According to the latest and the most extensive medical studies:
-Mammary tumors are 3 times more common at a female dog, than at a woman. Of those tumors, 53% are malignant at dogs, and 90% are malignant at cats.
-Female dogs which are sterilized before they are able to breed, have a 200 times lower risk to have breast cancer(7-9 months) ,and the ones which are sterilized after the first time they are able to breed have a 12,5 times lower risk than unneutered female dogs.
(source: The Merck Veterinary Manual, Eighth Edition, 2003, Whitehouse Station, NJ, USA)
Moreover,the ovariohysterectomy (the elimination of the ovaries and uterus) completely eliminates the risk of ovarian cancer or of pyometra (pus in the uterus). Pyometra is a very common disease common at unsterilized dog females, it’s difficult to diagnose in its early stages and fatal at the final stage.
In conclusion, it’s best for the health of any female(cat or dog) to be sterilized between 3 and 6 months(after immunization and before the first heat).
Recent studies recommend sterilizing males between same age.
Early sterilization doesn’t have adverse health effects .Side effects of sterilizing prepubescent are minor: external genital organs reduce their size; growth of long bones sometimes lasts several months.
Important advantages of sterilization are:
-at neutered tomcats, reducing the penile bulb makes the risk of urethral blockage to be lower than at non-castrated tomcats.
-neutered male dogs aren’t likely to have prostate disease, occurring especially in old age.
-neutered males are protected from the risk of testicular cancer.
-there is a statistically proven correlation between being sterilized animals and a low level of aggression.
It is true that sterilization itself does not automatically solve certain problems of an animal behavior: they are treated with training and education effort. But it is certainly statistically verified that incidents of dogs biting people involve in 6 cases out of 10 unsterilized dogs(See, for example, http://www.dogbitelaw.com/PAGES/statistics.html)
Statistically, castrated animals live longer than unsterilized ones.
Reproduced from www.ortovet.ro with the agreement of mister Liviu Gaita.