Until cases recognized, MERS a formidable bug
21 cases of person-to-person spread
The rapid transmission and high attack rate of Middle East Respiratory Syndrome (MERS) coronavirus in a hospital dialysis unit in Al-Hufuf, Saudi Arabia "raises substantial concerns about the risk of health careassociated transmission of this virus," a team of researchers recently reported.1
A team of experts that included SARS veteran Allison McGeer, MD, a microbiologist and infectious disease consultant at Mount Sinai Hospital in Toronto, reviewed an outbreak of MERS that occurred between April 1 and May 23, 2013. A total of 23 confirmed cases and 11 probable cases were part of a single outbreak involving several health care facilities. There was strong evidence of person-to-person transmission as patients moved between units and facilities.
"Epidemiologic and phylogenetic analyses support person-to-person transmission; however, it is not possible to be certain about whether there were single or multiple introductions from the community," the authors found. "Similarly, we are unable to determine whether person-to-person transmission occurred through respiratory droplets or through direct or indirect contact and whether the virus was transmitted when the contact was more than [1 meter] away from the case patient."
Because some patients presented with gastrointestinal symptoms, and transmission appeared to occur between rooms on the ward, the current WHO recommendations for surveillance and control should be regarded as the minimum standards, they noted. Hospitals should use contact and droplet precautions and should consider the follow-up of persons who were in the same ward as a patient with MERS infection, they recommended.
"One of the patients who was in the ICU at the time — who was exposed and got infected — was in fact a dialysis patient at another dialysis unit," McGeer says. "So by the time he got tracked down he was already sick and he had been dialyzed. He exposed people in the other dialysis unit, and subsequently two or three of those patients got sick."
As part of the investigation, medical records were reviewed for clinical and demographic information and determination of potential contacts and exposures. Case patients and contacts were interviewed and viral RNA was sequenced. Symptoms in the 23 MERS patients included fever in 20 patients (87%), cough in 20 (87%), shortness of breath in 11 (48%), and gastrointestinal symptoms in 8 (35%). Twenty of them presented with abnormal chest radiographs. As of June 12, a total of 15 patients (65%) had died, 6 (26%) had recovered, and 2 (9%) remained hospitalized.
"A total of 21 of the 23 cases were acquired by person-to-person transmission in hemodialysis units, intensive care units, or in-patient units in three different health care facilities," the authors found.
Super spreaders’ a la SARS
Sequencing data from four isolates revealed a single monophyletic clade, meaning all the organisms were closely related. Among 217 household contacts and more than 200 health care worker contacts, MERS infection developed in five family members (three with laboratory-confirmed cases) and in two health care workers (both with laboratory-confirmed cases).
"The apparent heterogeneity in transmission, with many infected patients not transmitting disease at all and one patient transmitting disease to seven others, is reminiscent of SARS," the team reported.
This so called "super spreader" phenomenon was further evidenced by a patient that transmitted MERS to three people and four patients who transmitted infection to two persons each. Super spreaders were also reported during the SARS pandemic, but McGeer questions whether the label really provides any actionable information.
"We know enough now about shedding of influenza and other respiratory viruses to know that there is a fair amount of heterogeneity in what kind of particle sizes you put out and what concentration of virus you put out," she says. "The difficulty is that it doesn’t help in the sense that I can’t really tell you by looking at a person whether they are going to be somebody that sheds lot of virus and particles of the right size. I don’t think we are really further ahead on what makes a super spreader,’ but there was evidence from that outbreak that there does seem to be heterogeneity in the way people transmit."
The incubation period of confirmed cases was 5.2 days, which may partially explain why cases occurred despite the use of infection control measures. For example, between April 14 and April 30, MERS infection was confirmed in nine patients who were undergoing hemodialysis. Of those, eight had an onset of disease before or within 24 hours after infection-control interventions were implemented on April 21.
"I don’t think we have evidence to say that transmission occurred through precautions," McGeer says.
The interventions included monitoring hand hygiene, implementing droplet and contact precautions for febrile patients, testing patients with fever for MERS, putting masks on all patients undergoing hemodialysis, not allowing patients with suspected MERS infection into the dialysis unit, enhancing environmental cleaning, and excluding visitors and nonessential staff.
"The 65% case fatality rate in this outbreak is of concern," the researchers conclude. "We and others have found that the severity of illness associated with MERS infection ranges from mild to fulminant. The clinical syndrome is similar to SARS, with an initial phase of nonspecific fever and mild, nonproductive cough, which may last for several days before progressing to pneumonia. Some patients with MERS infection also had gastrointestinal symptoms, a finding similar to that with SARS."
Reference
1. Assiri, A. McGeer A, Perl TM, et al. Hospital Outbreak of Middle East Respiratory Syndrome Coronavirus. N Engl J Med 2013;369:407-416.
