Prions — in plants? New concern for chronic wasting disease

Pri­ons — the infec­tious, deformed pro­teins that cause chronic wast­ing dis­ease in deer — can be taken up by plants such as alfalfa, corn and toma­toes, accord­ing to new research from the National Wildlife Health Cen­ter in Madison.

The research fur­ther demon­strated that stems and leaves from tainted plants were infec­tious when injected into lab­o­ra­tory mice.

The find­ings are sig­nif­i­cant, accord­ing to the researchers and other experts, because they reveal a pre­vi­ously unknown poten­tial route of expo­sure to pri­ons for a Wis­con­sin deer herd in which the fatal brain ill­ness con­tin­ues to spread. The dis­ease has also become a press­ing issue nation­wide: The national Cen­ters for Dis­ease Con­trol and Pre­ven­tion has iden­ti­fied the deer dis­ease in 17 states and pre­dicts it will spread to other states.

In Wis­con­sin, where the state Depart­ment of Nat­ural Resources has scaled back its efforts to slow the spread of CWD, some crit­ics say the new research should cause the agency to revisit its approach.

Michael Samuel, a CWD researcher and wildlife ecol­ogy pro­fes­sor at the Uni­ver­sity of Wisconsin-Madison who was not involved in the plant research, said the new study is sig­nif­i­cant. Pre­vi­ous stud­ies have shown the dis­ease can be trans­mit­ted animal-to-animal and via soil.

It’s impor­tant because it iden­ti­fies a poten­tial path­way,” Samuel said of the study.

Christo­pher John­son, who con­ducted the study, wrote in the abstract: “Our results sug­gest that pri­ons are taken up by plants and that con­t­a­m­i­nated plants may rep­re­sent a pre­vi­ously unrec­og­nized risk of human, domes­tic species and wildlife expo­sure to CWD.”

The research has not yet been sub­mit­ted for pub­li­ca­tion in a sci­en­tific journal.

The study focused on those pri­ons sim­i­lar to those caus­ing CWD in deer.

The dis­ease is one of a class of neu­ro­log­i­cal, prion-caused dis­eases known as trans­mis­si­ble spongi­form encephalopathies, includ­ing scrapie in sheep and goats, bovine spongi­form encephalopa­thy — or mad cow dis­ease — in cat­tle, and Creutzfeldt-Jakob dis­ease in humans. CWD was dis­cov­ered in Wisconsin’s deer herd in 2002 and has been found since the mid-1990s in north­east­ern Col­orado and south­east­ern Wyoming.

John­son is sched­uled to present his research at the annual meet­ing of The Wildlife Soci­ety in Mil­wau­kee in Octo­ber. John­son stud­ies CWD at the fed­eral wildlife dis­ease cen­ter, which is run by the U.S. Geo­log­i­cal Sur­vey. His ear­lier work found CWD pri­ons can linger in and be ampli­fied and trans­mit­ted by soil.

‘Major review’ needed?

James Kazmier­czak, the state pub­lic health vet­eri­nar­ian, said that a mol­e­c­u­lar species bar­rier, though lit­tle under­stood, appears to have so far pre­vented the CWD pri­ons from mak­ing peo­ple and cat­tle sick.

Also, Kazmier­czak said, data reported to the Wis­con­sin Divi­sion of Pub­lic Health show lit­tle devi­a­tion from the national rate — a lit­tle above one case per mil­lion peo­ple — in annual cases of Creutzfeldt-Jakob Dis­ease. Nor, he added, does data on more than 800 Wis­con­sin hunters who have con­sumed CWD-tainted veni­son show any human cases of prion brain disease.

Nation­wide, accord­ing to the CDC, “no strong evi­dence of CWD trans­mis­sion to humans has been reported.”

Even so, the threat of CWD trans­mis­sion by crop and food plants — and the newly dis­cov­ered poten­tial for expo­sure to humans and live­stock — has prompted some to say the state Depart­ment of Nat­ural Resources should recon­sider its CWD policy.

That is very dis­con­cert­ing,” George Meyer, exec­u­tive direc­tor of the non­profit Wis­con­sin Wildlife Fed­er­a­tion, said of the research.

My impres­sion,” said Meyer, a for­mer DNR sec­re­tary, “is (that) it should cause a major review of the very weak CWD strat­egy that is being pur­sued by the DNR.”

Dave Clausen, for­mer chair­man of the Nat­ural Resources Board and a vet­eri­nar­ian who has stud­ied CWD, has also crit­i­cized the DNR for being pas­sive on the disease.

He agreed with Meyer that the new research should give the agency pause. He said the poten­tial pres­ence of pri­ons in plants is not only a pub­lic health con­cern but “has big impli­ca­tions for our agri­cul­tural econ­omy, not just in this state but all across the country.”

Dis­ease has spread

Soon after the dis­cov­ery of CWD in Wis­con­sin, the DNR embarked on an aggres­sive effort to halt spread of the dis­ease by putting in place addi­tional and longer hunt­ing sea­sons, requir­ing hunters to shoot a female deer before tak­ing a buck, and hir­ing sharp­shoot­ers to kill deer.

But the ambi­tious pro­gram grew unpop­u­lar with hunters and landown­ers, and the num­ber of hunters par­tic­i­pat­ing in the state’s annual deer hunt declined. Mean­while, the dis­ease spread.

The DNR reports that preva­lence of the dis­ease has increased in all sex and age classes of deer. Dur­ing the past 11 years, for exam­ple, agency data esti­mates preva­lence in adult males has risen from 8 to 10 per­cent, to more than 20 per­cent. And in adult females, the preva­lence has grown from about 3 or 4 per­cent to about 9 percent.

In a dis­ease hot spot in south­west­ern Wis­con­sin, CWD preva­lence has increased to 27 per­cent among deer 2 ½ years or older, accord­ing to DNR sta­tis­tics. The growth was called “fright­en­ing” by Robert Rol­ley, a DNR researcher who worked on the study.

And the dis­ease has spread far beyond where it orig­i­nally showed up. Two years ago, the dis­ease was dis­cov­ered in a doe in Wash­burn County in north­east­ern Wis­con­sin. Test­ing has turned up no other infected deer in the area.

Admin­is­tra­tion scaled back CWD

Gov. Scott Walker promised hunters while cam­paign­ing that he would reeval­u­ate the agency’s approach to deer hunt­ing and the dis­ease. After his elec­tion, he hired Texas deer expert James Kroll for the job.

Kroll down­played the poten­tial impact of CWD, both in his report to Walker and in a July white paper on the dis­ease. He did not return phone calls seek­ing com­ment on the prion plant study.

In the white paper, Kroll cited stud­ies of the Wis­con­sin deer herd from 2003 to 2007 show­ing no increased mor­tal­ity rates from CWD. He also wrote, “it is my opin­ion CWD does not pose a threat to human health,” cit­ing stud­ies on the lack of transmission.

He rec­om­mended the agency take a “more pas­sive approach” to the illness.

As a result of hunter con­cerns and Kroll’s report, the DNR has elim­i­nated many of the extra hunt­ing sea­sons and reg­u­la­tions intended to reduce herd size and slow the spread of the dis­ease. Test­ing for the dis­ease has also dropped off.

Research unlikely to prompt revision

Tom Hauge, who directs the DNR’s wildlife man­age­ment pro­gram, said the new research is unlikely to cause the agency to reeval­u­ate its CWD program.

Cur­rent man­age­ment is grounded in the real­ity of the present con­di­tions,” Hauge said. “There is no sci­ence to indi­cate that human health is at risk to date. And live­stock to date have not been impacted. That real­ity has shaped the socioe­co­nomic response.”

Hauge also said the cur­rent polit­i­cal atmos­phere has been a fac­tor. He said the spe­cial CWD reg­u­la­tions “wore thin on peo­ple” and that “man­i­fested itself in a guber­na­to­r­ial campaign.”

Until that land­scape changes,” Hauge said, “we have to live with the real­i­ties we face right now.”

Con­cerns raised, but ques­tions remain

Tami Ryan, who heads the DNR’s Wildlife Health Sec­tion, helped orga­nize The Wildlife Soci­ety ses­sion at which John­son will present his find­ings. She said she invited him because the agency is inter­ested in learn­ing more about the research. She called the ini­tial work “very impor­tant research” but said she wants to see more data, espe­cially on whether lab ani­mals can become infected by eat­ing tainted plant mate­r­ial rather than just via injection.

I’m also inter­ested in the con­t­a­m­i­na­tion level,” Ryan said. “What is the con­cen­tra­tion and fre­quency of expo­sure that would result in infec­tion? Is this as great a risk as com­ing into con­tact with another infected ani­mal? A level of risk assess­ment is necessary.”

For the moment, she said, “I don’t hear alarm bells.”

John­son said he is test­ing whether ani­mals can become infected by eat­ing CWD-laden plant tis­sues. He also said future work will address the ques­tions raised by Ryan about the prion con­cen­tra­tions in plants nec­es­sary to cause infection.

We’re just scratch­ing the sur­face here,” John­son said.

But Bryan Richards, CWD project leader at the National Wildlife Health Cen­ter, said even the find­ings to date should be taken seri­ously by state and fed­eral wildlife and pub­lic health agencies.

The poten­tial for expo­sure is unde­ni­able,” Richards said.

Can humans get CWD?

Clausen said the plant research should be con­sid­ered in the con­text of other CWD stud­ies. He said research has shown the mol­e­c­u­lar bar­ri­ers that seem to have pro­tected humans from infec­tion may be more porous than some believe.

In 2004, for exam­ple, a CDC study pub­lished in the sci­en­tific jour­nal Emerg­ing Infec­tious Dis­eases con­cluded that the trans­mis­sion of bovine spongi­form encephalopa­thy, or mad cow dis­ease, to humans indi­cates that “the species bar­rier may not com­pletely pro­tect humans from ani­mal pro­tein diseases.”

The arti­cle also cited lab stud­ies in which CWD pri­ons were found to infect human prion pro­teins. Still, the arti­cle con­cluded, “lim­ited inves­ti­ga­tions have not iden­ti­fied strong evi­dence for CWD trans­mis­sion to humans.”

Another study, led by Marcelo Bar­ria from the Mitchell Cen­ter for Alzheimer’s Dis­ease and Related Brain Dis­or­ders at the Uni­ver­sity of Texas and pub­lished in the March 2011 issue of the Jour­nal of Bio­log­i­cal Chem­istry, showed that CWD pri­ons in the lab­o­ra­tory can be manip­u­lated over gen­er­a­tions to change and become more infec­tious to humans.

Our find­ings lead to a new view that the species bar­rier should not be seen as a sta­tic process but rather a dynamic bio­log­i­cal phe­nom­e­non that can change over time when prion strains mature and evolve,” the researchers concluded.

Such sci­ence, Clausen said, should raise ques­tions about a man­age­ment approach to CWD that does not stem the spread of the dis­ease and, as a result, increases the risk of human and ani­mal exposure.

The DNR’s offi­cial approach to envi­ron­men­tal con­t­a­m­i­na­tion with CWD pri­ons has been a yawn and a shrug. Whether con­cerns from con­sumers, pub­lic health offi­cials or agri­cul­tural inter­ests will change that is an open ques­tion at this time,” Clausen said.

John Stauber, an activist and co-author of the book “Mad Cow USA,” said the new research should be espe­cially sober­ing in a nation he believes is ignor­ing a pos­si­ble dan­ger­ous pub­lic health threat.

The impli­ca­tions of prion dis­eases poten­tially (spread­ing) via con­t­a­m­i­nated agri­cul­tural plants is mind-boggling,” Stauber said. “Imag­ine peo­ple, wildlife or live­stock eat­ing a cereal or veg­etable that could years or decades later cause an incur­able, fatal brain disease.

The best sci­en­tists have always warned that with pri­ons, all bets are off. There is no other deadly dis­ease agent as bizarre or invis­i­ble. Unfor­tu­nately, fed­eral and state food and wildlife agen­cies have been ignor­ing the prion threat, down­play­ing its human health risks, cut­ting back on research, and pre­tend­ing this will all go away. It won’t.”

This project was sup­ported by The[1]  Joyce Foun­da­tion. The non­profit Wis­con­sin Cen­ter for Inves­tiga­tive Jour­nal­ism ( col­lab­o­rates with Wis­con­sin Pub­lic Radio, Wis­con­sin Pub­lic Tele­vi­sion, other news media and the UW-Madison School of Jour­nal­ism and Mass Communication.

All works cre­ated, pub­lished, posted or dis­sem­i­nated by the Cen­ter do not nec­es­sar­ily reflect the views or opin­ions of UW-Madison or any of its affiliates.


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