Why conduct research and distribute oral rabies vaccine too Permission from the State to distribute oral rabies vaccine is contingent on Fairfax County conducting research that adds to the body of knowledge regarding oral rabies vaccine use. Although the safety and effectiveness of the oral rabies vaccine have been established, vaccine distribution strategies are still a matter of research.

What is being learned through the Fairfax County Oral Rabies Vaccine Pilot Program?

Under the direction of Dr. Francois C. Elvinger, program manager of the Fairfax County Oral Rabies Vaccine Pilot Program, the following research has emerged:

  • Lori Thiele, pilot program coordinator, conducted a major study in spring 2001 to evaluate the effect of 'fragmented' distribution of oral rabies vaccine within the target area. Oral rabies vaccine distribution in urban and suburban environments is constrained by factors that are different from those encountered in rural areas where most oral rabies vaccine is distributed. These include greater potential for human exposure to the vaccine, greater competition for the vaccine bait by companion animals, and property access and land use restrictions that limit vaccine bait placement. As a result, uniform vaccine distribution density even across uniform raccoon activity zones may not be feasible. It is hoped that the immunity islands created within the target area by a 'fragmented' distribution scheme will lead to satisfactory vaccination of raccoons. To examine this, a group of contiguous suburban parks with a combined surface area of 1,000 acres was delineated into eight sections using squares formed by the County tax grid. Alternate tax squares totaling 441 acres received oral rabies vaccine at the rate of 1 dose per acre and the other four (a total of 559 acres) were not vaccinated. Of the one hundred vaccine doses monitored across the four vaccinated squares, 87 doses were contacted within four to five days of placement. Of the 100 raccoons then live-trapped starting 24 days after vaccine distribution, 13 had blood titers demonstrating prior exposure to the rabies antigen (titers ranged from 1:25 to 1:1400), with eight positive raccoons trapped in the four vaccinated squares and the remaining five trapped in the unvaccinated squares. The averages of the positive titers were not significantly different (P=0.88) when vaccinated and non-vaccinated areas were compared, suggesting that raccoons exposed to oral rabies vaccine move beyond the area in which oral rabies vaccine is distributed. See linked map of the 2001 test results(JPG file, 109K). Two interesting additional findings came from the study:
    • Based on trapping 144 raccoons (44 prior to vaccine distribution and 100 following with a combined 28% trap success), the raccoon population on the 796 acres of parkland considered in the estimate is 221 or about one raccoon per three acres (0.28 raccoons per acre).

    • Of the 44 raccoons live-trapped and bled in six of the eight squares prior to vaccine distribution, six had titers demonstrating prior exposure to the rabies antigen (titers ranged from 1:13 to 1:110). This was a larger number of raccoons than expected and a finding that deserves further study.

  • Although it is known that the spread of rabies in Anne Arundel and Pinellas Counties, two suburban and urban counties in Maryland and Florida, was effectively halted through oral rabies vaccination, the proportion of raccoons that need to become vaccinated for a program to be effective is still a matter of conjecture. Information from these counties suggests that only less than half of the raccoons need to be vaccinated for the disease to be effectively stopped. In the Fairfax County pilot program, 37 percent of 89 blood samples and 45 percent of 66 blood samples collected after the first and second vaccine distributions indicated exposure to oral rabies vaccine. The course of rabies in a raccoon population that is vaccinated with oral rabies vaccine can be modeled to establish an effective herd immunity level for rabies to allow a better calculation of the resources needed to organize an effective program.

  • There is a lot of competition by wildlife and companion animals for the bait that contains the oral rabies vaccine. Opossums and foxes, and in urban areas especially cats and dogs like to eat the bait, and thus deprive raccoons of the vaccines. A study conducted by Jaime Landolfi, a veterinary student in her summer internship program, attempted to quantify competition by putting out hair sampler devices (hair salons) to identify animals that picked up bait at central point feeding stations, i.e. at those locations from which County citizens reported raccoon activity (i.e., or involuntary feeding). Raccoons at those locations took up most of the vaccines, however, an important competitor for bait were cats.

  • Karen Wolf, a veterinary student and summer intern, evaluated wildlife movement and competition for bait in culverts that are built under highways like the Fairfax County Parkway and the Interstate that pass through the county. She evaluated the potential for highways to function as barriers to wildlife movement and consequently also to the spread of rabies. She documented that there was a lot of traffic by raccoons and other wildlife through culverts and that not only raccoons but other wildlife species ate baits when in these culverts. This could be expected to spread the immunity against rabies on the outskirts of the target area, but also shows the potential for rabid animals to migrate back into the target area.

  • Elizabeth Embree Schmidt, a junior veterinary and graduate student, for her thesis project is studying the submissions for rabies testing and incidence of rabies throughout Fairfax County. From 1995 through 1999, the County laboratory tested 234 to 465 animals per year. This constitutes a significant use of Health Department resources, and the study focuses on spatial and temporal submission patterns and the relationships between tested animal species and positive tests. The outcome of her study may allow better prediction of where and when cases may be detected at the laboratory and help in the planning of County resources in the control of public exposure to rabies.


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