The Middle East Respiratory Syndrome (MERS) coronavirus has become enzootic in dromedary camels in Saudi Arabia and diverged into five different variants, according to a Chinese-led study published in the Friday issue of the journal Science.
In a second study published in the same journal, researchers designed a vaccine shown to be effective in protecting dromedaries against the virus.
Over the past three years, several MERS outbreaks have been reported in the Middle East and most recently in South Korea, with a fatality rate of roughly 35 percent.
Arabian camels are a common host for the MERS virus, and one of the most likely sources of human infection, the researchers said. The virus can diversify in the animals and then be passed to people, but little is known about its prevalence there and the route by which it is transmitted to humans.
To gain more insights, researchers took samples from more than 1,300 camels in Saudi Arabia, the country most affected by MERS, between May 2014 and April 2015.
The overall infection rate of the MERS virus among this sample was 12 percent with a peak during the winter season, December 2014 to January 2015, at 21 to 23 percent, said the study, led by Professor Yi Guan and Assistant Professor Huachen Zhu at the University of Hong Kong, in collaboration with King Abdulaziz University, Saudi Arabia; and scientists from Mainland China, Australia and Egypt.
The MERS virus was predominantly shed from the respiratory tracts of camels, with over 25 percent of nasal swabs positive for coronaviruses, and only one percent of samples from digestive tracts positive. "Thus, air-borne transmission is the most likely way to spread the virus," they concluded.
Genetic sequencing identified five different lineages of the virus, all of which have the ability to infect both humans and camels, said the study.
Viruses that led to the South Korean outbreak and the recent human infections in the Middle East were from lineage 5, which was generated by recombination between viruses of lineages 3 and 4.
"This novel recombinant virus lineage appeared in Saudi Arabian camels as early as in July 2014, while human infections with viruses of this lineage were only reported from February 2015 onwards," Zhu said.
"The human MERS coronavirus identified in South Korea early this summer shows extremely high similarity to a camel virus sampled in March 2015 in Riyadh, indicating the origin of Korean viruses is from camels of the Middle East," he added.
The researchers also found two other coronaviruses co-circulating with the MERS coronavirus in the camels, including one closely related to the human 229E coronavirus that causes common colds in humans.
The results showed that around 6.9 percent of Saudi Arabian camels were simultaneously infected by two or three coronavirus species, and over half of the MERS coronavirus-positive camels were also infected with at least one other coronavirus.
Co-infections of different coronavirus species occur frequently in camels, highlighting the role of dromedary camels as an important host for coronaviruses, they said.
In addition, young dromedary camels, under one year old, played an important role in maintaining and spreading this virus.
In the second study, European researchers found after administering a candidate vaccine both nasally and intramuscularly, all camels developed detectable levels of antibodies against the MERS virus within three weeks.
Upon infection with the virus, these vaccinated camels experienced only mild clinical symptoms and were found to have significantly lower levels of the virus compared to those who did not receive the vaccine.
"This is nonetheless a very significant step forward in the fight against this pathogen," study author Joaquim Segales, lecturer at the Universitat Autonoma de Barcelona, said in a statement.
"Now we need to delve more deeply into the duration of the immunity and dosage before applying it in real situations."
