Comparison of growth in neutered Domestic Shorthair kittens with growth in sexually-intact cats

Abstract

The first aim of these studies was to compare growth patterns of healthy kittens neutered during growth with growth standards created for sexually-intact kittens. A second aim was to clarify the impact of neutering in kittens on body composition and body shape. Study 1 was a retrospective observational study comparing median growth trajectories of healthy, client-owned domestic shorthair (DSH) kittens in optimal body condition and neutered at different ages, with previously-created growth standards from a similar, sexually-intact, population. The neuter groups contained between 3.0k and 9.3k cats. For all neuter groups in both sexes, the median growth trajectory inclined upwards after the procedure, with this being more marked in female than in male kittens. This upwards inclination was less marked for kittens neutered later during growth in both sexes, with the effect being least in kittens neutered after 28-29 weeks. Study 2 was an analysis of new body composition and zoometric data from a previously-published randomised study, comparing growth-related measures between 11 pairs of sexually-intact and neutered (at 19 weeks age) female DSH cats in a research population. Before neutering, the growth pattern in neutered kittens and sexually-intact kittens was similar, but neutered kittens were heavier by 52 weeks (mean difference in fold change vs. 10 weeks 1.34, 95-CI: 1.07-1.72), had a greater fat mass (mean difference in fold change vs. 10 weeks 1.91, 95-CI 1.09-3.21) and greater lean mass (mean difference in fold change vs. 10 weeks 1.23, 95-CI: 1.03-1.48). Abdominal girth (mean difference in fold change vs. 10 weeks 1.20, 95-CI: 1.04-1.39) and rib cage length (mean difference in fold change vs. 10 weeks 1.18, 95-CI: 1.02-1.36) were also greater, but there were no differences in other zoometric measurements. Veterinarians should consider the potential impact that neutering has on gain of adipose tissue, especially early neutering in female kittens. Bodyweight should be monitored closely during growth and especially after neutering to prevent inappropriate weight gain.

Fig 1. Median male growth trajectory by neuter group on z-score scale.

Neuter Groups 1–4 represent, respectively, neutering ages of up to 20 weeks (0.7k cats), 20–23 weeks (2.1k cats), 23–28 weeks (2.5k cats) and >28 weeks (3.0k cats). Groups calculated from the lower quartile, median and upper quartile of ages at all neutering procedures performed on DSH cats between April 1994 and November 2016. The solid blue line represents the mean trajectory, whilst the blue-shaded ribbon represents the interquartile range. The grey shaded area represents the neutering age range for the group, and the solid grey vertical line shows the median observed neutering age. Grid lines represent the standard growth centiles, such that a growth trajectory following the centile curves would be horizonal in these plots. In all groups, there was an upwards inclination in growth trajectory, which was most marked in neuter groups 1 and 2.

Fig 2. Median female growth trajectory by neuter group on z-score scale.

Neuter Groups 1–4 represent, respectively, neutering ages of up to 21 weeks (1.0k cats), 21–25 weeks (2.7k cats), 25–29 weeks (3.0k cats) and >29 weeks (3.8k cats). Groups were calculated from the lower quartile, median and upper quartile of ages at all ovariohysterectomy procedures performed on DSH cats between April 1994 and November 2016. The solid blue line represents the mean trajectory, whilst the blue-shaded ribbon represents the interquartile range. The grey shaded area represents the neutering age range for the group, and the solid grey vertical line shows the median observed neutering age. Grid lines represent the standard growth centiles, such that a growth trajectory following the centile curves would be horizonal in these plots. In all groups, there was an upwards inclination in growth trajectory, which was marked in neuter groups 1–3, but only modest in neuter group 4.

Fig 3. Estimated change in z-score post-neutering for a cat at median weight.

Shown for different combinations of sex (female: Red; male: Blue) and neuter group, with 95% confidence intervals and Tukey post-hoc comparison groups. Neuter Groups 1–4 represent, respectively, neutering ages representing quartiles in the general neutered population–up to 20 weeks (448 cats), 20–23 weeks (451 cats), 23–28 weeks (507 cats) and >28 weeks (364 cats) for males, and up to 21 weeks (628 cats), 21–25 weeks (653 cats), 25–29 weeks (515 cats) and >29 weeks (497 cats) for females. The lower horizontal dotted line shows the point of zero change (meaning the cat remained on its initial centile line) and the upper horizontal dotted line represents an increase equal to one standard growth centile interval. The annotations give the Tukey post-hoc comparison groups, which indicate whether differences exist between estimated means for different combinations of neuter group and sex. Only comparisons between neuter groups of the same sex, and between sexes within the same neuter group were tested, and the groups within the relevant neuter group and relevant sex are as indicated in the annotations; within sets of means sharing the same neuter group or sex, those with different letters/numbers for that sex/group are significantly different at the 5% significance level. In all groups, there was a significant increase in z-score post neutering, and this was significantly larger in female than male.

Fig 4. Estimated means and simultaneous 95% credible intervals for each measure, for each group and timepoint, shown on the original scales.

The 11 sexually-intact cats are shown in red, whilst the 11 paired cats that were neutered are shown in blue; the grey dashed line indicates age of neutering. Panel A shows mass measurements, whilst and panels B and C show zoometric measurements. Weight, fat mass and lean mass measured by dual-energy X-ray absorptiometry (Hologic QDR-1000 W; Hologic, Inc., Waltham, MA, USA); height measured at the points of the scapula; ribcage measured at the deepest part of the thorax; length measured from manubrium of the sternum to a parallel point below the anus; girth measured around the narrowest point of the waist; forelimb bone measured from olecranon to carpus; elbow width, measured across the humeral condyles; hindlimb bone measured from patella to tarsus; chest depth measured mid-thorax from spine to the deepest part.

Fig 5. The estimated mean and 95% simultaneous credible intervals (difference between sexually-intact and neutered kittens (11 cats per group), in the fold-change for each measure, between 11 wk (when both groups were still sexually intact) and each subsequent timepoint.

Black dots represent the mean values, whilst error bars represent the 95% simultaneous credible intervals; the vertical grey dashed line indicates age of neutering. Differences are shown as the fold change of the neutered group divided by the fold change of the sexually-intact group, so that values >1 (shown by the horizontal dashed line) indicate that the neutered group had a greater difference in fold change. Panel A shows mass measurements, whilst and panels B and C show zoometric measurements. For details of the measurements see the legend for Fig 4.

Fig 6. Estimated mean fat mass (green) and lean mass (lilac) at the initial and final measurement points for the sexually-intact and neutered kittens (11 cats per group).

The intervals shown are simultaneous 95% credible intervals, whilst the green and lilac portions of each column represent fat and lean mass, respectively.