Research Article: Modeling the effect of surgical sterilization on owned dog population size in Villa de Tezontepec, Hidalgo, Mexico, using an individual-based computer simulation model

Date Published: June 1, 2018

Publisher: Public Library of Science

Author(s): Luz Maria Kisiel, Andria Jones-Bitton, Jan M. Sargeant, Jason B. Coe, D. T. Tyler Flockhart, Erick J. Canales Vargas, Amy L. Greer, Carlos E. Ambrósio.


Surgical sterilization programs for dogs have been proposed as interventions to control dog population size. Models can be used to help identify the long-term impact of reproduction control interventions for dogs. The objective of this study was to determine the projected impact of surgical sterilization interventions on the owned dog population size in Villa de Tezontepec, Hidalgo, Mexico. A stochastic, individual-based simulation model was constructed and parameterized using a combination of empirical data collected on the demographics of owned dogs in Villa de Tezontepec and data available from the peer-reviewed literature. Model outcomes were assessed using a 20-year time horizon. The model was used to examine: the effect of surgical sterilization strategies focused on: 1) dogs of any age and sex, 2) female dogs of any age, 3) young dogs (i.e., not yet reached sexual maturity) of any sex, and 4) young, female dogs. Model outcomes suggested that as surgical capacity increases from 21 to 84 surgeries/month, (8.6% to 34.5% annual sterilization) for dogs of any age, the mean dog population size after 20 years was reduced between 14% and 79% compared to the base case scenario (i.e. in the absence of intervention). Surgical sterilization interventions focused only on young dogs of any sex yielded greater reductions (81% – 90%) in the mean population size, depending on the level of surgical capacity. More focused sterilization targeted at female dogs of any age, resulted in reductions that were similar to focusing on mixed sex sterilization of only young dogs (82% – 92%). The greatest mean reduction in population size (90% – 91%) was associated with sterilization of only young, female dogs. Our model suggests that targeting sterilization to young females could enhance the efficacy of existing surgical dog population control interventions in this location, without investing extra resources.

Partial Text

The overabundance of dogs in developing countries poses significant public health, animal health, and animal welfare concerns [1–3]. Reproduction control is one of several methods than can be used for controlling the growth of dog populations [3]. Surgical sterilization is the most common type of dog reproduction control and remains the most frequently performed pet contraception procedure in veterinary practice [4]. Surgical sterilization can not only help to limit the increase of the dog population by preventing the birth of unwanted puppies [3], but offers other benefits including the prevention of some canine diseases such as mammary neoplasia or benign prostatic hyperplasia [4–8]. Furthermore, surgical sterilization can have an impact in dog’s unwanted sexual behaviors such as a reduction in roaming, mounting, urine marking, and sexual aggression, when performed in young animals (6–16 weeks) [7]. While surgical sterilization is commonly used in developed countries, the high cost and limited resources (including veterinary surgeons) in many developing countries makes surgical sterilization an infeasible method of population control for large-scale application [9].

The control of dog reproduction is one of several measures that can be used to control the size of dog populations [3]. Surgical sterilization is the most commonly used method of pet contraception [4]. Several mathematical models have been developed to evaluate the effect of sterilization programs for both owned and free-roaming dog populations in different parts of the world, including Italy [14], Brazil [15–17], and India [18–19]. No models have been developed to evaluate the effect of ongoing surgical sterilization programs aimed at owned dogs in Mexico. Currently, the government sterilization program in Villa de Tezontepec has a maximum surgical capacity of 21 surgeries per month (equivalent to the sterilization of 8.6% of the current owned dog population per year) and focuses on dogs of mixed ages and sexes. Our model suggests that the long-term deployment of the mixed age surgical sterilization intervention at the lowest surgical capacity examined (in line with the existing program), combined with the current proportion of owned dogs that are confined (45%), and an initial population of sterilized dogs (37% females and 14% males), would only reduce the mean dog population by approximately 14.4% after 20 years. Our baseline model projections are comparable to those reported by Baquero et al. (2016) [15], where an annual probability of sterilization of 12% and 8% for females and males resulted in a 17% reduction in the owned dog population reduction after 30 years. We used our model to examine the impact of modifying the existing surgical capacity, as well as the ages and sexes of the dogs receiving the intervention. Our findings suggest that if the number of sterilizations were to double (42 surgeries/month: 18.7% of the owned dog population sterilized annually), the owned dog population could decrease by 46.7% in the same 20-year time period. However, such surgical increases may be difficult to sustain over the long term for a single region, due to the costs and required program resources.

All models are simplifications of reality and therefore, as with any model-based analysis, ours has limitations. This model only considered the owned dog population in this region and did not consider stray (non-owned, free-roaming dogs) or feral (dogs living in a “wild and free state” without intentional or direct food or shelter provided by people) dog populations [33]. Therefore, this model only describes population dynamics for a subset of all dogs within this community. Consequently, the results of these model simulations do not reflect the total impact of the interventions on the overall dog population size for this community. Current surgical sterilization interventions in this region are exclusively targeted at owned dogs and the outcome of this model can help inform current programs for the owned dog population in this region. For this reason, we believe it was appropriate to model only the owned dog population to evaluate the effect of surgical sterilization interventions in this subset of the dog population. For this model, we assumed that all dogs that were unconfined (completely or partially allowed to roam), within their age group, had equal risk of non-age-related mortality, and that all unconfined female dogs had the same pregnancy risk; however, in reality, dogs allowed to roam all day might have higher risk of mortality and pregnancy than those that are allowed to roam only part of the day. In addition, it was assumed that all dogs that immigrated to the hypothetical population did so as puppies (less than 8 weeks old). Even though empirical data from this region suggests that this was a reasonable assumption, this may have influenced the effectiveness of interventions being studied. This model did not consider the growth of the human population in the 20-year period, which could influence the community capacity used for this model. One of the implications of capping the owned dog population size in the model (i.e. establishing a community capacity) was not being able to detect the growth of the owned dog population over time beyond the community capacity, both in the absence and presence of dog population control interventions (i.e. surgical sterilization control). Another limitation of this model is that it was not spatially specific; therefore, we do not know if the effect of the interventions varies by neighbourhood. We also assumed that participation in the sterilization intervention was equally distributed across region. In reality, heterogeneous socioeconomic levels could impact the participation of dogs in the subsidized sterilization program and the enrolment might not be evenly distributed in Villa de Tezontepec. Lastly, another considerable limitation of our model is that it did not take in account the owners’ decision making, such as the willingness to breed their dogs or to participate in dog population control initiatives. Owned dog populations and their management are highly influenced by owner decision making. For example, dog owners that breed their dogs for profit would be less likely to participate dog reproduction control interventions; this was not considered in the model. This model could be expanded to incorporate the free-roaming, non-owned dog population to evaluate population control methods for the total dog population in this location, if appropriate data resources were to become available.

Controlling the growth of the owned dog population is very important for the sustainability of the federally subsidized rabies vaccination program in Mexico [10]. Our model findings suggest that the sterilization of young, sexually immature dogs, especially females, could be more successful at reducing the owned dog population size over a 20-year time horizon compared to the current strategy of sterilization focused on dogs of any age and sex. Reducing the proportion of owned female dogs becoming pregnant also had a significant effect on the overall owned dog population size, suggesting that if owners were discouraged from breeding their female dogs, this alone could reduce the owned dog population size. Computer simulation models can help governments and other decision-makers explore options for optimizing the limited resources allocated for dog population management programs.




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