Study: High intensity sterilization of free-roaming cats reduces populations with least death of cats

Over a 10-year period, free-roaming cat populations managed using high-intensity Trap-Neuter-Return (TNR) experienced over 30 times fewer preventable cat deaths compared to taking no action, while reducing cat populations. Dr. Julie Levy, Fran Marino Endowed Professor of Shelter Medicine Education at UF, co-authored the study, which was published in the most recent issue of the peer-reviewed open-access journal Frontiers in Veterinary Science.

The authors simulated the impacts and associated costs of seven different management strategies for reducing cat populations and evaluated the long-term impact of different management strategies of deaths of cats and kittens during that reduction.

The publication is a collaborative effort of experts in wildlife conservation and cat protection. The working group was convened by the Alliance for Contraception in Cats & Dogs (ACC&D). According to Dr. John Boone, wildlife biologist and ACC&D Board Vice Chair, “The effectiveness of trap-neuter-return (TNR) programs often is debated, but less commonly is defined well. TNR groups most often track numbers of sterilizations performed and cats entering or euthanized in shelters as measures of effectiveness. These metrics are important, but they do not measure reduction in numbers of outdoor cats or illustrate how management translates into ‘lives saved’ in an outdoor cat population.”

In addition to population end points, the model estimated preventable deaths (cats killed by lethal management plus kittens dying before adulthood) occurring over a 10-year period, a metric that has not been evaluated thoroughly to date. Diverse management scenarios that were evaluated included TNR, removal and euthanasia, episodic culling, and taking no action. All active management scenarios were tested at both high- and low-intensity, using a published simulation model of FRC population dynamics (Miller et al., 2014) developed from empirical data.


The results may change the way we think about TNR efforts and cat welfare. The study’s main findings include:

  • With sufficient applied intensity, TNR can reduce preventable deaths drastically (up to 31 times less than a “no action” scenario) while also reducing population size
  • The ability of TNR to reduce preventable deaths and population size largely disappears when implemented at lower intensities, though individual treated cats benefit
  • Because cats are highly fecund, kitten deaths comprise a large majority of preventable deaths under all management scenarios, including removal where trapped cats are then euthanized
  • More than with any other option, TNR outcomes (especially population size reduction) can be improved by concurrent efforts to reduce abandonment and conduct non-lethal removals that emphasize adoption
  • Culling (waiting until populations rebound before conducting a removal effort) is likely to be ineffective for population management regardless of intensity, while the effectiveness of steady removal varies with intensity. High intensity
  • TNR offers a unique combination of maximum reduction in preventable deaths while also achieving moderate population size reduction

Study co-author and veterinary epidemiologist Margaret Slater of the ASPCA said, “If a TNR effort is sufficiently intensive to quickly suppress most reproduction, benefits compound into far fewer kitten deaths over time, along with gradual population size reduction. A lower-intensity effort – while helping individual cats – does not reach the thresholds necessary to achieve these greater benefits. We suggest front loading TNR efforts and adopting cats where possible to save lives while also reducing the number of outdoor cats.”

Boone JD, Miller PS, Briggs JR, et al. A Long-Term Lens: Cumulative Impacts of Free-Roaming Cat Management Strategy and Intensity on Preventable Cat Mortalities. Frontiers in Veterinary Science. 2019;6. doi:10.3389/fvets.2019.00238.