This study was conducted under the supervision of Katherine Houpt, a leading animal behaviouralist from Cornell University, Ithaca, NY. It is the first study to follow early age desexed cats beyond 4.5 years of age. The title of the paper for those interested is;

Long-term risks and benefits of early age gonadectomy in cats.
Spain, Scarlett and Houpt
JAVMA 2004, 224:3;372-379

4.3 – 15.4 million cats are estimated to be euthanased per year in the United States! These numbers represent cats that were never adopted or adopted but relinquished later (namely for behavioural problems). The study was conducted to compare early age desexing with desexing at a traditional age in terms of retention in the adoptive household (>3 months from adoption) overall mortality rate, and incidence of common medical and behavioural problems via follow up for up to 11 years after surgery.

The retrospective cohort study was conducted using records obtained from the SPCA in Erie County, NY. Cats between the ages of 6 months and 12 months were eligible for the study and they were desexed prior to adoption (there were many other criteria used for eligibility but this requires more detailed description - something that I feel is not necessary for this post).

Adopters of sampled cats were contacted and sent a detailed questionnaire that contained 93 questions grouped into 5 sections;
(1) Whether the cat was still owned by the adopter, and if not what happened to the cat
(2) The cat’s behaviours and activities and, where appropriate, whether the owner considered the behaviour a serious problem.
(3) The cat’s medical history including medical problems, vaccinations, and diet
(4) Demographic information about the owner’s household
(5) Which vet administered care to the cat and supplied medical records.
47 outcomes were evaluated that included medical problems and behavioural problems (far too many to list here!). All of the outcomes were analysed and the appropriate statistical test used to determined significance.

Results

Medical outcomes
Among all cats, desexing prior to 5.5 months of age was associated with significantly reduced incidence of feline asthma and gingivitis when compared to those desexed between 5.5 and 12 months of age (this can be explained by the fact that androgens can transform immune function, so early desexing reduces the level of androgens and therefore reduces the incidence of these two inflammatory conditions). Early age desexing was also associated with lower incidence of abscesses among males and the statistical analyses showed that early desexing had a protective effect – that is, after 3.5 months of age, the benefit of early desexing diminished with time such that after 5-6 years, there was no meaningful benefit.

Medical outcomes not related to age of desexing included; overall mortality rate, relinquishment rate, obesity, FLUTD and urethral obstruction of male cats, arthritis, chronic gastroenteritis, chronic renal failure, diabetes, FeLV, FIP, FIV, long bone fractures, pneumonia, urolithiasis (bladder stones), and vaginitis.

Many vets will delay early castration because of concerns about urethral obstruction due to anatomical changes in the penis. This study, together with many others cited, found no association with early desexing and FLUTD, and if there was changes in penis size, then it did not lead to an increase in the incidence of FLUTD or urethral obstruction. They therefore concluded that this should not be used as a reason to delay early castration.

There are some studies that have found that early desexing causes a delay in the closure of the long bone growth plates, but in this study it was shown that if this occurs, then it does not contribute to long bone fractures (long bone fractures were rare in this study anyway and thus physeal fractures are not a problem for desexed cats in general!)

The article also briefly mentions other studies describing safe anaesthetic and surgical protocols for early age desexing. These studies indicate desexing at 7 weeks of age is associated with more rapid anaesthetic recovery and fewer perioperative complications when compared to those desexed at 7 months. The article also cites other studies that show most vets still express concerns about the safety of early desexing and they perceive risk to be at least one problem associated with the practice of early age desexing.

Behavioural outcomes
Desexing before 5.5 months of age was associated with decreased occurrence of hyperactivity and increased occurrence of shyness around strangers. In male cats, not females, early age desexing was also associated with reduced occurrence of aggression towards vets, sexual behaviours, and urine spraying. Interestingly amongst males only, there was an increased risk of hiding frequently. This was also one of the main reasons adopters relinquished their cats!