Investigating the Ecology, Control, and Prevention of Terrestrial Rabies in Free-Ranging Wildlife
PROJECT GOAL: Study the ecology of wildlife and evaluate risk factors that may be involved with the transmission of rabies among wildlife and rabies virus trafficking across landscapes and to develop methods and strategies that reduce or eliminate such transmission.
Project Accomplishments 2010
Landscape Genetics of Raccoons: Implications for ORV Programs— In the United States, raccoons ( Procyon lotor) are reservoirs for the raccoon rabies virus variant, which can infect humans and other wildlife species. To combat this threat, APHIS Wildlife Services conducts an ORV program in many eastern States. In a recent study, NWRC, Wildlife Services' Pennsylvania State Office, and the Centers for Disease Control and Prevention (CDC) determined the genetic structure of raccoons in Pennsylvania and the geographic features (e.g., ridges and valleys), if any, that may hinder or enhance raccoon gene flow (i.e., movement) and the transportation of the rabies virus by this mammal.
As part of this study, researchers collected a total of 185 raccoon DNA samples from one ridge site and two adjacent valleys in southwestern Pennsylvania. Raccoon genetic structure within and among the sites was characterized at nine microsatellite loci. Results indicated that there was little genetic difference among the raccoons sampled, and that random mating was occurring within the population over a reasonably broad geographic area (e.g., sites up to 36 kilometers apart). However, distance was found to impact some of the genetic difference, which suggests that adequate widths of ORV zones are critical for success. The researchers concluded that geographic features within this landscape influence gene flow in raccoons only to a limited extent, and geographic features such as ridges are not long-term barriers to the spread of rabies virus. These results may be of value to inform future ORV efforts in Pennsylvania and other eastern states with similar landscapes.
Field Evaluation of Infrared Thermography for Screening Rabid Animals—NWRC researchers are exploring the use of infrared thermography (IRT) as a field tool to detect rabies in trapped raccoons. NWRC and APHIS Wildlife Services' field operations personnel evaluated IRT on 311 animals captured during trap, vaccinate, and release efforts in Ohio in 2009. Of these captures, 292 were raccoons, some of which were caught more than once. Thirty-two of the trapped animals that showed possible signs of being rabid were euthanized, and their brains were tested to confirm whether they had rabies. Although four of the animals were classified as positive for rabies based on the IRT screening, all laboratory tests were negative for rabies. The ability of the IRT to correctly identify negative raccoons ranged from 85 to 98% (depending upon the operator).
The researchers will continue this study through 2010 in an area of higher rabies prevalence to identify field conditions that may influence evaluations and conduct IRT field evaluations on rabies-positive animals.
In this study, researchers placed remote download GPS collars on 10 raccoons (
Procyon lotor). These collars offer advantages over traditional very high frequency (VHF) telemetry and conventional “store on board” global positioning system (GPS) collars, particularly in urban environments, because they have a remote download capability that allows researchers to retrieve data without collecting the collar. To date, raccoons have restricted their space use to small green-spaces when available, but also occasionally inhabited nearby abandoned houses. Restricted movements by raccoons in urban areas suggest that rabies may move more slowly in these locations than in surrounding areas with higher levels of habitat connectivity. However, these results do not suggest that urban areas should be disregarded when conducting ORV baiting operations. Small, tree-covered habitat patches—particularly those that border urban housing areas—should be hand-baited in an effort to prevent rabies spread between raccoons and from raccoons to domestic animals and humans.
Vitrification of Raboral V-RG® for Improved Vaccine Stability—In the United States, the canine variant of rabies has been eliminated, but wildlife reservoirs for other variants of rabies are a constant source of infection for pets, livestock, and humans. Raboral V-RG, an oral rabies vaccine, is the vaccine currently used in the United States to immunize free-ranging wildlife for rabies. However, biological materials such as the Raboral V-RG require low temperatures for stability. The stability of live viruses in vaccines for field use is of crucial importance. The longer the vaccine remains viable, the better the odds are for a target animal to encounter the vaccine and become immunized.
NWRC scientists have been working on a method called vitrification that may help APHIS Wildlife Services' ORV program increase rabies vaccination rates for wild, free-ranging wildlife. Vitrification is the process of preparing materials in a matrix of compounds, usually sugars or polymers, in a manner that, upon drying, results in the formation of a glass rather than a crystallized product. Vitrification of Raboral V-RG provides protection from a loss of viability at elevated temperatures. This protection extends through temperatures likely to be encountered in outdoor and non-refrigerated storage conditions. NWRC scientists observed that, even at temperatures of 50° C or higher for 24 hours, substantial amounts of viable vaccine virus remained in the samples. At 37° C, for the duration of the three-week study, the vaccine virus was protected and essentially no loss was observed. This is in stark contrast to the standard liquid suspension presently being used where viability decreased measurably over time and no detectable viable vaccine virus remained after three weeks. These results suggest that the vaccine virus would remain stable for even longer periods of time in a vitrified format.