As communities across the U.S. consider how best to manage their population of unowned, free-roaming cats, the trap-neuter-return (TNR) method is receiving increased attention. The following analysis reviews the impact of TNR on free-roaming cat populations and animal shelters in a wide variety of contexts.
What is TNR? Why is it sometimes called TNVR?
TNR is a non-lethal technique for managing unowned, free-roaming cats. The cats are humanely trapped, spayed or neutered by a licensed veterinarian, ear-tipped (the universal sign that they have been sterilized), and returned to where they were trapped.
Many TNR programs also vaccinate cats against the rabies virus and three common viruses in cats (feline viral rhinotracheitis, calicivirus, and panleukopenia virus) through what’s known as the FVRCP vaccine. Programs that routinely vaccinate cats against rabies are sometimes referred to as trap-neuter-vaccinate-return (TNVR) programs, to emphasize this important public health component.
In addition, some TNR/TNVR programs will find homes for adoptable cats and kittens (when doing so is feasible) and coordinate with designated caregivers who provide ongoing care for the cats (e.g., food, water, shelter, veterinary care).
Are TNR programs effective?
A TNR program’s effectiveness can generally be measured in two ways: directly or indirectly. Direct measurement is more difficult because it involves a rigorous census before and after implementation, demonstrating a reduction in an area’s population of free-roaming cats (or at least a reduction in population growth).
Indirect measurement typically involves the use of an animal shelter’s intake data showing a reduction in the number of “stray” cats coming into the shelter from a specific area of the community where targeted TNR efforts have been implemented. Such reductions serve as an imperfect proxy for reductions in a community’s overall population of free-roaming cats.
Two key benefits of TNR
As the various peer-reviewed studies cited below demonstrate, TNR can be an effective method for both reducing feral cat populations and reducing the number of cats and kittens entering local animal shelters.
Reducing feral cat populations
Implemented with sufficient intensity (i.e., sterilizing enough cats in a given area), TNR has been shown to stabilize, reduce, and even eliminate free-roaming cat populations at a local level. Computer modeling suggests that population reductions are achievable if at least 30% of the unsterilized portion of a given population is sterilized every six months .
TNR has been shown to stabilize, reduce, and even eliminate free-roaming cat populations at a local level.
A detailed study of a TNR program in rural North Carolina revealed a 36% average reduction in six sterilized colonies over the first two years; the population of three unsterilized colonies increased by 47% over the same period . Additional reductions among the sterilized colonies were documented during four- and seven-year follow-up censuses . Studies conducted in more densely populated areas have yielded comparable results. In Chicago, Illinois, for example, a neighborhood TNR program (20 colonies) resulted in an average 54% decrease from initial population levels and an average 82% decrease from peak levels .
A TNR program at the University of Central Florida led to a population reduction of 66% over the first six years tracking data was available, from 68 to 23 cats . Over the next 17 years, the program further reduced the population of free-roaming cats by 57%, from 23 to 10 cats. As is often the case with TNR programs, adoption played a key role, with 92 of the 204 cats (45%) recorded on campus being adopted through the program .
In Key Largo, Florida, ongoing TNR efforts led to a 55% reduction in community cat numbers over 14 years, from 455 to 206 cats . TNR reduced and, after 17 years, eliminated, an estimated 300 cats from the Newburyport, Massachusetts, waterfront .
In Key Largo, Florida, ongoing TNR efforts led to a 55% reduction in community cat numbers over 14 years.
Since at least 2005, a TNR program has been in place on the campus of the University of KwaZulu-Natal’s Howard College (in Durban, South Africa), which is recognized as an “urban conservancy… interspersed with conservation-sensitive natural bush habitat and a nature reserve on the northern border” . Sterilization efforts led to a 38% reduction in the number of cats on campus (from 55 to 34) after four years [9,10].
Surveys of colony caregivers have also demonstrated significant population reductions. Caregivers in Rome, Italy, for example, reported a 22% decrease from initial population levels across the 103 colonies in their care (program duration: 2–6 years). Median colony size was reduced from 12 (range: 4–50) to 10 cats (range: 2–40) per colony . A similar survey in urban areas of Australia found a reduction in colony size from a median of 11.5 cats to 6.5 cats over 2.2 years .
Caregivers in Rome, Italy, reported a 22% decrease from initial population levels across the 103 colonies in their care.
A TNR program on the campus of a Louisiana hospital reduced the original population of free-roaming cats by 25% (from 40 cats to 30) and the overall population by 10% (due to the arrival of six new cats) over three years. No new litters of kittens were reported during this same time period. Prior to the program’s implementation, several cats had been removed annually but “a noticeable reduction in overall numbers was never achieved” .
Beginning in 2001, hysterectomy has been used to control the population of feral cats at the Rio de Janeiro zoo. Between 2001 and 2004, “the estimated population became stable, showing a trend to decrease” . Prior to the implementation of this sterilization program, cats had been on the property for 10 years and “resisted different efforts of population control” . (Note: TNR programs typically sterilize females cats via ovariohysterectomy; hysterectomy, by contrast, leaves female cats hormonally intact but unable to reproduce.)
Shelter intake reduction
Researchers in Alachua County, Florida, documented a 66% reduction in shelter intake of cats from a “target” zip code of focused TNR efforts; intake from the remainder of the county decreased 12% over the same two-year period . San José (California) Animal Care and Services observed a 14% reduction in feline intake four years after implementing its return-to-field (RTF) program .
Researchers in Alachua County, Florida, documented a 66% reduction in shelter intake of cats from a “target” zip code of focused TNR efforts.
Studies documenting the results of six large-scale, three-year shelter-based programs integrating TNR and RTF programs have found a median reduction of 32% (range: 1–45%) in feline intake.
Of particular interest was the 40% median reduction in the number of kittens entering the shelters involved. Although one shelter observed a 12% increase, the authors suggest that the general trend of decreased kitten intake is likely evidence of “an impact (the extent of which is, admittedly, unknown) on reproductive capacity in the [shelters’] service areas” [17,18].
Are stray or feral cats dangerous to humans and public health?
Wildlife impacts of free-roaming cats: Estimates vs. evidence
Cat health and welfare after TNR: Considerations and concerns
Miller, P. S.; Boone, J. D.; Briggs, J. R.; Lawler, D. F.; Levy, J. K.; Nutter, F. B.; Slater, M.; Zawistowski, S. Simulating Free-Roaming Cat Population Management Options in Open Demographic Environments. PLoS ONE 2014, 9 (11), e113553.
Stoskopf, M. K.; Nutter, F. B. Analyzing Approaches to Feral Cat Management—One Size Does Not Fit All. Journal of the American Veterinary Medical Association 2004, 225 (9), 1361–1364.
Nutter, F. B. Evaluation of a Trap-Neuter-Return Management Program for Feral Cat Colonies: Population Dynamics, Home Ranges, and Potentially Zoonotic Diseases, North Carolina State University, Raleigh, NC, 2005.
Spehar, D. D.; Wolf, P. J. A Case Study in Citizen Science: The Effectiveness of a Trap-Neuter-Return Program in a Chicago Neighborhood. Animals 2018, 7 (11).
Levy, J. K.; Gale, D. W.; Gale, L. A. Evaluation of the Effect of a Long-Term Trap-Neuter-Return and Adoption Program on a Free-Roaming Cat Population. Journal of the American Veterinary Medical Association 2003, 222 (1), 42–46.
Spehar, D. D.; Wolf, P. J. Back to School: An Updated Evaluation of the Effectiveness of a Long-Term Trap-Neuter-Return Program on a University’s Free-Roaming Cat Population. Animals 2019, 9 (10).
Kreisler, R. E.; Cornell, H. N.; Levy, J. K. Decrease in Population and Increase in Welfare of Community Cats in a Twenty-Three Year Trap-Neuter-Return Program in Key Largo, FL: The ORCAT Program. Frontiers in Veterinary Science 2019, 6 (7).
Spehar, D. D.; Wolf, P. J. An Examination of an Iconic Trap-Neuter-Return Program: The Newburyport, Massachusetts Case Study. Animals 2017, 7 (11).
Tennent, J.; Downs, C. T. Abundance and Home Ranges of Feral Cats in an Urban Conservancy Where There Is Supplemental Feeding: A Case Study from South Africa. African Zoology 2008, 43 (2), 218–229.
Jones, A. L.; Downs, C. T. Managing Feral Cats on a University’s Campuses: How Many Are There and Is Sterilization Having an Effect? Journal of Applied Animal Welfare Science 2011, 14 (4), 304–320.
Natoli, E.; Maragliano, L.; Cariola, G.; Faini, A.; Bonanni, R.; Cafazzo, S.; Fantini, C. Management of Feral Domestic Cats in the Urban Environment of Rome (Italy). Preventive Veterinary Medicine 2006, 77 (3–4), 180–185.
Tan, K.; Rand, J.; Morton, J. Trap-Neuter-Return Activities in Urban Stray Cat Colonies in Australia. Animals 2017, 7 (6), 46.
Zaunbrecher, K. I.; Smith, R. E. Neutering of Feral Cats as an Alternative to Eradication Programs. Journal of the American Veterinary Medical Association 1993, 203 (3), 449–452.
Mendes-de-Almeida, F.; Faria, M. C. F.; Landau-Remy, G.; Branco, A. S.; Barata, P.; Chame, M.; Pereira, M. J. S.; Labarthe, N. The Impact of Hysterectomy in an Urban Colony of Domestic Cats (Felis Catus Linnaeus, 1758). International Journal of Applied Research in Veterinary Medicine 2006, 4 (2), 134–141.
Levy, J. K.; Isaza, N. M.; Scott, K. C. Effect of High-Impact Targeted Trap-Neuter-Return and Adoption of Community Cats on Cat Intake to a Shelter. The Veterinary Journal 2014, 201 (3), 269–274.
Edinboro, C. H.; Watson, H. N.; Fairbrother, A. Association between a Shelter-Neuter-Return Program and Cat Health at a Large Municipal Animal Shelter. Journal of the American Veterinary Medical Association 2016, 248 (3), 298–308.
Spehar, D. D.; Wolf, P. J. The Impact of an Integrated Program of Return-to-Field and Targeted Trap-Neuter-Return on Feline Intake and Euthanasia at a Municipal Animal Shelter. Animals 2018, 8 (4).
Spehar, D. D.; Wolf, P. J. Integrated Return-to-Field and Targeted Trap-Neuter-Vaccinate-Return Programs Result in Reductions of Feline Intake and Euthanasia at Six Municipal Animal Shelters. Frontiers in Veterinary Science 2019, 6 (77).