CWD prions found in moose, deer, reindeer muscles in Norway, highlighting potential risk to people- Mary Van Beusekom, MS

Researchers who late last week reported finding chronic wasting disease (CWD) prions in the muscles of infected moose, red deer, and reindeer in Norway say the discovery raises questions about the risk of human exposure through eating the meat of infected cervids (members of the deer family).

CWD is a fatal neurodegenerative cervid disease caused by prions, infectious proteins that trigger abnormal folding in normal proteins, especially in the central nervous system (CNS). Infected animals shed CWD prions in body fluids, which can spread to other cervids through direct contact or the environment.

The authors note that previous studies of cervids infected with North American CWD prion strains have detected prions in lymphoid tissues, peripheral nerves, muscles, blood, and excreta. But this was the first identification of European CWD prion strains in cervid muscle. The study was published in Emerging Infectious Diseases.

“The tissue distribution of PrPSc [CWD prions] in reindeer, with a CWD strain similar to cases found in North America, was therefore not surprising,” the researchers wrote. “However, the findings of PrPSc in peripheral tissues in moose and red deer by PMCA [protein misfolding cyclic amplification] were less expected, especially in muscles, given the sporadic occurrence and lack of evidence, to date, for contagiousness of these new CWD strains.”

North American, Norwegian CWD prion strains differ
First identified in 1967 in Colorado, CWD has now been found in 35 US states, Canada, Finland, Norway, South Korea, and Sweden. CWD prion strains seen in reindeer and moose in Nordic countries differ substantially from those found in North American cervids and have been characterized as sporadic rather than contagious.

“CWD sporadically detected in moose and red deer in 3 Nordic countries demonstrated pathologic and strain characteristics different from CWD in reindeer, including an unexpected lack of prions outside the central nervous system as measured by standard diagnostic tests,” the Norwegian Veterinary Institute–led research team wrote.

Studies have shown two distinct patterns of prion progression in animals, the authors noted. The one observed with contagious CWD prion strains—North American CWD—involves the replication of prions in tissues from the lymphoreticular system (organs and tissues involved in immune responses) followed by spread to the peripheral nervous system and then the brain. With the other pattern, seen in less-contagious or noninfectious prion diseases such as atypical scrapie in sheep and goats, replication mainly takes place in the CNS.

Importance of highly sensitive testing
For the study, the investigators looked for prions in the lymph nodes, nerves, and muscles of four moose, two reindeer, and one red deer that tested positive through Norway’s CWD surveillance program via enzyme-linked immunoassay (ELISA) tests. Three healthy reindeer, three moose, and one red deer served as controls.

This work emphasized the importance of using ultrasensitive test to identify cases of CWD that conventional methods would have missed.
Jason Bartz, PhD

Western blot testing confirmed the presence of CWD prions in the brain, lymph nodes, and nerves in reindeer, but only in the brain of moose and red deer. PMCA, a more sensitive test, revealed prions in the lymphoreticular system of moose and red deer with CWD and in the muscles of both of these species and in infected reindeer.

Of all muscles tested, those from the head were more likely to be positive than those from other areas of the body. Those results confirm the variability of CWD prion distribution in isolates from different cervid species in Norway and demonstrate that the moose samples had two different prion types.

“This work emphasized the importance of using ultrasensitive test to identify cases of CWD that conventional methods would have missed,” Jason Bartz, PhD, a professor at Creighton University, told CIDRAP News.

CWD prions from the tissues of one moose lymph node and muscle sample showed infectivity when experimentally transmitted to two of four bank voles. “The negative transmission observed with 2 PMCA-positive tissues could be to the result of the presence of very low levels of infectivity, below the detection limit of the vole bioassay,” the authors wrote.

Although no human CWD cases have been identified, the results highlight the potential risk of human exposure to even small numbers of prions through handling and consuming infected cervids, they said.

“The presence of prions in peripheral tissues indicates that CWD may have a systemic nature in all Norwegian cervid species, challenging the view that prions are exclusively localized in the CNS in sporadic CWD of moose and red deer,” the researchers concluded. “Our findings expand the notion of just how widely distributed prions can be in cervids affected with CWD and call into question the capability of emerging CWD strains in terms of infectivity to other species, including humans.”

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