The gonad chronicles, part 2: Veterinary research explores neutering's elusive impact - DVM
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The gonad chronicles, part 2: Veterinary research explores neutering's elusive impact
Research suggests that neutering may help modulate unwanted behaviors and eliminate or reduce the risk of some diseases—but exacerbate other health issues. So consider the favorable and adverse effects of neutering to help clients make informed decisions.


Reasons for neutering

Neutering (castration or ovariohysterectomy) is usually recommended for aggressive dogs and cats, not because it is therapeutic but because it prevents a heritable behavior component from being passed on. If heritability is complex or has a large environmental component to expression, neutering dogs known to be aggressive may not substantially decrease genetic risk within the population of breeding dogs as much as we would like.

Owners may choose to neuter their dog for many different reasons, which can vary by sex. Common reasons for neutering male dogs include: the dog marks with urine, the dog has an enlarged prostate, the dog has prostatic or testicular neoplasia, or the working dog is more focused on other dogs' odors than on his job. Common reasons for neutering females include a desire to decrease the risks of mammary neoplasia to the fullest extent possible. In some cases, the client cannot or will not monitor for pyometra or doesn't want to deal with the behavioral and physical consequences of heat cycles.

The most common reason for ovariohysterectomy (OHE) is to spare dogs the risk of mammary neoplasia, which is extremely common in dogs, but only malignant about half the time.5 In countries where dogs are not routinely spayed, the risk for mammary neoplasia may be as high as 53 percent.6 Neutering female dogs before their first heat appears to decrease the risk of mammary neoplasia to 0.5 percent. If clients wait until the second heat, the risk is 8 percent, and after the second heat, the risk increases to 26 percent.7

While spaying after the second heat does not reduce risk, spaying at the time the mammary tumors are removed appears to have a beneficial effect on survival.8,9 Some authors don't feel that the data is strong enough to say there is any mammary sparing effect because of problems with bias in the data.10 The patterns for OHE and mammary neoplasia are similar in cats, but 85 to 90 percent of feline mammary tumors are malignant.5,11

Repeated heat cycles increase the risk of pyometra.12 In one study, by 10 years of age, 25 percent of the females studied developed pyometra. There are strong breed attributes that may increase or decrease this risk, but OHE eliminates this risk altogether.13

The risk of prostatic disease in male dogs increases with age. Benign hypertrophy may affect at least 60 percent of dogs older than 7 years.14 Castration both prevents and treats benign prostatic hyperplasia. Testicular tumors are fairly rare and most are prevented or treated by castration. However, most prostate cancer, when diagnosed, is advanced and does not respond to androgen deprivation, so monitoring is important for a favorable outcome.15

Other health issues

Additional studies have raised concerns about neutering's adverse effects on other health issues. Castration may increase the risk of bladder transitional cell carcinoma16 and prostate carcinoma,16,17 an effect augmented by breed. At least one study suggests that neutering may facilitate tumor growth while having no effect on initiating cancer development.18 Neutered dogs have been reported to have a high risk for osteosarcoma compared with intact dogs,19,20 but the studies are complicated by other factors and the neutered dogs actually lived longer than the intact dogs.

Spayed females have been reported to have an increased risk of hemangiosarcoma, when compared with intact females,21,22 a finding that may be heavily influenced by breed, but not supported by the PLoS One study that stimulated this debate.23 Transitional cell carcinoma may affect more neutered than intact females, but it is rare, although more common in females.24

Early neutering, as practiced on shelter dogs to reduce the number of unwanted animals, may have different effects on health than neutering later in life. Early neutering increases long bone growth, which may pose risks for later skeletal conformations, potentially affecting hip dysplasia,25-28 but these findings are not supported by all studies,29 and even when present, the effect may be relatively weak.28

If OHE for female shelter puppies can be delayed until 3 months of age, risks for temporal, related cystitis and later urinary incontinence may be decreased,28 but this may matter less than anticipated.29 If the risk is that the pup would not otherwise be neutered, neutering of shelter dogs is always preferred. One retrospective case series evaluating the true prevalence of urinary incontinence in spayed female dogs that were neutered across ages produced two findings: acquired urinary incontinence had a low prevalence (N=29 or 5.12 percent in 566 dogs), and there was no effect of age at OHE between the continent and incontinent dogs.30

Chemical castration with implantable deslorelin, a gonadotropin-releasing hormone (GnRH) agonist, is occasionally used because it renders animals functionally sterile, a condition that is reversible with removal of the implant.31 Deslorelin decreases testosterone and luteinizing hormones to undetectable levels in dogs and so may also affect testosterone-modulated behaviors. When alpha-agonists fail, deslorelin may successfully treat estrogen-dependent urinary incontinence in female dogs.32

In ferrets, those treated with deslorelin showed decreased fighting associated with sexual behavior and increased play behavior when compared with ferrets who had been surgically castrated.33 One study that examined play, fear and aggressive behaviors in dogs treated with deslorelin compared with those surgically castrated found no differences between groups.34

When given to cats, deslorelin rendered them infertile, caused decreases in testicular volume and disappearance of penile spines.35 Cats that were implanted experienced decreased libido, decreases in mounting and mating behaviors, and decreased spraying, but food intake increased, a potential concern given the obesity epidemic in cats.


Source: DVM360 MAGAZINE,
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