Scientists have found more evidence that many camels in the Middle East have been exposed to the Middle East respiratory syndrome coronavirus (MERS-CoV) or a close relative, increasing the suspicion that camels may have spread the virus to humans.
In serologic tests on 110 dromedary camels in Egypt, one test showed that 94% of them had antibodies to MERS-CoV, and a second test revealed antibodies in 98%, according to a report in today's issue of Eurosurveillance. Tests of humans, water buffaloes, cows, and other domestic animals in Egypt and Hong Kong showed no MERS-CoV antibodies.
"The antibody titres were very high" in both sets of tests, "suggesting that the virus infecting these camels was MERS-CoV virus itself or a very closely related virus," says the report by a team of Chinese, Egyptian, and American scientists.
The findings echo those published last month by a team from the Netherlands and Germany, who tested 50 dromedaries in Oman and found that all had antibodies to MERS-CoV or a close relative. They also found that 14% of a sample of camels in the Canary Islands had similar antibodies.
Which animals harbor the MERS virus and which ones passed, or are passing, it to humans remains a mystery. Although the antibody findings indicate that camels probably have been exposed to MERS-CoV or a very similar virus, scientists have not yet isolated the virus itself from camels or any other animals.
Two weeks ago another group reported finding a fragment of DNA in an Egyptian tomb bat that matched MERS-CoV, but the fragment was so small that experts said it could have represented a related virus, not the identical species.
For the new study, the investigators devised a new serologic test that they say is safe and specific and therefore useful for large-scale seroepidemiologic studies in animals. The test, called a pseudoparticle neutralization (ppNT) assay, involves a nonreplicating HIV virus that has been engineered to produce the spike protein of MERS-CoV.
The team also used what they call the gold-standard test, microneutralization (MN), but they note that it requires the use of live MERS-CoV virus and therefore Biosafety Level 3 (BSL-3) containment. In contrast, the pseudoparticle test involves no dangerous live virus and can be used in less stringent BSL-2 labs.
Samples from Egypt and Hong Kong
The authors collected animal and human serum samples in Egypt and Hong Kong, with the Hong Kong sera serving as unexposed controls. In Egypt, they gathered samples from 110 dromedaries, 8 water buffaloes, 25 cows, 5 sheep, and 13 goats.
Most of the camels had been imported from Sudan and were 5 to 7 years old. The group also used 815 human samples that had been collected for a study on influenza virus.
In Hong Kong, the authors used samples from 260 swine and 204 bird samples, including wild northern pintails and Eurasian widgeons, plus 528 archived human samples collected in 2011 and 2012.
In screening tests, both MN and the ppNT test showed a high prevalence of MERS-CoV antibodies (a titer of at least 1:20) in the camels: 93.6% and 98.2%, respectively, the report says. The antibody titers ranged up to 1,280 or higher in the MN test and up to 10,240 in the ppNT assay. None of the other animal samples or the human samples showed any signs of antibodies to MERS-CoV.
Of five camel samples that were negative on the MN test, four had weakly positive results on the ppNT, with antibody titers ranging from 40 to 160.
Given that several other coronaviruses are extremely common in humans, the lack of MERS-CoV antibodies in the human samples indicates that the MN and ppNT assays are specific for MERS-CoV, the investigators write.
No cross-reaction with SARS virus
The team also found that the camel sera with high MERS-CoV antibody levels did not cross-react with the SARS (severe acute respiratory syndrome) coronavirus.
"Taken together, these data indicate that a MERS-CoV or a highly related virus is endemic in dromedary camels imported for slaughter in Egypt," they state. "These findings provide independent confirmation of the results recently reported by Reusken et al [the Dutch-German team], who found very high antibody titres to MERS-CoV in dromedary camels."
Because the camels used in the study had been brought to Egypt from Sudan and other African countries, it is unclear where they originally acquired their infection, the report says. Given the similar findings from camels in Oman and the Canary Islands, "it is likely that this coronavirus is widespread in North and East Africa and the Arabian Peninsula."
If future research confirms that bats and dromedary camels harbor MERS-CoV, "we will have a scenario of a virus reservoir in bats with a peridomestic animal such as the camel as intermediate host, which may in fact be the immediate source of human infection," the investigators say.
They add that in some MERS-CoV index cases, the patients had a history of exposure to camels. "Given that the MERS-like coronavirus in camels appears to be ubiquitous, it remains to be explained why MERS in humans appears relatively rare," they observe.
Perera RA, Wang P, Gomaa MR, et al. Seroepidemiology for MERS coronavirus using microneutralisation and pseudoparticle virus neutralization assays reveal a high prevalence of antibody in dromedary camels in Egypt, June 2013. Eurosurveill 2013 Sep 5;18(36) [Full text]
Aug 8 CIDRAP News story on earlier camel findings
Aug 30 CIDRAP News story on report of possible MERS-CoV in bat