Middle Eastern respiratory syndrome coronavirus (MERS-CoV), first identified in the fall of 2012 in a Saudi Arabian patient, has since infected over 160 individuals, causing 71 deaths. Identifying the source of infection is important for efforts to prevent further infections. Recently two studies revealed the presence of antibodies to the virus in dromedary camels in Jordan and Saudi Arabia, two countries where large clusters of infections have occurred. Detection of the viral RNA genome in clinical specimens by polymerase chain reaction (PCR) now provides additional evidence that MERS-CoV can infect camels.
Samples were obtained from all camels (n=14) on a farm where two individuals with laboratory confirmed MERS-CoV infection had been in contact with animals. Nasal swab specimens from three camels were positive when assayed by PCR using MERS-CoV specific primers. Nucleotide sequence analysis revealed that the virus from one camel clustered with sequences obtained from the two farm-associated MERS-CoV infections. Sera from all camels on the farm reacted with MERS-CoV in immunofluorescence and neutralizing assays.
These observations provide strong evidence that MERS-CoV can replicate in camels. However, the authors were not able to isolate infectious virus from camel specimens. MERS-CoV has been previously cultured from human clinical specimens, and it is known what types of cells should be used for virus isolation. Levels of virus in the camel specimens might be too low to detect by culturing, or alternatively only fragments of viral genomes might be present, especially if the infection is over.
Proof that infectious MERS-CoV virus is present in camels will require isolation of infectious virus in cultured cells. If PCR is routinely used to diagnose viral infections such as influenza, why is it not sufficient to conclude that MERS, CoV is present in camels? The answer is that this is not a routine case – the investigators are attempting to determine the origin of MERS-CoV and therefore demonstrating infectious virus is essential. You can bet that the investigators are hard at work attempting to isolate infectious virus from the camels.
The authors note that because of the nucleotide sequence similarity between the camel and human viruses, is not possible to determine if the camels were infected by humans, or if humans infected the camels. It is also possible that camels and humans were infected by a third source. Analysis of outbreaks in which the viruses have undergone more extensive sequence divergence should permit establishment of the chain of transmission. If I had to speculate, I would say the virus is going from camels to humans. So far there has been little evidence of seropositivity in humans outside of the known cases, while many camels have antibodies that react with the virus.