Middle East Respiratory Syndrome / MERS

CDC Yellow Book 2024

Travel-Associated Infections & Diseases

Author(s): Claire Midgley, Marie Killerby, Aron Hall

INFECTIOUS AGENT: Middle East respiratory syndrome coronavirus (MERS-CoV)


Arabian Peninsula

Parts of North, West, and East Africa


Travelers in or near the Arabian Peninsula who have contact  with camels or patients with known MERS infection, or who visit health care  facilities


Practice general hygiene measures, including regular hand washing, especially after contact with camels and people who are ill

Immunocompromised people and those with underlying health conditions should avoid close contact with camels

Follow safe food and water precautions and avoid drinking raw camel milk or camel urine, or eating improperly cooked meat, including camel meat


Contact state or local health department

Infectious Agent

Middle East respiratory syndrome coronavirus (MERS-CoV) is a single-stranded, positive-sense RNA virus that belongs to the family Coronaviridae, genus Betacoronavirus, which causes Middle East respiratory syndrome (MERS).


MERS-CoV is a zoonotic virus known to transmit sporadically from the host reservoir, camels (specifically dromedaries [Camelus dromedarius]), to humans. Limited human-to-human transmission chains can subsequently occur, usually via close contact in health care or household settings.

Camel-to-Human Transmission

Transmission from camels to humans occurs via direct contact (e.g., grooming, petting), and possibly via indirect contact (e.g., contact with camel feces or camel products, being in settings where camels are present). Precise transmission mechanisms are unknown, but camel handlers with prolonged direct contact with live camels are thought to have the greatest risk for infection.

MERS-CoV has been detected in camels in North, West, and East Africa, and in or near the Arabian Peninsula, but most camel-to-human transmission has been reported within the Arabian Peninsula. Limited recent evidence suggests acute (RNA positive) infection among camel workers in parts of East Africa, and evidence for past (antibody positive) infection in camel workers in parts of North and East Africa. The extent of camel-to-human transmission on the African continent is not fully understood, however, and no current evidence of subsequent human-to-human transmission in Africa has been reported.

Person-to-Person Transmission

After camel-to-human transmission, MERS-CoV can be spread from person to person, resulting in outbreaks in households and in health care settings. MERS-CoV does not seem to pass easily between people and generally requires very close contact (e.g., in households, including sharing a bedroom with or caring for a person known to be infected with MERS-CoV, or when providing unprotected patient care in health care settings).

Large health care–related outbreaks have been documented, with transmission to other patients, visitors, and health care personnel; transmission has been reported in emergency departments, inpatient wards, and outpatient dialysis units. Superspreading events in health care facilities (generally, a single case linked to ≥5 subsequent cases) often have involved severely ill patients and have been associated with late recognition of people infected with MERS-CoV, crowding, delayed implementation of infection-control practices, and performance of aerosol-generating procedures prior to adopting airborne precautions.

Some evidence shows that symptomatic people, especially those with more severe illness, play a major role in human-to-human transmission, but little evidence supports transmission from asymptomatic people. Sustained community transmission of MERS-CoV has not been shown. A few MERS cases have been reported without camel, health care, or known MERS case exposure, indicating that MERS-CoV transmission pathways are not fully understood.


First reported in September 2012, illnesses with onset as early as April 2012 were subsequently documented. To date, all MERS cases reported to the World Health Organization (WHO) have been linked to travel to, or residence in, countries in or near the Arabian Peninsula where risk of infection is ongoing, including Bahrain, Iran, Jordan, Kuwait, Lebanon, Oman, Qatar, Saudi Arabia, United Arab Emirates, or Yemen.

Cases among travelers to the Arabian Peninsula have been reported from North Africa, Asia, and Europe; subsequent transmission to travelers’ contacts also has been documented. More than 1,300 people in the United States have been evaluated for MERS after travel to the Arabian Peninsula. To date, only 2 patients in the United States have tested positive for MERS-CoV infection; both were travelers who arrived from the Arabian Peninsula in May 2014, and no secondary transmission was identified for either case.

Most reported MERS cases have been linked to human-to-human transmission within health care facilities; people who have direct contact with camels or close contact with symptomatic MERS patients are at risk for MERS-CoV infection.

Camel-Associated Risk

While recent evidence has highlighted the potential for camel-to-human transmission within parts of North and East Africa, no subsequent human-to-human transmission has been reported after such exposures. The risk for MERS-CoV infection in North, West, or East Africa is thought to be minimal, but travelers to the region who have direct camel contact, including those who work with camels, could be at risk.

Health Care–Associated Risk

Health care personnel and others who visit or work in facilities experiencing known MERS-CoV transmission are at risk for exposure and infection. In addition, depending on their activities, travelers to, in, or near the Arabian Peninsula, including tourists, medical tourists, or business travelers, also could be at risk for infection. Close contacts of ill travelers who come from the Arabian Peninsula represent another group at potential risk for infection.

One of the largest health care–associated outbreaks (186 cases, 38 deaths), occurred in the Republic of Korea in 2015 because of delayed recognition of a single infected business traveler returning home from the Arabian Peninsula. Health care–associated transmission also has occurred in France and the United Kingdom from cases exported from the Arabian Peninsula. Rapid detection and isolation of patients with MERS seeking medical care is critical to preventing secondary transmission in health care facilities.

Clinical Presentation

MERS is associated with a spectrum of illness that ranges from asymptomatic infection to mild upper respiratory tract illness to severe acute respiratory failure and multiple organ dysfunction. High mortality has been observed with MERS; ≈35% of confirmed cases have been fatal. For people who develop symptomatic illness, the incubation period is ≈2–14 days; median incubation period is slightly more than 5 days. Disease is most often characterized by cough, fever, and shortness of breath. Other nonspecific symptoms include abdominal pain, nausea, vomiting, and diarrhea; arthralgias and myalgias; chills; headache; and sore throat. Initial symptoms can progress to pneumonia. Chest radiographs have shown variable pulmonary involvement.

In addition to acute and often severe respiratory compromise, serious complications of MERS include acute renal injury and cardiovascular collapse. Abnormal laboratory findings can include elevated liver function tests, lymphopenia, and thrombocytopenia.

More severe illness and poorer outcomes have been observed in older adults, people who are immunocompromised, and people with underlying medical conditions (e.g., cardiovascular disease [including hypertension], diabetes mellitus, chronic kidney disease, chronic lung disease). Individuals with >1 underlying condition are at increased risk for poor outcomes.


Several diagnostic assays have been developed to detect acute infection with MERS coronavirus, including real-time reverse transcription PCR (rRT-PCR). These assays can reliably distinguish MERS coronavirus from other human coronaviruses, including severe acute respiratory syndrome coronavirus 2, the virus that causes coronavirus disease 2019 (COVID-19). Notably for MERS, specimens collected from the lower respiratory tract (e.g., bronchoalveolar lavage, endotracheal aspirates, sputum) are the priority for testing, although upper respiratory tract and serum specimens also can be used. To increase the likelihood of detecting MERS coronavirus, collect specimens from multiple sites and at multiple time points over the course of the illness.

In the United States, most state public health laboratories are approved to test for MERS-CoV using an rRT-PCR assay developed by the Centers for Disease Control and Prevention (CDC). Coordinate testing through state and local health departments, who will, in turn, contact CDC for additional diagnostic support as needed. For details on who should be evaluated as a person under investigation for MERS-CoV infection, see Box 5-04 and the CDC website. Consult with public health departments for case-patients with equivocal clinical presentations or exposure histories (e.g., uncertain health care exposure).

Because the risk for MERS-CoV transmission from camels in North, West, and East Africa is not yet fully understood, consider MERS evaluation for travelers coming from these regions who develop severe respiratory illness (fever and pneumonia or acute respiratory distress syndrome) ≤14 days of direct camel contact.

Box 5-04 Patients in the United States who should be evaluated for Middle East respiratory syndrome coronavirus (MERS-CoV) infection

SEVERE ILLNESS: criteria for evaluation
Patient has fever and pneumonia or fever and acute respiratory distress syndrome, no alternative diagnosis, and ≥1 of the following epidemiologic risk factors

  • Within 14 days before symptom onset, a history of travel from countries in or near the Arabian Peninsula


  • Within 14 days before symptom onset, history of close contact with a person who themselves developed fever and acute respiratory illness within 14 days of travel to countries in or near the Arabian Peninsula


  • Is a member of a cluster of patients with severe acute respiratory illness of unknown etiology

MILDER ILLNESS: criteria for evaluation
Patient has fever and symptoms of respiratory illness (e.g., cough and/or shortness of breath, not necessarily pneumonia), no alternative diagnosis, and ≥1 of the following epidemiologic risk factors

  • Within 14 days of symptom onset, a history of being in a health care facility in a country or territory in or near the Arabian Peninsula where recent health care–associated cases of MERS have been identified


  • Within 14 days of symptom onset, a history of direct camel contact in or near the Arabian Peninsula


  • Within 14 days of symptom onset, a history of close contact with a person with confirmed MERS-CoV infection case while that person was ill


Treatment is currently limited to supportive care; no specific therapies for patients with MERS have yet been approved. As of April 2022, antiviral and monoclonal antibody therapies are in development or under investigation for potential use. In preclinical trials, some FDA-approved therapeutic options for adult patients with COVID-19 are also demonstrating efficacy against MERS-CoV.


Although no vaccine or preventive drug has been approved for use in humans, several are under investigation. Because of the risk for nosocomial transmission resulting in sizeable hospital outbreaks, rapid detection and isolation of patients with MERS is critical. Standard, contact, and airborne infection-control precautions are recommended for hospitalized patients being evaluated for or diagnosed with MERS.

Travelers should practice general hygiene precautions (e.g., frequent handwashing; avoiding touching their eyes, noses, and mouths; avoiding contact with sick people). Additionally, WHO recommends as a general precaution that anyone visiting places where camels are present practice general hygiene measures, including regular handwashing before and after touching animals. In line with food hygiene practices, WHO recommends people avoid drinking raw camel milk or camel urine or eating meat (including camel meat) that has not been properly cooked. WHO also recommends that people at higher risk for severe MERS illness avoid close contact with camels.

CDC website: www.cdc.gov/coronavirus/mers

The following authors contributed to the previous version of this chapter: John T. Watson, Susan I. Gerber

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