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ACC&D position statement:
Modeling study advocating use of vasectomies and hysterectomies in feral cat management

To compare alternative methods of reducing outdoor cat abundance, the Alliance for Contraception in Cats & Dogs (ACC&D) recently convened a team with broad-ranging expertise to develop a stochastic simulation model for free-roaming cats. Our preliminary results and resulting recommendations were presented at ACC&D’s 5th International Symposium in June 2013, and a full treatment of our findings is currently being prepared for publication. 

Given its involvement in this project, ACC&D has received inquiries about the recently published study "Estimation of effectiveness of three methods of feral cat population control by use of a simulation model" by Robert J. McCarthy, DVM, MS, DACVS; Stephen H. Levine, PhD; and J. Michael Reed, PhD in the August 15, 2013 issue of the Journal of the American Veterinary Medical Association (JAVMA) (vol. 243, no. 4, pp. 502-11). These authors developed a different simulation model to argue for the use of vasectomy vs. castration for male cats and hysterectomy vs. ovariohysterectomy for female cats (i.e. TVHR vs. traditional TNR). They concluded that TVHR is far more effective than TNR in reducing population size, and that TNR could in fact be counterproductive and lead to population growth in some circumstances. 

ACC&D recognizes that free-roaming cat population dynamics are complex, and believes that our overall understanding will benefit from the insights and perspectives of multiple groups of investigators with different approaches. That said, we also note that some of the results presented by McCarthy et al. are not consistent with results obtained in our own modeling study. More specifically, we found that if TNR is performed with sufficient intensity and for a sufficient duration, it can be effective in reducing population size, as long as dispersal (newly abandoned cats or other cats immigrating) into the treated population does not exceed a defined threshold level. Reasons for this discrepancy in results could include differences in the selection of model parameters, and the adoption of different assumptions in the absence of definitive data, particularly with regard to the behavioral impacts of TVHR vs. TNR and their relevance to population-level fecundity. 

The evidence for the success of TVHR in this study depends on assumptions related to the reproductive physiology and behavioral ecology of domestic cats that, in our view, do not seem to be supported by available empirical evidence and remain to be better understood. Our committee also felt it was an important consideration that TVHR would likely not address nuisance behaviors, a common reason residents are intolerant of cats and population management efforts are undertaken, and that results seemed to be achieved in this model at the expense of cat welfare. 

Model results should not be regarded as definitive conclusions, but rather as “structured projections” that can be used to suggest high-priority research needs and assist in the creation of management plans. We look forward to productive discussions, collaborations, and field trials designed to better clarify these critically important issues. Determining what intervention(s) is most effective in reducing free-roaming cat populations is important from the perspectives of both humane population control and optimal use of limited resources.

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