at ,6 months (5.4%), 6–11 months (5.1%), and
12–23 months (4.3%) compared to intact females (1.7%).
The risk of hip dysplasia for those dogs spayed between
2 and 8 years was 0.
Another study of 1 ,733 boxers that were studied for 8 years
found that dogs that were spayed/castrated and then diagnosed
with hip dysplasia within 6 months of gonadectomy were 1.5
times more likely to develop lameness, and that those housed
on slippery floors were 1.6 times more likely to develop hip
dysplasia.77 Dogs from litters with high preweaning mortality
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rates had twice the risk of hip dysplasia. Obesity is another
factor that may contribute to hip-dysplasia development and
it is well documented that after spaying/castrating dogs tend
to gain weight, although the timing of gonadectomy does
not appear to influence the risk of becoming overweight.78
These studies demonstrate the need for keeping pets at a
healthy weight, and also show that hip dysplasia is indeed a
multifactorial disease. Gonadectomy is just one factor that
may play a role in the development of hip dysplasia.
Studies have also suggested that gonadectomy may influ-
ence the risk of cranial cruciate ligament injuries. In particular,
four studies of note have specifically examined this issue. In
the first, golden retrievers spayed/castrated before 1 year of age
were found to have a significantly higher risk of cranial cruci-
ate ligament rupture.55 Males and females gonadectomized
before 1 year of age had a risk of 5 .1% and 7.7%, respectively,
compared to a risk of 1.4% in late-castrated males and 0 in
late-spayed females and intact males and females (Table 2).
In the second retrospective study of 10,000 dogs with cranial
cruciate ligament rupture, those dogs that had undergone
gonadectomy (timing unknown) had a higher risk of rupture
than intact dogs.79 The third study, also a retrospective study,
of 3,218 dogs treated in one practice found that gonadecto-
mized dogs (timing unknown) had a 2.1-fold-greater risk of
ligament rupture compared to intact dogs.80 Based upon these
studies, it would appear that gonadectomy does play a role in
cranial cruciate ligament rupture, although the effect of body
condition was not examined in these studies. A fourth study in
Labrador retrievers found that males castrated before 6 months
of age had a higher risk (7.6%) of developing cranial cruciate
ligament rupture compared to intact males (2.3%) (Table 2).56
This study did examine body-condition score (BCS). A mean
BCS of both castrated and intact males with cranial cruciate
ligament rupture was 6 compared to a BCS of 5 in the castrated
and intact males without rupture. No differences were seen
among female dogs with regard to the risk of cranial cruciate
ligament rupture and timing of spaying compared to intact
females.
Elbow dysplasia in male Labrador retrievers is seen
more frequently in males castrated at ,6 months (4.2%)
and between 2 and 8 years (2.2%) compared to intact males
(0.6%). Elbow dysplasia was not seen in any males castrated
from 6 to 23 months of age in a study of 1,500 Labrador
retrievers.56 No differences were seen in female Labradors
at any spaying age interval compared to intact dogs.
Concerns have been expressed by some veterinarians
and laypersons about an apparent increased risk of capital
physeal fractures in adult castrated male cats and its possible
relationship to castration. The fact that physeal closure is
delayed in cats (and dogs) that are gonadectomized early in
life81,82 has been suggested as the explanation for the increased
risk in these cats. In the largest series of cases reported, 25
of 26 cats with spontaneous capital physeal fractures were
castrated males.83 Fourteen of 16 cats for which the age at
castration was known were castrated before 6 months of age.
When compared to age- and sex-matched control cats, body
weight at the time of initial exam was significantly greater
in the affected cats (12.3±2.6 lb [5.6±1.2 kg]) than control
cats (9.9±6.4 lb [4.5±2.9kg]). The authors suggested that an
abnormally high body weight was a risk factor for a slipped
capital physeal fracture, as was neuter status, age, and delayed
physeal closure (delayed in cats castrated at 7 weeks or
7 months of age). Another study of cats with slipped capital
physes similarly found that nine of ten cats for which weights
were recorded were considered to be obese or overweight.84
Additionally, in that study, three of 13 cats were Siamese cats
(23%), which was considerably greater than the percentage
of Siamese cats in the control population (5%).
Although this type of fracture is rare, good client educa-
tion is necessary, particularly with male cats that are castrated
before a year of age, to prevent these cats from becoming over-
weight. It is well documented that cats gain weight following
castrating (or spaying), so caloric reduction must be imple-
mented to prevent cats from becoming overweight or obese.82
While another alternative is to wait until after a year of age to
castrate male cats, many male cats will develop objectionable
behaviors if castration is delayed beyond puberty.85
Behavior
Concerns regarding the effect of gonadectomy on behavior
of dogs have been recently expressed. Following castration
in dogs, there is a 90% decrease in roaming, a 70% decrease
in mounting behavior, and a 60% decrease in aggression
with other male dogs.86 In another study however, male dogs
castrated before 5.5 months were found to have aggression
toward family members, barking or growling at strangers,
and excessive barking that bothered family members.48 These
three behaviors were highly associated with one another.
Other studies have reported similar mixed findings.87,88
Cats also demonstrate behavioral changes associated
with gonadectomy.89 With male cats, castration before
5.5 months is associated with decreased sexual behavior and
urine spraying, decreased aggression toward veterinarians,
and increased frequency of hiding behaviors. In both male
and female cats, gonadectomy before 5.5 months was
associated with a decreased occurrence of shyness around
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Optimal age to spay/castrate dogs and cats
strangers. Another study of 800 kittens randomly assigned
to prepubertal gonadectomy (8–12 weeks) or traditional-age
gonadectomy (6–8 months) found no difference in behavioral
problems between groups after adoption.90 Kittens were
followed frequently for 24 months after adoption, and the
occurrence of inappropriate elimination, fearful behavior,
non-play-related aggression, and destruction did not differ
based upon timing of gonadectomy.
Longevity
The effect of gonadectomy on longevity in dogs has been
expressed as a concern for dogs based upon studies from the
human literature, in which females exposed to estrogens during
the course of their lives tend to live longer than males who do
not have this exposure.91–93 Additionally, age-associated declines
in testosterone levels are strongly independent predictors of
mortality in older men.94 These concerns appear unfounded
in dogs, however. A study of 40,139 dogs from the Veterinary
Medical DataBases looking at life span and neuter status found
that life expectancy increased by 26.3% in females and by
13.8% in males that were castrated.95 This effect was consistent
among different size classes of dogs. Dogs that were spayed/
castrated were more likely to die of immunomediated diseases
and neoplasia, and less likely to die of trauma, infectious dis-
eases, vascular diseases, and degenerative diseases.
A retrospective study of 119 exceptionally aged rottweil-
ers (13 years) were compared with 186 rottweilers with
an average life span for the breed (9.4 years) to determine
whether neuter status played a role in living to an excep-
tional age.96 The study found that in females, neuter status
did play a role in life span, with females having the longest
ovary exposure (6.1–8.0 years) being 3.2 times more likely
to reach an exceptional age compared to dogs that had
shorter ovary exposure. There were multiple problems with
the study, including a lack of randomization, and different
time frames studied for the two populations, among others,
that may have played a role in the outcome. If the findings
of this study were found to be repeatable, it would appear
that females of the rottweiler breed may possibly respond
differently than the population of dogs as a whole (based on
the study of over 40,000 dogs).
While the rottweiler study raises some interesting ques-
tions in that breed, results from the larger study of over
40,000 dogs would pertain to the dog population as a whole.
Unless other large, well-designed studies prove otherwise,
the effect of neuter status on longevity should be considered
heavily by veterinarians who are counseling their clients as
to the risks/benefits of gonadectomy.
Discussion
It becomes clear when examining the literature that there are
numerous considerations that play into decision making as
to the timing of spaying or castrating, especially with dogs.
With cats, the overwhelming evidence would suggest that
gonadectomy is safe in cats at any age over 6 weeks. Since
cats of both sexes will gain weight after gonadectomy, it is
crucial to maintain cats at a healthy weight so as to avoid
weight-associated endocrine disease and potential for slipped
capital physes in males.
In the dog, the timing of gonadectomy is less defined.
Since longevity is increased in dogs that are gonadectomized,
and since some life-threatening diseases (mammary neoplasia,
pyometra, prostatic abscess, etc) can be avoided, those benefits
would appear to take precedence over many of the concerns
of other diseases that occur with lesser frequency. If possible,
females, should not be spayed until they are 3–4 months of
age to avoid an increased risk of urinary incontinence. For
many breeds, the literature suggests it is safe to castrate males
at any age above 6–8 weeks of age.
Exceptions to these guidelines noted may include concern
about osteosarcoma in the rottweiler, and possibly hip dyspla-
sia in male golden retrievers, cranial cruciate ligament rupture
in both male and female golden retrievers, and lymphoma
in male golden retrievers. In these breeds, veterinarians may
wish to wait until just after a year of age before gonadectomy
if these concerns take precedence over longevity benefits and
decreased risk of female reproductive diseases and neoplasia.
With the female golden retriever, however, if attempting to
decrease the risk of hemangiosarcoma and mast-cell tumor,
then the dogs should undergo ovariohysterectomy before
1 year of age. The male Labrador retriever may be castrated
at approximately 2 years of age to have the lowest risk of
both cranial cruciate ligament disease and elbow dysplasia,
although those castrated between 6 and 11 months of age
have similar risk for both orthopedic diseases, as do intact
dogs. To lower the risk of hip dysplasia in female Labrador
retrievers, ovariohysterectomy may be performed at 2 years of
age. In the vizsla breed, to further decrease the fairly low risk
of lymphoma, hemangiosarcoma and mast-cell tumor, dogs
should undergo gonadectomy before 12 months of age.
When the older literature, as well as newer studies, is
examined, it is important to recognize that there are limita-
tions to these studies, particularly with breed-specific studies.
As demonstrated by the golden and Labrador retriever studies
(very similar breeds), as well as the vizsla study, data from
one specific study cannot be extrapolated to other breeds or
the general population. Additionally, there are potentially
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178
Howe
confounding factors in these studies, such as the retrospective
nature of the studies, population studied, housing and envi-
ronment, feeding and exercise protocols, and weight, among
others. In many studies in the literature, author bias may have
played a role in the interpretation of data. Where possible, it
is important to look at actual study data to draw independent
conclusions. As more research is conducted into identifying
breed-specific risks and benefits of spaying/castrating, veteri-
nary decision making may become more complicated in the
individual animal of certain breeds, particularly in light of
mixed results regarding timing of gonadectomy to decrease
the risk of various conditions within given breeds of dogs.
Conclusion
Gonadectomy has both benefits and risks. This is particularly
true in the dog, where there seem to be more potential risks
than in the cat. It is important for the veterinarian to weigh these
risks against the benefits gained for each patient that presents
for spaying or castrating. While the benefits likely heavily out-
weigh the risks for most patients, the considerations discussed
in this paper should be kept in mind when determining the best
age to spay or castrate pet dogs of certain breeds. One should
use caution, however, when trying to extrapolate findings
from one breed to other breeds, or to the general population of
dogs as a whole, since studies from a single breed often have
conflicting results when compared to findings from studies of
other breeds or the general dog population.
Disclosure
The author reports no conflicts of interest in this work.
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