Key Findings
The model reveals that there is no single “best” strategy for dog population management; the effectiveness of each approach varies based on specific goals.
While culling (Lethal Control) can achieve the lowest population at certain capture rates, Female-only CNVR consistently provides a more durable reduction in population. However, if too few sterilization surgeries are completed, it may be completely ineffective in reducing the population.
Additionally, both Female-only and Male-inclusive CNVR reduce population turnover, leading to higher vaccination coverage and fewer puppies, which are often linked to dog bite incidents.
Summary
Free-roaming dogs present significant public health risks due to rabies transmission, cause environmental damage, and raise animal welfare concerns. Popular management techniques such as culling, vaccination, and catch-neuter-vaccinate-release (CNVR) have shown inconsistent results, highlighting the need for a comprehensive, flexible model to evaluate multiple control strategies under varied conditions.
In the study titled “Assessing multiple free-roaming dog control strategies in a flexible agent-based model,” the researchers proposed four hypotheses:
- CNVR strategies will outperform Lethal Control in achieving greater population reduction and longer population rebound times to pre-intervention levels.
- CNVR strategies will result in fewer young dogs in the population compared to Lethal Control.
- Longer intervention durations and higher capture rates will lead to greater population reductions in both CNVR and Lethal Control strategies, as more dogs are sterilized or culled. However, due to reduced reproductive capacity, CNVR interventions will maintain population reduction benefits longer post-intervention than Lethal Control for all evaluated durations.
- Vaccines that provide longer immunity will achieve higher maximum coverage in dog populations, as each vaccination will offer a longer period of protection for a given dog.
To confirm their hypotheses, the researchers used an open-source agent-based model. In this model, each individual dog is represented as an agent with unique survival, reproduction, and other life history characteristics. The researchers always began by allowing the model to “burn in,” meaning no intervention was made for a 10-year period. After this, the researchers tested and compared four interventions: Lethal control, Female-only CNVR, Male-inclusive CNVR, and Vaccine-only. It should be noted that under male-inclusive CNVR, both males and females are caught and vaccinated, but only females will be sterilized.
Each strategy was evaluated for its impact on several key metrics, including population reduction, total dog deaths, percentage of puppies, and maximum vaccination coverage. These metrics offered a comprehensive assessment of each strategy’s effectiveness in reducing and maintaining lower dog population sizes.
To ensure robustness and reliability, the process was repeated 25 times, yielding the following results:
I. Population Reduction
Over a 10-year period, with a monthly capture rate of 300 dogs, the study found significant differences in population reduction among the four interventions. Female-only CNVR resulted in the lowest mean population size (21,302 dogs), closely followed by Lethal Control (21,560 dogs), Male-inclusive CNVR (24,594 dogs), and Vaccine-only control (25,486 dogs). Interestingly, the difference between Male-inclusive CNVR and Vaccine-only was not statistically significant, indicating that the sterilization efforts in the Male-inclusive CNVR strategy were ineffective from a population reduction perspective.
II. Population Rebound
The researchers also evaluated the time required for the population to rebound once interventions ended. Female-only CNVR had the longest permanence of population control (PSPR) post-intervention, lasting 4.66 years before the population rebounded to 90% of its initial size. This was followed by Lethal Control at 2.10 years and Male-inclusive CNVR at 1.63 years.
III. Total Deaths During Intervention
In terms of total dog deaths during the intervention period, Female-only CNVR had the lowest number of deaths, followed by Male-inclusive CNVR, Lethal Control, and finally Vaccination-only. The researchers concluded that CNVR methods led to less suffering, as indicated by fewer deaths. Notably, the deaths under Lethal Control were lower than those under Vaccination-only. The researchers explained that reducing the population size through active culling also decreases the number of background natural deaths that tend to occur in large populations at carrying capacity.
IV. Puppy Percentage
Lethal Control produced the highest percentage of puppies, followed by Vaccination-only, Male-inclusive CNVR, and Female-only CNVR. The mean puppy percentages were 25.6%, 23.8%, 18.2%, and 15.6%, respectively. This is significant because many dog bites may be instigated by mothers perceiving a threat to their puppies, so fewer puppies should enhance public safety. Additionally, the suffering and high mortality rates of free-roaming puppies can be distressing for the public, making a lower percentage of puppies beneficial from both a safety and welfare perspective.
V. Vaccine Coverage and Vaccine Protection Duration
Female-only and Male-inclusive CNVR strategies resulted in higher maximum vaccination coverage compared to the Vaccination-only strategy, with average coverages of 27.2%, 27.5%, and 24.5%, respectively. This is due to the lower turnover in the population where CNVR is carried out. .
The study also tested two vaccine immunity durations: 12 months and 36 months. Longer vaccine protection periods resulted in higher maximum vaccination coverage and lower lapsed immunity percentages. This extended protection reduced the need for frequent re-vaccination, making the intervention more sustainable.
VI. Impact of Capture Rate and Population Size
The researchers also tested how varying capture rates — 100, 300, and 500 dogs per month — affected the results. They found that higher capture rates were inversely related to the mean population size at the end of the 10-year intervention. Capture rates of 100, 300, and 500 dogs per month resulted in mean population sizes of 27,265, 21,187, and 18,982, respectively. Notably, Lethal Control combined with a capture rate of 500 dogs per month resulted in the lowest population size (16,885 dogs), demonstrating that this combination was significantly more effective than others. This finding underscores the critical role of capture rate in population control strategies.
VII. Intervention Duration
Lastly, the study compared the effects of 5, 10, and 15-year intervention periods. The authors found that intervention duration alone does not affect the time it takes for the population to rebound to 90% of its previous level. They concluded that dog population control should be maintained permanently over time, rather than as a one-time, fixed-period approach.
However, regardless of the intervention period, the strategy of Female-only CNVR always resulted in longer-lasting population reduction benefits than Lethal Control. In other words, it takes longer for the population to return to 90% of its previous level with Female-only CNVR compared to Lethal Control, highlighting the effectiveness of the CNVR strategy in maintaining reduced population sizes over time.
This flexible agent-based model demonstrated that no single strategy universally outperformed the others. Factors such as community acceptance, logistical feasibility, and financial constraints play crucial roles in determining the most appropriate intervention. Female-only CNVR has a more long-lasting impact on the population, but depending on the capture rate, it may not always result in the lowest possible dog population. Both Female-only and Male-inclusive CNVR have the additional benefit of reducing population turnover, which helps maintain better vaccination coverage and reduces the number of puppies. Puppies are particularly concerning, as they are often associated with dog bite incidents and suffer from high mortality. Consequently, the number of deaths is also lower under both CNVR strategies compared to Lethal Control and Vaccination-only.
The capture rate of 300 dogs per month (roughly 1% of the starting population) used in the model is insufficient for vaccination coverage to reach an annaul 70% vaccination rate, the benchmark recommended by WHO to stop rabies transmission. This is an important consideration when deciding on the final strategy. The researchers also noted the potential for combining different strategies to enhance effectiveness and called for further studies to refine and optimize these combinations.
Miscellaneous
Data From Study:
Dog Population Management / Modelling / Flexible Agent Model (2023)
Year of Publication:
2023
External Link:
Yoak, A.J., Calinger, K.M. & Hiby, E. Assessing multiple free-roaming dog control strategies in a flexible agent-based model. Sci Rep 13, 19826 (2023).
https://doi.org/10.1038/s41598-023-47076-x