I recently attended a seminar put on by the WSVMA on this topic. The panel of speakers included two boarded veterinary surgeons, one boarded veterinary behaviorist and one veterinary reproductive expert. At the end of the day I realized there are no easy answers to these questions, and the decision as to when or whether to spay and neuter needs to be an informed decision made by the dog owner.
Here are some facts which came to light during the discussion. Keep in mind that the knowledge in this area is based on retrospective studies, some of which have a limited sample size.
- Dogs that are spayed or neutered generally live longer. Of course it may not be that the spaying or neutering is causing the increase in longevity. It could be that dogs that spayed or neutered dogs, on average, receive better care during their lifetime.
- Certain cancers are more common in dogs that are spayed or neutered. Examples are bone cancer, hemangiosarcoma, lymphoma, transitional cell carcinoma, and mast cell tumors. The incidence of these tumors is somewhat breed related. For example, osteosarcoma in the general dog population has an incidence of 0.2%, but in Rottweilers and other large breeds the incidence is much higher. So in Rottweilers a 2-3 fold increase in incidence off osteosarcoma due to spaying or neutering is much more significant than it is in the general population. Some of the findings for specific breeds are complex. For example, the incidence of hemangiosarcoma in Golden Retrievers spayed after 12 months of age is 4 times that of unspayed females or females spayed prior to 12 months of age.
- Mammary (breast) cancer is almost eliminated if spaying is done prior to the first heat cycle and is significantly reduced if performed prior to the second heat. Dogs spayed after the second heat have a 26% chance of developing mammary neoplasia during their lifetime, and 50% of these are malignant. Compare this to the low incidence of osteosarcoma which is 0.2% of the general population. A 2-3 times increase of a very low incidence cancer is still rare.
- It appears that there is a greater incidence of orthopedic disease in spayed and neutered dogs. The most common conditions affected are cranial cruciate ligament injury (CCL) and hip dysplasia (early arthritis of the hip joint). Most of these studies compare dogs spayed or neutered before 5 months of age to ones spayed or neutered after twelve months of age. It is felt that waiting to spay or neuter until after the dog is finished growing should reduce the negative effect. However, it is not clear when exactly the growth plates close. In general it is felt that in small to medium breeds the growth plates close prior to a year of age, but in large breeds it could be over a year.
- There appears to be no real health advantage to neutering male dogs except preventing certain forms of prostate disease. Prostate cancer is higher in neutered dogs. There may be some behavior benefits to neutering but one study indicated that early neutering is associated with certain behavioral problems e.g. separation anxiety, aggression.
- Spayed and neutered dogs appear to be at an increased incidence of hypothyroidism and other immune mediated diseases.
- Spayed dogs have a greater incidence of urinary incontinence. This is particularly true when spaying is done prior to three months of age.
- Uterine infection (pyometra) has a significant incidence in dogs that are not spayed. It is estimated that close to 23% of unspayed female dogs will develop pyometra by 10 years of age. And pyometra is a life threatening condition that requires early intervention.
In summary, it appears that the incidence of many cancers with a low incidence are increased by spaying and neutering, and one form of cancer (mammary cancer) with a high incidence is increased by failure to spay prior to the female’s second heat cycle. Unspayed dogs have a high risk of developing a life threatening uterine infection.
Spayed and neutered dogs may have an increased risk of developing certain immune mediated diseases, e.g. hypothyroidism. There are conflicting reports in the literature regarding the effects of spaying and neutering on behavior, but some studies indicate an increase in certain behavior problems in dogs spayed or neutered prior to 6 months of age. Other studies indicate that neutering male dogs may help with certain behavioral problems such as aggression, urine marking, and mounting behavior.
So there is really no clear cut answer. One of the panel participants, the reproductive expert, recommended ovarian sparing hysterectomies for female dogs and vasectomies for male dogs. But the ovarian sparing hysterectomy is a complicated surgery and does not address the mammary cancer issue, and there are frequently problems related to vasectomies in male dogs.
Several of the panelists felt the best summary article in this area for clients to read is “Determining The Best Age At Which To Spay of Neuter” by Margaret Root-Kustritz, DVM, PhD.
The conclusion of this paper reads:
“For female dogs, the high incidence and high frequency of mammary neoplasia, and the significant effect of spaying on decreasing its incidence make ovariohysterectomy prior to the first heat the best recommendation for non-breeding animals. The demonstrated increase of urinary incontinence in bitches spayed before 3 months of age and the possible effect of CCL injury in bitches spayed before 6 months of age, suggest that spaying bitches after 6 months of age but before their first heat cycle is most beneficial. For bitches of breeds predisposed by ovariohysterectomy to highly malignant tumors and for breeding animals, spaying at a later age may be more beneficial.
For male dogs, castration decreases incidence of disorders with little health significance and may increase incidence of disorders of much greater health significance. For non-breeding animals, evaluation of breed and subsequent predispositions to disorder by gonadectomy should guide when and if castration is recommended.”
Carl D. Anderson, DVM