Free-roaming dogs play a critical role in the ecology of Echinococcus granulosus sensu lato (s.l.), the parasitic tapeworm that causes cystic echinococcosis (CE) in humans and animals. CE is a zoonotic disease that poses a significant threat to human and animal health, with canines serving as definitive hosts and livestock as intermediate hosts (1, 2). Infection remains a significant public health concern in regions that engage in livestock production, such as the central Andes in Peru (3). Previous studies have emphasized the prevalence of CE in regions like Junin due to the close proximity and interaction between dogs and livestock, which are central to the economy and agricultural practices of these areas (4, 5).

Echinococcus granulosus has an indirect life cycle which requires infection of multiple hosts to successfully survive from an egg to sexual maturity. The reproductive stage of E. granulosus parasitizes the gastrointestinal tract of its definitive host, wild and domestic canids [CDC-DPDx (6–8)]. Eggs shed in feces are highly environmentally stable and may persist outside of a host for extended periods of time (8, 9). Parasite eggs hatch upon consumption by an intermediate host, migrate into the bloodstream, and become lodged in viscera where parasite larvae contribute to slow-growing cystic lesions [CDC-DPDx (6, 8, 10)]. To complete its life cycle, E. granulosus larva must be consumed by scavenging canids feeding upon infected offal. While herbivorous species such as sheep tend to serve as the intermediate host, unfortunately, humans that consume E. granulosus s.l. eggs may also become infected and develop cysts in various organs, such as the lungs and liver (11–13). Without medical screening, CE may go undetected for years, or until cysts grow large enough to contribute to life-threatening disease that warrants surgical intervention (4, 14–17). That said, surgical interventions may not be readily available to the most vulnerable populations due to reduced medical access in developing agricultural economies (18–20). The persistence of CE in endemic regions poses a serious threat to human health and wellbeing. Halting transmission requires a clear understanding of animal reservoirs, environmental conditions that favor parasite persistence, and human behaviors that facilitate spillover. This challenge calls for a truly One Health approach—where animal, human, and environmental health overlap and where integrated interventions are most urgently needed (21).

The district of Canchayllo, in Junin, Peru, is a rural region located in the Central Andes. This region is known for its agricultural economy, and specifically, many families in this district engage in livestock management. The interdependence between sheep, herders, and herding dogs is central to this lifestyle. Unfortunately, these conditions provide a favorable environment for the transmission of E. granulosus s. l. Previous studies have estimated the prevalence of human CE in the central highlands of Peru at 5–7% based on ultrasound-based population surveys (3, 19). However, we believe that prevalence is likely higher given the protracted period of asymptomatic disease, reduced medical access of at-risk populations, high degree of contact between herders, dogs, and their livestock, and poor understanding of local disease epidemiology (19, 22).

Little is known about the contribution of free-roaming dogs to the epidemiology of CE in the Central Andes in Peru due to the limited number of integrative research initiatives bridging human and veterinary health. This study sought to better understand E. granulosus s.l. spillover risk, defined here as the transmission of parasite eggs from infected dogs to humans as an accidental, dead-end host, by examining parasite prevalence and ranging behavior in free-roaming dogs. While we refer to this phenomenon as spillover throughout the manuscript, we acknowledge that this term is usually used to describe cross-species transmission events involving biological barriers that prevent sustained transmission. In this case, humans serve as a dead-end host as they are susceptible to infection upon exposure, but do not permit subsequent infection of other hosts. Our goal was to establish an environmentally focused understanding of regional features that may be contributing to local disease transmission. To this end, we tracked free-roaming dogs to understand the movement of the definitive host of E. granulosus on the landscape and performed copro-ELISA to identify E. granulosus s.l. positive animals.

Most studies on echinococcosis in dogs have focused on working dogs. In contrast, our study aimed to better understand pet dogs, which may also play an important role in transmission to humans. Therefore, our first aim was to estimate the intensity of space use (e.g., probability of dog presence in specific areas) to inform spatially-targeted dog interventions. Specifically, we investigated the spatial association between E. granulosus s.l.-infected dog home ranges and proximity to their owners’ houses and a local slaughterhouse. In addition, knowing that free-roaming dogs often defecate along the borders of their territory to establish a “property line” (23), our second aim was to estimate the home range and core areas used by these dogs as a proxy for the spatial extent of spillover risk (e.g., transmission from dog feces to humans). We use our findings to increase our understanding of free-roaming dog ecology and spillover risk to inform interventions aimed at reducing human infection.

Echinococcus granulosus s.l. infection prevalence was alarmingly high in our canine population, with 85% positivity. Importantly, the total area covered by E. granulosus-infected dog home ranges overlap with all households in the study communities. This is a remarkable finding considering that we only sampled 20 dogs and suggests that widespread environmental contamination of E. granulosus egg-containing feces may be a significant driver of local disease persistence and a high exposure risk for individuals living in this area. Although dogs tend to defecate repeatedly in specific locations within their territory (23), contamination is not necessarily limited to these areas. Several studies have shown that Echinococcus eggs are highly resistant in the environment and can be dispersed passively by biotic and abiotic vectors, such as wind and coprophagous insects (27–29). These mechanisms allow egg dissemination in a radial pattern from the original site of deposition, potentially expanding the contamination zone beyond the dog’s most heavily used areas. This highlights both the importance of considering both direct deposition sites and secondary dispersal routes when assessing environmental risk.

Our observations of the slaughterhouse revealed that, despite a perimeter wall and gate, it remained accessible to free-roaming dogs through open entrances and rear access points. A few dogs were seen consuming discarded viscera inside the premises, with no formal system for offal disposal or personnel restricting access. Thus, the facility represents a potential source of infection. But we found no significant association between infection status, home range size, or proximity to the slaughterhouse. In fact, dogs in our study rarely visited the slaughterhouse, and when they did, these visits did not coincide with operational hours, suggesting that this facility is unlikely to be a major source of infection. However, many families in the study region do not rely on the slaughterhouse, and instead slaughter livestock near their homes, a practice often accompanied by feeding discarded offal deemed unfit for human consumption to dogs (30–32). This pattern is consistent with our findings, as all dog home ranges overlapped with owners’ households, and even those with the smallest home ranges that do not overlap with the slaughterhouse were positive. Although our questionnaire did not directly ask about at-home slaughter, informal conversations with participants and other community members, as well as direct field observations, confirmed that this is a widespread practice in both study communities. This risky behavior likely plays an important role in the high observed prevalence, yet remains largely unaddressed by health programs in Peru.

Previous intervention strategies to control E. granulosus transmission in the central highlands of Peru, including Junín, have primarily stemmed from government-led mass treatment campaigns focused on periodic deworming of dogs using praziquantel (33–36). These efforts were often accompanied by educational messaging discouraging the feeding of offal to dogs and encouraging improved slaughterhouse hygiene (37, 38). However, such campaigns have faced significant limitations, including inconsistent implementation due to limited resources, lack of follow-up, low community participation, and limited integration of veterinary and public health services (39). In many rural communities like Canchayllo, access barriers and limited engagement of local stakeholders have further weakened the effectiveness of these interventions (40, 41). Given our findings—particularly the persistence of infection in dogs and their frequent movement between households and communal areas—these past strategies have been insufficient to disrupt the parasite’s life cycle. A more sustainable and targeted approach is urgently needed, including enhanced surveillance and context-specific education campaigns grounded in a One Health framework (42).

An important finding of this study was that the HRs of all dogs and all but one dog’s core area overlap with their households, indicating that they spend most of their time near their owners’ homes. This information supports that existing intervention strategies based on delivering anthelmintic drugs to individual households have the potential to reduce the environmental presence of infectious E. granulosus eggs. Delivering deworming agents to dog owners would offer a simple solution to ensure that dogs receive the necessary preventative care to curtail the zoonotic cycle that places owners at increased risk of infection. Given that dogs are often found near home, there would be plenty of opportunities for public health inspectors and dog owners to find the dogs and to offer them consumable preventative treatment at necessary intervals to provide protection against reinfection. While this would not address pre-existing environmental contamination, it would be a critical step in reducing further spillover risk to humans.

Our study has several strengths, but also some limitations. While we distributed our sampling across the different study communities, we did not have access to a household roster to conduct true random sampling. Additionally, the tracking period for animals varied and was limited to less than a week. It is possible that seasonal events (e.g., livestock fairs) influence movement patterns in ways that could affect E. granulosus s.l. transmission. We did not use purgatives, which may have increased the chances of false negatives; however, given the high prevalence we observed, we believe this did not significantly affect our results. Finally, we did not explore in detail the practice of home slaughtering by dog owners, which may help explain some of our findings.

Our findings suggest that in areas where E. granulosus s.l. is highly endemic (43), humans may face a high risk of infection regardless of the infection status of their own dog due to environmental contamination and overlap by nearby positive, free-roaming dog home ranges. Additionally, dogs experience high infection rates regardless of direct access to slaughterhouses, highlighting the importance of informal and home-based slaughtering practices. The consistent overlap between dogs’ core areas and their owners’ households underscores the potential for household-targeted interventions. Our findings support other studies that propose integrated control strategies that go beyond slaughterhouse management and include regular deworming of dogs, safe disposal of livestock offal, community education on zoonotic transmission routes, and improved surveillance of canine infections. Targeting interventions at the household and community level is likely to be essential for reducing human risk and breaking the transmission cycle of this neglected but persistent zoonosis. Targeted interventions could be informed by the spatial ecology of free-roaming dogs, which is closely intertwined with human behavior, the environment, and livestock management systems. While our study contributes valuable data to the understanding of free-roaming dog ecology, it represents only one component of a broader, complex system. Additional research is essential to build a comprehensive evidence base capable of supporting integrated, One Health–oriented approaches to the control of this persistent zoonotic disease.