Journal Scan: Long-term strategies to manage free-roaming cats

Ian Dyball/stock.adobe.comTrap-neuter-return (TNR) is commonly used to manage free-roaming cat populations. Through sterilization, vaccination and other aspects of care, TNR programs help to mitigate common issues associated with unowned outdoor cats, including shelter crowding, predation on wildlife, disease transmission and high mortality. Although lethal management methods, including culling, are sometimes employed as alternatives to TNR, these methods can be controversial.

In a recent study, the Alliance for Contraception in Cats & Dogs compared various methods of feline population control by focusing both on long-term population reduction and animal welfare concerns. Their findings were published in Frontiers in Veterinary Science.

Why they did it

Most TNR programs gauge their effectiveness by the number of cats sterilized over time. Also important to consider, according to the study, are the birth and death rates in a given population of cats, as well as the recruitment rate of cats into the population from surrounding areas. Because existing studies on free-roaming cats typically examine short-term implications of various management strategies, the study authors chose instead to focus on long-term effects observed after several years of continuous population management.

What they did

The authors used computer modeling software to estimate how various population management strategies affected free-roaming cat populations over a simulated 10-year period.

The tested scenarios included:

taking no action  

low- or high-intensity removal of 25% or 50% of cats, respectively, every 6 months via trapping and euthanasia

low- or high-intensity episodic culling of 25% or 50% of cats, respectively, whenever the population reached carrying capacity

low- or high-intensity TNR of 25% or 75% of intact cats, respectively, every six months

For each management strategy, the investigators recorded the number of cats removed or sterilized, number of kitten births and number of deaths from natural and preventable causes. Preventable deaths were defined as cats removed through lethal management plus kittens failing to survive beyond six months of age. The final population size after 10 years of management was determined, as well as the cumulative number of preventable deaths during that time period.

The software program also estimated recruitment rates of cats from surrounding areas. The authors started with a simulated population size of 50 cats and a surrounding neighborhood population of 200 cats, then repeated the simulation with increasing population sizes to test whether results were scalable.

What they found

Management strategies had profound effects on both population size and mortality rates within the simulated cat populations, and the major findings held true for starting population sizes ranging from 50 to 5,000 cats.

Final population size after 10 years was highest in the scenario where no management action was taken. Episodic culling techniques reduced the population size slightly after 10 years, suggesting this method alone is relatively ineffective at population reduction. High-intensity TNR and low-intensity removal methods each reduced the population by half over 10 years, whereas high-intensity removal caused the greatest population reduction of all methods (86% reduction after 10 years).   

Lack of management resulted in a higher rate of preventable deaths than all other scenarios, including those involving lethal removal techniques. With no management employed, a starting population of 50 cats produced an estimated 1,145 kittens over 10 years, 1,001 of which died before reaching adulthood. Kitten survival rates were low for all management scenarios, ranging from 13% to 25%, a likely reflection of the many stressors that free-roaming kittens face, including competition for resources and risk of predation.

When compared with taking no action, high-intensity TNR reduced the number of kitten births and deaths by 97% over 10 years, making it the best option for suppressing reproduction. By comparison, low-intensity TNR and high-intensity removal methods each reduced the number of preventable deaths by around 75%. However, the number of kitten births and deaths was higher with these two scenarios than with high-intensity TNR.

Interestingly, of all the examined methods, high-intensity TNR saw the highest rate of cat recruitment from surrounding neighborhoods. Based on this finding, the authors suggested that the benefits of TNR can be optimized by including methods to reduce abandonment, such as adoption.

The authors acknowledged that simulation methods such as those used in this study “are approximations of reality.” However, the examined parameters and generated data were consistent with those from several studies examining actual cat populations.

Take-home points

This study illustrated how TNR and other population management efforts affect not only the existing population, but also multiple future generations of cats. Based on their findings, the authors recommended use of high-intensity sterilization as the most effective tool for combined population reduction and prevention of future kitten deaths. While other methods, including low-intensity sterilization and lethal removal, were moderately to highly effective at reducing populations, the high estimated rate of preventable deaths (consisting of kittens, euthanized cats or both) made these methods less efficient, according to the authors.  

They concluded by stating, “We cannot maximize our prospective goals (i.e. saving lives tomorrow) by focusing only on maximizing our proximate goals (i.e. saving lives today).”

Reference

1. Boone JD, Miller PS, Briggs JR, Benka VAW, Lawler DF, Slater M, Levy JK, Zawistowski S. A long-term lens: cumulative impacts of free-roaming cat management strategy and intensity on preventable cat mortalities. Front Vet Sci. 2019;6(238):1-9.

Dr. Stilwell provides freelance medical writing and aquatic veterinary consulting services through her business, Seastar Communications and Consulting. In addition to her DVM, obtained from Auburn University, she holds a MS in fisheries and aquatic sciences and a PhD in veterinary medical sciences from the University of Florida.