Abstract

Since 1988, Texas has experienced the onset of an expanding epizootic of canine rabies (678 cases) in South Texas and gray fox rabies (779 cases) in West Central Texas. To contain these rabies epizootics, the Texas Department of Health’s Zoonosis Control Division, Texas Animal Damage Control Service, Texas National Guard, and a group of enthusiastic volunteers from all over the state completed the largest distribution of vaccine-bait units in the world. It was also the first of its kind for coyotes and gray foxes. Over 5.9 million edible baits containing an oral rabies vaccine were airdropped over 98,000 square miles from 1995 through 1997.

Vaccine-bait combinations contained 2.0 ml Raboral V-RG vaccine at a minimum field dosage of 10^7.4 virus particles in a plastic container (sachet) within a hollow extruded bait. Coyote baits were produced using a fish meal-based formula, while gray fox baits were dog food-based. The baits were manufactured by Rhone-Merieux, Inc., Athens, GA, and contained tetracycline at a level of 150 mg/bait as a biomarker.

The Ontario Ministry of Natural Resources provided three Twin Otter aircraft outfitted with automated bait distribution equipment. Aerial distribution of bait was conducted over the entire target area at a bait density of 70 baits/square mile. Bait placement occurred in January and early February of each year. Distribution during winter is critical to program success, as the reduction in natural food availability makes the vaccine/bait units more attractive to the target animals.

Post-vaccination surveillance programs began with the collection of coyotes and gray foxes from the baited area 45 days after completion of each drop. Blood samples were sent to the US Army Veterinary Laboratory at Fort Sam Houston, San Antonio, Texas for RFFIT (Rapid Fluorescent Focus Inhibition Test) rabies viral antibody testing. Canine tooth samples were sent to Dr. David Johnston of Ontario, Canada for aging and biomarker analysis.

The goals of Oral Rabies Vaccination Program—including creating zones of vaccinated coyotes and gray foxes along the leading edge of the epizootics and containing the expansion of the epizootics—have been met to date. Of all rabies cases statewide, 29% were the Texas fox variant of rabies virus in 1996 versus 41% in 1995. Of all rabies cases statewide, 6% were the canine variant in 1996 versus 24% in 1995. These cases included spillover of both the Texas fox and canine variants to a wide variety of species, such as domestic cats, raccoons, livestock, and bobcats.