Missing information in animal surveillance of MERS-CoV

Chantal Reusken and colleagues¹ have identified neutralising antibodies against Middle East respiratory syndrome coronavirus (MERS-CoV) in dromedary camels, suggesting the role of camels in harbouring the virus and infecting human beings. The study findings suggest that bats and camels jointly play an important part in the persistence and evolution of MERS-CoV, resulting in spillover to the human population.1, 2 Theoretical epidemiologists refer to such combinations of animal hosts as the reservoir community.³

To confirm that the two animal species constitute a reservoir community, two conditions should be objectively examined.³ First, the combination of reservoir hosts should permit persistence of the virus in natural settings (eg, by frequently transmitting the virus to other hosts), which is the necessary condition. Second, in the absence of the reservoir community, transmission cannot be maintained (by definition), which is referred to as the sufficient condition.3, 4 If other animal hosts can also maintain transmission, the reservoir community will have to include these hosts. In view of these conditions, although the identification of antibodies in camels is remarkable, an explicit assessment of the epidemiological role of camels has yet to be made to elucidate the mechanism of emergence in human beings.

How can a complete view of reservoir dynamics be achieved? The series of transmission in a population could be traced with serological techniques. Rather than doing a cross-sectional survey (to take a snapshot of prevalence), the identification of the reservoir host requires understanding of the incidence (ie, rate of new infections) to measure transmissibility. Serial cross-sectional surveys or large-scale follow-up of cohorts (of susceptible camels, not of those already infected) would be required. Alternatively, implementation of a transmission experiment with uninfected camels could also be useful, because the transmissibility of infected camels can be objectively quantified. Even a simple one-to-one transmission experiment could inform about the presence of substantial transmissibility.⁵

Of course, identification of the route of transmission from camels to human beings would also be of utmost importance, and could directly lead to prevention. In addition to such preventive effort, clarification of reservoir dynamics should be given a high priority, because identification of the major route of animal-to-human transmission, without an understanding of the overall picture of persistence and evolution in different hosts, is difficult.