Wednesday, 7 September 2016

MERS-CoV: alpacapalooza...

In the search for the animals that may be another reservoir for, or just support infection by, Middle East respiratory syndrome coronavirus (MERS-CoV), a few studies have looked at furry little alpacas (Vicugna pacos). 

We already know from Eckerle and colleagues' work that cells derived from alpacas have the required receptor molecule that MERS-COV uses (DPP4) and can support replication of MERS-CoV in the lab,[1] But are cells in flasks different from furry animals in the wild?

Three articles came out in the June 2016 issue of Emerging Infectious Diseases looking at alpacas and MERS-CoV. There's no way to tell who submitted when in this journal.

Colorado State University team sought an easier - but still relevant - animal model than camels to work with.[2] They infected 3 (A1-A3) alpacas with 107 plaque forming units (PFU; a cell-specific measure of the amount of infectious virus in a diluted sample) of the HCoV-EMC/2012 variant of MERS-CoV via 3ml of diluted virus per nostril then housed 3 more uninfected alpacas (A4-A6) with them, 3 days later.  

After 70 days, A1-A6 were infected again ("challenged"), the same way. Three other alpacas were infected the same way but euthanized 5 days after infection and their tissues collected for analysis. Nasal swabs were collected before infection and then daily from all animals for 5 days post-infection and on day 10. A4-A6 were also swabbed 3 times per week to day 19. This could all have been a bit more clearly demonstrated using a timeline by the way.

None of the infected animals had a fever or observable nasal discharge and their appetites and activity remained constant. Infectious MERS-CoV was shed by A1-A3 to day 5 and transmission occurred to the A4-A6 arrivals; A4 was found to shed virus for 1 day, A5 shed no virus and A6 shed across 8 days. Lots of variation even in a controlled environment like this.

When A1-A6 were challenged with a fresh infection. A1-A3 had developed antibodies which protected them from infection as no virus was shed. A4-A6 shed a little infectious virus for at most 2 days. A4-A6 also developed neutralizing antibodies but took longer to do so. 

A wild transmission (from inoculated A1-A3 to naive A4-A6) of MERS-CoV which was of a lower dose than the original inoculum, seemed to elicit a milder antibody response as well. 

Virus was found in the nose, larynx and trachea of A7-A9 but not the lungs.

The Australian/Singapore team also sought an animal model with a better temperament and more manageable size than the camel.[3] 

Under biosafety level 3 conditions, they used a camel MERS-CoV variant (Al-Hasa_KFU-HKU13/2013) and exposed each animal to 106 50% tissue culture infectious doses (TCID50; another cell-specific measure of infectious virus quantity), monitored for 21 days then challenged. Blood as well as nasal, oral, rectal and urogenital swabs were collected over time and tested by sensitive RT-PCR, culture and for the presence of neutralizing antibodies.

The animals once again did not develop a fever (animal No.2 had a raised temperature though) or respiratory illness. 

Infectious virus was isolated from oral upper and deep nasal swabs but not from urogenital or rectal swabs. RNA detection by RT-PCR followed this pattern but after challenge and in the presence of antibodies which had appeared from day 10-12, viral RNA could not be detected anywhere in any animal. 

Neutralizing antibody did not appear until 21 days in animal No. 1, 10 days in No. 2 and still hadn't appeared at day 35 in No. 3. It was apparently unnecessary though, for protection from reinfection in this study of alpacas.  

These 2 studies used 106-107 cell-specific quantities of MERS-CoV to infect the alpacas, but, when sought, less was produced - except in one of the Australian/Singapore animals where the peak of 106 TCID50 detected equalled the input dose. This finding suggests the inoculum may not be relatable to real-world amounts of virus produced from an infected animal source. It may however, just be how much is needed to get an model system infected.

The authors all agreed that alpacas could be a good model for MERS-CoV in camels and that animal infections supported the finding of the initial alpaca cell culture work. But that culture link is quite not so straightforward. 

Deliberately HCoV-EMC/2012 variant infected goats, sheep and horses - also animals whose cells had supported MERS-CoV in the laboratory - showed little or no sign of viral replication and the animals mostly remained healthy (some nasal discharge was seen from 2 of 4 horses).[5] Despite some signs of neutralizing antibody developing in goats and a sheep, the same Colorado team were not convinced that any of these animals would be likely hosts for MERS-CoV in the wild. Cells in a flask are not always the most realistic model for animal transmission I guess.

The final alpaca article was from a Qatar/Netherlands team who tested alpacas in a region of Qatar where MERS-CoV is found to be enzootic among camels.[4] 

Hobby alpaca and camel herds were the subjects of this study. They had been kept about 200m apart in the same farm and cared for by the same animal workers. Blood samples were tested from 15 alpacas and 10 dromedary camels; nasal swabs were collected from the 10 camels but nasal, rectal and oral samples were only collected from a subset of the alpacas for antibody testing and sensitive RT-PCR. 

MERS-CoV neutralizing antibody was present in 15 of 15 alpacas and 9 of the 10 camels according to a 90% plaque-reduction neutralization test. This indicated past natural infections had occurred. Antibodies were also detected that suggested past infection by dromedary betacoronaviruses and camelid alphacoronaviruses but this was not unusual or unexpected. No swabs were positive by RT-PCR so no animals were acutely infected. The authors did not know when, how or how often MERS-CoV may have naturally infected the alpacas.

So we can add alpacas to camels on the short list of animals that can host MERS-CoV infection.

  1. Replicative Capacity of MERS Coronavirus in Livestock Cell Lines
  2. Infection, Replication, and Transmission of Middle East Respiratory Syndrome Coronavirus in Alpacas
  3. Experimental Infection and Response to Rechallenge of Alpacas with Middle East Respiratory Syndrome Coronavirus
  4. MERS-CoV Infection of Alpaca in a Region Where MERS-CoV is Endemic
  5. Inoculation of Goats, Sheep, and Horses with MERS-CoV Does Not Result in Productive Viral Shedding