Transmission and shedding of Middle East respiratory syndrome coronavirus from dromedary camels, alpacas, and domestic livestock
Date
2016
Authors
Adney, Danielle, author
Bowen, Richard, advisor
Mathiason, Candace, committee member
Olea-Popelka, Francisco, committee member
Zabel, Mark, committee member
Journal Title
Journal ISSN
Volume Title
Abstract
The Middle East respiratory syndrome coronavirus (MERS-CoV) is a novel betacoronavirus discovered in 2012 that causes severe respiratory disease that can result in death in infected humans. Human-to-human transmission can occur, although zoonotic transmission from dromedary camels plays an important role in transmission. A large percentage of dromedaries in the Middle East and Africa have antibodies specific for MERS-CoV, indicating widespread transmission among camels. In vitro studies indicate that other livestock may be susceptible to infection and transmit virus, however, field studies have not detected any seropositive goats, horses, sheep, or horses. Due to the requirement for specific high-containment facilities research on the role of dromedaries has primarily been limited to field surveys. Here we report experimental infection of dromedary camels with a human isolate of MERS-CoV. The objectives of this study were to characterize clinical disease, shedding, and tissue burdens in infected animals. Experimental infection with the virus resulted in a mild, transient upper respiratory tract infection accompanied by shedding of large amounts of infectious virus. While infectious virus was only detected for a short time viral RNA was detected for much longer, indicating that field studies only using PCR may not be sampling animals able to transmit virus. Due to their cost, size, and temperament, dromedaries are not conducive to high-containment studies, and we hypothesized that alpacas may be a suitable replacement for some studies. This dissertation reports shedding and transmission of MERS-CoV in experimentally infected alpacas (n = 3) or those infected by direct contact (n = 3). Infectious virus was detected in all experimentally infected animals and in 2 of 3 in-contact animals. All alpacas seroconverted and were rechallenged 70 days after the original infection in order to understand if previous infection results in protective immunity. Experimentally infected animals were protected against reinfection, and those infected by contact were partially protected. Necropsy specimens from immunologically naive animals (n = 3) obtained on day 5 postinfection showed virus in the upper respiratory tract. These data demonstrate efficient virus replication and animal-to-animal transmission and indicate that alpacas might be useful surrogates for camels in laboratory studies. Current understanding of MERS-CoV infection suggests that camels become infected as calves or during transport to slaughterhouses, and we hypothesized that vaccinating naïve animals could decrease viral shedding upon exposure. We vaccinated three dromedary camels and two alpaca with a MERS-CoV S protein subunit vaccine. Vaccinated animals developed neutralizing antibodies to MERS-CoV with titers ranging from <10 to 2560 at week 16. Vaccinated animals, as well as two control camels and two control alpaca were challenged with a human isolate of MERS-CoV and were monitored for clinical signs and viral shedding. Vaccinated animals displayed partial protection associated with low or moderate antibody titers whereas the vaccinated alpaca developed high levels of antibody and were protected against infection following viral challenge. In vitro assays indicate that sheep, goats, and horses are susceptible to infection with MERS-CoV. However, unlike dromedaries and alpacas, infected sheep, goats, and horses all failed to shed more than trivial quantities of virus and are therefore unlikely to serve as a source of transmission to humans or other animals. The research presented in this dissertation provides experimental evidence linking dromedaries to MERS-CoV infection. Here, we present the first experimental infection of dromedary camels with MERS-CoV. We describe clinical disease, viral shedding, organ burden, and seroconversion in these animals. It is hoped that these studies will help shape camel sampling as well as allow for a more complete understanding of current field sampling data. Because of the difficulties associated with dromedary camels, we developed a more tractable model of reservoir infection. We found that alpaca model can be a useful substitution for camels in some studies and as well as demonstrate its strengths and weaknesses in a vaccination study. Finally, the data presented here indicate that sheep, goats, and horses do not shed large amounts of infectious virus after challenge with MERS-CoV. This indicates that these animals do not play an important role in viral transmission and that surveillance and vaccine efforts should focus primarily on dromedary camels.