DISCUSSION

Novel aspects of the study

The novelty of this study lies in the fact that it represents the longest (>5 years) CNVR study conducted in a culturally challenging religious city in Asia and one of the few studies that simultaneously evaluated owned and free-roaming populations, including community, stray, and feral dogs, as well as both sexes. Furthermore, this study introduces and validates the concept of using a buffer zone to reduce dumping pressure, which is novel in the context of domestic dog population management despite being widely adopted in epidemiology and wildlife management, including wild dog populations. The study also investigated the relationship between the number of dogs per household in low-income communities and the likelihood of sterilization and regular ARV.

Shifts in counts and sex ratios of owned and community dogs from 2021 to 2025 and the increasing willingness of low-income communities to adopt female dogs

Using capture effort and clinical intake as proxies provides only limited insights into the actual dog population within the Anuradhapura Municipality. However, based on the nationally accepted dog:human ratio of 1:8 and the current human population of 63,276, the estimated dog population is approximately 7910, suggesting that the observations reasonably represent the local context.

Table 10 summarizes the annual numbers of sterilized dogs from 2020 to 2025 as percentages of the estimated population.

Year | Sterilized dog count | Percentage
2020 | 669 | 8.46%
2021 | 2716 | 34.34%
2022 | 1953 | 24.69%
2023 | 1085 | 13.72%
2024 | 1376 | 17.39%
2025 | 1139 | 14.40%

The cumulative number of sterilized dogs over the 6-year period, which included only sexually mature adults, reached 10,334, exceeding the estimated population by 2424 animals (30.64%). When younger age groups are considered, the true population is likely to exceed this estimate. Given the high dog density observed before and during the initial stages of the project, it is probable that the actual dog:human ratio in Anuradhapura was considerably higher than the national estimate. This discrepancy highlights the need for region-specific population estimates when planning dog population management programs.

Although total dog counts declined over time, except for a sudden increase of 21.1% in 2023, no significant temporal trend was observed after adjustment for differences in the number of catching days. It should be emphasized, however, that the reduction in the number of catching days was intentionally implemented by project management in response to the apparent decline in free-roaming dog numbers. Consequently, including the number of days as an offset could partially mask genuine population reductions. Conversely, the absence of a significant increase over a 5-year period itself suggests successful intervention, because the absence of sterilization would normally result in annual population growth. Previous studies have estimated annual increases ranging from 2.5% over 20 years at a 30% female sterilization rate [20] to 8% over 20 years at a 45% female sterilization rate [39]. Although these estimates originate from temperate regions and are highly context dependent, population growth in tropical countries such as Sri Lanka is likely to be higher due to more frequent estrous cycles. The slight declines observed in community dogs (3.7%), community males (4.7%), and community females (3.1%) were not statistically significant. In contrast, the marginal increase observed in owned dogs (14.7%) suggests improving responsible pet ownership. Interestingly, female ownership increased significantly by 24.1%, whereas the total male population declined by 9.7%.

These sex-specific changes were also reflected in the changing sex ratios. The overall male:female ratio declined significantly, whereas no significant changes were observed in either the owned or community categories. This apparent contradiction may represent Simpson's paradox, suggesting that the overall shift was generalized rather than driven by a specific subgroup. Because many owned male dogs roam freely without collars, some males classified as free-roaming during capture may actually have had owners.

Although the exact causes of the decline in male numbers remain unclear, a decreasing preference for adopting male puppies may have contributed. Male migration patterns, discussed later in relation to the buffer zone, may also have influenced male abundance within the municipality. In contrast, the significant 24.1% increase in the number of owned females strongly suggests increased female adoption. Historically, female puppies were more likely to be abandoned and less likely to be adopted, resulting in higher mortality caused by poor welfare and road accidents. Since the cost of sterilization currently exceeds 5000 LKR (~16 USD), many low-income households cannot afford these procedures. Because owners of female dogs bear the financial burden associated with unwanted litters, male dogs have traditionally been preferred. In addition, perceptions that intact males are better guard dogs and cultural beliefs among Sri Lankan Buddhists that sterilization is sinful and may affect fertility in the next life may contribute to resistance against male sterilization.

This preference for males remains evident from the survey findings. Within the municipality, there were 518 owned males and 491 owned females, whereas outside the municipality, there were 376 owned males and 287 owned females. Furthermore, only approximately 42% of adult males were sterilized in both regions, whereas the percentage of sterilized adult females exceeded 78%. The first author recalls numerous occasions when owners refused to sterilize male dogs because they wished to preserve natural sexual behavior and were unconcerned about unwanted pregnancies. Nevertheless, the contribution of intact owned males to reproduction among free-roaming females remains unknown.

The demographic changes observed were reflected in the shift in the male:female ratio from 1:2.1 to 1:3.6. Arguably, the 1:0.95 sex ratio among owned dogs within the municipality revealed by the questionnaire survey was more balanced than the 1:0.76 ratio observed outside the municipality, suggesting that improved access to sterilization services enhances the willingness of owners to adopt female dogs. In addition, the Vets for Future team's deliberate efforts to educate the public about the advantages of female dogs, including their perceived loyalty to families, may have contributed to this shift.

Previous dog population management studies have rarely examined the effects of interventions on sex ratios, and many programs have focused exclusively on females [28, 40]. Furthermore, the female-biased free-roaming population observed in this study contrasts with findings from other South Asian regions reporting approximately equal sex ratios [31, 41]. However, a preference for male pets has been documented in Bhutan, where the reported sex ratio of 1.4:1 is comparable to the present study's findings.

Impact of the CNVR effort on different age groups

Age-specific analyses did not reveal statistically significant trends at α = 0.05. Nevertheless, puppies aged <3 months exhibited an estimated annual decline of 24.7%, whereas the reduction among sexually active adults aged 1–6 years was only 6.24%. The 15.4% decline among dogs >6 years old is likely attributable to natural mortality, because this age group was not specifically targeted by the intervention.

Interestingly, older puppies aged 3 months–1 year showed an estimated 33.1% increase. Because the population captured increasingly consisted of owned dogs and female ownership increased significantly, the expansion of this age group, which is most likely to comprise newly adopted animals, may be viewed positively, as early sterilization prevents unintended pregnancies. Previous modeling studies have suggested that sterilizing females before 1 year of age at rates of approximately 26% may be sufficient to arrest long-term population growth [20]. Although these estimates are context dependent, prioritizing newly adopted young animals may strengthen future interventions.

Contribution of the buffer zone to controlling the free-roaming dog population within the city

During the first year of the project, the authors recognized that the food resources available to free-roaming dogs were stable due to surplus food from temples, food waste left by pilgrims, and deliberate feeding by volunteers and visitors. Consequently, immigration and deliberate dumping were considered the principal mechanisms responsible for maintaining dog numbers within the city. Therefore, the project boundary was extended by 3 km beyond the municipal boundary in the second year, based on the assumption that this distance approximated the maximum roaming range of free-roaming dogs. Although populations within Zones 1 and 2 stabilized by the third year, the population in Zone 3 did not decline as expected, leading to more intensive buffer zone coverage during 2024 and 2025.

Interestingly, the effects of the buffer zone were sex-specific. Significant effects were detected for Zone 1 males and Zone 3 females, whereas no effects were observed for either sex in Zone 2. The vacuum effect created by sterilization activities within the buffer may have drawn Zone 3 females outward while simultaneously reducing organized dumping of females and unwanted litters into the municipality. The absence of a similar effect among males in Zone 3 may be attributable to the overall decline in males and their lower vulnerability to abandonment.

The lack of detectable effects in Zone 2 and the selective effect on Zone 1 males remain difficult to explain. Nevertheless, several hypotheses may provide a framework for future investigations.

First, although temples within Zone 2 were originally assumed to be the primary dumping sites, truck drivers heading to markets within Zone 1 may preferentially abandon females and female puppies nearby. Once the buffer intervention reduced the supply of sexually mature females, intact males may have migrated outward in search of receptive partners, whereas resident females remained within familiar territories. Notably, the intact dogs remaining in Zone 1 after several years of intervention likely represent particularly timid individuals that repeatedly escaped capture.

Second, because Zone 2 is approximately 1.4 times as large as Zone 1 while maintaining comparable population levels, its lower density and abundant food supply may reduce competition and minimize vacuum effects.

Third, Zone 2 may contain hidden source populations. The Anuradhapura cemetery, located in the southeastern part of Zone 2, is home to a population of highly aggressive feral dogs that are extremely difficult to capture. This large refuge may counteract the buffer zone's influence.

Regardless of the underlying mechanism, these findings support the original hypothesis that peripheral regions immediately beyond target boundaries should be included in dog population management programs to reduce immigration and "seal the boundary." The discovery of sex-specific and zone-specific effects highlights the importance of conducting preliminary studies on dog behavior, migration patterns, and ecological niches before designing intervention zones.

A post hoc DAG (Figure 15) was developed to incorporate these unexpected findings and to provide a conceptual framework for future studies. The model integrates the three hypotheses described above and emphasizes the need for microgeographic and niche-based ecological investigations before implementing spatial interventions.

Although buffer zones are widely used in forestry, wildlife management, and disease control programs, this study represents, to the authors' knowledge, the first direct application of this concept to domestic dog population management. Interestingly, the interim Animal Policy of Australia, under Section 10(1)(b) of the Natural Resource Management Act 2004, describes the use of a 35-km external buffer zone around the dog fence within which landowners are expected to eliminate dingoes to protect livestock [36]. Although dingoes and free-roaming domestic dogs belong to the same species (_Canis _familiaris), the distinct behavioral characteristics of Sri Lankan free-roaming dogs make the present application of the buffer zone concept particularly novel.

ARV and sterilization coverage among owned dogs within and outside the city limits

The survey results revealed that >60% of the adult dogs owned were sterilized. The proportions of sterilized adult females within and outside the municipality were 78% and 86%, respectively, exceeding those reported in comparable interventions conducted elsewhere in South Asia [31]. It should be emphasized that, despite the widespread assumption that sterilizing 70% of a free-roaming population is sufficient to achieve self-sustaining population reduction, the proportion of females requiring annual sterilization depends on the potential population growth rate. This growth rate is determined by adult survival (the proportion of mature females surviving from one breeding season to the next), fecundity (the proportion of mature females producing a litter each breeding season), juvenile survival (the proportion of female pups surviving until sexual maturity), and litter size [35]. These variables are themselves influenced by factors such as food availability, disease outbreaks, and accidents, many of which are unpredictable. Therefore, it is difficult to recommend an ideal sterilization percentage, and continued efforts toward achieving complete sterilization of the free-roaming population may ultimately be required to minimize the suffering of street animals.

Figure 15: Directed acyclic graph illustrating hypothesized spatial behavioral mechanisms underlying buffer interventions.

Although many dog population management models prioritize female sterilization and largely overlook males, the authors emphasize the importance of sterilizing males for several reasons. First, a sexually mature male dog in Sri Lanka can impregnate multiple females during a breeding season, thereby contributing substantially to population growth. Furthermore, because a considerable proportion of owned and free-roaming females are sterilized, sexually intact males are more likely to experience sexual frustration, which may increase aggression. Such males may extend their roaming range in search of receptive females, resulting in more intense competition among males and increased pressure on the remaining unsterilized females.

Male dogs also contribute substantially to public nuisance through urine marking, territorial fighting, barking, and involvement in motorbike accidents. Sterilization of community males can reduce territorial aggression and nuisance to residents, whereas sterilization of owned males can reduce roaming and fighting behavior, potentially improving their suitability as companion animals.

At present, the authors do not possess sufficient data to estimate the proportion of the free-roaming population that has been sterilized. Nevertheless, zone-specific analyses indicate that the annual number of unsterilized free-roaming dogs requiring surgery has stabilized. Consequently, the authors recommend maintaining CNVR activities twice annually to preserve the gains achieved, given that dogs typically exhibit two reproductive cycles each year. In addition, stronger legal frameworks promoting the sterilization of owned dogs and requiring registration of breeders should be established to ensure that puppies born to owned females do not become unwanted and subsequently abandoned.

Survey data further confirmed that 94% of owned dogs within the municipality and 83% of those outside the municipal boundary had received at least one ARV. The proportions of regularly vaccinated dogs were 74% and 69%, respectively. Since the World Health Organization recommends maintaining vaccination coverage of at least 70%, these findings represent a major achievement in a culturally complex and economically disadvantaged urban environment. The authors therefore propose continuing vaccination campaigns at a frequency similar to that of sterilization programs.

Influence of the number of dogs per household on the likelihood of regular ARV and sterilization

Transportation of pets to and from clinics remains a major challenge for low-income households lacking access to personal vehicles. This problem becomes more pronounced in households with multiple dogs, because owners frequently have to leave one or more animals behind. The challenge is particularly severe for sterilization procedures, because postoperative animals require safer and more spacious modes of transportation than routine vaccination visits. These observations led to the initial hypothesis that larger household dog populations would be associated with poorer vaccination and sterilization outcomes. Aggregate-level analyses supported this hypothesis, revealing reductions of 21.4% in sterilization rates and 21.1% in annual vaccination rates for every additional dog per household. This finding challenges the view held by some local health authorities that ARV alone would be sufficient for rabies elimination, as higher dog densities within households were associated with lower vaccination compliance.

However, closer examination of the data revealed that poor outcomes were not always associated with households owning more dogs. Household-level analyses provided another example of Simpson's paradox. Although within-household heterogeneity remained considerable, between-location heterogeneity was highly significant, with adjusted sterilization probabilities ranging from 20.1% to 88.6% in the worst and best-performing locations, respectively. Although the effects of household and neighborhood demographics remain to be explored, factors such as distance to the nearest clinic, availability of transportation, owner education, and attitudes are likely to play important roles. Identifying locations with poor sterilization performance was an important outcome of this analysis and highlights areas requiring priority attention in future campaigns.

Additional pets within households are likely to result from unplanned litters that could not be rehomed or from owners adopting friendly free-roaming dogs despite limited financial capacity to provide veterinary care. Since owned dogs account for the majority of dog bites in Sri Lanka, vaccination of owned dogs should receive equal priority to that of free-roaming dogs in efforts aimed at eliminating dog-mediated human rabies.

The authors recognize the compassionate attitudes of owners who adopt multiple animals and propose that vulnerable communities should receive home visits and additional clinic sessions in areas with poor performance.

Incidence of TVT and skin infections

The increasing number of skin treatment cases probably reflects greater community reporting rather than increased dog density, and therefore does not provide reliable evidence of the success of the dog population management program. Nevertheless, the trend suggests increasing public awareness of animal welfare when quality veterinary services are provided free of charge.

Public awareness of TVT and skin diseases such as mange is likely to be low among the general public in Sri Lanka, and reliance on home remedies remains common. Positive word-of-mouth communication, reinforced by successful recoveries among friends' and neighbors’ pets, encourages owners to seek veterinary care for their own animals and to report affected free-roaming dogs to veterinary teams. Thus, improved access to veterinary care may enhance public participation in animal welfare activities.

Program economics and economic returns

The economic analysis relied primarily on national estimates because regional data were unavailable. Nevertheless, the findings indicate that dog sterilization and vaccination generate economic benefits compared with the overall costs of rabies. Furthermore, accommodation costs could be largely eliminated in the long-term if government veterinarians and dedicated regional dog-catching teams were able to operate at a similar capacity.

Plans for continuation

The perceived success of the program is reflected in the Provincial Council's decision to expand coverage to the entire North Central Province beginning in 2026. Although a different veterinary team will be responsible for vaccination and sterilization, the authors intend to remain involved in surveillance.