The suspicion that Middle Eastern respiratory syndrome coronavirus (MERS-CoV) may be widespread in Middle Eastern camels grew today with reports that dromedary camels in Jordan and Saudi Arabia were found to have antibodies to the virus or one closely related to it.
In one of two studies published in Eurosurveillance, Jordanian and European researchers reported that 11 of 11 camels tested in Jordan had MERS-CoV–like antibodies. Tests in goats, sheep, and cows were negative, with the exception of preliminary tests in six sheep, which were contradicted by further tests.
In the other study, another international team reported that 280 of 310 dromedary camels from various parts of Saudi Arabia had antibodies to MERS-CoV or a very similar virus, but further tests suggested that some of the results represented cross-reactions to bovine coronavirus (BCoV). Sheep, goats, cattle, and chickens all tested negative.
This past summer researchers reported finding MERS-CoV like antibodies in camels in Oman, Egypt, and the Canary Islands. More recently, scientists reported finding the virus itself in a camel owned by a MERS-CoV patient in Saudi Arabia and in three camels linked to two human cases in Qatar.
However, it remains unclear whether camels are a source of MERS-CoV in humans, because no one has yet demonstrated a close genetic match between a camel MERS-CoV isolate and a human isolate. Nonetheless, the new findings seem to strengthen the evidence that many camels in the Middle East have been exposed to the pathogen.
In the Jordanian study, scientists gathered serum and fecal samples from 11 camels, 150 goats, 126 sheep, and 91 cows between June and September of this year. Most of the animals were in the Zarqa governorate, site of the first known human outbreak of MERS-CoV in April 2012. (The outbreak was identified retrospectively in the fall of that year.)
The serum samples were tested for antibodies that react with MERS-CoV, SARS-CoV (severe acute respiratory syndrome coronavirus), and human coronavirus OC43. The latter is closely related to BCoV, which circulates in all four livestock species tested.
All 11 camels and 6 of the 126 sheep tested positive for MERS-CoV antibodies, versus none of the cows or goats. Also, 4 of 11 camels, 23 of 91 cows, 128 of 150 goats, and all the sheep samples reacted with OC43.
To confirm the findings, the researchers tested all the camel and sheep samples in a MERS-CoV neutralization assay. This revealed that all the camels had neutralizing antibodies, but none of the sheep did. A further confirmatory test yielded similar results.
The authors also tested the camel and sheep fecal samples, using polymerase chain reaction methods, but found no trace of MERS-CoV.
"These observations indicate MERS-CoV or a highly related virus circulated in dromedary camels in a region where transmission to humans occurs," the report says. The authors comment that BCoV is known to circulate in camels, but add that previous studies have excluded cross-neutralization between MERS-CoV and BCoV and that their study further confirms this.
"Our observation strengthens our earlier study in which MERS-CoV neutralising antibodies were found in dromedary camels in Oman where human cases have been reported as well," the researchers write. "Until the virus that elicits these antibodies in camels is detected, sequenced and compared to the viruses sequenced from human patients, it remains unclear whether this livestock species is indeed infected with MERS-CoV and thus represents an immediate source for human infection.
"However, our observations should be used to focus virological and serological studies in livestock, especially dromedary camels and sheep, and including humans handling these animals and their products."
In the Saudi study, researchers collected serum samples from 310 dromedary camels, 100 sheep, 45 goats, 50 cattle, and 240 chickens. The livestock were in Riyadh, Al Ahsa, and other areas of the country where human MERS cases have occurred.
Using a MERS-CoV pseudoparticle neutralization test, the authors found that 280 of 310 camels (90%) were seropositive, whereas all the other animals tested negative. The camel results differed by age: 47 of 65 (72%) of camels under 1 year old tested positive, compared with 233 of 245 (95%) of older camels.
The researchers did further tests on 56 camel samples in an effort to rule out positive results that represented cross-reactions to BCoV. They found that 19 of these had at least fourfold higher MERS-CoV microneutralization titers than BCoV microneutralization titers, indicating that these were "MERS-CoV specific" results. Others of the 56 samples were classified as indeterminate or BCoV-specific.
The authors say the MERS-CoV-specific findings confirm that some camels are being infected by a virus different from BCoV and possibly identical or closely related to MERS-CoV. "If we consider just the 'MERS-CoV' specific reactions, we observe that these animals are getting infected within the first year of life," they add.
They say further that the identity of the MERS-CoV–like virus suggested by the serologic findings can be established only by characterizing an actual isolate from the animals.
"Studies involving follow-up of herds of camels from time of calving though the first year of life with serial blood samples together with oral and rectal or fresh faecal swabs would better define the ecology of the MERS-CoV-like virus infecting these animals and provide virus for genetic characterization," the report states.
"Such studies are a priority to determine whether dromedary camels are in fact a source of human MERS-CoV infection or whether they are being infected by a ubiquitous novel coronavirus closely related to MERS-CoV."
Reusken CB, Ababneh M, Raj VS, et al. Middle East respiratory syndrome coronavirus (MERS-CoV) serology in major livestock species in an affected region in Jordan, June to September 2013. Eurosurveill 2013 Dec 12;50 (18): pii=20662 [Full text]
Hemida MG, Perera RA, Wang P, et al. Middle East respiratory syndrome (MERS) coronavirus seroprevalence in domestic livestock in Saudi Arabia, 2010 to 2013. Eurosurveill 2013 Dec 12;50(18):pii=20659 [Full text]
Dec 2 CIDRAP News story on Qatar camel findings