Wednesday 14 October 2015

MERS-CoV on the farm...

I'm going to spend a few posts catching up on some excellent papers showing the role of camels in harbouring and transmitting the Middle East respiratory syndrome coronavirus (MERS-CoV). There also seems to be some confusion remaining about what we know, what we don't know, and also how bats fit in to the picture. I'll finish the next few posts with a summary.

Please do check out my previous listing of the literature around MERS-CoV and camels.[1]

I'll republish an updated literature list in the summary post as well.

First up, an article from the scientific literature which was published by Hemida et al. in the July 2014 edition of Emerging Infectious Diseases, but would have gone online much earlier (CDC don't list that date for some reason I cannot fathom).[2]

This authors first remind us that MERS-CoV RNA has been detected in humans and dromedary camels (DCs) before and that DC infection has been shown to precede human infection in one study (well, two but they both analyse the same camels and humans).[3]

This study collected and froze nasal, oral or rectal swabs and blood samples from DCs on 2 farms in Al-Ahsa in the Kingdom of Saudi Arabia (KSA). The authors then looked for MERS-CoV RNA and antibodies.
  • Farm A:
    • 70 DCs
      • 4: 1 month of age
      • 8: approximately 1 year of age
      • 58: adults
    • Sampled 5 times between NOV2013 & FEB2014
    • Herd never grazed in the desert (so wasn't exposed to other camels)
    • November 30th 2013 results
      • 10 DCs were MERS-CoV RNA positive; 8 of 9 DCs that had both nasal and faecal samples tested were only positive in the nasal swab, 1 DC only in the faecal swab
    • December 2013 results
      • No positive DCs December 4th; the following resulted from December 30th
      • 7 of 8 calves and 2 of 3 adults
      • 12 adults with sera collected before this testing were seropositive - this include 2 that were MERS-CoV RNA positive suggesting DCs can be reinfected
      • 2 seronegative 1-year old calves had the highest nasal loads of MERS-CoV RNA suggesting maternal antibody may not be protective
      • 4 DCs had the equivalent of a human cold - cough, sneeze, discharge, elevated temperature and were off their food
    • February 14th 2014 results
      • No MERS-CoV RNA was detected in DCs
    • All 3 MERS-CoV RNA-positive DC calves who had sera collected on December 30th and February 14th, were MERS-CoV RNA negative in the February sample (thus an acute not chronic infection in camels) and all had a four-fold or great rise in antibody titer
  • Farm B:
    • 20 DCs
      • 3: calves
      • 17: adults
    • Sampled once, FEB-2014
    • Herd sometimes grazed in the desert
    • No MERS-CoV RNA was detected in DCs
Samples were tested by 2 MERS-CoV specific real-time RT-PCRs and a broadly reactive coronavirus conventional RT-PCR. MERS-CoV positive samples were re-extracted (nucleic acids were purified from another aliquot of the original sample) and re-tested to confirm.

Conventional (Sanger) full genome sequencing was also conducted generating 3 genomes from Farm A, KFU-HKU 13, KFU-HKU 19Dam (faecal swab) and KFU-HKU 1. These were identical in sequence across the full 30,100 nucleotide genome and across the spike gene of 4 more viruses.

Virus isolation using the Vero E6 cell line was successful from 2 nasal swabs and 1 faecal swab - all with high amounts of viral RNA (culture is nowhere near as sensitive as PCR-based detection methods) - collected on December 30, 2013.
  • A genome sequence from the faecal swab and the 2nd passage of culture isolate from the same faecal swab were directly compared - 3 nucleotide changes were identified, 2 of which led to an amino acid change (spike and membrane proteins)
So we learned from this study that DC MERS-CoV (genetically near identical to virus found in humans) doesn't mutate within a given DC herd (genetically stable in DCs), but does change a little upon cell culture in the laboratory. That change is not unexpected as cell lines in a flask are not camel/human cells in a complex microenvironment in the body. It's also not the first time such mutation has been seen.

We can also see that not all farms in a region of KSA have MERS-CoV when one does but that infections spreads within and around the herd - not persisting once it has moved through. However this herd and others in the region is one from which DCs can be moved to the via Buraidah in the KSA to the United Arab Emirates. Imports and exports and movement to shows and festivals being a problem when your animal is carrying an infectious agent - just as it is when an infected human hops on a plane and travels to Nigeria, or South Korea or the United States...or anywhere. We saw that adult DCs could probably be reinfected despite a pre-existing antibody response. But we learned nothing about the cell-mediated immune response - a gap in our knowledge that extends to the human immune response to MERS-CoV infection also.

While the peak of infection at Farm A occurred in late December in this study, only a limited time periods was sampled and too few farms to know if this is the pattern throughout the Arabian Peninsula, or just chance in Al Ahsa in 2013/2014. But there is another study that has looked a little longer and I'll review that soon. 

Sadly, there were no human farmers involved. The study would have been made more valuable if it had also followed any and all humans in contact with these camels over this period as well. More examples of camel-to-human infection would be great to have since there are still those who don't "believe" camels play a role in MERS. Of course, it's not belief that's needed, it's the willingness to sit down and listen to the scientific facts we have at hand. And that comes down to finding a way to pitch the facts in a way that works for each type of audience.

  2. MERS Coronavirus in Dromedary Camel Herd, Saudi Arabia
    Hemida MG, Chu DK, Poon LL, Perera RA, Alhammadi MA, Ng HY, Siu LY, Guan Y, Alnaeem A, Peiris M.
    Emerg Infect Dis. 2014 Jul;20(7):1231-4

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