A large cross-sectional serostudy of blood donors in Saudi Arabia has revealed a significant number of subclinical infections with Middle East respiratory syndrome coronavirus (MERS), which pose a likely source of infection in previously unexplained outbreaks of MERS. Notably, the prevalence of antibodies against MERS coronavirus (MERS-CoV) was significantly higher in persons with regular exposure to camels than in those without. This suggests that MERS-CoV is sporadically introduced into the population via camel contact.

The study, published in The Lancet Infectious Diseases, involved over 10,000 human serum samples from donors from all 13 provinces of Saudi Arabia. 227 sera from shepherds and abattoir workers with occupational exposure to camels were additionally tested. Samples were screened using recombinant ELISA and confirmed by recombinant immunofluorescence assay as well as plaque reduction neutralisation test.

Anti-MERS-CoV antibodies were found in 0.15% of subjects from the general population, in 2.3% of shepherds and in 3.6% of slaughterhouse workers. This represents a 15- to 23-fold higher prevalence in the camel-exposed individuals than in non-exposed persons. The seropositive individuals showed no signs of severe illness, indicating subclinical infection. The mean age of seroconverted persons was significantly younger than that of laboratory-confirmed primary MERS cases, and men demonstrated a much higher antibody prevalence than women. Antibody positivity was found in 6 of the 13 provinces, with an emphasis in central, more rural provinces compared to coastal provinces. Projected from this study, it is estimated that near to 45,000 subjects over 15 years old might be seropositive for MERS-CoV in Saudi Arabia.

MERS-CoV is an emerging pathogen which is responsible for an outbreak of severe acute respiratory illness predominantly in the Arabian Peninsula with a high number of fatalities. From September 2012 to mid April 2015, there have been 1,106 laboratory-confirmed cases with 421 deaths (WHO). The virus is transmitted from person to person via aerosols and smear infection and can also be acquired from camels. The wide presence of low rates of MERS-CoV antibodies across Saudi Arabia, including in provinces which have not yet reported outbreaks, is consistent with the concept of a predominantly zoonotic disease which does not readily spread to humans. Subclinical infections, especially in the healthy, young, male population, may constitute an infection source for MERS patients with no recalled camel contact.

Müller MA et al., Lancet Infect Dis doi: 10.1016/S1473-3099(15)70090-3