A field evaluation of baits for delivering oral rabies vaccines to raccoons (Procyon lotor)

A Global Perspective on Oral Vaccination of Wildlife against Rabies

The long-term mitigation of human-domestic animal-wildlife conflicts is complex and difficult. Over the last 50 yr, the primary biomedical concepts and actualized collaborative global field applications of oral rabies vaccination to wildlife serve as one dramatic example that revolutionized the field of infectious disease management of free-ranging animals. Oral vaccination of wildlife occurred in diverse locales within Africa, Eurasia, the Middle East, and North America. Although rabies is not a candidate for eradication, over a billion doses of vaccine-laden baits distributed strategically by hand, at baiting stations, or via aircraft, resulted in widespread disease prevention, control, or local disease elimination among mesocarnivores. Pure, potent, safe, and efficacious vaccines consisted of either modified-live, highly attenuated, or recombinant viruses contained within attractive, edible baits. Since the late 1970s, major free-ranging target species have included coyotes (Canis latrans), foxes (Urocyon cinereoargenteus; Vulpes vulpes), jackals (Canis aureus; Lupulella mesomelas), raccoons (Procyon lotor), raccoon dogs (Nyctereutes procyonoides), and skunks (Mephitis mephitis). Operational progress has occurred in all but the latter species. Programmatic evaluations of oral rabies vaccination success have included: demonstration of biomarkers incorporated within vaccine-laden baits in target species as representative of bait contact; serological measurement of the induction of specific rabies virus neutralizing antibodies, indicative of an immune response to vaccine; and most importantly, the decreasing detection of rabies virus antigens in the brains of collected animals via enhanced laboratory-based surveillance, as evidence of management impact. Although often conceived mistakenly as a panacea, such cost-effective technology applied to free-ranging wildlife represents a real-world, One Health application benefiting agriculture, conservation biology, and public health. Based upon lessons learned with oral rabies vaccination of mesocarnivores, opportunities for future extension to other taxa and additional diseases will have far-reaching, transdisciplinary benefits.

Iophenoxic acid derivatives as markers of oral baits to wildlife. New tools for their detection in tissues of a game species and safety considerations for human exposure

The bait-marker iophenoxic acid (IPA) and its derivatives are increasingly used for evaluating and optimizing the cost-effectiveness of baiting campaigns on wildlife, particularly on game species such as the wild boar. We aimed to determine whether concentrations of the three main IPA derivatives ethyl, methyl and propyl-IPA measured on thoracic liquid extracts (TLE) of hunted wild boars may be representative of two exposure doses, 40 and 200 mg, from 20 to 217 days after ingestion. Then we developed a method of detection of the three IPA derivatives by LC/ESI-MS-MS in muscle and liver to evaluate the suitability of these two other tissues for monitoring the marked bait consumption and for measuring available residues in the meat of marked animals. Three semi-captive wild boars received 40 mg of each IPA derivative, three received 200 mg, and three, as controls, did not receive IPA. Blood serum was sampled 20, 197 or 217 days after IPA exposure according to animals and to the derivative. Wild boars were shot by gun after the different times of serum sampling times, and TLE, muscle and liver were sampled. Our results suggest that TLE is not a relevant tissue for quantitatively expressing IPA exposure. Due to interference, no analytical method was validated on TLE containing digestive material. On the other hand, quantifications in the muscle and particularly in the liver could discriminate wild boars that had ingested the two IPA doses from 20 days until 7 months after exposure, especially for the two long term markers ethyl and propyl-IPA. So IPA quantifications in the liver sampled on hunted animals appear to be a reliable tool for monitoring bait consumption in the field at a large scale. Nevertheless, whatever the ingested dose, ethyl- and propyl-IPA concentrations measured in the muscle and the liver of tested animals until 217 days after exposure, remained higher than 0.01 mg/kg, the Maximal Residue Limit (MRL) is recommended for molecules for which no toxicological data are available. Based on the range of IPA residues available in these two tissues, implications for humans consuming marked animals are discussed.

Crystallographic analysis reveals the structural basis of the high-affinity binding of iophenoxic acid to human serum albumin

Background

Iophenoxic acid is an iodinated radiocontrast agent that was withdrawn from clinical use because of its exceptionally long half-life in the body, which was due in part to its high-affinity binding to human serum albumin (HSA). It was replaced by Iopanoic acid, which has an amino rather than a hydroxyl group at position 3 on the iodinated benzyl ring and, as a result, binds to albumin with lower affinity and is excreted more rapidly from the body. To understand how iophenoxic acid binds so tightly to albumin, we wanted to examine the structural basis of its interaction with HSA.

Results

We have determined the co-crystal structure of HSA in complex with iophenoxic acid at 2.75 Å resolution, revealing a total of four binding sites, two of which - in drugs sites 1 and 2 on the protein - are likely to be occupied at clinical doses. High-affinity binding of iophenoxic acid occurs at drug site 1. The structure reveals that polar and apolar groups on the compound are involved in its interactions with drug site 1. In particular, the 3-hydroxyl group makes three hydrogen bonds with the side-chains of Tyr 150 and Arg 257. The mode of binding to drug site 2 is similar except for the absence of a binding partner for the hydroxyl group on the benzyl ring of the compound.

Conclusions

The HSA-iophenoxic acid structure indicates that high-affinity binding to drug site 1 is likely to be due to extensive desolvation of the compound, coupled with the ability of the binding pocket to provide a full set of salt-bridging or hydrogen bonding partners for its polar groups. Consistent with this interpretation, the structure also suggests that the lower-affinity binding of iopanoic acid arises because replacement of the 3-hydroxyl by an amino group eliminates hydrogen bonding to Arg 257. This finding underscores the importance of polar interactions in high-affinity binding to albumin.

Liquid chromatography–electrospray ionization mass spectrometry for direct identification and quantification of iophenoxic acid in serum

A liquid chromatographic-electrospray ionization mass spectrometric technique was developed for direct quantitation of iophenoxic acid (IA) in serum. IA was spiked into canine, feline, bovine, equine, and porcine sera, extracted, and quantified using negative ion monitoring following chromatographic separation on a Luna C18(2) 3 microm (100 mm x 2.1mm) reversed-phase column. The limit of detection was 25 ng/mL and the limit of quantification was 50 ng/mL. Inter- and intra-assay accuracy (86-113% and 87-115%, respectively) and precision (1.8-7.7%) were calculated. Analysis of serum collected from feral pigs, raccoons, and opossums following ingestion of IA-marked baits confirmed the appropriateness of this method for bait acceptance studies.

Persistence of elevated rabies prevention costs following post-epizootic declines in rates of rabies among raccoons (Procyon lotor)

Determining the benefits to cost relationships among different approaches to rabies control and prevention has been hindered by the inherent temporal variability in the dynamics of disease among wildlife reservoir hosts and a tangible and objective measure of the cost of rabies prevention. A major and unavoidable component of rabies prevention programs involves diagnostic testing of animals and the subsequent initiation of appropriate public health responses. The unit cost per negative and positive diagnostic test outcome can be reasonably estimated. This metric when linked to methodologies subdividing the epizootic process into distinct temporal stages provided the requisite detail to estimate benefits derived from rabies control strategies. Oral rabies vaccine (ORV), for prevention of the raccoon-associated variant of rabies, has been distributed in Ohio and adjoining states in an effort to develop an immune barrier to the westward spread of epizootic raccoon rabies. The costs of ORV delivery have been quantified. Herein, the cost structures required to assess the benefits accrued by prevention were developed. A regression model was developed effectively predicting (r2=0.70) the total number of rabies diagnostic tests performed by 53 counties in five northeastern (NE) states from 1992 to 2001. Five temporal stages sufficed to capture the range of variability in the raccoon rabies epizootic process. Unit costs, dollars per diagnostic test outcome, were calculated for negative and positive results from published reports. Ohio counties were matched to NE counties based on similar socioeconomic characters. A "pseudo-epizootic" of raccoon rabies was introduced into Ohio and the costs savings from ORV were derived as the excess costs imposed by epizootic spread throughout the state. At 46 km/year (range modeled, 30-60 km/year), the pseudo epizootic spread, and reached the enzootic stage, in all Ohio counties by year 13 (range modeled, 11-17 years). Cumulative excess costs for Ohio ranged between $11 and $21 million; counties of low socioeconomic status experienced the greatest relative excess costs. The costs for rabies prevention activities reached apices during the epizootic stage of raccoon rabies (2.7-10.8 times baseline) an unforeseen finding indicated elevated costs persisted (1.7-7.2 times baseline) into the enzootic stage.

Assessing the costs and benefits of an oral vaccine for raccoon rabies: a possible model

Any cost-benefit analysis of the use of an oral vaccine to control raccoon rabies should include calculating both costs and benefits in terms of $/unit area. Further, cost savings must be adjusted to match the stages of an epizootic: pre-epizootic, epizootic, and post-epizootic. A generic model, which can be adapted to different sites, illustrates the use of threshold analysis to link distribution costs, cost savings, bait density, and vaccine price. Initial results indicate the need to lower the cost of the vaccine, continue research to determine optimal bait densities, and examine distribution plans that do not require continued protection of areas in which raccoon rabies was eliminated through previous vaccination programs.