TNR and TVHR as methods to control nuisance from feral cats

Ireland, T., & Neilan, R. M. (2016). A spatial agent-based model of feral cats and analysis of population and nuisance controls. Ecological Modelling, 337, 123-136.

Free-roaming feral cats are common in areas of concentrated human habitation, and can pose considerable threats of nuisance and damage to native ecosystems. Trap-neuter-return (TNR) and trap-vasectomy-hysterectomy-return (TVHR) are two humane methods for the reproductive control of feral cat populations. Both TNR and TVHR render a cat infertile, but cats that have undergone TVHR continue to produce hormones that drive mating behaviors. We built a stochastic agent-based computational model for simulating the survival, reproduction, and movement of individual feral cats and the use of TNR and TVHR to modify cats’ reproductive abilities and behaviors. Daily movement of cats between colonies is implemented based on the distance between colonies and landscape properties (e.g. rural, urban). Spatially targeted TNR and TVHR policies are evaluated using two management goals: (1) reduce total population size and (2) reduce nuisance attributed to feral cats. Nuisance includes spraying and noise, both of which are associated with un-neutered males, as well as population abundance. Results indicate that both TNR and TVHR have the potential to greatly reduce population size. Effectiveness of each control depends on the capture rate, number of colonies targeted, size of each colony, and movement of individual cats between colonies. Results show that on average TVHR performs moderately better than TNR at reducing population size, but TNR substantially outperforms TVHR in reducing multiple nuisance measures.

Focusing on specific age classes in spay programs

Lancaster, E. P., Lenhart, S., Ojogbo, E. J., Rekant, S. I., Scott, J. R., Weimer, H., & New Jr, J. C. (2016). Modeling Interventions in the Owned Cat Population to Decrease Numbers, Knox County, TN. Journal of Applied Animal Welfare Science, 1-12.

To find management strategies for controlling the owned cat population in Knox County, TN, the authors formulated a mathematical model using biological properties of such nonhuman animals and spay actions on certain age classes. They constructed this discrete-time model to predict the future owned cat population in this county and to evaluate intervention strategies to surgically sterilize some proportion of the population. Using the predicted population size and the number of surgeries for specific scenarios, they showed that focusing on specific age classes can be an effective feature in spay programs.

Reply to McCarthy et al. (2013) on methods to control feral cats

Lepczyk, C. A., Conant, S., Duffy, D., Unit, P. C. S., Honolulu, H., Bird, D. M., & Calver, M. (2013). Feral cat management. Zoonoses Public Health, 60, 189-195.

Reply to McCarthy et al. (2013) 

TNR vs. TVHR to control feral cats

McCarthy, R. J., Levine, S. H., & Reed, J. M. (2013). Estimation of effectiveness of three methods of feral cat population control by use of a simulation model. Journal of the American Veterinary Medical Association,243(4), 502-511.

Objective—To predict effectiveness of 3 interventional methods of population control for feral cat colonies.

Design—Population model.

Sample—Estimates of vital data for feral cats.

Procedures—Data were gathered from the literature regarding the demography and mating behavior of feral cats. An individual-based stochastic simulation model was developed to evaluate the effectiveness of trap-neuter-release (TNR), lethal control, and trap-vasectomy-hysterectomy-release (TVHR) in decreasing the size of feral cat populations.

Results—TVHR outperformed both TNR and lethal control at all annual capture probabilities between 10% and 90%. Unless > 57% of cats were captured and neutered annually by TNR or removed by lethal control, there was minimal effect on population size. In contrast, with an annual capture rate of ≥ 35%, TVHR caused population size to decrease. An annual capture rate of 57% eliminated the modeled population in 4,000 days by use of TVHR, whereas > 82% was required for both TNR and lethal control. When the effect of fraction of adult cats neutered on kitten and young juvenile survival rate was included in the analysis, TNR performed progressively worse and could be counterproductive, such that population size increased, compared with no intervention at all.

Conclusions and Clinical Relevance—TVHR should be preferred over TNR for management of feral cats if decrease in population size is the goal. This model allowed for many factors related to the trapping program and cats to be varied and should be useful for determining the financial and person-effort commitments required to have a desired effect on a given feral cat population.



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TNR evaluation: demography, home range and potential zoonosis

Nutter, F. B. 2005. Evaluation of a trap-neuter-return management program for feral cat colonies: population dynamics, home ranges, and potentially zoonotic diseases. PhD dissertation, North Carolina State University. Raleigh, NC. 

Management of feral cats is controversial, and alternatives to lethal control methods are gaining popularity. To evaluate the effectiveness of sterilization programs, nine feral cat colonies were divided into groups of three, managed either by spaying females and castrating males, spaying females and vasectomizing males, or leaving all cats intact. Colonies were followed intensively for four years, and intermittently for three additional years. Most cats were trapped in fewer than ten trap nights each. Breeding females produced a mean of 1.4 litters/year and 3 kittens/litter. Kitten mortality was 75% by 6 months of age. Feral and pet domestic cats had similar baseline health status and prevalences of FIV, FeLV, Cryptosporidium, Giardia, and Toxocara cati, but feral cats had higher prevalences of Bartonalla henselae and Toxoplasma gondii. Castrated male and spayed female cats survived longer than intact male and female cats. Survival times of vasectomized males were equivalent to those of intact males. Control colonies decreased in size and remained stable in composition, while intact colonies increased in size and had high turnover. One neutered colony went extinct and several others had fewer than five cats at the end of the project. Home ranges of both intact and neutered cats were small, usually less than 1 ha. Vasectomized males had larger home ranges than either intact or castrated males, probably because they were searching for intact females. Community-level stakeholder meetings were successful in building consensus among groups, and a basic decision tree for feral cat management was developed. Computer simulation modeling using VORTEX software suggested that harvesting breeding colonies every one or two years at 50% to 100% can keep colonies small, but will not lead to long-term reductions in cat numbers. Models of neutered colonies suggested that 75% to 80% sterilization is necessary to cause population decrease and eventual extinction. The mean estimated time to extinction of 12.8 years fits well with ongoing observations of steady decline in sterilized colonies.