Bait Uptake Rates Research Articles

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.

Fox baiting in agricultural landscapes: preliminary findings on the importance of bait-site selection

Context Little is known about the importance of bait-site selection during lethal fox-baiting programmes. Improved bait placement may increase the efficacy of baiting and help reduce fox impacts on wildlife and livestock. Aims To determine whether bait uptake by the red fox (Vulpes vulpes) differed among five landscape elements (roadsides, fence lines, open paddocks, creek lines and remnant vegetation) and at sites with high or low habitat (ground cover) complexity. Methods We measured bait uptake at 300 bait stations distributed evenly among the landscape elements in agricultural landscapes in northern Victoria, Australia. Bait uptake was also compared between sites with low and high habitat complexity in districts subject to no fox control and annual fox control. Key results Among landscape elements, bait uptake was significantly higher in roadside vegetation and along vegetated creek lines than it was along fence lines and in open paddocks (P < 0.05 in each case). Within roadside vegetation, bait uptake was significantly (P = 0.001) lower at sites with a high habitat complexity than at sites with low complexity, particularly in areas subject to annual fox control. Conclusions Bait placement influences bait-uptake rates considerably and greater consideration should be placed on bait-site selection during fox-baiting programmes. Habitat complexity limited bait uptake, which may indicate a reduced capacity of foxes to find baits in complex habitats. Implications Our results should help improve bait-site selection in agricultural landscapes and may increase the efficacy of fox baiting to the benefit of native fauna and livestock.

Bait consumption by Sooty Terns: implications for island eradication programmes

SummaryRat Rattus spp. predation is widely recognised as a major threat to insular seabird populations. Rodent eradication programmes have advanced over the last two decades with purported benefits to ecosystems and seabird conservation. However, the danger of non-target poisoning to seabirds has rarely been examined, despite hundreds of insular eradication efforts. We initiated research to assess the risk of rodenticide bait to nesting Sooty Terns Sterna fuscata on Wake Atoll Complex. Placebo bait with a fluorescing dye was used in a control treatment design to track exposure and ingestion. Camera-based observations and faecal samples, post-mortem inspections and live chick observations suggested that bait exposure and consumption occurred, but that it was infrequent. Results indicated that poison bait eradication should occur outside the chick rearing phase to minimise risks to nesting seabirds. Our study also documents previously unreported environmental fluorescence, which may increase false positive results in pre-eradication assessments of bait palatability, bait uptake rates, and bait application methods that use inactive fluorescing test baits. We recommend that future conservation efforts begin with an evaluation of environmentally-based fluorescence before formal biomarker studies.

Evaluating seasonal bait delivery to badgers using rhodamine B

In the UK and Ireland, research on the control of bovine tuberculosis in badgers includes the development of a palatable bait for oral delivery of a vaccine and a means of its deployment in the field. In the present study, we carried out field deployment of bait according to the established method of bait marking in early spring and early summer to compare the effects of seasonality on bait uptake rates. All baits contained rhodamine B (RhB) which was subsequently detected in the hair and whiskers of captured badgers. During the 8 days of bait feeding at 14 badger setts, 99% of baits deployed in spring, and 100% of those deployed in summer were removed. The presence of RhB in captured badgers indicated high rates of uptake amongst adult badgers in spring (93%) and summer (98%). Only cubs captured in summer showed evidence of having taken bait (91%). Between 67% and 100% of each social group was estimated to have taken bait. The detection of RhB in 96% of badgers captured at outlier setts, where bait was not fed, suggested that deployment at main setts alone may be sufficient to target a relatively high proportion of the badger population. The number of baits deployed per marked badger suggested that a similar level of uptake might be achievable using fewer baits. The results clearly demonstrate the potential value of the bait-marking methodology for delivering vaccine baits to badgers during spring and summer, but that deployment in early summer is necessary to target cubs.

Potential Food Item Distractions during Raccoon ORV Baiting Campaigns on Cape Cod, Massachusetts: Would You Like Fries With That?

USDA APHIS Wildlife Services has been a primary cooperator in the Cape Cod Oral Rabies Vaccination Program (CCORVP) in southeastern Massachusetts since 2001. The CCORVP (1994 - present) was originally designed to reduce the incidence of terrestrial rabies adjacent to the Cape Cod Canal in order to prevent its spread on to peninsular Cape Cod. However, since the barrier breach in 2004, CCORVP is now focused on rabies control in this coastal resort area southeast of Boston, MA. An integral component of wildlife rabies management is oral vaccination with vaccine-laden baits. Consequently, maximizing bait uptake rates is critical to achieving sufficient population immunity to reduce rabies prevalence and achieve control. To that end, knowledge of raccoon food habits, especially at ORV bait delivery times (spring and fall on Cape Cod) is crucial. We undertook a study of raccoon food selection in southeastern Massachusetts to assess the presence of and interest in competing food resources. We collected intact stomachs from raccoons found dead or euthanized for rabies testing within the CCORVP zone during 2006 (n = 33) and 2007 (n = 109) for analysis. Stomach contents were dominated by vegetation (80%), followed by invertebrates (43%), non-food items (41%), vertebrates (35%), and hair (primarily raccoon based on appearance) (15%). Food item occurrence appears to be related to age. We present findings, potential management implications, and suggestions for assessing food use related to rabies control in other locations.

Acceptance of baits, designed to carry oral rabies vaccines, by foxes in Britain

The spread of classical rabies (RABV) in the red fox (Vulpes vulpes) within Europe has been reversed during the last few decades by the widespread use of vaccine baits. This approach has resulted in most of western Europe being essentially free of terrestrial rabies, and has been associated with significant increases in fox density. The few remaining foci of disease in western Europe are now situated within areas of increasing density of foxes. In Great Britain, rabies contingency planning is centred on the use of vaccine baits, following the successes in Europe. However, in Britain fox density in both rural and urban areas has generally been higher than in continental ‘rabies-endemic’ Europe. One consequence of a high fox density is that a greater proportion of the population needs to consume the vaccine baits and become immunised, to reduce the absolute density of susceptible animals and thus eradicate the disease. Therefore, bait uptake rates in Great Britain, in both rural and high-density urban areas, should be of interest in contingency planning for the control of future outbreaks of rabies in Europe. We report here on studies in Britain on the uptake of vaccine baits by foxes, and how this should influence both British and European rabies contingency planning.

Effects of bait type and deployment strategy on uptake by free-living badgers

Baits are increasingly used in wildlife management to deliver orally administered vaccines and contraceptives. The efficacy and cost-effectiveness of vaccination or fertility-control campaigns can be substantially affected by bait uptake rates. This study assessed whether bait type and deployment strategy affected bait uptake by free-living badgers (Meles meles L.). Six social groups of badgers were presented with three bait types (meat, fruit, cereals) and two deployment strategies (dispersed single baits versus aggregated multiple baits at fixed baiting stations) for six weeks. In each social group, the type of bait and deployment strategy were rotated every week so that by the end of the test every group had experienced all combinations. On three days, biomarkers (ethyl iophenoxic acid, propyl iophenoxic acid and rhodamine B) were added to the baits to determine the proportion of badgers ingesting these baits. The results indicated that both bait type and deployment strategy affected the proportion of baits eaten by badgers and the number of badgers gaining access to baits. Meat and fruit baits were taken significantly more frequently than cereals, and dispersed meat baits had the highest rates of disappearance. Biomarker levels suggested that the proportion of badgers that gained access to all baits was substantially lower when baits were aggregated, although small sample sizes prevented statistical assessment of this effect. The results suggest that dispersed single baits are likely to be consumed in greater proportions by a higher number of individual badgers than multiple baits at fixed stations.

Efficacy of bait distributional strategies to deliver canine rabies vaccines to coyotes in southern Texas.

This study sought to develop a baiting strategy to deliver an oral rabies vaccine to free-ranging coyotes (Canis latrans) in southern Texas. To determine bait longevity, dog foodlard baits were placed (n = 50) on- and off-roads during July 1994 and January 1995. Coyote visitation and uptake rates did not differ between on-road and off-road placement of baits. To evaluate bait stations as possible visual cues, baits were placed out both with (n = 50) and without (n = 50) bait stations. A visual cue of a bait station did not affect coyote response to baits. Bait longevity was shorter during July (< or = 4 days) than January because of consumption of baits by imported fire ants (Solenopsis invicta). The effect of two different bait densities on coyote acceptance rates was determined on six 93.5 km2 study areas in southern Texas. Three study areas received a bait density of 19 baits/km2 (50 baits/mi2) and the remaining three study areas received 58 baits/km2 (150 baits/mi2). Coyote bait uptake rates, based upon the proportion of coyotes marked with either tetracycline hydrochloride or rhodamine B or both, were 83% (n = 99 coyotes) and 87% (n = 101 coyotes) for the 19 and 58 baits/km2 densities, respectively. Bait uptake rates did not differ (P > 0.54) between the two bait densities. Rodents and rabbits, which were fed baits containing tetracycline hydrochloride and a simulated oral rabies vaccine sachet containing rhodamine B, did consume the bait but not the rhodamine B sachet. These animals then were killed and fed to captive coyotes (n = 9). Canine teeth were extracted from coyotes and processed for tetracycline determination. Each coyote tested negative for tetracycline. Therefore, it was unlikely that coyote bait consumption rates were overestimated because of coyotes secondarily marking themselves by ingesting prey items that consumed baits.

Responses of captive and free-ranging coyotes to simulated oral rabies vaccine baits.

The objective of this study was to develop a bait for delivering an oral rabies vaccine to free-ranging coyotes (Canis latrans) in southern Texas. Captive trials were conducted from January to April, 1994, to determine bait preferences and behavioral responses of coyotes (n = 42) to selected baits and attractants. Baits were hollow rectangular cubes made of polymer dog food or fish meal. Attractants had sweet (watermelon), fruity (raspberry), sulfurous (synthetic WU), and lard (beef lard) fragrances. Captive coyotes did not exhibit a preference for either bait bases or attractants; however, coyotes chewed dog food baits 1.6 times more than fish meal baits. Average proximity of coyotes eliciting a response to baits was 2.2 +/- 1.3 m (mean +/- SE). Captive coyotes readily accepted dog food baits containing 2 ml of liquid rhodamine B, a biological marker. Rhodamine B staining of the oropharyngeal region was evident in each captive coyote. Results from the field evaluation of baits and attractants were consistent with that of the captive trials. Of 2,070 bait station-nights conducted from February to April, 1994, coyotes comprised the greatest single species visitation and uptake rates with 31% and 28%, respectively. Bait uptake rates of free-ranging coyotes did not differ among bait-attractant combinations. Coyotes took baits 93% of the time they encountered a bait, regardless of bait type.