Author affiliation: University of Oxford and Ethiopian Wolf Conservation Programme, Tubney, UK (J. Marino, E.F.R. Preston, C. Sillero-Zubiri); University of Oxford and Ethiopian Wolf Conservation Programme, Robe-Bale, Ethiopia (M. Abute, A. Hussein, E. Bedin); Ethiopian Wildlife Conservation Authority, Addis Ababa, Ethiopia (F. Regassa); Ethiopian Public Health Institute, Addis Ababa (A. Deressa); Animal and Plant Health Agency, Weybridge, UK (A.C. Banyard, A.R. Fooks).
Infectious diseases pose a growing threat to wildlife, livestock, and human populations globally (1,2). The recent SARS-CoV-2 pandemic starkly illustrates the serious implications of zoonotic disease transmission, emphasizing how pathogens can leap from wildlife to humans, highlighting a disturbing narrative concerning the dynamics between domestic and wild animal populations. The interplay of these populations is crucial, as this often results in the emergence of infectious diseases that can severely compromise wildlife species, sometimes with catastrophic consequences (2). The risk posed by infectious diseases is particularly acute in small, vulnerable populations living closely alongside humans and their domesticated animals; the likelihood of stochastic demographic shifts—especially concerning population decline or outright extinction—escalates dramatically due to disease spillover events (1,2). Domestic dogs frequently pose a significant threat to wild carnivores, being implicated in numerous cases of disease transmission that hinder conservation efforts (3–7). This is exacerbated by increasing populations of free-ranging dogs in rural and wild areas, leading to a heightened frequency of disease spillovers into wildlife.
Rabies poses a significant threat across many mammal species, extending even to humans and livestock. The persistence of the rabies virus among domestic dog populations often constitutes an alarming public health and conservation issue (8). This virus is especially concerning for a number of threatened carnivores, including both endangered African wild dogs (Lycaon pictus) (10–12) and the Ethiopian wolves (Canis simensis) (13–16). Unlike many viral pathogens, rabies almost invariably leads to fatal outcomes following productive infection. Additionally, other diseases affecting both domestic and wild animals have emerged as global threats to endangered species, the most notable being canine distemper virus (CDV). CDV is characterized by its high infectiousness coupled with the severe immunosuppression it causes, often leading to secondary opportunistic infections that dramatically increase morbidity and mortality rates in affected populations (17). Unlike rabies, which presents with predictable fatality, CDV may circulate mildly, with outcomes influenced significantly by factors such as viral and host genetics, nutritional status, and susceptibility across species lines (18). Recent years have seen concerning spillovers of CDV from domestic dogs linked to marked declines in various wild carnivore populations globally. Outbreaks have notably impacted African wild dogs (5,6,19,20), lions (Panthera leo) (19,21), black-footed ferrets (Mustela nigripes) (22), among others.
Across rural Africa, the interface between domestic livestock and wild wildlife creates numerous opportunities for the transmission of infectious viral diseases, with Ethiopia notably noted for having some of the highest incidences of rabies in the world (29). In this precarious context, the potential for population declines or even extinctions looms large due to concurrent disease outbreaks affecting wildlife. While several case studies have extensively evaluated the effects of disease on populations and proposed management strategies to mitigate these risks (30–32), they predominantly focus on single diseases, leaving a significant gap in our understanding of how multiple diseases may simultaneously affect wildlife. Diseases can interact in various ways within the same population, as evidenced by the distinct reservoir species and dynamics associated with CDV and rabies in African wild dogs (6,7). Over the years, carnivore species have suffered outbreaks of these diseases, sometimes impacting different populations or occurring at different times (5,6,10–12,16,19–21,23), yet instances of concurrent infections remain notably underreported, with only brief observations recorded in northern raccoons (Procyon lotor), red foxes (Vulpes vulpes), and striped skunks (Mephitis mephitis) across the United States (33–35). The prevailing absence of essential knowledge surrounding the demographic outcomes resulting from the dual threats posed by these viral infections represents a significant gap, particularly as the frequency of CDV occurrence and its related illnesses in wild carnivores continues to rise (36,37). Most studies reporting co-infection narrowly focus on individual mortalities rather than addressing the broader demographic impacts, often limiting findings to isolated cases without linking them to distinct disease outbreaks (38) or examining instances lacking clear connections to outbreaks (39).
The potential for concurrent outbreaks deserves careful consideration, particularly for populations at risk, especially those of significant conservation concern (16,40). Without ongoing disease monitoring throughout outbreak periods—particularly where multiple pathogens are present—these concurrent outbreaks may remain unnoticed, masking their potential impacts on vulnerable populations. Here, we document alarming concurrent outbreaks of rabies and distemper in Ethiopian wolves, detailing the spatial dynamics and temporal spread of mortality and the resulting implications for the overall health of the host population. Each disease has significantly influenced the survival prospects for the wolves residing in the Bale Mountains, which remains home to the largest population and accounts for over half of the 500 remaining Ethiopian wolves; prior outbreaks have already led to the extinction of one of the smallest populations in the area (16).
Monitoring of the Ethiopian wolves in the Bale Mountains has been continuous since 1997, focusing especially on two core subpopulations, the Sanetti Plateau and the Web Valley. The neighboring subpopulations of Morebawa (connected to the Web Valley through the Genale Corridor) and Chafadalacha were also included in monitoring efforts. These subpopulations consist of geographically separated groups of wolf packs, isolated from others by habitat bottlenecks or physical barriers. A dedicated team of 6–8 trained monitors conducts frequent observations of Ethiopian wolf packs, employing methods such as on-foot and horseback monitoring during daylight hours, adhering to standardized protocols (15,41). Surveillance for disease also forms a core part of this routine field monitoring; carcasses are discovered occasionally or are reported by park rangers and local community members, triggering more intensive follow-up searches. Established disease alert networks, encompassing local communities and park staff, further enhance disease detection efforts across local dog populations. Oral rabies vaccination campaigns were executed in 2018 and 2019, particularly focusing on Morebawa (10 packs) and the Web Valley (2 packs), with no vaccinations administered to packs in the Sanetti Plateau prior to the outbreak (Ethiopian Wolf Conservation Programme, unpub. data). Additionally, a limited trial of a CDV vaccine targeting wolves in Chafadalacha (2 packs) and Morebawa (1 pack) was conducted.
In March 2019, an Ethiopian wolf carcass was located within the Bale Mountains, prompting a suspicion of infectious disease as the cause of mortality; this discovery resulted in heightened monitoring efforts for carcasses throughout the area. Detailed necropsy examinations were conducted whenever possible on the carcasses discovered, with tissue samples collected specifically from organs such as lymph nodes, lungs, spleen, and brain. Care was taken to select tissues appropriate for the identified pathogens in testing, with the estimated time of death categorized according to the stages of decomposition: within one day, one to two days, within one week, within one month, and greater than one month.
We meticulously mapped the spread of infectious diseases across the Ethiopian wolf populations in the Bale Mountains, utilizing GPS coordinates of carcasses with adjustments made based on estimated times of death. Complementing this mapping effort, population dynamics were assessed through intensive monitoring conducted across the packs in the two core areas, specifically Web Valley and Sanetti Plateau, including a thorough tally of neighboring packs (eight in Web Valley and five in Sanetti Plateau). The stability of these social groups and their territories facilitated reliable total counts; direct sightings of complete packs were common, especially during communal greetings in early morning and evening hours, during territorial patrols, and around dens during breeding seasons. We adhered to established observational protocols throughout the year for focal monitoring of the selected packs, allowing for accurate assessments regarding their sizes, composition, and reproductive success. This careful observation enabled us to classify wolves into age and sex categories while identifying individual wolves based on ear mints or specific morphological traits (41). Changes to population dynamics resulting from the outbreak were evaluated by comparing both overall wolf numbers and numbers segmented into age and sex categories before and after the incidents, based on timing of discovery of the first and last carcass located.
From March 2019 to November 2019, we recorded 57 carcasses with an additional 5 wolves presenting with advanced clinical signs synonymous with infectious agents (Table 1; Appendix Table 1). Wolf deaths were noted across at least 19 packs spanning four subpopulations: Sanetti Plateau, Chafadalacha, Web Valley, and Morebawa. Samples for laboratory analysis were collected from 19 of these carcasses; unfortunately, the decomposition state or scavenging rendered many carcasses unsuitable for testing. Results indicated that seven animals were positive for the rabies virus, while thirteen were detected with CDV, and one juvenile tested positive for both pathogens. Among the confirmed cases, fatalities attributed to distemper outnumbered those attributable to rabies. Although we were unable to extrapolate these findings to broader population terms, testing less than half of the detected carcasses remains significant when juxtaposed with data gleaned from previous studies.
Figure 1
Figure 1 illustrates the retrieved carcasses of Ethiopian wolves across various months and subpopulations during the study of concurrent rabies and canine distemper outbreaks in Ethiopia during 2019. The estimated date of death was determined from postmortem examinations.
Figure 2
Figure 2 depicts the geographic locations of Ethiopian wolf carcasses found during the 2019 study, with shades of blue indicating the kernel density distribution of carcass discoveries.
Figure 3
Figure 3 represents the number of retrieved carcasses per month during the study of concurrent rabies and canine distemper outbreaks in the Ethiopian wolf population during 2019, with the estimated time of death determined from postmortem observations.
The timing and locations of encountered carcasses indicate specific pathways through which these diseases dispersed across the subpopulations (Figures 1–3). Initial infections appeared centered in the Sanetti Plateau, where the first two carcasses were recovered in March and April 2019; however, CDV was not confirmed until May 2019 after the discovery of the carcass. The overall mortality rate peaked in July 2019—five months following the outbreak’s start—by which point the spread of disease had advanced southward to Chafadalacha and the more remote Web Valley (Figures 2 and 3). At the outbreak’s peak, both CDV and rabies were commonly detected in the Web Valley, while rabies virus was not seen in the Sanetti Plateau until a month later (Figure 3). This dispersion of both rabies and distemper appeared to happen bi-directionally, suggesting independent origins for these initial infection clusters (Figure 2).
During the period from August to October 2019, disease spread followed along the Genale habitat corridor linking the Web Valley with the Morebawa subpopulation, with at least one confirmed death from distemper noted toward the conclusion of the outbreak (Figures 1, 2). It is worth mentioning that monitoring efforts in Morebawa were comparatively limited relative to those in Web Valley and Sanetti Plateau.
The total encountered carcasses (n = 57) served as a partial and indirect indicator of the wider mortality trend during the outbreaks (Table 1). We evaluated local population effects from total counts of Ethiopian wolves across the two major monitoring areas by comparing pack compositions pre- and post-outbreak in both the Sanetti Plateau (5 packs) and Web Valley (8 packs) subpopulations (Table 2). Notably, we recognized a total of 64 wolves unaccounted for in these two areas, with 50 carcasses having been assessed: 17 missing in the Sanetti Plateau (a 60% decline) and 47 in the Web Valley (a 53% decline).
This mortality analysis revealed insights on 29 carcasses classified by age and 42 by sex (Table 1). A higher number of carcasses belonged to adult wolves (greater than 2 years), followed by subadults (1–2 years), and then juveniles (Table 1), exhibiting an uneven sex distribution (17 male carcasses, 11 female, and 29 with unknown sex). Among the 13 carcasses confirmed to be affected by CDV, 6 were adults, 4 were subadults, and 3 were juveniles; the sex distribution comprised 6 females and 7 males. For the rabies-positive individuals, results indicated that three adults and four juveniles tested positive, while no subadults were found to be affected, albeit with a small sample size (n = 7). The individual testing positive for both diseases was identified as a juvenile female.
Population declines, inferred from alterations in pack compositions, aligned with the age distribution of carcasses discovered: of the wolves missing from the population, 54 were adults, 40 were subadults, and 16 were classified as juveniles. In the Web Valley subpopulation, mortality rates were noted at 53% among adult wolves and 73% among subadults. In contrast, in the Sanetti Plateau subpopulation, mortality rates were reported at 41% among adults and 80% among subadults. A noticeable trend revealed that females were more significantly unaccounted for than their male counterparts (38 females in the Web Valley vs. 35 males in the Sanetti Plateau).
This intensive monitoring of the Ethiopian wolf population has yielded critical confirmation of concurrent outbreaks alongside co-infections involving rabies and CDV within this endangered species, providing important new insights regarding the impacts at the population level. The outcomes align with escalating concerns that concurrent outbreaks may exert a more significant toll on the survival of Ethiopian wolves compared to isolated outbreaks of single diseases (16,40). Such findings could carry implications for additional species that are vulnerable to similarly contracting these diseases, including African wild dogs, lions, and other carnivorous mammals. Research addressing viral infections across wild carnivore populations has remained relatively scarce for various reasons, thereby underscoring the crucial nature of the findings derived from concurrent outbreaks as they pertain to disease surveillance and control, despite certain limitations.
Our effort to broaden the monitoring undertaking across the extensive population of these wolves and over an extended time period has significantly aided in the detection of multiple pathogens, which may have otherwise gone unnoticed; we only found a single prior incidence of co-infection in another individual (33). A fundamental reason that concurrent outbreaks remain infrequently reported hinges on the fact that rabies and CDV are not uncommon among wild carnivores and can affect many of the same species. If monitoring is concentrated solely within one subpopulation, it is plausible for early signs of one disease to be overlooked, or the deaths might be erroneously thought to stem from just one disease. Our study places a premium on sustained and rigorous monitoring efforts that facilitated the detection of mortality occurrences across the landscape, resulting in the recovery of 57 wolf carcasses while yielding no evidence of deaths observed in any other species. The detection of most carcasses is intrinsically imperfect; timely identification of the sex or age of carcasses can also be hindered by delayed detection. The timeline for death following rabies virus infection in canine species typically falls within two weeks post-infection. Conversely, clinical manifestations of CDV can arise within several days following infection, with outcomes leaning toward either clearance and survival from infection or deterioration into a severe condition characterized by acute leukopenia, ultimately resulting in secondary infections and potentially death. Although variability exists within the timelines of CDV infections, instances culminating in mortality generally align closely with those observed for rabies virus infections. It is important to remember that effective carcass detection relies heavily on proactive monitoring efforts.
Population declines as pronounced as those noted in this study inevitably raise alarms for any threatened species, particularly those existing within small population sizes such as the Ethiopian wolf. Historically, consecutive outbreaks of distemper and rabies prompted a situation leading to one of the smallest populations of this species becoming functionally extinct in areas such as Delanta (16). The ongoing oral vaccination of Ethiopian wolves against rabies holds significant promise for mitigating extinction risks; this approach serves as a particularly crucial intervention in places where rabies remains entrenched among terrestrial carnivores across broad landscapes (45). While measures to control rabies have developed substantially, largely due to the virus’s relevance to human and agricultural health (46), tools aimed at managing distemper in wildlife remain less advanced. CDV does not typically infect humans and thus tends to receive less focus; while canine distemper vaccines have been around for years, they are seldom utilized within wildlife contexts, and their overall effectiveness remains uncertain, predominantly resonating from the complexities surrounding CDV dynamics in natural ecosystems (18). A trial of a CDV vaccine is currently underway (Ethiopian Wolf Conservation Programme, unpub. data).
To conclude, this investigation underscores the grave risk of co-infections through multiple pathogens for susceptible populations of threatened carnivores. With CDV and rabies viruses capable of impacting populations simultaneously, the consequences can be profound, even exacerbated by partial vaccination efforts. Vaccination strategies implementing multivalent rabies/CDV vaccines should be prioritized wherever feasible, especially considering the increased danger concurrent outbreaks pose; the compounded effects of both diseases must be integrated into future vaccination program models and assessments of extinction risks among susceptible endangered populations. Existing models applied to Ethiopian wolf populations require updates to encompass emerging data regarding potential multiple hosts of CDV, insights on acquired immunity (49), and effectiveness evaluations of current oral rabies vaccinations (50). As demonstrated in the case of the Ethiopian wolf, the implementation of efficient monitoring systems for endangered wildlife populations at risk of co-infection is essential to ensure the timely detection of concurrent outbreaks, enabling accurate assessment of their true impacts and incorporation into predictive extinction models.
Dr. Marino is the science director of the Ethiopian Wolf Conservation Programme and a research fellow at the Wildlife Conservation Research Unit, University of Oxford. She has worked with Ethiopian wolves since 1999 and recorded and monitored several disease outbreaks in the population during this time. Dr. Preston is a lecturer in Wildlife Ecology at the Royal Veterinary College. She worked with the Ethiopian Wolf Conservation Programme during the concurrent outbreaks.
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What are the primary threats to the population of Ethiopian wolves and how do they relate to broader ecosystem health?
Ongoing concerns regarding population declines among species like the Ethiopian wolf, timely and effective monitoring and management strategies are essential. The dual threats posed by rabies and canine distemper virus illustrate the complex challenges faced in conserving these endangered wolves and highlight the urgent need for a comprehensive approach to health management in wildlife populations.
The findings of this study not only contribute critical data regarding disease impacts on Ethiopian wolves but also raise significant implications for the conservation of other vulnerable carnivore species facing similar threats from infectious diseases. As wildlife health is intricately linked to broader ecosystem health, advancing our understanding of disease dynamics and implementing effective vaccination programs can be crucial in ensuring the survival of threatened species across diverse habitats.
Future research should focus on the development and application of vaccination strategies, improved surveillance for early detection of disease outbreaks, and increased collaboration among conservationists, wildlife health professionals, and local communities. By fostering a holistic approach, we can enhance the resilience of Ethiopian wolves and potentially transform our strategies for managing other at-risk carnivore populations in a changing environment.
