High Rate of Circulating MERS-CoV in Dromedary Camels at Slaughterhouses in Riyadh, 2019

Abstract

MERS-CoV is a zoonotic virus that has emerged in humans in 2012 and caused severe respiratory illness with a mortality rate of 34.4%. Since its appearance, MERS-CoV has been reported in 27 countries and most of these cases were in Saudi Arabia. So far, dromedaries are considered to be the intermediate host and the only known source of human infection. This study was designed to determine the seroprevalence and the infection rate of MERS-CoV in slaughtered food-camels in Riyadh, Saudi Arabia. A total of 171 nasal swabs along with 161 serum samples were collected during the winter; from January to April 2019. Nasal swabs were examined by Rapid test and RT-PCR to detect MERS-CoV RNA, while serum samples were tested primarily using S1-based ELISA Kit to detect MERS-CoV (IgG) antibodies and subsequently by MERS pseudotyped viral particles (MERSpp) neutralization assay for confirmation. Genetic diversity of the positive isolates was determined based on the amplification and sequencing of the spike gene. Our results showed high prevalence (38.6%) of MERS-CoV infection in slaughtered camels and high seropositivity (70.8%) during the time of the study. These data indicate previous and ongoing MERS-CoV infection in camels. Phylogenic analysis revealed relatively low genetic variability among our isolated samples. When these isolates were aligned against published spike sequences of MERS-CoV, deposited in global databases, there was sequence similarity of 94%. High seroprevalence and high genetic stability of MERS-CoV in camels indicating that camels pose a public health threat. The widespread MERS-CoV infections in camels might lead to a risk of future zoonotic transmission into people with direct contact with these infected camels. This study confirms re-infections in camels, highlighting a challenge for vaccine development when it comes to protective immunity.

Keywords: ELISA; MERS-CoV; RT-PCR; camel; seroprevalence; slaughterhouse; transmission.

Figure 1

Molecular detection of MERS-CoV in slaughterhouse camels in Riyadh, 2019. (A): Camel nasal swab samples tested by RT-PCR (n = 171) and presented as Ct values. Negative control and positive control samples were tested in this assay. Samples are considered positive if their values is below the dotted line, which represent the cutoff of the assay. (B): A comparison between the RT-PCR and the Rapid test representing the sensitivity and specificity of the Rapid test.

Figure 2

Genetic analysis of spike gene sequences isolated from slaughterhouse camels in Riyadh, 2019. (A): Phylogenetic tree based on nucleotide sequences of spike gene of the 13 isolates of this study. (B): Pairwise heatmap presentation of the 13 sequences (C): Heatmap to compare between MERS-CoV spike gene sequences obtained from (I) the current study (n = 13) and (II) archived sequences obtained from GenBank (n = 250). The color scale is shown to indicate the distance value between sequences.

Figure 3

Seroprevalence of MERS-CoV in slaughterhouse camels in Riyadh, 2019. (A): Anti-MERS-CoV antibody levels in the tested camel sera using S1-based ELISA Kit. Samples are considered positive when ELISA ratio is above 1.1. (B): Neutralizing anti-MERS-CoV antibodies in camel sera were confirmed in all samples that are positive by ELISA. This is shown as 50% inhibitory concentration of MERS pseudotyped viral particles in log scale. (C): Correlation between ELISA ratios and titres of neutralizing antibodies by MERSpp assay. Pearson correlation coefficient of 0.80, R squared of 0.65, and a p-value of <0.0001.