Prairie dog responses to vector control and vaccination during an initial Yersinia pestis invasion

We evaluated the invasion of plague bacteria Yersinia pestis into a population of black-tailed prairie dogs (Cynomys ludovicianus; BTPDs) in South Dakota. We aimed to ascertain if Y. pestis invaded slowly or rapidly, and to determine if vector (flea) control or vaccination of BTPDs assisted in increasing survival rates. We sampled BTPDs in 2007 (before Y. pestis documentation), 2008 (year of confirmed invasion), and 2009 (after invasion). We estimated annual BTPD re-encounter rates on three 9-ha plots treated annually with deltamethrin dust for flea control and three 9-ha plots lacking dust. In 2007 and 2008, approximately half the adult BTPDs live-trapped were injected subcutaneously with either an experimental plague vaccine (F1-V fusion protein) or placebo formulation; the remaining individuals were not inoculated. From 2007 to 2009, we sampled 1559 BTPDs on 2542 occasions. During 2007-2008, the prevalence and intensity of fleas on BTPDs were 69-97% lower on the dusted vs. no dust plots. From 2007 to 2008, the annual re-encounter rate of non-inoculated BTPDs was 150% higher on the dusted vs. no dust plots. During the same interval on the dusted plots, the re-encounter rate was 55% higher for vaccinated adult female BTPDs vs. nonvaccinated adult females, but the annual re-encounter rate was 19% lower for vaccinated adult males. By late August 2008, BTPDs were nearly extirpated from the no dust plots. During 2007-2008 and 2008-2009 on the dusted plots, which persisted, the BTPD re-encounter rate was 41% higher for vaccinated vs. non-vaccinated adult females but 35% lower for vaccinated adult males. Yersinia pestis erupted with vigor as it invaded. Flea control enhanced BTPD survival but did not offer full protection. Flea control and F1-V vaccination seemed to have additive, positive effects on adult females. Annual re-encounter rates were reduced for vaccinated adult males; additional experimentation is needed to further evaluate this trend.

Plague mitigation for prairie dog and black-footed ferret conservation: Degree and duration of flea control with 0.005% fipronil grain bait

Sylvatic plague, a primarily flea-borne zoonosis, is a significant threat to prairie dogs (Cynomys spp., PDs) and their specialized predators, endangered black-footed ferrets (Mustela nigripes, BFFs). Host-fed fipronil baits have proven effective in controlling fleas on PDs for the purposes of plague mitigation and BFF conservation. Currently, annual treatments are the norm. We tested the long-term efficacy of fipronil bait treatments with black-tailed PDs (C. ludovicianus, BTPDs) and BFFs in South Dakota, USA. During 2018-2020, we provided BTPDs on 21 sites with grain bait formula, laced with 0.005% fipronil (50 ​mg/kg); 18 non-treated sites functioned as baselines. In 2020-2022, we live-trapped, anesthetized, and combed BTPDs for fleas. Flea control was significant for at least 639-885 days. Flea abundance on the treated sites was < 0.5 fleas/BTPD for ∼750 days. During 2020-2022, we sampled BFFs for fleas on 4 BTPD colonies treated with fipronil grain bait and 8 non-treated colonies. Flea control was significant with BFFs, but flea abundance began to rebound within ∼240 days post-treatment. When feasible, the combination of insecticide treatments, such as fipronil baits, and BFF vaccination against plague provide a "two-pronged" protection approach for these endangered carnivores. If fipronil bait treatments are less effective with predatory BFFs than PDs, as found herein, the "two-pronged" approach might be used to protect BFFs and biennial fipronil bait treatments might be used to protect PDs. If BFF vaccination is not possible, or few BFFs can be vaccinated, annual fipronil bait treatments might be used as a precaution to protect BFFs. Flea densities might be surveyed to determine when/where more frequent treatments seem useful.

Comparison of Flea Sampling Methods and Yersinia pestis Detection on Prairie Dog Colonies

Scientists collect fleas (Siphonaptera) to survey for Yersinia pestis, the bacterial agent of plague. When studying fleas parasitizing prairie dogs (Cynomys spp.), two primary methods are used: (1) combing fleas from live-trapped prairie dogs and (2) swabbing fleas from burrows with cloth swabs attached to metal cables. Ideally, burrow swabbing, the cheaper and easier method, would explain flea burdens on prairie dogs and provide reliable information on plague prevalence. In a linear regression analysis of data from 1-month intervals (June-August 2010-2011) on 13 colonies of black-tailed prairie dogs (Cynomys ludovicianus, BTPDs) in New Mexico, flea abundance on swabs explained 0-26% of variation in BTPD flea burdens. In an analysis of data (May-August 2016) from six colonies of BTPDs in Montana, flea abundance on swabs explained 2% of variation in BTPD flea burdens. In an analysis of data from a short-term interval (July 23-27, 2019) on four colonies of BTPDs in Montana, flea abundance on swabs explained 0.1% of variation in BTPD flea burdens. In an analysis of data from 1-week intervals (August-October 2000) on four colonies of white-tailed prairie dogs (Cynomys leucurus, WTPD) in Utah, swabbing data explained 0.1% of variation in WTPD flea burdens. Pools of fleas from two WTPD colonies were tested for Y. pestis by mouse inoculation and isolation; 65% from WTPDs tested positive, whereas 4% from burrows tested positive. Data herein also show that results from burrow swabbing can misrepresent flea species composition and phenology on prairie dogs. Burrow swabbing is useful for some purposes, but limitations should be acknowledged, and accumulated data should be interpreted with caution.

Fipronil Pellets Reduce Flea Abundance on Black-Tailed Prairie Dogs: Potential Tool for Plague Management and Black-Footed Ferret Conservation

In western North America, sylvatic plague (a flea-borne disease) poses a significant risk to endangered black-footed ferrets (Mustela nigripes) and their primary prey, prairie dogs (Cynomys spp.). Pulicides (flea-killing agents) can be used to suppress fleas and thereby manage plague. In South Dakota, US, we tested edible "FipBit" pellets, each containing 0.84 mg fipronil, on free-living black-tailed prairie dogs (Cynomys ludivicianus). FipBits were applied along transects at 125 per ha and nearly eliminated fleas for 2 mo. From 9-14 mo post-treatment, we found only 10 fleas on FipBit sites versus 1,266 fleas on nontreated sites. This degree and duration of flea control should suppress plague transmission. FipBits are effective, inexpensive, and easily distributed but require federal approval for operational use.

Species Redescriptions and New Species of the Onthophagus mexicanus Species Group (Coleoptera: Scarabaeidae), with Notes on Distribution and Rodent-dung Beetle Associations

The Onthophagus mexicanus species group includes at least 18 species: O. anewtoni Howden and Génier, O. arnetti Howden and Cartwright, O. browni Howden, O. cartwrighti Howden, O. championi Bates, O. concinnus Castelnau, O. cynomysi Brown, O. eulophus Bates, O. guatemalensis Bates, O. hecate (Panzer), O. mcclevei Howden and Génier, O. medorensis Brown, O. mexicanus Bates, O. orpheus (Panzer), O. polyphemi Hubbard, O. pseudoguatemalensis sp. n., O. totonacus sp. n. and O. velutinus Howden and Cartwright. Onthophagus pseudoguatemalensis sp. n. and O. totonacus sp. n. are described from Mexico (Jalisco and Veracruz, respectively). Onthophagus cartwrighti, O. championi, O. eulophus and O. guatemalensis are redescribed, while lectotypes are designated herein for O. championi and O. eulophus. The distribution of O. cartwrighti is clarified; a new country record is provided for O. championi (Honduras); new state records are reported for O. championi (Oaxaca and Veracruz, Mexico) and O. guatemalensis (Oaxaca, Mexico). The accurate distribution of O. eulophus remains unknown since its original description. Updated distribution maps are included for all the species within the group. An updated determination key to species of the O. mexicanus species group is provided. The rarity of O. eulophus and O. totonacus in the entomological collections is thought to be a consequence of their trophic habits; both species are suggested to be inquilines of rodent nests or burrows.

Plague transforms positive effects of precipitation on prairie dogs to negative effects

Rodents characteristically benefit from increased precipitation, especially in typically dry habitats; “good years” of high precipitation improve their forage and water balance. However, Yersinia pestis (plague), a flea-borne pathogen of mammals that was introduced to western North America, has the greatest negative impact on at least some species of rodents during years of above-average precipitation. In the absence of plague mitigation, negative effects of plague in wet years might overwhelm the otherwise beneficial effects of increased moisture. In Montana and Utah, USA, where plague now occurs enzootically, we investigated the influence of precipitation on finite rates of annual population change (2000–2005) for 3 species of prairie dogs (Cynomys spp.) in replicated plots treated with deltamethrin dust and in non-treated plots for paired comparisons. There was a significant interaction between precipitation and treatment. When we reduced plague vector fleas, prairie dog visual counts tended to increase with increasing precipitation. Simultaneously, there was a negative relationship between counts and precipitation on paired plots where plague was not managed, suggesting that plague transformed and reversed the otherwise beneficial effect of increased precipitation. Are the good years gone for prairie dogs? Even if the good years are not gone, they are perhaps relatively scarce compared to historic times prior to the invasion of plague. This scenario might apply to other ecosystems and may pose broad conservation challenges in western North America.

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Negative plague effects overwhelm positive precipitation effects on prairie dogs.

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“Good years” of primary production may be gone in the introduced ranges of plague.

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Plague management with flea-control addresses this conservation challenge.

Epizootic Plague in Prairie Dogs: Correlates and Control with Deltamethrin

The plague bacterium, Yersinia pestis, is a generalist pathogen of flea (Siphonaptera) vectors and mammalian hosts. In colonies of prairie dogs (PDs, Cynomys spp.), Y. pestis causes occasional epizootics, killing ≥90% of PDs within weeks to several months. We evaluated the effectiveness of deltamethrin, a pyrethroid insecticide, as a tool for preventing plague epizootics among three PD species. Specifically, we studied PD population growth on paired plots treated with deltamethrin for flea control or left untreated as baselines. We also evaluated PD population growth relative to flea abundance and PD density. All epizootics occurred on nontreated plots. Epizootics occurred on plots with very low PD densities as well as high densities. Mean population change, assessed by comparing visual counts of PDs in years before and during epizootics, was +88% for treated plots and -97% for nontreated plots. For comparison, an experimental oral vaccine against plague had an average change in population index or estimate during epizootics of -69% on vaccine plots compared with -83% for associated nontreated (placebo) plots. In our study and on plots not treated with deltamethrin, PD population growth was negatively correlated with flea abundance in the year before the epizootic, lending support to the hypothesis that flea abundance plays a critical role in plague transmission under natural conditions. Generally speaking, deltamethrin is a highly effective tool for plague management on PD colonies. That said, continued study is needed to refine deltamethrin treatments and to develop a more integrated strategy for plague management.

Ecology and Management of Plague in Diverse Communities of Rodents and Fleas

Plague originated in Asia as a flea-borne zoonosis of mammalian hosts. Today, the disease is distributed nearly worldwide. In western United States of America, plague is maintained, transmitted, and amplified in diverse communities of rodents and fleas. We examined flea diversity on three species of prairie dogs (Cynomys spp., PDs) and six species of sympatric small rodents in Montana and Utah, United States of America. Among 2896 fleas, 19 species were identified; 13 were found on PDs and 9 were found on small rodents. In Montana, three flea species were found on PDs; the three species parasitize PDs and mice. In Utah, 12 flea species were found on PDs; the 12 species parasitize PDs, mice, voles, chipmunks, ground squirrels, rock squirrels, and marmots. Diverse flea communities and their willingness to parasitize many types of hosts, across multiple seasons and habitats, may favor plague maintenance and transmission. Flea parasitism on Peromyscus deer mice varied directly with elevation. Fleas are prone to desiccation, and might prosper at higher, mesic elevations; in addition, Peromyscus nest characteristics may vary with elevation. Effective management of plague is critical. Plague management is probably most effective when encompassing communities of rodents and fleas. Treatment of PD burrows with 0.05% deltamethrin dust, which suppressed fleas on PDs for >365 days, suppressed fleas on small rodents for at least 58 days. At one site, deltamethrin suppressed fleas on small rodents for at least 383 days. By simultaneously suppressing fleas on PDs and small rodents, deltamethrin should promote ecosystem resilience and One Health objectives.

Managing plague on prairie dog colonies: insecticides as ectoparasiticides

Human health practitioners and wildlife biologists use insecticides to manage plague by suppressing fleas (Siphonaptera), but insecticides can also kill other ectoparasites. We investigated effects of deltamethrin and fipronil on ectoparasites from black-tailed prairie dogs (Cynomys ludovicianus, BTPDs). In late July, 2018, we treated three sites with 0.05% deltamethrin dust and 5 sites with host-fed 0.005% fipronil grain. Three non-treated sites functioned as experimental baselines. We collected ectoparasites before treatments (June-July, 2018) and after treatments (August-October, 2018, June-July, 2019). Both deltamethrin and fipronil suppressed fleas for at least 12 months. Deltamethrin had no detectable effect on mites (Arachnida). Fipronil suppressed mites for at least 12 months. Lice (Phthiraptera) were scarce on non-treated sites throughout the study, complicating interpretation. Concentrating on eight sites where all three ectoparasites where found in June-July, 2018 (before treatments), flea intensity was greatest on BTPDs carrying many lice and mites. These three ectoparasites co-occurred at high numbers, which might facilitate plague transmission in some cases. Lethal effects of insecticides on ectoparasite communities are potentially advantageous in the context of plague management.

FLEA PARASITISM AND HOST SURVIVAL IN A PLAGUE-RELEVANT SYSTEM: THEORETICAL AND CONSERVATION IMPLICATIONS

Plague is a bacterial zoonosis of mammalian hosts and flea vectors. The disease is capable of ravaging rodent populations and transforming ecosystems. Because plague mortality is likely to be predicted by flea parasitism, it is critical to understand vector dynamics. It has been hypothesized that paltry precipitation and reduced vegetative production predispose herbivorous rodents to malnourishment and flea parasitism, and flea parasitism varies directly with plague mortality. We evaluated these hypotheses on five colonies of Utah prairie dogs (UPDs; Cynomys parvidens), on the Awapa Plateau, Utah, US, in 2013-16. Ten flea species were identified among 3,257 fleas from UPDs. These 10 flea species parasitize prairie dogs, mice, rats, voles, ground squirrels, chipmunks, and marmots, all known hosts of plague. The abundance of fleas on individual UPDs (1,198 observations) varied inversely with UPD body condition; fleas were most abundant on lightweight, malnourished UPDs. Flea abundance on UPDs was highest in dry years that were preceded by wet years. Increased precipitation and soil moisture in the prior year might generate humid microclimates in UPD burrows (that could facilitate flea survival and reproduction) and paltry precipitation in the current year could predispose UPDs to malnourishment and flea parasitism. Annual re-encounter rates for UPDs (1,072 observations) were reduced in wetter years preceded by drier years; reduced precipitation and vegetative production might kill UPDs, and increased flea densities in drier years could provide conditions for plague transmission (and UPD mortality) when moisture returns. Re-encounter rates were reduced for UPDs carrying at least one flea compared to UPDs with no detected fleas. These results support the hypothesis that reduced precipitation in the current year predisposes UPDs to flea parasitism. Our results also suggest a link between flea parasitism and UPD mortality. Given documented connections between flea parasitism and plague transmission, our results point toward an effect of flea parasitism on plague-related deaths for individual UPDs, a phenomenon rarely investigated in nature.

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 management of prairie dog colonies: degree and duration of deltamethrin flea control

Plague is a flea-borne disease of mammalian hosts. On the grasslands of western North America, plague stifles populations of Cynomys spp. prairie dogs (PDs). To manage plague, PD burrows are treated with 0.05% deltamethrin dust that can suppress flea numbers and plague transmission. Here, we evaluate the degree and duration of deltamethrin flea control with three PD species at six sites across four U.S. states. Data were simultaneously collected at paired plots. Burrows from one randomly assigned member of each pair were treated with deltamethrin; non-treated plots served as experimental baselines. Flea control was strong ≤two months after treatment, remained moderate one year later, and was statistically detectable for up to two years at some sites. Flea abundance was lower in plots with higher rates of deltamethrin application. After burrow treatments, flea abundance increased over time, reaching >one per PD within 255 to 352 days. Nevertheless, annual treatments of burrows with deltamethrin provided PDs with substantial protection against plague. Even so, deltamethrin should be further evaluated and combined with other tools under an integrated approach to plague management. Integrated plague management should help to conserve PDs and species that associate with them, including the endangered black-footed ferret (Mustela nigripes).

RESISTANCE TO DELTAMETHRIN IN PRAIRIE DOG ( CYNOMYS LUDOVICIANUS) FLEAS IN THE FIELD AND IN THE LABORATORY

Sylvatic plague poses a substantial risk to black-tailed prairie dogs ( Cynomys ludovicianus) and their obligate predator, the black-footed ferret ( Mustela nigripes). The effects of plague on prairie dogs and ferrets are mitigated using a deltamethrin pulicide dust that reduces the spread of plague by killing fleas, the vector for the plague bacterium. In portions of Conata Basin, Buffalo Gap National Grassland, and Badlands National Park, South Dakota, US, 0.05% deltamethrin has been infused into prairie dog burrows on an annual basis since 2005. We aimed to determine if fleas ( Oropsylla hirsuta) in portions of the Conata Basin and Badlands National Park have evolved resistance to deltamethrin. We assessed flea prevalence, obtained by combing prairie dogs for fleas, as an indirect measure of resistance. Dusting was ineffective in two colonies treated with deltamethrin for >8 yr; flea prevalence rebounded within 1 mo of dusting. We used a bioassay that exposed fleas to deltamethrin to directly evaluate resistance. Fleas from colonies with >8 yr of exposure to deltamethrin exhibited survival rates that were 15% to 83% higher than fleas from sites that had never been dusted. All fleas were paralyzed or dead after 55 min. After removal from deltamethrin, 30% of fleas from the dusted colonies recovered, compared with 1% of fleas from the not-dusted sites. Thus, deltamethrin paralyzed fleas from colonies with long-term exposure to deltamethrin, but a substantial number of those fleas was resistant and recovered. Flea collections from live-trapped prairie dogs in Thunder Basin National Grassland, Wyoming, US, suggest that, in some cases, fleas might begin to develop a moderate level of resistance to deltamethrin after 5-6 yr of annual treatments. Restoration of black-footed ferrets and prairie dogs will rely on an adaptive, integrative approach to plague management, for instance involving the use of vaccines and rotating applications of insecticidal products with different active ingredients.

Evaluation of Yersinia pestis Transmission Pathways for Sylvatic Plague in Prairie Dog Populations in the Western U.S

Sylvatic plague, caused by the bacterium Yersinia pestis, is periodically responsible for large die-offs in rodent populations that can spillover and cause human mortalities. In the western US, prairie dog populations experience nearly 100% mortality during plague outbreaks, suggesting that multiple transmission pathways combine to amplify plague dynamics. Several alternate pathways in addition to flea vectors have been proposed, such as transmission via direct contact with bodily fluids or inhalation of infectious droplets, consumption of carcasses, and environmental sources of plague bacteria, such as contaminated soil. However, evidence supporting the ability of these proposed alternate pathways to trigger large-scale epizootics remains elusive. Here we present a short review of potential plague transmission pathways and use an ordinary differential equation model to assess the contribution of each pathway to resulting plague dynamics in black-tailed prairie dogs (Cynomys ludovicianus) and their fleas (Oropsylla hirsuta). Using our model, we found little evidence to suggest that soil contamination was capable of producing plague epizootics in prairie dogs. However, in the absence of flea transmission, direct transmission, i.e., contact with bodily fluids or inhalation of infectious droplets, could produce enzootic dynamics, and transmission via contact with or consumption of carcasses could produce epizootics. This suggests that these pathways warrant further investigation.

Plague bacterium as a transformer species in prairie dogs and the grasslands of western North America

Invasive transformer species change the character, condition, form, or nature of ecosystems and deserve considerable attention from conservation scientists. We applied the transformer species concept to the plague bacterium Yersinia pestis in western North America, where the pathogen was introduced around 1900. Y. pestis transforms grassland ecosystems by severely depleting the abundance of prairie dogs (Cynomys spp.) and thereby causing declines in native species abundance and diversity, including threatened and endangered species; altering food web connections; altering the import and export of nutrients; causing a loss of ecosystem resilience to encroaching invasive plants; and modifying prairie dog burrows. Y. pestis poses an important challenge to conservation biologists because it causes trophic-level perturbations that affect the stability of ecosystems. Unfortunately, understanding of the effects of Y. pestis on ecosystems is rudimentary, highlighting an acute need for continued research.

Prevalence of the generalist flea Pulex simulans on black-tailed prairie dogs (Cynomys ludovicianus) in New Mexico, USA: the importance of considering imperfect detection

If a parasite is not detected during a survey, one of two explanations is possible: the parasite was truly absent or it was present but not detected. We fit occupancy models to account for imperfect detection when combing fleas (Siphonaptera) from black-tailed prairie dogs (Cynomys ludovicianus) during June-August 2012 in the Vermejo Park Ranch, New Mexico, USA. With the use of detection histories from combing events during monthly trapping sessions, we fit occupancy models for two flea species: Oropsylla hirusta (a prairie dog specialist) and Pulex simulans (a generalist). Detection probability was <100% for both species and about 21% lower for P. simulans. Pulex simulans may be especially difficult to detect because it is about half the size of O. hirusta. Monthly occupancy (prevalence) for P. simulans was estimated at 24% (June, 95% confidence interval = 19-30), 39% (July, 32-47), and 56% (August, 49-64) in new prairie dog colonies, and 43% (32-54), 61% (49-71), and 79% (70-87) in old colonies. These results suggest P. simulans can attain high prevalence on prairie dogs, especially in old colonies. If P. simulans is highly prevalent on prairie dogs, it may serve as a "bridge vector" between Cynomys and other mammalian hosts of the plague bacterium Yersinia pestis, and even function as a reservoir of Y. pestis between outbreaks.