OUTFOXING RABIES: AIRDROP VACCINE PROGRAM CHECKS SPREAD OF DEADLY DISEASE

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FORT STOCKTON — A squadron of state-chartered aircraft flies a series of “bombing” runs across South and West Texas each January. Flying at 500 feet, they drop millions of cubes the size of “fun-size” candy bars over 40 counties.

They battle a deadly, unseen enemy, but the only human casualty is an occasional airsick passenger.

For the past nine Januarys, the state has been fighting—and winning—the battle to prevent the spread of rabies into urban populations in South and West Texas.

The program has been so successful that it has virtually eliminated the type of rabies spread by coyotes, which back in 1995 posed a serious public health threat to San Antonio.

“Something had to be done and fast,” said Gary Nunley, director of Texas Cooperative Extension’s and USDA-APHIS’s Wildlife Services unit, after rabid coyotes started cropping up in Atascosa County, just 45 miles from San Antonio.

The greatest fear was that the canine strain of rabies that coyotes carry would jump to the huge, free-ranging dog population in and around San Antonio.

“A buddy of mine is chief of animal control in San Antonio,” said Dr. James Wright, a Texas Department of State Health Services veterinarian based in Tyler. “He indicated there are a jillion unvaccinated dogs roaming the streets of San Antonio. If canine rabies had reached Bexar County, we would literally have had rabid animals on playgrounds.” State health officials were anxious to avoid a repeat of a 1988 South Texas rabies outbreak that caused two deaths and 3,000 people to undergo rabies exposure treatments.

Casting about for a solution, state officials focused on an imaginative idea that had proved successful with red fox rabies problems in Europe and Canada. Large animal populations could be inoculated against rabies by feeding them baits laced with vaccine. But only way to get the baits to animals ranging in some of the roughest country in Texas was to drop them from the air.

“That’s when we started the state’s Oral Rabies Vaccination Program,” Wright said. “Our first efforts were to make a barrier between where the rabid coyotes were and where they were not. Every year since then we’ve moved the barrier farther south and west until now we have completely saturated South Texas with the bait. Today, we just don’t see the coyote variant of the virus anymore because we’ve got everything down there vaccinated.”

The coyote bait drops started in 1995. Gray fox baits were added the next year because a fox rabies epidemic was rapidly spreading across the Edwards Plateau.

Results have been dramatic.

The health department reports that, because of the program, the number of canine-strain rabies cases decreased from 122 in 1994 to the last single case along the border in 2001. Gray fox variant rabies cases dropped from 244 in 1995 to 61 by 2003.

Now the planes maintain a bait zone along the border to stop any possible canine rabies influx from Mexico. The gray fox bait zone varies from year to year across the Edwards Plateau. The plan is to encircle the gray fox rabies variant with baited buffer zones that will soon come together like a drawstring to protect the entire Texas Hill Country.

The Department of State Health Services is the vaccination program’s lead agency. It works in partnership with Extension Wildlife Services, USDA-APHIS Wildlife Services and the Texas National Guard. Tom Sidwa of Austin, director of the program, said the idea behind the drops is not to protect the health of foxes or coyotes, but to protect the health of people.

“While this program addresses wildlife concerns, rabies control is not just one thing,” he said. “It’s a strategy that includes good stray-animal control programs and a major reliance on domestic-animal vaccines.

“We still must have conscientious pet owners vaccinating their pets. If your dog or cat is unvaccinated and it encounters a rabid animal, you’re likely to be exposed.”

“The drops are done in January when coyote and fox food sources are scarce,” Nunley said. “It’s also a good time of year because fire ants, which also like the baits, are usually less active. Zapata is the first drop location, followed by Fort Stockton and finally Junction.”

The “drop season” starts long before the arrival of four specially equipped King Air planes chartered from a firm in Virginia. Sidwa first orders the baits from a vaccine manufacturer in Georgia; this is done early because there’s a lengthy process necessary to certify the vaccines’ potency.

The logistics of setting up housing, fuel, airports and a host of other things that accompany any large field operation are addressed next. Sidwa said extensive preparation goes into locating the zones, drawing the computerized flight lines, and feeding this data into each plane’s computer. An automated program takes the planes to the bait-drop line and keeps them on it. Data downloaded from each plane every night tell Sidwa where the planes have been and what they’ve dropped. Everything is tracked.

Bob Sims, district Extension Wildlife Services supervisor at Kerrville, and his crew load the bait on the planes at all three sites. Crews use a conveyor system to put the bait directly into each plane’s bin. These circular bins dispense the baits using accurate metering and counting mechanisms.

Though 60 people may be involved at a given location, each plane’s flight crew is small. There’s a navigator, whose main job is to keep the bait from hitting people, roadways and structures, the bait disburser who sees that the bait is feeding through the machinery properly and the contract pilot.

“Our biggest nemesis is the weather,” Sidwa said. “Visibility is a concern because we have to be able to see three miles in distance and 1,000 feet in altitude. The other thing is high winds. The planes fly only about 500 feet off the ground. It can be a pretty rough ride for the crews.

“But if it’s possible and safe to fly, we’ve got to get the bait on the ground.”

The baits themselves are 1 1/4 inches square by 3/4 inch thick. A plastic bag containing the vaccine is placed inside the bait and sealed there with wax. Coyote bait is made of fish meal. Gray fox bait is made of dog food with molasses and vanilla flavoring added. While eating the bait, the animal bites into and ruptures the plastic bag, completing its vaccination.

“To be effective, the vaccine must coat the animal’s tonsils,” Nunley said.

The planes drop 70 baits per square mile in South Texas for coyotes and 100 baits per square mile in West-Central Texas for gray fox—about 2.7 million baits each year across roughly 31,000 square miles. Funding for the $4 million program comes from the State of Texas and U.S. Department of Agriculture.

Imprinted on each bait is a warning and the toll-free phone number of the command center at each site.

Though millions of baits have been dropped over thousands of acres during the program’s nine years, Nunley said, incidents have been few.

“Once some folks pulled in a catfish with a bait in its mouth,” he recalls. “The bait had fallen into a stock tank, and the fish had picked it up. The fishermen called us using the number on the bait. Another time we accidentally hit a guy with a bait, but he wasn’t hurt and was a good sport about it.”

Ground surveillance begins once the bait drop is over. Extension Wildlife Services trappers monitor animals taken in the course of routine predator control duties, taking blood samples to check the animals’ immunity levels. They also take samples of teeth, which are cross-sectioned and checked for the presence of tetracycline. The chemical is added to the vaccine and puts down layers that show up in the teeth.

“Blood and teeth samples can determine if the animal picked up the bait and didn’t get immunized or picked up the bait and was immunized,” Nunley said. “The tests are very definitive, and a high percentage of the animals have been found to be immunized.”

The Department of State Health Services reports no animal cases of canine or gray fox rabies outside of the original containment zones since the program began, and no human cases of rabies have occurred, either.

Those involved with the program hope to duplicate their success by treating gray foxes on the Edwards Plateau. Baits are also being developed for rabies variants carried by skunks.

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