Camels carry a heavy viral load

Despite thousands of years of close proximity to humans, camels generally haven’t been considered a major source for human diseases. 

However, the MERS coronavirus (MERS-CoV) which caused a disease outbreak in Saudi Arabia in 2012 is known to have originated in bats and incubated in camels before infecting humans. Camels are thought to serve as a major reservoir of MERS-CoV, and a 2014 study identified a range of mammalian viruses in pooled camel faecal samples.

In the new study, a team led by researchers from the Abu Dhabi Food Control Authority and the US Centres for Disease Control and Prevention (CDC) sequenced DNA from nasopharyngeal samples taken from 108 camels known to carry MERS-CoV.

They found sequences related to mammalian viruses from 13 genera in 10 families, including viruses known to infect humans and other animals, as well as some potentially novel camel viruses. Many of the camels were infected with viruses from two or three different genera, and MERS-CoV was found with another coronavirus, alpha-CoV, in more than 90% of the samples.

The high co-infection rates raise the risk that viruses might recombine, borrowing genetic material from each other, gaining the ability to infect a different host species. 

This risk is exacerbated by the fact that humans and camels often mix in live animal markets which are home to many other species such as cattle, sheep, and goats and even occasionally chickens, dogs, or cats.

“We need to educate people who regularly handle camels to maintain distance and exercise caution. There are only limited studies like this on camel viruses, and we recommend running wide surveillance to get a better picture of the viruses carried in camels,” says Suxiang Tong of the CDC, one of the scientists who led the study. 

Next, the team will repeat their analysis in camels without MERS-CoV in order to evaluate the viral load of healthy camels. 

They intend to sequence the genomes of some of the novel viruses to better understand them, and they are planning a longitudinal study in camel farms to better understand the virus transmission dynamics in a population. 

Kwok-Yung Yuen, a professor of microbiology and infectious disease at the University of Hong Kong, who led the 2014 study, describes the high co-infection rate of an alphacoronavirus with the MERS coronavirus, which is a betacoronavirus, as “really intriguing”.

“As the study was done at the live animal market where a high concentration of camels and perhaps other animals are brought together, cross infection between stressed animals in such overcrowded situations may explain this high co-infection rate,” he says, noting that co-infection may help generate new genotypes or new viruses which may cross the species barrier. 

He adds: “As in the case of wild life markets for SARS and live poultry markets for avian influenza A H5N1 and H7N9, the importance of epidemic centres for generating emerging infectious disease outbreaks should not be underestimated.”

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