A newly identified form of prion disease may have been quietly spreading in the brains of African camels for decades, according to a report published in the June issue of
The spread of the new, fatal neurodegenerative disease—similar to the well-known “mad cow disease” caused by misfolded proteins in the brain—is a major concern for communities and public health.
This “makes it necessary to assess the risk for humans and develop evidence-based policies to control and limit the spread of the disease in animals and minimize human exposure,” the authors of the new report conclude.
The Italian and Algerian researchers behind the report were first clued into the new prion disease after accounts from slaughter houses described camels with telltale neurological symptoms. These include tremors, aggressiveness (biting and kicking), hyperreactivity, distinct up and down head movements, a hesitant and faltering gait, loss of limb control, occasional falls, and difficulty getting up. The researchers confirmed the suspicion after inspecting three camel brains collected from symptomatic animals that arrived at a slaughterhouse in Ouargla, Algeria. All three had clear signs of prion deposition and damage.
Generally, prions are infectious, deformed versions of the normal prion protein (), which functions on the outside of healthy cells in the brain. The gnarled versions prompt normal prion proteins to deform and malfunction, setting off a cascade that leads to telltale clumps of contorted proteins. This leads to progressive damage, marked by a sponge-like appearance of brain tissue as well as the behavioral symptoms.
But that wasn’t all researchers saw in their camel samples. They also collected lymph tissue from one of the camels and found it too showed signs of prion disease. In other types of prion disease, infectious proteins zooming around the lymphatic system suggests that the disease is infectious, jumping from animal-to-animal. Though they only had lymph tissue from one animal, they found evidence of prion disease in every lymph node they tested, heightening concern that the disease was spread from another animal.
Looking further into slaughterhouse records, the researchers found more reports of similarly sick camels. In 2015, 20 out of 937 camels showed the neurological symptoms, while 51 of 1,322 did so in 2016. That suggests a prevalence of around 3 percent. And accounts from camel breeders and slaughterhouse personnel suggested that such cases reach back into the 1980s.
Reaching back to the 1980s raises some eyebrows, linking to the timing of another prion disease epidemic. Prions cause a variety of diseases in other animals, including Creutzfeldt-Jakob disease (a prion disease in humans), scrapie (affecting sheep and goats), and chronic wasting disease (found in mule, deer, elk, and reindeer). But, the most well-known is bovine spongiform encephalopathy (BSE aka mad cow disease). A massive epidemic of BSE began in the United Kingdom in 1986, which eventually lead to health experts diagnosing more than 180,000 cattle with the fatal disease and slaughtering millions more in eradication efforts, which in total cost an estimated $7.1 billion.
Unlike the situation with the camel prion disease, BSE doesn’t easily jump from animal-to-animal. Experts blamed the disease’s spread on contaminated feed, instead. At the time, it was common to grind up remnant cow parts into a protein-packed powder and add it to the feed of other cows. If any of the powder came from cows sick with BSE, it could contain infectious prions—regardless of how the feed was treated. Prions can withstand heat, irradiation, enzymes that normally degrade proteins, and protein-altering chemicals, such as formaldehyde.
Likewise, people (and some other animals, such as cats) who ate meat from infected cattle could also contract the disease. An estimated 231 people worldwide are known to have gotten sick with a fatal prion disease linked to contaminated beef consumption, according to the US Food and Drug Administration. The practice of using cattle protein in cattle feed was banned in the US, UK, and elsewhere amid the epidemic.
While molecular data showed that the camels had prions that seemed slightly different from those found in BSE, the researchers couldn’t rule out the possibility that the camels’ prion disease originated in BSE-contaminated meat from abroad.
For now, it’s too early to tell. “The origin of [camel prion disease] is unknown,” they conclude. “It might be a disease unique to dromedaries or a malady deriving from transmission of a prion disease from another species.” They also note that camels tend to scavenge in the “widespread” waste dumps in the Sahara Desert, near oil extraction plants. It also can’t be ruled out that they ate prion-contaminated garbage, the authors note.
Regardless of the origin, the researchers say the disease requires “urgent” surveillance and further research to mitigate risks to humans and animals.
, 2018. DOI:10.3201/eid2406.172007 (About DOIs).