As devastating as all prion diseases are, the good news for us is that most have proven notoriously difficult to transmit between unrelated animal groups. Take the much publicized mad cow disease outbreak in the UK during the 1990s.
“Experts at the time feared hundreds of thousands of human cases, but that just didn’t materialize,” says the CDC’s Dr. Ermias Belay, lead author of a recent paper in the journal Emerging Infectious Diseases on the potential for CWD jumping to the human population. To date, says Belay, the total number of human deaths from mad cow disease (approximately one for every 10,000 infected beef carcasses that entered the food chain in Great Britain) was in no way proportional to the panic generated by media coverage. “There seems to be a substantial species barrier,” he says, “that prevents the agent from jumping from cows to humans.”
If anything, the interspecies barrier with CWD appears as strong, if not more so. Despite intensive epidemiological and laboratory studies, no human cases (or cervid-to-cattle transmission) have yet been documented. Several highly publicized prion disease deaths in young hunters and/or consumers of venison, for instance, have been traced back to familial, genetic, or otherwise “normal” human prion disease. Indeed, the closest researchers have come to triggering CWD in a primate is by injecting diseased deer tissue directly into the brains of squirrel monkeys.
Unfortunately for the nation’s deer herds, the high improbability of cervid-to-human transmission–and the complacency this has engendered–has proven something of a two-edged sword. “When CWD first appeared in Wisconsin, it triggered a huge panic,” acknowledges Harry Campbell, co-owner of a Milwaukee-based marketing firm specializing in outdoor recreation. “Deer licenses and retail sales dropped precipitously, and the whole hunting industry seemed in peril. Today, that panic is gone completely. The basic reason: nobody died.”
Clearly this “people are safe” message has spread through much of the hunting community nationwide. In the wake of the 2005 sportsmen’s feast in Oneida County, for example (an instance in which 350 people were served steak, chili, stew, and sausage from a local game-farmed white-tail that subsequently tested positive for CWD), public health officials found themselves astonished by the indifference they saw in individuals who knew they’d eaten CWD positive venison. “We set up a hot line inviting people who’d attended the banquet to call us for more information,” recalls Fanelli. “I anticipated a ton of calls from worried individuals, but I only got two. There just didn’t seem to be a lot of concern out there.” –Jim Thornton