ESTES PARK – The mule deer buck, his impressive rack silhouetted against the Continental Divide, pursued a doe across a grassy resort lawn in a timeless Rocky Mountain drama.

The buck’s thick neck was swollen by hormones released during the rut. His sleek coat and thick antlers spoke of a successful year foraging. His command of prime real estate during the rut was proof of his competitive edge.

But a tiny transmitter wired to one antler showed the buck had been tested by the Colorado Division of Wildlife for chronic wasting disease. On this morning, veterinarian Lisa Wolfe learned the buck, designated E-23, was infected with the fatal brain ailment.

The deer jumped at the sting of the tranquilizer dart hitting his flank, then quickly resumed his pursuit of the doe. A few minutes later he wobbled and sank to the snow.

Wolfe prepared a final injection.

Euthanizing the deer was part of a Division of Wildlife “test-and-cull” experiment to see if removing sick deer immediately after testing reduces infection rates more effectively than the current strategies.

“Based on our models on how the disease spreads, it should have an effect,” said Mike Miller, the Colorado Division of Wildlife’s wildlife veterinarian. “If it doesn’t, then we need to rethink our model.”

The experiment offers a number of potential advantages over the division’s other techniques – removing animals that appear sick, culling groups of deer in areas of high infection and random population reduction.

Researchers hope the test-and-cull method catches infected deer earlier. As infected animals get sicker, they become increasingly contagious, shedding more and more of the abnormal prion protein thought to cause the disease.

The experiment also overcomes a common criticism of the agency’s “hot spot” culling technique, in which whole groups of untested deer in high infection areas are killed. Critics complain that uninfected deer will be killed by this method. Opposition in Boulder has prevented hot- spot culling on county open space.

Charles Southwick, a retired University of Colorado biology professor, is among the most vocal critics of hot-spot culling. Southwick says that because it is impossible to kill every sick animal, some remain to reinfect survivors. And removing so many deer may encourage others to migrate into the vacant territory, he said, exposing more animals to infection.

But Southwick supports the Estes Park experiment.

“I know it’s expensive … but I am strongly in favor of this approach,” said Southwick.

Since December 2002, Division of Wildlife teams have systematically tranquilized the deer outside Rocky Mountain National Park, removing snips of their tonsils for biopsy. The deer are tagged for later identification. Some are also fitted with radio collars designed to fall off later in the winter.

Over 34 days between December and May, the division tested 51 of the estimated 89 male and 130 of the estimated 261 female mule deer wintering in Estes Park.

A smaller number were tested inside Rocky Mountain National Park by park staff.

Fifteen positive cases were found among the town deer, including 5 percent of the does and 18 percent of the bucks. Some biologists think bucks are more susceptible because they inspect multiple does during the rut, increasing their exposure risk.

Over seven days, 13 of the positive cases were redarted and given lethal injections. The remaining two died of other causes.

The removal of the 13 positive cases should be enough to decrease the infection rate identified in the current testing season, Miller said.

By next year, agency and park biologists hope have tested every deer in the herd of about 450 that wanders between the town and the park. Such comprehensive testing has never been done before.

So many town deer already sport the telltale test buttons that biologists have trouble finding new animals to test.

Eventually, the team will begin retesting deer to see how quickly the infection is spreading through the healthy population.

Only two of the 50 deer tested this fall have been positive. Until final results are analyzed, Miller said, he hesitates to draw firm conclusions “but the results are encouraging.”

A paper describing the results of last season’s program, written by Wolfe, Miller and University of Wyoming pathologist Elizabeth Miller, will be published in the Wildlife Society Bulletin in 2004.

But the technique has its drawbacks, especially cost. At $300 per test, the program is twice to three times as expensive as other types of culling.

And the Estes Park deer are about as accommodating as wild mule deer can be. The herd essentially winters in town, where they become so used to people that it is easy for Miller and Wolfe, who is his wife, to approach for a clean shot with their dart guns.

The technique wouldn’t work in national forests or county open space.

Another important factor is that Estes Park supports the program, Miller said.

“It is expensive and this is the ideal situation to apply this approach,” he said. “But if the measuring stick is a reduction in prevalence, it might not be as expensive as it appears now.”

While Miller defends hot-spot culling as the most cost-effective current technique, he said test- and-cull may be an alternative for urban communities.

But one-third to one-half of the cost could be eliminated when new, rapid field tests become available. Much of the current cost, Miller said, stems from the use of telemetry gear to relocate infected animals after test results come back.

Field tests would give biologists results while deer were still sedated.

Miller and Wolfe are already well into the second year of the program. On Monday, they darted and biopsied two young bucks and resampled a doe in the Fall River Estates neighborhood with state wildlife technicians Tracey and Michael Sirochman.

After lunch, the team received a call that every game warden dreads: two bucks were tangled in a fence on Riverside Drive, fighting with each other.

When the division teams arrived, the scene was grim: the older buck, its magnificent rack wound with wire, had been battered into submission by its rival. But the younger buck, also in its prime, was entangled and couldn’t retreat. When Wolfe approached the younger buck with the dart gun, its churning hooves sent dirt flying as he drove his antlers forward again and again.

Wolfe quickly darted the victor. The biologists and two onlookers swarmed over the younger deer to keep it away from its rival.

As Michael Sirochman worked to cut the wire entangling the young buck’s rack, the animal’s tine ripped up through Sirochman’s coat and gashed his cheek.

The loser also had to be sedated to free it from the wire, but Wolfe gave it only a half dose to minimize chemical stress. She quickly drew blood and snipped a tonsil sample. But it was too late for the big buck, which soon stopped breathing.

Everyone was dejected. But Wolfe moved on to the younger buck without a word.

As Miller dragged the old buck over to the agency truck, he noted that it was relatively thin – a possible indicator of CWD. Ear tags showed the buck was tested a year ago, but it could have developed the disease since. CWD also makes deer less wary, which could explain how the buck became entangled in the fence.

But fences, swingsets and hammocks are among the hazards faced by deer and elk that move to town.

Tests on the buck would be back in a couple of days. Until then Miller could only speculate.

“Hell of a way for him to go,” he said quietly, unfolding a tarp over the carcass.