Middle East respiratory syndrome coronavirus neutralising serum antibodies in dromedary camels: a comparative serological study

Abstract

Background: A new betacoronavirus-Middle East respiratory syndrome coronavirus (MERS-CoV)-has been identified in patients with severe acute respiratory infection. Although related viruses infect bats, molecular clock analyses have been unable to identify direct ancestors of MERS-CoV. Anecdotal exposure histories suggest that patients had been in contact with dromedary camels or goats. We investigated possible animal reservoirs of MERS-CoV by assessing specific serum antibodies in livestock.

Methods: We took sera from animals in the Middle East (Oman) and from elsewhere (Spain, Netherlands, Chile). Cattle (n=80), sheep (n=40), goats (n=40), dromedary camels (n=155), and various other camelid species (n=34) were tested for specific serum IgG by protein microarray using the receptor-binding S1 subunits of spike proteins of MERS-CoV, severe acute respiratory syndrome coronavirus, and human coronavirus OC43. Results were confirmed by virus neutralisation tests for MERS-CoV and bovine coronavirus.

Findings: 50 of 50 (100%) sera from Omani camels and 15 of 105 (14%) from Spanish camels had protein-specific antibodies against MERS-CoV spike. Sera from European sheep, goats, cattle, and other camelids had no such antibodies. MERS-CoV neutralising antibody titres varied between 1/320 and 1/2560 for the Omani camel sera and between 1/20 and 1/320 for the Spanish camel sera. There was no evidence for cross-neutralisation by bovine coronavirus antibodies.

Interpretation: MERS-CoV or a related virus has infected camel populations. Both titres and seroprevalences in sera from different locations in Oman suggest widespread infection.

Funding: European Union, European Centre For Disease Prevention and Control, Deutsche Forschungsgemeinschaft.

Figure 1

Reactivity of livestock sera with three coronavirus S1 antigens Fluorescent intensities per antigen at a serum dilution of 1/20. Black lines indicate median. Dashed line is cutoff of the assay. RFU=relative fluorescence units. SARS-CoV=severe acute respiratory syndrome coronavirus. HCoV=human coronavirus. MERS-CoV=Middle East respiratory syndrome coronavirus.

Figure 2

MERS-CoV and human coronavirus OC43 or bovine coronavirus cross-reactivity Combinations of the mean fluorescent intensities of reactions of sera with MERS-CoV and human coronavirus OC43 antigens from 105 Spanish dromedary camels (A). plaque reduction neutralisation tests for bovine coronavirus and MERS-CoV (B): two representative sera are shown (numbers 15 and 5, corresponding to camel ID numbers in table 2) in dilutions of 1/40, 1/160, and 1/640 as well as the virus input control. All samples were tested in duplicates (only one well shown) and titres were expressed as the serum dilution resulting in a plaque reduction of at least 90%. IgG reactivity of both camel sera to MERS-CoV antigen and human coronavirus OC43 antigen in a two-step dilution series in the microarray (C). IgG reactivity of two two-step serially diluted Omani dromedary camel sera with human coronavirus EMC antigen and human coronavirus OC43 antigen in the microarray (D). RFU=relative fluorescence units. MERS-CoV=Middle East respiratory coronavirus.