Two men die after eating deer meat infected with Prion disease

Scientists released a report on two human cases of prion disease in people who ate deer meat, which they say may imply zoonotic transmission.^[] The Centers for Disease Control and Prevention (CDC) has told reporters that the kind of zoonotic transmission hinted at in the report is unlikely.

Prion diseases encapsulate a range of rare, deadly diseases that infect humans or animals. Despite being under the same family of disease, the various prion diseases tend to be very different from one another and have different modes of transmission.

Human-infecting prion diseases

Classic Creutzfeldt-Jakob disease (CJD)

CJD is a type of prion disease that infects humans. The disease impacts the brain and can cause dementia along with other problems.^[] While rare in all age groups, it more commonly strikes older adults in their late 60s. Very few cases occur in those younger than 30. It can occur sporadically without known causes, familially from inherited genes, or iatrogenically from contact with prions in healthcare settings. CJD has no vaccine or cure, and, once symptoms arise, can lead to death within months to a year.

Vector Creutzfeldt-Jakob Disease (CJD)

CJD can be at times confused with another type of prion disease: variant CJD (vCJD).^[] The CDC notes that despite their nearly identical names, these are separate conditions with different infection risks. Unlike CJD, vCJD more commonly infects younger people. It can cause symptoms of pain when touching things, as well as mental symptoms.

Also unlike CJD, people can contract vCJD via zoonotic transmission from cows. Specifically, vCJD infections have been discovered in people who ate meat from cows infected with the disease bovine spongiform encephalopathy (BSE). BSE is another animal-based prion disease, sometimes known as mad cow disease. vCJD is even more rare than CJD, and only four cases have been reported in the United States, according to the CDC.

Animal-infecting prion diseases

Chronic wasting disease (CWD)

CWD is a type of prion disease that infects cervids, or popular game animals.^[] In animals, studies have found that CWD can cause symptoms like “weight loss over weeks or months, behavioral changes, excessive salivation, difficulty swallowing, polydipsia, and polyuria.”^[] Most infected animals die within several months of disease onset. Studies have not proven that CWD can infect humans, but the CDC has noted a potential risk due to the fact that BSE can infect humans. If CWD were to infect humans, the CDC says, hunters and people who eat meat from CWD-infected animals could be at the highest risk of developing the disease. 

Bovine spongiform encephalopathy (BSE)

BSE is a type of prion disease that infects cows. People who eat meat from BSE-infected cows can be at risk for vCJD.^[]

Potential for CWD to transmit to humans

Representatives from the CDC did not attribute the hunters’ cases of CJD to zoonotic transmission of CWD. 

"A history of hunting and/or eating venison does not mean that someone got CJD that way,” Ryan A. Maddox, PhD, an epidemiologist at the CDC who studies prion diseases including CWD, told USA TODAY about the report.^[] 

“Many Americans hunt and even more eat venison,” Maddox added. “Some will develop sporadic CJD by chance and others will not."

However, Maddox told USA TODAY that he agrees with researchers that investigations are needed to assess the potential risks of CWD on people's health.

In a study published this May, the National Institutes of Health (NIH) found that it was unlikely for cervid animals with CWD to infect humans.^[] Researchers used a lab-based study in which they exposed healthy human organoids to infected CWD prions from various cervid animals for 7 days. Following the exposure, the researchers observed the organoids for up to 6 months. By the end of the study, none of the human organoids had become infected with CWD. The researchers said the results indicated a “substantial resistance or barrier to the propagation of infection,” according to a news release about the study. However, the researchers noted limitations, including the possibility of genetic susceptibility in some people that was not accounted for and that new strains of CWD with a lower barrier to infection could emerge in the future.

How can doctors protect people from prion disease?

While patients may be unlikely to contract CJD from deer infected with CWD, they can still develop the condition through other means. Given how deadly CJD is, it is important for doctors to stay vigilant about risks.

In a 2022 interview with medical information outlet Consultant360, Maddox encourages neurologists to take advantage of the National Prion Disease Pathology Surveillance Center, which provides tools to accurately diagnose prion disease.^[]

If suspecting a case in a younger patient, he suggests, be mindful of genetic factors that may play a role. However, he reminds doctors that CJD is rare and primarily affects the older, aging population. 

He also encourages neurologists to do what they can to reduce surgical transmission risks. Maddox recommends three ways to reduce risk: using disposable instruments, utilizing “stringent sterilization methods” outlined by the CDC, and quarantining instruments “until prion disease is no longer in the differential.”

Ruling out more common, treatable diseases is also important before diagnosing someone with human prion diseases like CJD as there is still no cure for humans with the condition.