A Recombinant Raccoon Poxvirus Vaccine Expressing both Yersinia pestis F1 and Truncated V Antigens Protects Animals against Lethal Plague

Safety and immunogenicity of orally administered poxvirus vectored constructs in the white-footed mouse (Peromyscus leucopus)

Globally, zoonotic spillover is becoming more frequent and represents a growing public health concern. Reservoir-targeted vaccination offers an intriguing alternative to traditional vaccine practices by establishing protection in wild populations that maintain the natural pathogen cycle. As an important pathogen reservoir, Peromyscus leucopus Rafinesque or the white-footed mouse has been the target of several experimental vaccines. However, strategies are limited by the method of administration, need for repeated dosing, or safety of constructs in the field. To address these concerns, we evaluated two highly attenuated poxviruses, raccoonpox virus (RCN) and modified vaccinia Ankara (MVA) virus as potential oral vaccine vectors in white-footed mice. Following oral administration, P. leucopus showed no adverse signs. A single oral dose elicited robust immune responses in mice to the foreign influenza hemagglutinin protein expressed by poxvirus vaccine vectors. Serum hemagglutinin inhibition antibody titers were detected by day 7 post immunization and persisted until study termination (77 days post immunization). This study establishes the safety and immunogenicity of recombinant MVA and RCN poxviruses in P. leucopus and demonstrates the suitability of these vectors as part of a reservoir-targeted vaccine strategy for white-footed mice.

Oral Sylvatic Plague Vaccine Does Not Adequately Protect Prairie Dogs (Cynomys spp.) for Endangered Black-Footed Ferret (Mustela nigripes) Conservation

The plague bacterium Yersinia pestis is lethal to endangered black-footed ferrets (Mustela nigripes, BFF) and the prairie dogs (Cynomys spp., PD) on which they depend for habitat and prey. We assessed the effectiveness of an oral sylvatic plague vaccine delivered in baits to black-tailed PD (Cynomys ludovicianus, BTPD) from 2013 to 2017 on the Charles M. Russell National Wildlife Refuge (CMR) in northcentral Montana. We permanently marked BTPD on four paired vaccine (N = 1,349 individuals) and placebo plots (N = 926; 7,027 total captures). We analyzed capture–recapture data under a Cormack–Jolly–Seber model to estimate annual apparent survival. Overall, survival averaged 0.05 lower on vaccine plots than on paired placebo plots. Immediately before noticeable die-offs and detecting plague on pairs CMR1 and CMR2, 89% of BTPD sampled on vaccine plots had consumed at least one bait and the immune systems of 40% were likely boosted by consuming baits over multiple years. Survival to the following year was 0.16 and 0.05 on the vaccine plots and 0.19 and 0.06 on the placebo plots for pairs CMR1 and CMR2, respectively. These rates were markedly lower than 0.63, the overall average estimate on those same plots during the previous 3 years. PD populations subjected to such large die-offs would not be expected to sustain a BFF population. An overriding limitation to achieving sufficient protection rests with vaccine delivery constraints. Late summer/fall bait distribution results in the highest bait uptake rates. However, the PD birth pulse each spring can double the size of populations in most years, greatly reducing the proportion of vaccinates in populations and diminishing potential herd immunity benefits. In addition to nonvaccinated juveniles and PD that do not consume bait, incomplete vaccine protection and time required for immunity to develop leaves a large majority of PD populations vulnerable to plague for 6–7 months or more each year.

Outside the Box: Working With Wildlife in Biocontainment

Research with captive wildlife in Animal Biosafety Level 2 (ABSL2) and 3 (ABSL3) facilities is becoming increasingly necessary as emerging and re-emerging diseases involving wildlife have increasing impacts on human, animal, and environmental health. Utilizing wildlife species in a research facility often requires outside the box thinking with specialized knowledge, practices, facilities, and equipment. The USGS National Wildlife Health Center (NWHC) houses an ABSL3 facility dedicated to understanding wildlife diseases and developing tools to mitigate their impacts on animal and human health. This review presents considerations for utilizing captive wildlife for infectious disease studies, including, husbandry, animal welfare, veterinary care, and biosafety. Examples are drawn from primary literature review and collective 40-year experience of the NWHC. Working with wildlife in ABSL2 and ABSL3 facilities differs from laboratory animals in that typical laboratory housing systems, husbandry practices, and biosafety practices are not designed for work with wildlife. This requires thoughtful adaptation of standard equipment and practices, invention of customized solutions and development of appropriate enrichment plans using the natural history of the species and the microbiological characteristics of introduced and native pathogens. Ultimately, this task requires critical risk assessment, understanding of the physical and psychological needs of diverse species, creativity, innovation, and flexibility. Finally, continual reassessment and improvement are imperative in this constantly changing specialty area of infectious disease and environmental hazard research.

SENTINEL COYOTE PATHOGEN SURVEY TO ASSESS DECLINING BLACK-FOOTED FERRET (MUSTELA NIGRIPES) POPULATION IN SOUTH DAKOTA, USA

As part of the national recovery effort, endangered black-footed ferrets (Mustela nigripes) were reintroduced to the Cheyenne River Sioux Reservation in South Dakota, US in 2000. Despite an encouraging start, numbers of ferrets at the site have declined. In an effort to determine possible causes of the population decline, we undertook a pathogen survey in 2012 to detect exposure to West Nile virus (WNV), canine distemper virus (CDV), plague (Yersinia pestis), tularemia (Francisella tularensis), and heartworm (Dirofilaria immitis) using coyotes (Canis latrans) as a sentinel animal. The highest seroprevalence was for WNV with 71% (20/28) of coyotes testing antibody-positive. Seroprevalence of CDV and plague were lower, 27% and 13%, respectively. No evidence of active infection with tularemia or heartworm was seen in the coyotes sampled. As this study did not sample black-footed ferrets themselves, the definitive cause for the decline of this population cannot be determined. However, the presence of coyotes seropositive for two diseases, plague and CDV, lethal to black-footed ferrets, indicated the potential for exposure and infection. The high seroprevalence of WNV in the coyotes indicated a wide exposure to the virus; therefore, exposure of black-footed ferrets to the virus is also likely. Due to the ability of WNV to cause fatal disease in other species, studies may be useful to elucidate the impact that WNV could have on the success of reintroduced black-footed ferrets as well as factors influencing the spread and incidence of the disease in a prairie ecosystem.

Differential plague susceptibility in species and populations of prairie dogs

Laboratory trials conducted over the past decade at U.S. Geological Survey National Wildlife Health Center indicate that wild populations of prairie dogs (Cynomys spp.) display different degrees of susceptibility to experimental challenge with fully virulent Yersinia pestis, the causative agent of plague. We evaluated patterns in prairie dog susceptibility to plague to determine whether the historical occurrence of plague at location of capture was related to survival times of prairie dogs challenged with Y. pestis. We found that black-tailed prairie dogs (Cynomys ludovicianus) from South Dakota (captured prior to the detection of plague in the state), Gunnison's prairie dogs (Cynomys gunnisoni) from Colorado, and Utah prairie dogs (Cynomys parvidens) from Utah were most susceptible to plague. Though the susceptibility of black-tailed prairie dogs in South Dakota compared with western locations supports our hypothesis regarding historical exposure, both Colorado and Utah prairie dogs have a long history of exposure to plague. It is possible that for these populations, genetic isolation/bottle necks have made them more susceptible to plague outbreaks.

Sylvatic plague management and prairie dogs - a meta-analysis

Yersinia pestis, a bacterial pathogen that causes sylvatic plague, is present in the prairie dogs (Cynomys spp.) of North America. Epizootics of sylvatic plague through transmission in vectors (fleas) commonly completely extirpate colonies of prairie dogs. Wildlife managers employ a wide variety of insecticidal treatments to suppress plague and conserve prairie dog colonies. I compiled and statistically compared the available literature describing methods of plague control and their relative effectiveness in managing plague outbreaks by using meta-analyses. Natural log response ratios were used to calculate insecticide-induced vector mortality and vaccine-conferred survival increases in prairie dogs in 37 publications. Further, subgroupings were used to explore the most effective of the available vector suppression insecticides and plague suppression vaccines. After accounting for the type of treatment used and the method by which it was applied, I observed plague reduction through use of both insecticides and vaccines. Insecticides resulted in a significant reduction of the abundance of vectors by 91.34% compared to non-treated hosts (p<0.0001). Vaccines improved survival of prairie dog hosts by 4.00% (p<0.0001) compared to control populations. The use of insecticides such as deltamethrin and carbaryl is recommended to stop actively spreading epizootics, and dual antigen oral vaccines to initially suppress outbreaks.

Plague vaccine: recent progress and prospects

Three great plague pandemics, resulting in nearly 200 million deaths in human history and usage as a biowarfare agent, have made Yersinia pestis as one of the most virulent human pathogens. In late 2017, a large plague outbreak raged in Madagascar attracted extensive attention and caused regional panics. The evolution of local outbreaks into a pandemic is a concern of the Centers for Disease Control and Prevention (CDC) in plague endemic regions. Until now, no licensed plague vaccine is available. Prophylactic vaccination counteracting this disease is certainly a primary choice for its long-term prevention. In this review, we summarize the latest advances in research and development of plague vaccines.

Impact of Sylvatic Plague Vaccine on Non-target Small Rodents in Grassland Ecosystems

Oral vaccination is an emerging management strategy to reduce the prevalence of high impact infectious diseases within wild animal populations. Plague is a flea-borne zoonosis of rodents that often decimates prairie dog (Cynomys spp.) colonies in the western USA. Recently, an oral sylvatic plague vaccine (SPV) was developed to protect prairie dogs from plague and aid recovery of the endangered black-footed ferret (Mustela nigripes). Although oral vaccination programs are targeted toward specific species, field distribution of vaccine-laden baits can result in vaccine uptake by non-target animals and unintended indirect effects. We assessed the impact of SPV on non-target rodents at paired vaccine and placebo-treated prairie dog colonies in four US states from 2013 to 2015. Bait consumption by non-target rodents was high (70.8%, n = 3113), but anti-plague antibody development on vaccine plots was low (23.7%, n = 266). In addition, no significant differences were noted in combined deer mice (Peromyscus maniculatus) and western harvest mouse (Reithrodontomys megalotis) abundance or community evenness and richness of non-target rodents between vaccine-treated and placebo plots. In our 3-year field study, we could not detect a significant positive or negative effect of SPV application on non-target rodents.

Factors Influencing Uptake of Sylvatic Plague Vaccine Baits by Prairie Dogs

Sylvatic plague vaccine (SPV) is a virally vectored bait-delivered vaccine expressing Yersinia pestis antigens that can protect prairie dogs (Cynomys spp.) from plague and has potential utility as a management tool. In a large-scale 3-year field trial, SPV-laden baits containing the biomarker rhodamine B (used to determine bait consumption) were distributed annually at a rate of approximately 100-125 baits/hectare along transects at 58 plots encompassing the geographic ranges of four species of prairie dogs. We assessed site- and individual-level factors related to bait uptake in prairie dogs to determine which were associated with bait uptake rates. Overall bait uptake for 7820 prairie dogs sampled was 70% (95% C.I. 69.9-72.0). Factors influencing bait uptake rates by prairie dogs varied by species, however, in general, heavier animals had greater bait uptake rates. Vegetation quality and day of baiting influenced this relationship for black-tailed, Gunnison's, and Utah prairie dogs. For these species, baiting later in the season, when normalized difference vegetation indices (a measure of green vegetation density) are lower, improves bait uptake by smaller animals. Consideration of these factors can aid in the development of species-specific SPV baiting strategies that maximize bait uptake and subsequent immunization of prairie dogs against plague.

Human case of bubonic plague resulting from the bite of a wild Gunnison's prairie dog during translocation from a plague-endemic area

Plague is a zoonotic disease (transmitted mainly by fleas and maintained in nature by rodents) that causes severe acute illness in humans. We present a human plague case who became infected by the bite of a wild Gunnison's prairie dog, and a good practical example of the One Health approach that resulted in a rapid public health response. The exposure occurred while the animal was being transported for relocation to a wildlife refuge after being trapped in a plague enzootic area. This is the first report of a human plague case resulting from the bite of a Gunnison's prairie dog. Additionally, we present an observation of a longer incubation period for plague in captive prairie dogs, leading to a recommendation for a longer quarantine period for prairie dogs during translocation efforts.

Vaccines for Conservation: Plague, Prairie Dogs & Black-Footed Ferrets as a Case Study

The endangered black-footed ferret (Mustela nigripes) is affected by plague, caused by Yersinia pestis, both directly, as a cause of mortality, and indirectly, because of the impacts of plague on its prairie dog (Cynomys spp.) prey base. Recent developments in vaccines and vaccine delivery have raised the possibility of plague control in prairie dog populations, thereby protecting ferret populations. A large-scale experimental investigation across the western US shows that sylvatic plague vaccine delivered in oral baits can increase prairie dog survival. In northern Colorado, an examination of the efficacy of insecticides to control fleas and plague vaccine shows that timing and method of plague control is important, with different implications for long-term and large-scale management of Y. pestis delivery. In both cases, the studies show that ambitious field-work and cross-sectoral collaboration can provide potential solutions to difficult issues of wildlife management, conservation and disease ecology.

Sylvatic Plague Vaccine Partially Protects Prairie Dogs (Cynomys spp.) in Field Trials

Sylvatic plague, caused by Yersinia pestis, frequently afflicts prairie dogs (Cynomys spp.), causing population declines and local extirpations. We tested the effectiveness of bait-delivered sylvatic plague vaccine (SPV) in prairie dog colonies on 29 paired placebo and treatment plots (1-59 ha in size; average 16.9 ha) in 7 western states from 2013 to 2015. We compared relative abundance (using catch per unit effort (CPUE) as an index) and apparent survival of prairie dogs on 26 of the 29 paired plots, 12 with confirmed or suspected plague (Y. pestis positive carcasses or fleas). Even though plague mortality occurred in prairie dogs on vaccine plots, SPV treatment had an overall positive effect on CPUE in all three years, regardless of plague status. Odds of capturing a unique animal were 1.10 (95% confidence interval [C.I.] 1.02-1.19) times higher per trap day on vaccine-treated plots than placebo plots in 2013, 1.47 (95% C.I. 1.41-1.52) times higher in 2014 and 1.19 (95% C.I. 1.13-1.25) times higher in 2015. On pairs where plague occurred, odds of apparent survival were 1.76 (95% Bayesian credible interval [B.C.I.] 1.28-2.43) times higher on vaccine plots than placebo plots for adults and 2.41 (95% B.C.I. 1.72-3.38) times higher for juveniles. Our results provide evidence that consumption of vaccine-laden baits can protect prairie dogs against plague; however, further evaluation and refinement are needed to optimize SPV use as a management tool.

Burrow Dusting or Oral Vaccination Prevents Plague-Associated Prairie Dog Colony Collapse

Plague impacts prairie dogs (Cynomys spp.), the endangered black-footed ferret (Mustela nigripes) and other sensitive wildlife species. We compared efficacy of prophylactic treatments (burrow dusting with deltamethrin or oral vaccination with recombinant "sylvatic plague vaccine" [RCN-F1/V307]) to placebo treatment in black-tailed prairie dog (C. ludovicianus) colonies. Between 2013 and 2015, we measured prairie dog apparent survival, burrow activity and flea abundance on triplicate plots ("blocks") receiving dust, vaccine or placebo treatment. Epizootic plague affected all three blocks but emerged asynchronously. Dust plots had fewer fleas per burrow (P < 0.0001), and prairie dogs captured on dust plots had fewer fleas (P < 0.0001) than those on vaccine or placebo plots. Burrow activity and prairie dog density declined sharply in placebo plots when epizootic plague emerged. Patterns in corresponding dust and vaccine plots were less consistent and appeared strongly influenced by timing of treatment applications relative to plague emergence. Deltamethrin or oral vaccination enhanced apparent survival within two blocks. Applying insecticide or vaccine prior to epizootic emergence blunted effects of plague on prairie dog survival and abundance, thereby preventing colony collapse. Successful plague mitigation will likely entail strategic combined uses of burrow dusting and oral vaccination within large colonies or colony complexes.

Oral vaccination of wildlife against rabies: Differences among host species in vaccine uptake efficiency

Oral vaccination using attenuated and recombinant rabies vaccines has been proven a powerful tool to combat rabies in wildlife. However, clear differences have been observed in vaccine titers needed to induce a protective immune response against rabies after oral vaccination in different reservoir species. The mechanisms contributing to the observed resistance against oral rabies vaccination in some species are not completely understood. Hence, the immunogenicity of the vaccine virus strain, SPBN GASGAS, was investigated in a species considered to be susceptible to oral rabies vaccination (red fox) and a species refractory to this route of administration (striped skunk). Additionally, the dissemination of the vaccine virus in the oral cavity was analyzed for these two species. It was shown that the palatine tonsils play a critical role in vaccine virus uptake. Main differences could be observed in palatine tonsil infection between both species, revealing a locally restricted dissemination of infected cells in foxes. The absence of virus infected cells in palatine tonsils of skunks suggests a less efficient uptake of or infection by vaccine virus which may lead to a reduced response to oral vaccination. Understanding the mechanisms of oral resistance to rabies virus vaccine absorption and primary replication may lead to the development of novel strategies to enhance vaccine efficacy in problematic species like the striped skunk.

Using Off-the-Shelf Technologies to Mass Manufacture Oral Vaccine Baits for Wildlife

Technology and infrastructure costs can limit access to oral vaccination tools for wildlife disease control. We describe vaccine bait mass manufacturing employing off-the-shelf technologies. Our approach has helped advance scaling-up of plague vaccination campaigns, but components of this production system could be translated into other wildlife vaccination applications.

Plague Vaccine Development: Current Research and Future Trends

Plague is one of the world’s most lethal human diseases caused by Yersinia pestis, a Gram-negative bacterium. Despite overwhelming studies for many years worldwide, there is no safe and effective vaccine against this fatal disease. Inhalation of Y. pestis bacilli causes pneumonic plague, a fast growing and deadly dangerous disease. F1/LcrV-based vaccines failed to provide adequate protection in African green monkey model in spite of providing protection in mice and cynomolgus macaques. There is still no explanation for this inconsistent efficacy, and scientists leg behind to search reliable correlate assays for immune protection. These paucities are the main barriers to improve the effectiveness of plague vaccine. In the present scenario, one has to pay special attention to elicit strong cellular immune response in developing a next-generation vaccine against plague. Here, we review the scientific contributions and existing progress in developing subunit vaccines, the role of molecular adjuvants; DNA vaccines; live delivery platforms; and attenuated vaccines developed to counteract virulent strains of Y. pestis.

Infectivity of attenuated poxvirus vaccine vectors and immunogenicity of a raccoonpox vectored rabies vaccine in the Brazilian Free-tailed bat (Tadarida brasiliensis)

Bats (Order Chiroptera) are an abundant group of mammals with tremendous ecological value as insectivores and plant dispersers, but their role as reservoirs of zoonotic diseases has received more attention in the last decade. With the goal of managing disease in free-ranging bats, we tested modified vaccinia Ankara (MVA) and raccoon poxvirus (RCN) as potential vaccine vectors in the Brazilian Free-tailed bat (Tadarida brasiliensis), using biophotonic in vivo imaging and immunogenicity studies. Animals were administered recombinant poxviral vectors expressing the luciferase gene (MVA-luc, RCN-luc) through oronasal (ON) or intramuscular (IM) routes and subsequently monitored for bioluminescent signal indicative of viral infection. No clinical illness was noted after exposure to any of the vectors, and limited luciferase expression was observed. Higher and longer levels of expression were observed with the RCN-luc construct. When given IM, luciferase expression was limited to the site of injection, while ON exposure led to initial expression in the oral cavity, often followed by secondary replication at another location, likely the gastric mucosa or gastric associated lymphatic tissue. Viral DNA was detected in oral swabs up to 7 and 9 days post infection (dpi) for MVA and RCN, respectively. While no live virus was detected in oral swabs from MVA-infected bats, titers up to 3.88 x 10⁴ PFU/ml were recovered from oral swabs of RCN-infected bats. Viral DNA was also detected in fecal samples from two bats inoculated IM with RCN, but no live virus was recovered. Finally, we examined the immunogenicity of a RCN based rabies vaccine (RCN-G) following ON administration. Significant rabies neutralizing antibody titers were detected in the serum of immunized bats using the rapid fluorescence focus inhibition test (RFFIT). These studies highlight the safety and immunogenicity of attenuated poxviruses and their potential use as vaccine vectors in bats.

Models for managing wildlife disease

Modelling wildlife disease poses some unique challenges. Wildlife disease systems are data poor in comparison with human or livestock disease systems, and the impact of disease on population size is often the key question of interest. This review concentrates specifically on the application of dynamic models to evaluate and guide management strategies. Models have proved useful particularly in two areas. They have been widely used to evaluate vaccination strategies, both for protecting endangered species and for preventing spillover from wildlife to humans or livestock. They have also been extensively used to evaluate culling strategies, again both for diseases in species of conservation interest and to prevent spillover. In addition, models are important to evaluate the potential of parasites and pathogens as biological control agents. The review concludes by identifying some key research gaps, which are further development of models of macroparasites, deciding on appropriate levels of complexity, modelling genetic management and connecting models to data.

Age at Vaccination May Influence Response to Sylvatic Plague Vaccine (SPV) in Gunnison's Prairie Dogs (Cynomys gunnisoni)

Gunnison's prairie dogs (Cynomys gunnisoni) have been considered at greater risk from Yersinia pestis (plague) infection in the montane portion of their range compared to populations at lower elevations, possibly due to factors related to flea transmission of the bacteria or greater host susceptibility. To test the latter hypothesis and determine whether vaccination against plague with an oral sylvatic plague vaccine (SPV) improved survival, we captured prairie dogs from a C. g. gunnisoni or "montane" population and a C. g. zuniensis or "prairie" population for vaccine efficacy and challenge studies. No differences (P = 0.63) were found in plague susceptibility in non-vaccinated animals between these two populations; however, vaccinates from the prairie population survived plague challenge at significantly higher rates (P < 0.01) than those from the montane population. Upon further analysis, we determined that response to immunization was most likely associated with differences in age, as the prairie group was much younger on average than the montane group. Vaccinates that were juveniles or young adults survived plague challenge at a much higher rate than adults (P < 0.01 and P = 0.02, respectively), but no difference (P = 0.83) was detected in survival rates between control animals of different ages. These results suggest that host susceptibility is probably not related to the assumed greater risk from plague in the C. g. gunnisoni or "montane" populations of Gunnison's prairie dogs, and that SPV could be a useful plague management tool for this species, particularly if targeted at younger cohorts.

Apparent field safety of a raccoon poxvirus-vectored plague vaccine in free-ranging prairie dogs (Cynomys spp.), Colorado, USA

Prairie dogs (Cynomys spp.) suffer high rates of mortality from plague. An oral sylvatic plague vaccine using the raccoon poxvirus vector (designated RCN-F1/V307) has been developed for prairie dogs. This vaccine is incorporated into palatable bait along with rhodamine B as a biomarker. We conducted trials in August and September 2012 to demonstrate uptake and apparent safety of the RCN-F1/V307 vaccine in two prairie dog species under field conditions. Free-ranging prairie dogs and other associated small rodents readily consumed vaccine-laden baits during field trials with no apparent adverse effects; most sampled prairie dogs (90%) and associated small rodents (78%) had consumed baits. Visual counts of prairie dogs and their burrows revealed no evidence of prairie dog decline after vaccine exposure. No vaccine-related morbidity, mortality, or gross or microscopic lesions were observed. Poxviruses were not isolated from any animal sampled prior to bait distribution or on sites that received placebo baits. We isolated RCN-F1/V307 from 17 prairie dogs and two deer mice (Peromyscus maniculatus) captured on sites where vaccine-laden baits were distributed. Based on these findings, studies examining the utility and effectiveness of oral vaccination to prevent plague-induced mortality in prairie dogs and associated species are underway.