Friday, July 25, 2008

To Spay or not to Spay??

Dog undergoing spay surgery at a vet clinic

For those of you who have considered neutering or spaying your dog, consider the following report on the health effects of doing so... the document is a bit lengthy, but worth reading if you own a red setter!!

Long-Term Health Risks and Benefits Associated with Spay / Neuter in Dogs
Laura J. Sanborn, M.S.
May 14, 2007

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
Rutgers University
New Brunswick, NJ 08901

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.

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%)>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%)>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.

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 direct
impact 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.
Page 7 of 12

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 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 disease32. Obese dogs are more likely to be diagnosed with hypothyroidism,
diabetes mellitus, pancreatitis, ruptured cruciate ligament, and neoplasia (tumors)32.

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 Sweden (where spaying is very uncommon) found that 23% of all female dogs
developed pyometra before 10 years of age44. Bernese Mountain dogs, Rottweilers, rough-haired Collies,
Cavalier King Charles Spaniels and Golden Retrievers were found to be high risk breeds44. Female dogs
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

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 develop perianal fistulas than are other

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.
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)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 of immature dogs delays the closure of the growth plates in bones that are still growing,
causing those bones to end up significantly longer than in intact dogs or those spay/neutered after
maturity50. 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.
Spay/neuter is associated with a two fold increased risk of cranial cruciate ligament rupture51. Perhaps this
is 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.”
Page 9 of 12
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
Compared to intact dogs, spayed/neutered dogs were found to have a 3.1 fold higher risk of patellar

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

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.
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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