Sterilization Campaigns Linked to Falling Free-Roaming Dog Numbers in Thailand

Thailand is home to an estimated 9.91 million dogs, most of them free-roaming, and managing their numbers has long been a source of tension between animal welfare goals, public health priorities and everyday community life. Researchers from the University of Glasgow, working with the Thai non-governmental organization Jai Dog Rescue, tracked dog numbers, sterilization coverage, puppy prevalence and community perceptions over several years in the province of Nakhon Nayok. Their conclusion is striking: a comparatively modest but well-targeted sterilization effort was associated with measurable and sustained declines in free-roaming dogs, changes large enough that local residents noticed them on their own streets.

The programme at the heart of the study is built around Catch-Neuter-Vaccinate-Return, or CNVR, a widely used approach in which unowned or community-owned dogs are trapped, surgically sterilized, vaccinated and returned to the exact location where they were found. Jai Dog Rescue has operated in Nakhon Nayok since 2021, combining CNVR campaigns with sterilization of owned dogs, community education, rescue and rehabilitation, and support for local dog carers. The organization’s strategy relies on an intense pulse of activity: mobile clinics are stationed in a subdistrict until dog catchers report that roughly 80 percent of accessible free-roaming dogs have been sterilized, after which the clinic moves on.

A permanent clinic at the organization’s headquarters then absorbs a slower trickle of dogs brought in by community members and catchers, maintaining coverage after the initial campaign ends. To measure whether this effort actually changed the population, the researchers relied on annual transect surveys conducted by Jai Dog staff between October 2021 and September 2025 in two subdistricts, Khok Kruat and Tha Chang. The province was divided into walkable polygons of roughly 2.5 to 5 square kilometres, and a subset was randomly selected for repeated monitoring. Staff walked, cycled or drove the same routes each year, recording every dog seen into a mobile application, along with its age, sex, sterilization status, health and indicators of ownership. Sterilization status was determined visually through ear notches or, in males, the absence of gonads. The analysis ultimately included 549 dog observations, split almost evenly between the two subdistricts, with 54 percent of dogs showing signs of ownership and 46 percent categorized as unowned.

The statistical analysis used generalized linear mixed models to test whether sterilization effort predicted changes in four outcomes: dogs observed per kilometre, the probability that an individual dog was sterilized, the presence of lactating females and puppies, and visible health status. The results were consistent across all three ways of quantifying effort. Dogs observed per kilometre declined significantly as sterilization effort increased. At the peak of the intervention, the model predicted 10.35 dogs per kilometre in Khok Kruat and 2.95 in Tha Chang; four years later those figures had fallen to 5.22 and 1.49 respectively. The probability that a sighted dog was a puppy also declined significantly with time since the intervention peak, dropping from 0.03 in Khok Kruat and 0.10 in Tha Chang before the campaign to 0.003 and 0.01 three years afterwards.

Perhaps the most operationally interesting finding concerns the timing of sterilizations. When effort was broken down year by year, the number of sterilizations conducted one year before a survey showed the strongest association with the probability that an observed dog was sterilized, followed by the counts from three and two years prior. With the highest observed effort three years earlier but nothing since, the modelled probability of a female dog being sterilized was 0.52; with the highest effort just one year earlier, it rose to 0.79. This compounding effect suggests that the initial high-intensity pulse combined with ongoing maintenance sterilizations kept coverage stable, with no rebound in the dog population observed during the years of monitoring. Female dogs were consistently more likely to be sterilized than males, echoing earlier Thai surveys in which owners often believe male dogs do not require neutering.

The transect data were triangulated with a one-time Knowledge, Attitudes and Practices survey administered in 2025 to 316 households across two districts: Pak Phli, where Jai Dog had been working since 2022, and Ongkharak, a control district where the organization had not operated. Of the 330 households approached, 316 complete responses were included, a non-response rate of 4.24 percent. The two districts appeared demographically similar, with comparable rates of dog ownership, around 55 percent overall, and similar religious and gender composition. Respondents in the intervention district reported an average of 2.8 dogs on their street, or soi, compared with 6.1 in the control district, an unadjusted difference of 3.31 dogs that was statistically significant. Residents of the intervention area were also more likely to say that dog numbers had decreased or stayed the same since before the COVID-19 lockdowns, while control-district respondents more often reported stable or increasing numbers.

Perceptions of the dogs themselves differed just as sharply. In the intervention district, most respondents were neutral or positive about the number of free-roaming dogs on their street, whereas in the control district most were neutral or negative, with fewer than 20 percent expressing positive views. Reports of nuisance behaviours, led across both districts by barking, garbage-raiding and dog fights, were lower where Jai Dog worked, and dog bites were reported by 1.2 percent of intervention households compared with 9.2 percent of control households. Owned dogs in the intervention district were also significantly more likely to be sterilized, at 71 percent versus 45 percent in the control district, and 73.2 percent of intervention respondents preferred humane management options such as rehoming, vaccination and return, or CNVR, compared with 50.7 percent in the control district.

Not every indicator moved. The analysis found no significant effect of sterilization effort on the presence of lactating females or on the probability that a dog appeared healthy. The authors attribute this largely to sample size: only nine lactating females and 39 visibly sick dogs appeared in the entire dataset, too few to detect change. The health measure itself was also coarse, a binary judgement of healthy or unhealthy based on visual assessment from a distance, and baseline levels of visible illness were low. The puppy finding, while statistically significant in the full dataset, lost significance when second-day observations were removed, though sensitivity analyses including Firth logistic regression and leave-one-out tests showed the effect estimate remained stable, suggesting a genuine trend that larger samples would need to confirm.

The study’s design has clear strengths and acknowledged limitations. It offers multi-year data spanning the period before and after intervention, an intervention-control comparison, and the combination of objective transect counts with community perceptions, an approach the authors describe as complementary and directionally consistent. However, the KAP analysis was exploratory and unadjusted, so between-district differences cannot be interpreted as independent effects of the programme, and baseline differences between the subdistricts, including higher human density in Khok Kruat at 231 people per square kilometre versus 198 in Tha Chang, complicate direct comparisons. Sterilization numbers were divided by human population rather than dog population, an assumption the authors flag, and the transect findings apply at the subdistrict level. The study also did not examine rabies transmission directly, since rabies cases in the province are rare, although all dogs sterilized at the clinics were ear-notched and vaccinated, meaning transect counts double as a rough index of dogs receiving at least one vaccine dose. Reaching vaccination coverage above 70 percent remains the best-established way to curb rabies transmission, and sterilization reduces population turnover, helping maintain such coverage.

For the wider field of dog population management, the practical implications may be as valuable as the scientific ones. The authors show that a single day of transect observation per polygon per year was sufficient to detect significant declines in dog density and increases in sterilization coverage, a finding that could substantially reduce the cost of monitoring for other organizations, though it would not support mark-resight analyses that estimate absolute population size. The work also fills a documented gap in evaluations of dog population management in Thailand beyond Bangkok, and demonstrates that impact can be achieved with comparatively low cumulative effort, whereas much of the existing literature reports changes only after two to five years of intensive regular campaigns. The authors are careful to note that the observational design does not establish causality, but the coherence of the evidence, from declining transect counts to falling puppy prevalence to residents’ own accounts of quieter streets and fewer bites, paints a consistent picture. Sustained sterilization coverage, they conclude, offers benefits for both human and dog welfare, and simple, repeatable monitoring can prove it.

Subject of Research: Impacts of dog sterilization campaigns on free-roaming dog population dynamics in Thailand

Article Title: The impacts of dog sterilization campaigns in Thailand: understanding changes in dog population dynamics

Article References: Slote, B., Lau, A., Dower, M., Rees, E. M., Virhia, J., Chng, N. R., Hiby, E., Hampson, K., & Ferguson, E. A. (2026). The impacts of dog sterilization campaigns in Thailand: understanding changes in dog population dynamics. Frontiers in Veterinary Science, 13, Article 1926523. https://doi.org/10.3389/fvets.2026.1926523

Image Credits: AI Generated

DOI: 10.3389/fvets.2026.1926523

Keywords: free-roaming dogs, dog population management, CNVR, sterilization, Thailand, rabies, animal welfare, transect surveys, KAP survey, One Health, Nakhon Nayok, Jai Dog Rescue