What is the current recommendation for age of spaying and neutering? Is the traditional six months still followed?
Or, should it be done early; or once a pup is over a year old; or even later? There is a lot of opinion on this subject; the facts are less than clear. Let’s look at what we know.
The most common argument against sterilization at six months is the desire to have pups mature properly. Growth plates in the long bones close later in early-sterilized dogs, theoretically creating a longer-legged dog.
The problem is how this growth pattern is interpreted. A vocal supporter of late sterilization, canine sports medicine consultant Christine M. Zink, D.V.M., Ph.D., D.A.C.V.P., suggests that sterilization before six months of age has been linked to a high incidence of cruciate ligament rupture. Zink proposes that the delayed growth plate closure creates disproportionate bone growth. This, in turn, produces undue angulation of the stifle, which puts stress on the ligament, predisposing it to rupture.
This theory was attacked by veterinary surgeon Lisa Howe, D.V.M., Ph.D., who says that disproportionate bone growth has not been proven. The change in stifle angle has also not been verified. Another researcher, Laura J. Sanborn, M.S., suggests that the increased incidence of ligament rupture could be linked to obesity associated with the surgery.
Does early sterilization affect the incidence of hip dysplasia? A study showed that pups sterilized before 5-1/2 months of age had a 6.7-per-cent incidence. Those sterilized at the traditional age had a slightly lower, 4.7-per-cent incidence.
But another fact was revealed in the study. Howe points out that dogs sterilized later were three times more likely to be euthanized for the condition than the early age group. Could early sterilization be associated with a milder form of hip dysplasia?
The biggest reason cited by veterinarians for sterilization before the first heat is to avoid the risk of mammary cancer. The incidence of this cancer in intact (never spayed) bitches is between 30 and 50 per cent, depending on which study you look at.
If we tag the relative risk in the intact group at 1.00 (this is called an odds ratio), bitches spayed before their first heat have a miniscule risk of 0.005. It climbs to 0.08 for bitches that have one heat and 0.26 if they have two or more heats. Spaying before the first heat protects against mammary cancer.
When we look at bone cancer, the statistics favour late sterilization. In a study of 683 Rottweilers, a breed that has a high incidence of bone cancer, males and females sterilized before one year of age had a higher risk over those that were sexually intact. The time of sterilization was not studied. We don’t know if this effect is limited to Rottweilers or if it applies to other breeds.
In male dogs, Zink suggests that neutering has no protective influence on the incidence of prostatic cancer. Howe agrees with this. However, neutering does prevent other prostatic diseases such as benign prostatic hyperplasia, prostatic cysts and abscesses, as well as cystic hyperplasia.
In females, urinary incontinence occurs more often in spayed versus intact bitches. A higher incidence is seen in bitches spayed before three months of age. Based on this fact, sterilization is recommended after three months to minimize this hormonal incontinence. Other studies look at spaying before or after the first heat, but the results vary. Because this condition is generally easily treated, it is of minor importance.
Pyometra is a condition that occurs in older bitches. According to Howe, this can be seen in up to two-thirds of intact bitches over nine years of age. In Scandinavian countries, where only seven per cent of bitches are spayed, pet insurance statistics show that 25 per cent of insured bitches developed pyometra by 10 years of age. This life-threatening condition can be avoided with spaying.
Zink also suggests that hypothyroidism is more likely to be seen in spayed and neutered dogs. Howe concurs that this is true. However, the overall incidence of hypothyroidism is 0.2 per cent (some breeds are more prone than others). Discouraging sterilization to prevent a disease with a low incidence and good response to hormone supplementation, while other diseases such as mammary cancer and prostate problems have a high incidence, would not make sense.
The decision of when (or if) to sterilize needs to be made with a full understanding of the literature. We still don’t have all the data. And, because breeds differ, across-the-board recommendations are not possible.
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Originally published in the July 2011 issue of Dogs in Canada. Subscribe now and never miss an issue.
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