Saturday, March 18, 2017

EID Journal: Domestic Pig Unlikely Reservoir for MERS-CoV


Although the MERS Coronavirus was initially isolated in a Saudi patient in the summer of 2012, and it was suspected as being a zoonotic infection, it would take more than a year before camels were identified as a carrying antibodies to a MERS-like virus (see The Lancet Camels Found With Antibodies To MERS-CoV-Like Virus).

While highly suggestive, this wasn't proof that camels were the animal reservoir for MERS, only that they had been exposed to a `similar' virus.  

Over the next six months studies (see Dec. 2013 The Lancet: Identification Of MERS Virus In Camels and  EID Journal: MERS Coronaviruses in Dromedary Camels, Egypt) would help solidify the role of camels in the spread of the virus.  
Unknown, however, was what other animal reservoirs might exist. 

For the next 18 months - other than some close-but-no-cigar MERS-like coronaviruses found in bats - only humans and camels were found infected with the MERS virus.

That changed in early 2016 (see  EID Journal: MERS-CoV Antibodies In Alpacas - Qatar), when researchers reported detecting substantially elevated antibodies to MERS-CoV in Alpacas raised near (but not with) camels.

Alpacas are part of the same biological family (Camelidae) as dromedaries (as are Bactrian camels, llamas, vicuñas, and guanacos), so this finding isn't all that surprising.  

Since then, we've seen a number of studies where researchers have challenged various livestock animals with the MERS virus to see if they can be infected. Last August, in Study: Experimental Infection Of Goats, Sheep & Horses With MERS-CoV, we looked at a study that reassuringly found that:

Minimal or no virus shedding was detected in all of the animals. During the four weeks following inoculation, neutralizing antibodies were detected in the young goats, but not in sheep or horses.

But in December of 2016, in EID Journal: Livestock Susceptibility to Infection with MERS-CoV, researchers reported that pigs (not initially tested because they are not commonly raised in the Middle East) may need to be added to the short list of livestock that can carry the MERS virus.

They experimentally inoculated llamas, pigs, sheep, and horses with the MERS virus, and found that pigs & llamas both shed the virus from the nose, and  seroconverted.

Given the susceptibility of pigs to other coronaviruses (see  mBio: PEDV - Porcine Epidemic Diarrhea Virus - An Emerging Coronavirus and Swine Enteric Coronavirus Diseases), this has understandably raised concerns.

Today we've a dispatch published by the EID Journal which confirms that pigs can be experimentally infected with the MERS virus, but finds they do not develop clinical disease and only shed small quantities of the virus, making them unlikely to spread the disease.

Domestic Pig Unlikely Reservoir for MERS-CoV

Emmie de Wit, Friederike Feldmann, Eva Horne, Cynthia Martellaro, Elaine Haddock, Trenton Bushmaker, Kyle Rosenke, Atsushi Okumura, Rebecca Rosenke, Greg Saturday, Dana Scott, and Heinz Feldmann

Author affiliations: National Institutes of Health, Hamilton, Montana, USA (E. de Wit, F. Feldmann, E. Horne, C. Martellaro, E. Haddock, T. Bushmaker, K. Rosenke, R. Rosenke, G. Saturday, D. Scott, H. Feldmann); Columbia University, New York, New York, USA (A. Okumura)


We tested the suitability of the domestic pig as a model for Middle East respiratory syndrome coronavirus (MERS-CoV) infection. Inoculation did not cause disease, but a low level of virus replication, shedding, and seroconversion were observed. Pigs do not recapitulate human MERS-CoV and are unlikely to constitute a reservoir in nature.



Recently, Vergara-Alert et al. showed MERS-CoV shedding in pigs inoculated with 107 TCID50 of MERS-CoV and suggested that pigs could play a role as a reservoir for the circulation of MERS-CoV (12). In our hands, pigs inoculated with a 10-fold lower infectious dose of MERS-CoV were also successfully infected, but the low amount of virus replication in and shedding from the respiratory tract implies that the pig is unlikely to play a profound role as an intermediate host for MERS-CoV in nature.

Taken together, our data indicate that MERS-CoV can infect pigs, leading to a low level of replication in the pig respiratory tract, but does not cause clinical signs of disease. Furthermore, viral shedding from mucosal membranes of the upper respiratory tract was rather limited with no infectious virus measurable at any time postinoculation. Thus, the pig is not a suitable animal disease model for MERS-CoV infection.

The caveat, as with all studies of this type, is that MERS-CoV - like all viruses - continues to evolve and adapt, and what can be said about it today may not hold true forever.

But for now, at least, this is encouraging news.

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