MERS-coronavirus in dromedary camels

Middle Eastern respiratory syndrome coronavirus (MERS-CoV), first identified in the fall of 2012 in a Saudi Arabian patient, has since infected over 180 individuals, causing 77 deaths. Antibodies to the virus and the viral genome have been found in dromedary camels in Jordan and Saudi Arabia, implicating those animals as the source of human infections. A new study reveals that the virus has infected camels throughout Saudi Arabia since at least 1992.

Serum, blood, and rectal and nasal swabs were collected from dromedary camels in November-December 2013 from southwestern, western, northwestern, eastern, and central regions of the Kingdom of Saudi Arabia. Of 203 serum samples, 150 (74%) were found to contain antibodies to MERS-CoV. The number of seropositive animals varied from 5% to 95% depending on location and the age of the animals (in general, seropositivity was higher in adult camels compared with young camels). Antibodies against MERS-CoV were also detected in archived serum samples from 1992 through 2010.

Polymerase chain reaction was used to detect viral nucleic acids in clinical specimens from camels. Viral nucleic acid was most frequently detected in nasal  swabs; only three rectal samples were positive. More samples from juvenile camels contained viral nucleic acids (36/104, 35%) than from adults (15/98, 15%). No viral nucleic acids were detected in the blood of these animals.

Phylogenetic analysis of approximately 3 kb of viral nucleic acid sequence revealed <1% divergence from published MERS-CoV sequences.

These findings indicate that dromedary camels are a reservoir of MERS-CoV. The finding of higher seroprevalence in older camels suggests that younger animals are infected as they are introduced into herds in which the virus is circulating. Proving that infected camels are the source of human infections will require epidemiological investigations of human cases where the infection might have been acquired from camels. If camels indeed spread the virus to humans, it will be important to determine the route. As not all MERS-CoV cases have documented exposure to camels, there should be other routes of infection other than contact with camels, such as through contaminated material or person to person contact.

MERS-CoV has been circulating in dromedary camels in Saudi Arabia since 1992, but it is likely that the virus has been infecting these animals even longer. Camels do not appear to be adversely affected by MERS-CoV infection, a situation often seen when host and pathogen have co-evolved for long periods of time. Whether or not this speculation is correct will require additional work.

I spoke with two of the authors of this new study, W. Ian Lipkin and Thomas Briese, on a special episode of the science show This Week in Virology. You can find TWiV special – MERS-coronavirus in dromedary camels at www.microbe.tv/twiv. During this episode it was revealed that the investigators have propagated infectious MERS-coronavirus from nasal swabs of several dromedary camels.