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MERS transmissibility from camels to humans appeared low
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An international team of researchers determined that Middle East respiratory syndrome, or MERS, was not highly transmissible from dromedary camels to humans based on recent serologic evidence.
The researchers analyzed serum samples from individuals with varying degrees of exposure to an infected dromedary herd of approximately 70 animals in Al Hasa, Saudi Arabia, during peak calving season, from December 2013 to February 2014.
Serum samples were drawn from 191 people, who were grouped into five categories:
- Group 1: Four herdsmen who had daily contact with the infected herd through feeding, grooming and administration of treatment, and who also drank unboiled milk from infected camels;
- Group 2: Eight individuals, including veterinary staff, who had sporadic but regular contact with the infected animals, using only disposable gloves for protection;
- Group 3: 30 veterinary surgeons and clinical support staff at the Clinical Research Center at the Faculty of Veterinary Medicine and Animal Resources, King Faisal University, Saudi Arabia, who were not exposed to the infected herd but came into daily contact with various types of domestic livestock, including camels, using only disposable gloves for protection;
- Group 4: Three workers at a camel slaughterhouse in Al Hasa, where 25 to 35 camels were slaughtered daily, who used no protective equipment;
- and Group 5: 146 individuals in the Al Hasa region who had no workplace exposure to camels and served as negative controls in the study.
The researchers used a pseudoparticle neutralization assay to test the serum samples for MERS coronavirus (MERS-CoV) antibodies.
They found that none of the human blood samples had serologic positivity for infection, even when a 50% reduction in luciferase signal was utilized as the threshold for suspected antibody.
The virus infecting the herd was genetically similar to those previously seen in humans, according to the researchers.
“Our findings do not imply that dromedaries are not a source of infection for humans,” they wrote. “Spillover infection of humans may be more common in other settings in which humans are exposed over sustained periods to animals among which virus prevalence is higher.”
However, human disease does not appear to be directly proportional to the level of exposure to MERS-CoV in camels. Similar patterns of low transmissibility have been observed in humans with high exposure to poultry infected with avian influenza A(H5N1). The biological basis of this pattern is unknown, the researchers said.
It was previously established that MERS-CoV from the same dromedary herd had potential to infect ex vivo cultures from the human respiratory tract, suggesting groups 1 and 2 were repeatedly exposed over at least a 1-month period to camels shedding a virus that could potentially infect humans.
“We conclude that MERS-CoV was not highly transmissible from dromedaries to humans with various levels of exposure to this infected dromedary herd,” the researchers wrote. “Further studies on the mechanisms by which MERS-CoV is transmitted from dromedaries to humans, whether by direct or indirect routes, and the heterogeneity of human susceptibility to this virus are needed.”
Disclosure: The researchers report no relevant financial disclosures.
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