DENVER – Land conservation is a tricky business, and everything is a trade off, according to David Lucas, manager of The Rocky Mountain Arsenal National Wildlife Refuge, outside metro Denver. Native insects most likely perish from the chemical-laden flea spray wildlife managers use, but the endangered mammals that live on the refuge stay plague-free.

The plague is now easily curable in humans, but is lethal to the dogs. These Prairie dogs are the main prey source for the endangered Black-footed ferret, about a third of the world’s population of wild-born Black-footed ferrets live on this refuge.

“It’s all a big chess game,” says Lucas. Refuge employees administer a cocktail of vaccines and flea-killers to the dogs to prevent the plague.

Eight people in New Mexico caught the plague from infected Prairie dogs in the last two years, according to the New Mexico Department of Health.

But none of the dog colonies in the refuge have been infected, partly because of Lucas and his team.

They vaccinate the dogs with a pill filled with blue dye inside globs of peanut butter. They then check the scat and see if it is dyed blue. If it is, the dogs are successfully vaccinated, said Lucas.

Another method of prevention the team uses is spraying habitat with permethrin, commonly used in household flea spray. Fleas spread the plague to larger animals. Permethrin is commonly used worldwide, but it kills native bees and other insect species as well, according to the National Pesticide Institute.

The wildlife refuge was once a chemical plant operated by Shell, involved in manufacturing the now-banned pesticide DDT. Congress passed the Rocky Mountain Arsenal National Wildlife Refuge Act nearly two decades ago, spurring a cleanup of the site.

“We spent a billion dollars getting rid of one chemical. Now we’re spraying another one,” Lucas said. “It’s all a big process. Are we getting it totally right today? Probably not.”