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Keywords:

  • Canis simensis;
  • capture probability;
  • disease control;
  • Ethiopia;
  • immunity;
  • serology

Summary

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
  • 1
    As outbreaks of infectious diseases have emerged as a threat to small populations, conservation managers are increasingly making decisions regarding whether and how to intervene in such situations. Past controversies and lack of knowledge and firm guidelines may inhibit this process. We present data on a vaccination campaign against a rabies outbreak in endangered Ethiopian wolves as a case study of a disease-control intervention in a threatened population.
  • 2
    Ethiopian wolves on the periphery of the outbreak area were trapped to administer a dose of injectable rabies vaccine and to assess the magnitude and duration of the immune response. The expansion of an established population monitoring programme allowed us to assess the factors influencing the probability of capturing particular animals and to evaluate the overall success of the intervention.
  • 3
    All wolves sampled 1 month after vaccination had protective levels of serum antibody titres. A booster dose administered within 1–6 months appeared to be necessary to maintain these levels. Females were less likely to be trapped than expected, if dispersing females were included in the population. Animals captured in the first trapping session were more likely to be recaptured if the pack was trapped again.
  • 4
    The intervention was successful in halting the spread of the rabies outbreak and had few short-term impacts on the population of wolves and non-target species.
  • 5
    Synthesis and applications. Demographic, spatial and behavioural heterogeneities within populations may affect vaccine uptake or delivery and thus the efficacy of vaccine-based interventions. Managers of populations of threatened species should ensure that disease-control programmes are carefully designed to maximize information gained on all aspects of an intervention, and thus to evaluate its outcome and impact. Dissemination and discussion of results is crucial in order to apply what has been learnt to similar scenarios in the same or related populations.

Introduction

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Infectious diseases are significant causes of population declines in wildlife. Over the last two decades, pathogens have been identified as causes of population reductions and local extinctions in a wide range of taxa and habitats, including phocine distemper in seals (Heide-Jørgenson et al. 1992), chytridiomycosis in frogs and toads (Daszak et al. 1999), rabbit haemorrhagic disease (Villafuerte et al. 1994) and crayfish plague (Alderman 1996). Recent examples among carnivores include rabies in Blanford's fox Vulpes cana (Macdonald 1993), African wild dogs Lycaon pictus (Gascoyne et al. 1993; Hofmeyr et al. 2004) and Ethiopian wolves Canis simensis (Sillero-Zubiri, King & Macdonald 1996) and canine distemper virus in black-footed ferrets Mustela nigripes (Williams et al. 1988) and lions Panthera leo (Roelke-Parker et al. 1996). Disease is a specific threat to small, fragmented or threatened populations, with viral diseases in particular responsible for high mortality and local extinction of several such populations (Murray et al. 1999; Dobson & Foufopoulos 2001). These populations are vulnerable to pathogens that can infect multiple host species and that are typically transmitted from more abundant reservoir host populations (Murray et al. 1999; de Castro & Bolker 2005).

Although ecologists now acknowledge the role played by pathogens and their potential threats to endangered populations (Scott 1988; Daszak, Cunningham & Hyatt 2000; Woodford, Butynski & Karesh 2002; Kissui & Packer 2004), in many instances managers of such populations remain ill-equipped to handle infectious disease outbreaks. Action is often hampered by a lack of detailed quantitative ecological and epidemiological data with which to undertake thorough contingency planning and objective risk assessment. Such risks include both those from the pathogen and from the intervention itself. Examples of the latter include possible stress-related responses, physical injury, vaccine-induced mortality (Carpenter et al. 1976; Durchfield et al. 1990) and anaesthetic accidents. In addition, few disease-control interventions in endangered populations have been subject to a rigorous evaluation of the impact of such programmes, based on data collected before, during and after a disease outbreak. In some cases, the failure to monitor effectively and evaluate an intervention has generated considerable and damaging controversy (e.g. African wild dogs in the Serengeti ecosystem; Burrows 1992; Creel 1992; Burrows, Hofer & East 1994, 1995; Creel, Creel & Monfort 1997).

The evaluation of any vaccination-based disease control intervention should include an assessment of the protective effect generated by the vaccine in the target species, preferably under field conditions. Extrapolation from the immune responses seen in related species or captive conspecifics may be misleading (Gascoyne et al. 1993; Woodroffe et al. 2004). For example, prophylactic vaccination of African wild dogs through the administration of a single dose of inactivated rabies vaccine (effective in domestic dogs) has on several occasions failed to prevent the deaths of some animals from rabies (Kat et al. 1995; Scheepers & Venzke 1995; Hofmeyr et al. 2000). However, empirical evidence suggests that multiple vaccinations may be effective in this species (Hofmeyr et al. 2004).

Epidemiological theory recognizes the importance of heterogeneity arising from age-related, genetic, spatial and behavioural factors in the transmission of infectious diseases within populations (May & Anderson 1984). Similarly, heterogeneity in immune response to vaccination between various population subgroups, as a result of differences in the proportion of hosts that become protected (vaccine ‘take’), the degree of protection and the duration of immunity, has important implications for the design and efficacy of vaccination programmes (Halloran, Haber & Longini 1992; Woolhouse, Haydon & Bundy 1997). If vaccination can only be administered following the physical capture of individuals, as is the case for many wild species, differences in the ‘trappability’ (the probability of capturing an individual animal; Tuyttens et al. 1999) of individuals in different segments of the population may give rise to an additional source of heterogeneity.

Ethiopian wolves persist in several small, highly fragmented populations (Marino 2003) that are threatened by the repeated introduction of pathogenic viral diseases from surrounding domestic dogs (Laurenson, Shiferaw & Sillero-Zubiri 1997; Sillero-Zubiri & Marino 2004; Randall et al. 2006). The largest population, in the Bale Mountains National Park (BMNP), south-central Ethiopia, has suffered several dramatic declines as a result of mortality induced by epidemics of rabies (Sillero-Zubiri, King & Macdonald 1996; Randall et al. 2004) and canine distemper (Ethiopian Wolf Conservation Programme (EWCP), unpublished data; Laurenson et al. 1998). In 2003, rabies was again diagnosed in wolves in BMNP and an intervention based on the capture and vaccination of susceptible wolves was undertaken in an attempt to contain the outbreak. We describe the implementation and short-term outcome of the intervention in the BMNP Ethiopian wolf population, with particular emphasis on the immune response of Ethiopian wolves to parenteral rabies vaccination and on the trappability (and hence protection) of different segments of the population. We then analyse the factors that may have contributed to the success of the intervention, and use these to underpin recommendations to conservation managers confronted with disease threats to endangered populations.

Materials and methods

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

the outbreak

The BMNP, situated in south-central Ethiopia (6°54′ N, 39°42′ E), contains the largest remaining contiguous piece of Afro-alpine habitat (Yalden 1983), an ecosystem upon which the endemic Ethiopian wolves are reliant (Sillero-Zubiri et al. 2004). Prior to 2003, BMNP harboured at least 300–350 of the global population of approximately 500 wolves (Marino 2003; Randall et al., in press), largely in three linked subpopulations of relatively high density (Fig. 1). A detailed description of the 2003–04 rabies epidemic in the Ethiopian wolves of BMNP is given by Randall et al. (2004). Briefly, rabies was diagnosed on 28 October 2003 from wolves found dead in the Web Valley since mid-August. A parenteral vaccination campaign of susceptible wolves was instigated, with the objectives of (i) containing the virus to the Web Valley and (ii) reducing the BMNP population's extinction probability by protecting wolf packs in other key areas of the park. Trapping and vaccination started on 13 November 2003 in the Morebawa subpopulation (Fig. 1). The control phase of the intervention lasted until 14 January 2004, although further follow-up trapping sessions were conducted to assess the duration of antibody response. During the course of the intervention, 84 wolves were captured in over 5200 trapping hours.

image

Figure 1. Map of the Afro-alpine habitat in the Bale Mountains National Park, showing wolf pack territories in the rabies outbreak area (Web Valley) and the vaccination zones (Morebawa and the Sanetti Plateau). Figures show the percentage of each pack that was vaccinated during the control phase of the intervention.

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the intervention

Wolves were captured using rubber-lined Soft Catch leg-hold traps (Woodstream Corporation, Pennsylvania, USA), following Sillero-Zubiri (1996). Three leg-hold traps were buried in a 1-m2 trap-garden baited with meat, and three to six of the trap-gardens were placed within wolf pack territories in known, frequently visited areas. Traps were checked every 2–3 h during the day, and then at approximately 18:00, 22:00, 02:00 and 06:00. Wolves found in traps were covered with a blanket to induce passivity and allow physical restraint, and subsequently immobilized using a combination of medetomidine (0·09 mg kg−1 Domitor; Pfizer Animal Health, Pennsylvania, USA) and ketamine (1·5 mg kg−1 Ketaset; Fort Dodge Animal Health, Iowa, USA). Anaesthetized animals were vaccinated intramuscularly with an inactivated vaccine, with alternate animals given either a 1-mL or 2-mL dose at a single injection site (batch numbers 79056A and 71032A, Nobivac Rabies; Intervet, Milton Keynes, UK), and marked using coloured ear-tags (Rototag; Dalton ID Systems, Oxon, UK). Samples collected from each animal included serum, whole blood in EDTA, saliva in phosphate-buffered saline (PBS), hair, a tissue sample from the ear pinnae stored in 96% ethanol and, when available, faeces in ethanol and 5% formalin. The weight, body measurements and physical condition of all captured animals were recorded. One wolf in each of 12 packs (preferably an adult male; adult females were not collared) was radio-collared to assist post-intervention monitoring. Long-acting antibiotics (300 mg Clamoxyl LA; Pfizer Animal Health) were administered to all wolves. The effects of medetomidine were reversed using atipamazole (6·5 mg Antisedan; Pfizer Animal Health) and animals observed until full recovery (no ataxia evident) or until lost from sight. Late pregnant females (as evidenced by swollen mammary glands) were not anaesthetized because of the risk posed to the foetuses. Such animals had serum collected and were vaccinated whilst held beneath a blanket. This procedure typically took less time than anaesthetic induction. Serum was extracted after leaving blood samples to stand for 24 h, or after 12 h if centrifuged, and split into at least two samples. Serum samples were kept cool in the field before being transferred to –20°C.

serology

Nineteen primary-vaccinated animals were recaptured 25–34 days after initial vaccination, to assess humoral immune response and to deliver a booster dose of 1 mL vaccine. Further recapture efforts yielded serum samples from two primary-vaccinated animals and one boosted animal 6 months (154–188 days) post-vaccination, and four primary-vaccinated and one boosted animal at 1 year (344–373 days) post-vaccination. Blood was collected from recaptured animals whilst under physical restraint only.

Serum samples were sent to the Rabies Unit, Veterinary Laboratories Agency, Addlestone, UK. Both sets of serum samples from each animal were tested to ensure repeatability. All further analyses were based on the results of the first tests, unless otherwise stated. The fluorescent antibody virus neutralization (FAVN) test was used for the detection of rabies virus-specific antibodies, as described by Cliquet, Aubert & Sagné (1998). The FAVN test measures neutralizing antibodies as a modality of a protective immune response following vaccination. A titre of 0·5 IU mL−1 is recommended by the World Health Organization (1992) as the minimum measurable antibody level in domestic dogs concomitant with seroconversion. The serum antibody titres were loge-transformed and the effects of volume (1 mL, 2 mL), batch (79056A, 71032A), sex (male, female) and age class (subadult/juvenile, adult) on antibody titres were tested using analysis of variance (anova). Model simplification was done through backwards deletion of non-significant terms (P > 0·05), beginning with the least significant (Crawley 2002). Studentised residuals were examined to detect possible outliers in the model (Belsley, Kuh & Welsch 1980).

trappability

Factors affecting probability of trapping were assessed from the 0-, 1- and 12-month trapping sessions. Fifteen packs (containing 116 wolves) were targeted initially, with six of these packs (49 wolves) retargeted at 1 month. Four packs were targeted in the third session at 12 months, of which three packs (25 wolves) had been targeted on both previous occasions and one (six wolves) had been targeted only once. Thus 15 packs were trapped at least once, seven packs at least twice and three packs three times. Pack sizes and composition (age class and sex of pack members) at the time of trapping were known from population monitoring and behavioural observations made before, during and after the outbreak and intervention. For a complete overview of population monitoring methods, see Sillero-Zubiri (1994), Marino, Sillero-Zubiri & Macdonald (2006) and Randall et al. (in press). Individuals were classed as juveniles (≤ 12 months old), subadults (> 12 months but ≤ 24 months) or adults (> 24 months). ‘Floater females’ (dispersing adult females searching for breeding vacancies in packs, whose home ranges lie mainly in the interstices of established packs; Sillero-Zubiri & Gottelli 1995) were also present in the population. Previous estimates that floater females constitute 7% of the population (Sillero-Zubiri & Gottelli 1995) predicted the presence of eight floater females across the 15 trapped packs. Trapped wolves (n = 9) that originated from packs other than the 15 targeted were excluded from the analysis; however, five wolves that originated from targeted packs and were trapped in the territories of adjacent packs were included. Trapping data were analysed for the effects of age class, sex and previous capture on the probability of capture and vaccination (trappability) of wolves. This was initially done using chi-squared tests for unconditional associations, followed by fitting all variables in a generalized linear mixed model (GLMM) with a logistic link function, with pack fitted as the random effect (floater females were excluded from the GLMM as they do not form a discrete pack). Model simplification was done through backwards elimination. At the pack level we used data from the first trapping of each pack to examine factors affecting the proportion of wolves captured (using a generalized linear model with weighted regression and binomial errors; Crawley 2002) and the probability of capturing at least one adult female (Fisher's exact test and univariate GLM with binomial errors). Predictor variables included pack size (range 6–11), territory size (range 352–1509 ha, calculated as 99% minimum concave polygons, L. A. Tallents, unpublished data), number of adult females in pack and trap-hours (with one trap-hour defined as one trap-garden open for 1 h). Finally, capture success rates at each trapping occasion were measured as the number of trap-hours per wolf caught (within target packs) and the proportion of wolves trapped in each pack. All analyses were conducted using R (version 2.0.1, The R Foundation for Statistical Computing), with P= 0·05 indicating statistical significance.

Results

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

the intervention

Sixty-nine animals were vaccinated between November 2003 and February 2004 during the control phase of the intervention: 36 and 33 from the Morebawa and Sanetti subpopulations, respectively (Fig. 1). An additional eight animals were vaccinated during the follow-up recapture phase (March–November 2004) and seven animals were sampled from the Web Valley outbreak area 2 months after the last carcass was found. Population coverage was 37% over 16 packs in Morebawa and 48% over nine packs in Sanetti. Seven packs (five in Morebawa and two in Sanetti), unknown at the time of intervention, were not trapped; however, these wolves are included in calculations of overall population coverage.

serology

Of the 77 wolves captured for the first time in the intervention zone, 74 were regarded as seronegative (rabies virus neutralizing antibody titres < 0·5 IU mL−1) whereas three had titres of 0·87, 0·87 and 1·97 IU mL−1. These latter were all seronegative when duplicate serum samples were tested (with mean titres across the two tests of 0·52, 0·69 and 1·07 IU mL−1), suggesting that the initial seropositive results were the result of variation in the FAVN test, a common occurrence at low titres (Briggs et al. 1998). Of the seven animals captured in Web Valley post-outbreak, three tested seropositive (including retest of duplicate samples, with mean values of 0·77, 1·56 and 5·92 IU mL−1). The higher of these values could imply recent exposure to the virus, most probably through an abortive infection (de Diaz, Fuenzalida & Bell 1975). A saliva sample from one of these wolves was negative for rabies virus RNA on RT-PCR, but other saliva samples collected were too degraded for analysis. Two of these animals were still alive 1 year later, while the remaining animal was last seen 5 months after sampling.

Analysis of antibody titres at 1 month post-vaccination showed that all 19 sampled wolves seroconverted (median 4·50 IU mL−1, interquartile range 3·01–9·03 IU mL−1). An initial anova suggested a significant effect of volume on loge neutralizing antibody titres, with the 2-mL dose inducing a significantly higher titre (F = 4·55, residual d.f. = 17, P= 0·048); however, the Studentised residual for one of the observations was highly significant (maximal model, t= 6·04, d.f. = 14, P < 0·001) and remained so when the model was refitted with titres derived from retesting of all 19 duplicate serum samples (t = 3·58, d.f. = 14, P= 0·002). Excluding this animal, the anova revealed a significant effect of vaccine batch on post-vaccination titres, for both the original (F = 10·54, residual d.f. = 16, P= 0·005) and duplicate (F = 7·55, residual d.f. = 16, P= 0·014) serum samples. This result may, however, be confounded by geographical and temporal factors that led to disparities in vaccine transport and storage conditions: the two batches were employed in separate areas with different logistic constraints, and the batch that induced the lower titres was used later in the campaign.

The effect of volume was no longer significant when included in the model with batch (F = 2·16, residual d.f. = 15, P= 0·16), although animals vaccinated with 2 mL vaccine did have a higher mean titre than those that received only 1 mL (6·48 IU mL−1 compared with 4·05 IU mL−1). Neither age class (P = 0·27) nor sex (P = 0·56) had a significant effect.

Figure 2 shows the longevity of rabies neutralizing antibody titres in primary-vaccinated and boosted wolves. At 180 days, two wolves (one primary-vaccinated with a 2-mL dose and the other primary-vaccinated with a 1-mL dose and boosted with a 1-mL dose at 30 days) were seropositive, with titres of 0·87 IU mL−1 and 0·66 IU mL−1, respectively. A third animal that was primary-vaccinated with 1 mL but did not receive a booster was seronegative (0·29 IU mL−1). Of the five wolves sampled approximately 1 year post-vaccination, the four that received only a primary vaccination were seronegative (of these, three animals had received 2 mL and one 1 mL). The single animal that received a booster dose after an initial 1 mL was seropositive (3·42 IU mL−1).

image

Figure 2. Mean rabies neutralizing antibody titres (with 95% confidence intervals) of wolves that received a single injection with inactivated rabies vaccine on day 0 (primary-vaccinated, diamonds) and those that received a booster dose at approximately 30 days (boosted, squares). Dotted line shows the adopted cut-off for seropositivity (0·5 IU mL−1). Figures above the symbols are the number of wolves in each group.

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trappability

Males had higher odds of capture than females at first trapping [χ2 = 4·34, d.f. = 1, P= 0·04, odds ratio = 2·33, exact 95% confidence interval (CI) = 1·11–4·86], although this association was no longer significant if floater females (none of whom were captured and whose existence was extrapolated from previous studies) were ignored (χ2 = 1·69, d.f. = 1, P= 0·19). There was no association between age class and trappability, whether floater females were included (χ2 = 0·89, d.f. = 1, P= 0·35) or excluded (χ2 = 0·30, d.f. = 1, P= 0·59). These results held in the GLMM, with neither age class nor sex, nor their interaction, found to be significant (P > 0·15). The estimated variance of pack random effects in the GLMM was small (0·0004). The odds of being trapped a second time were greater for animals captured in the first trapping session (GLMM, t= 2·26, d.f. = 44, P= 0·03, odds ratio = 3·96, exact 95% CI = 1·20–13·04). However, the probability of an animal being captured a third time was not associated with it having been captured on the first (P = 0·13), second (P = 0·48) or either previous (P = 0·34) trapping occasion. The effect of trapping occasion on capture success rates is shown in Fig. 3.

image

Figure 3. Graph showing capture success rate as a function of the number of times packs were trapped. Figures above the graph show the number of wolves trapped (n), the percentage of wolves trapped that had not been trapped on any previous occasion (% new) and the number of wolves trapped as a proportion of all wolves present within targeted packs (success).

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Pack size, territory size and trap-hours were not significantly associated with the proportion of wolves captured within a pack (P > 0·37). In six of the 15 target packs, no adult female was trapped. An adult female was more likely to be trapped when there were two or more adult females in the pack (8/8 vs. 1/7, Fisher's exact test, P= 0·001). The probability of capturing at least one adult female was not significantly associated with total pack size (P = 0·84), trap-hours (P = 0·78) or territory size (P = 0·09).

effect of intervention on rabies outbreak

The last known rabies case was found in the Web Valley on 30 January 2004. Two carcasses were recovered on 18 and 22 November 2003 in the isthmus pack connecting the Web Valley to the Morebawa subpopulation (Fig. 1) and seven of eight individuals disappeared from the adjoining Weshema pack in Morebawa some time shortly after 17 January 2004. This Weshema pack had not been identified at the time of the intervention, although two pack members were vaccinated in adjacent territories. Of these, one individual who was vaccinated on 13 December 2003 disappeared after being resighted on 7 January 2004. The surviving individual received 2 mL vaccine on 15 November 2003, in the first week of the intervention. Despite increased levels of surveillance no other rabies-related deaths were detected in the intervention areas. In contrast, 40 carcasses were found in the Web Valley and surrounding areas during the course of the outbreak, and another 36 wolves disappeared in addition to the seven from Weshema, a total mortality of 76% in the Web Valley subpopulation (Randall et al. 2004).

risks to target and non-target species

During the intervention, 114 capture events occurred in more than 5200 trap-hours, with a capture event defined as the capture and sampling of a wolf. This definition thus includes animals recaught in further trapping sessions but excludes ‘trap-happy’ animals recaught within the same trapping session who were not resampled. Two capture events resulted in injury: one animal suffered a complete, uncomplicated fracture of the right tibia and fibula during physical restraint when two wolves were caught simultaneously, and a trap-inflicted injury to another wolf necessitated amputation of the medial digit of the right forefoot at the first interphalangeal joint. Both wolves were released and were still alive 1 year after capture. All but three captured wolves were alive 6 months after capture, a rate consistent with background mortality estimates (Randall et al. 2004). A full analysis of differential survival rates between handled and unhandled animals is underway. Trapping had minimal impact on non-target species: eight raptors, Aquila spp., were caught in traps and released without evident injury, while one Starck's hare Lepus starcki was killed because of a trap-induced femoral fracture. Six domestic dogs found in traps were vaccinated at the BMNP's request and released without incident.

Discussion

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

This paper presents several strands of evidence through which the success of the intervention can be assessed and general recommendations made. First, valuable information on the response of Ethiopian wolves to inactivated parenteral rabies vaccine was obtained. All 19 wolves recaptured 1 month after vaccination exhibited serum neutralizing antibody titres well above the accepted level of seroconversion. Of the wolves sampled 1 year after vaccination, the only individual with a high titre had received a booster dose of 1 mL at 1 month post-vaccination (Fig. 2). These results are comparable to the magnitude and duration of antibody responses reported in domestic dogs (Mansfield et al. 2004). Doubling the 1-mL dose (recommended for domestic dogs) at a single injection site did not produce significantly higher initial antibody titres in Ethiopian wolves at 1 month, nor did it appear to improve the longevity of antibodies, although the sample sizes were very small. Given these results, and that the immune response of Ethiopian wolves appears similar to that of domestic dogs, a prophylactic vaccination protocol of 1 mL inactivated vaccine, with a booster 1–6 months later, is advisable. Further boosters would undoubtedly be required to maintain immunity for life, although it should be noted that dogs are still protected against rabies virus challenge for 22–36 months after vaccination (Wandeler 2006), indicating that even in the absence of neutralizing antibodies individuals can mount a protective anamnestic immune response upon challenge.

The presence of three unvaccinated, seropositive wolves in the outbreak area raises the possibility that these animals may have survived exposure to the virus. It is speculated that this exposure was to a low dose of virus by non-bite transmission, most probably through the mucous membranes following oral–oral contact with an infected animal (Constantine 1962; Dutta 1998; Fooks et al. 2006). It is also plausible that the individual in the sample of 19 wolves (trapped outside the outbreak area) whose humoral immune response to vaccination was significantly higher than that of the others may have, although seronegative at the time of vaccination, retained sufficient immunologic memory from a previous exposure to mount a substantial and rapid humoral response to the vaccine.

Understanding the factors affecting the probability of capturing at least one adult female in each pack is important, as this may affect population recovery rates. In Ethiopian wolves, as in other co-operatively breeding canids, reproduction is essentially restricted to a single dominant female in a pack (Sillero-Zubiri, Gotelli & Macdonald 1996); therefore the pack (as defined by the presence of at least one reproductively capable female and male) is considered the fundamental breeding unit in the population. This has implications for population recovery following an epidemic: Marino, Sillero-Zubiri & Macdonald (2006) reported an inverse density-dependent growth rate in the Web Valley following the rabies epidemic there in 1992, with low population growth rate at low wolf densities most probably the result of the extinction of two of the five study packs (although the possibility of an Allee effect resulting in increased adult mortality or decreased juvenile recruitment within small packs was not ruled out; Courchamp, Clutton-Brock & Grenfell 1999) Failure to trap and vaccinate an adult female in a substantial proportion of packs, and failure to vaccinate any floater females who may otherwise have been able to fill breeding vacancies, may retard population recovery and keep populations at a level where they remain vulnerable to additional stochastic perturbations. Even if the pack continues to defend a territory, the loss of adult females could delay breeding until any surviving subadult females reach reproductive maturity. However, the validity of this hypothesis requires further investigation, as it will depend on the pattern of mortality seen in partially vaccinated packs, and on the response of a pack to the loss of its adult females. The interactions between number of adult females in a pack, territory size and quality and juvenile recruitment, and their implications for post-epidemic population recovery, also warrant further scrutiny.

The positive association between the probability of a wolf being trapped on the first and again on the second trapping occasion suggests that wolves may be inherently ‘trap-happy’ or ‘trap-shy’ such that repeated captures over time will yield the same individuals. While the trapping experience is almost certainly highly stressful, the sedation of animals with an agent known to cause a degree of retrograde amnesia (ketamine; Saha et al. 1990) may inhibit any potential deterrent effect. The lack of a similar association between the probability of capture on the third and any previous occasion and the marked decline in capture success between the second and third trapping sessions could be explained by the fact that wolves were not anaesthetized when trapped a second time, although sample sizes were small (Fig. 3).

All available empirical and theoretical evidence (Haydon et al. 2006) suggests that this intervention was effective in halting the spread of the outbreak. Rabies mortalities were recorded in all territories in Web Valley and the isthmus, up to the main vaccination area in Morebawa. The extent of the eastward spread of the virus towards the Sanetti Plateau is unknown, although no rabies-confirmed deaths occurred within the surveillance area there. Despite this success, and the minimal short-term impacts on the population, we believe that the adopted course of action was only justified in the face of a large-scale outbreak. Less invasive and more sustainable methods, such as the use of oral vaccines, are highly desirable and should be developed and tested as tools for future use, either prophylactically or during outbreak situations. The high prevalence of rabies in the surrounding domestic dog population and the apparent frequent incursion of rabies into the BMNP wolves makes the development of an effective oral vaccination protocol all the more urgent.

recommendations

Our experience in managing this outbreak leads us to suggest a number of recommendations that may be useful to other managers faced with disease epidemics in endangered populations. First, we are aware that the long-term population monitoring performed by the Ethiopian Wolf Conservation Programme was fundamental to the initial rapid detection of the outbreak and to the intervention outcome. The EWCP supplied valuable demographic and surveillance data and a team of experienced and dedicated field staff who were integral to the implementation of the intervention and post-intervention monitoring. Clearly, in other situations an existing monitoring system and experienced management team will assist any disease control programme and thus efforts should be made to ensure such expertise and programmes are in place.

Secondly, potential tools for disease control should be investigated and assessed, and risk analyses and predictive modelling of management options carried out in advance, where possible. In this regard we were assisted by existing theoretical models (Haydon, Laurenson & Sillero-Zubiri 2002) and action plans (Laurenson, Shiferaw & Sillero-Zubiri 1997) stemming from the pre-existing knowledge that domestic dog diseases pose a threat to Ethiopian wolves. However, oral baiting and vaccination trials had not been carried out, although permission had been requested. Managers of threatened species where the disease risk has not been identified or explored should perhaps devote more effort to risk assessment and contingency planning for disease outbreaks.

Thirdly, when considering an intervention programme, and with the potential controversy of such an approach, we suggest that it is essential to design carefully both the intervention itself and post-intervention monitoring, in order to maximize the information gained for that and future intervention programmes. The capacity and resources for population monitoring after intervention and the data required for subsequent outcome analysis should be carefully considered. In some situations, it may be that additional procedures that involve more handling will be necessary to assess the effectiveness of the intervention. These issues have financial and human resource implications, and must be built into emergency funding budgets. In addition, expert opinion should be canvassed so that policy can be decided in advance, although the details of each outbreak situation may preclude firm policy decisions in some cases. We therefore recommend that contingency policy, decision trees and plans be developed before a problem arises so that the course of action is already laid out for decision making and intervention as required.

Fourthly, we acknowledge that previous discussions, debates and failures greatly assisted our approach to the intervention. Such discussion has stimulated a more rigorous approach to disease management problems and has generally driven forward the issue by increasing general awareness, although in some situations misunderstanding of the issues has led to lack of decision-making. Thus we suggest that maintaining discussion and disseminating information, such as that gained in this outbreak, in accessible literature is crucial in enlightening debate and improving future decision-making in the face of wildlife disease outbreaks.

An integrated and collaborative approach is now essential to studying and controlling pathogens that affect wildlife. With ecological, behavioural, political and human sociocultural factors all having an effect on transmission and spread of disease, control programmes need to address all these aspects if they are to have a chance of success. We suggest that links with institutional and government authorities, including laboratories and other experts, should be in place prior to the occurrence of an outbreak. With increasing restrictions on shipping samples because of bioterrorism threats, developing and maintaining quality local or regional diagnostic laboratories must also be a priority.

In conclusion, we hope that this case study and our experience-based recommendations will encourage wildlife researchers and managers to collect and present data on disease management interventions. Only with a balanced understanding of the risks involved, and by learning from interventions and their consequences, can a considered, multidisciplinary approach to disease management in wildlife populations be achieved We strongly recommend that all efforts are made to carry out a priori both risk assessments and research into the feasibility of disease control methods.

Acknowledgements

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

We would like to acknowledge the invaluable contribution and support of staff of the Ethiopian Wolf Conservation Programme, The Oromia Agriculture and Resource Development Bureau and the Ethiopian Wildlife Conservation Department. Drs Theo Kanellos and Stuart Chalmers (Intervet) and members of the IUCN/SSC Canid and Veterinary Specialist Groups all gave freely of their advice during the planning and implementation of the vaccination. Funding for the intervention was supplied by Born Free Foundation, Frankfurt Zoological Society and Morris Animal Foundation. We wish to acknowledge the expertise of Mr Graham Parsons and Mrs Trudy Goddard (VLA, Weybridge) for undertaking the serology and Mr Robin Sayers (VLA, Weybridge) and Dr Darren Shaw (University of Edinburgh) for assistance with statistical analysis of the data. D. L. Knobel was supported by a grant from the Wellcome Trust, UK. A. R. Fooks and S. M. Brookes were financially supported by the UK Department for Environment, Food and Rural Affairs (Defra) (contract SEV 35000).

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  6. Discussion
  7. Acknowledgements
  8. References
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