Middle East Respiratory Syndrome (MERS)

01Pathophysiology

MERS-CoV is a lineage C betacoronavirus that was first isolated in Saudi Arabia in 2012. As of mid-2025, roughly 2,627 laboratory-confirmed cases and around 943 associated deaths have been reported globally, yielding a case fatality rate of approximately 36%. The overwhelming majority of cases have been reported from Saudi Arabia.

The virus's natural reservoir is the dromedary camel. Seroepidemiological studies show that nearly all adult camels in the Arabian Peninsula and Africa carry antibodies to MERS-CoV, reflecting long-standing endemic circulation. Human infection occurs through direct or indirect contact with infected camels (nasal secretions, milk, meat, urine, feces) or, less efficiently, through human-to-human transmission in close-contact or healthcare settings.

The cellular entry mechanism is central to understanding why MERS looks the way it does clinically. MERS-CoV uses its spike (S) glycoprotein to bind to dipeptidyl peptidase 4 (DPP4, also known as CD26), a transmembrane receptor expressed on the surface of epithelial cells in the lower respiratory tract, renal tubular cells, intestinal epithelium, liver cells, and immune cells including macrophages and T lymphocytes. This broad receptor distribution is the reason MERS produces not only pneumonia but also extrapulmonary organ damage, notably acute kidney injury, which is a distinguishing feature compared to other coronavirus infections.

Once the virus enters the cell, it replicates and triggers a dysregulated host immune response. Infection of macrophages via the S protein-DPP4 interaction suppresses their capacity to produce pro-inflammatory cytokines (TNF-alpha, IL-6) while augmenting production of the immunosuppressive cytokine IL-10. This immune evasion allows the virus to replicate unchecked in its early stages. Eventually, the host mounts a delayed but exaggerated inflammatory response, resulting in a cytokine storm, diffuse alveolar damage, and ARDS. The diffuse alveolar damage is the pathological substrate for the bilateral ground-glass opacities and consolidation seen on imaging.

The renal tropism of MERS-CoV is a high-yield concept. DPP4 is expressed on kidney epithelial cells, and acute renal failure in MERS patients occurs earlier (median onset around 11 days) than it did in SARS patients (around 20 days), suggesting that MERS-CoV may cause direct viral injury to the kidneys rather than purely secondary ischemic damage. This makes the combination of progressive respiratory failure plus acute kidney injury a signature pattern.

The incubation period is approximately 5 to 7 days (range 2 to 14 days). From the onset of illness to hospitalization, the median interval is roughly 4 days. Patients who require ICU admission often exhibit rapid progression from a febrile upper respiratory illness to pneumonia within one week.

02Classification and Clinical Manifestation

Clinical pearls to keep in mind:

• GI symptoms (diarrhea, vomiting, abdominal pain) are present in roughly one-third of patients and may occasionally be the presenting complaint, especially in immunocompromised individuals.

• Fever may be absent in the very young, the elderly, immunosuppressed patients, or those on antipyretics or certain medications. A vignette describing an afebrile pneumonia with travel history should still raise concern.

• Healthcare-associated transmission is a well-documented pattern. Clusters in hospital settings, particularly dialysis units and ICUs, are repeatedly described in outbreak reports.

03Diagnostic Workup

The best initial test for a suspected MERS case is real-time reverse transcription PCR (rRT-PCR) targeting the upE gene. The WHO recommends screening for MERS-CoV using RT-PCR of the upstream envelope gene (upE), followed by confirmation with ORF1a, ORF1b, or the nucleocapsid (N) gene. This is a two-step diagnostic algorithm: a positive upE screening must be confirmed with a second, independent genomic target before the case is considered laboratory-confirmed.

Lower respiratory tract specimens (sputum, bronchoalveolar lavage, tracheal aspirate) carry a higher viral load than nasopharyngeal swabs and are preferred. If only upper respiratory specimens are available and the result is negative but clinical suspicion remains high, repeat testing or lower tract sampling should be pursued.

Laboratory findings at presentation may include leukopenia, lymphopenia, thrombocytopenia, and elevated lactate dehydrogenase levels, particularly as illness severity increases. These are nondiagnostic but supportive in the right clinical context. Renal function tests may reveal rising creatinine in cases progressing toward multi-organ failure.

Chest imaging is abnormal in more than 80% of hospitalized MERS cases. Ground-glass opacity is found in over 60% of chest radiographs, with consolidation in approximately 20%. The most common pattern seen on both CXR and CT is ground-glass opacities and consolidation with peripheral predominance, eventually spreading to involve both lungs as the disease progresses. Lower lobes tend to be affected more than upper lobes early in the course of illness, and the radiographic appearance can progress rapidly. CT may additionally show air bronchograms, crazy-paving pattern, and interstitial thickening. In severe cases, imaging is consistent with ARDS.

Serology (ELISA for S1 protein, immunofluorescence, microneutralization assays) is not used for acute diagnosis. It requires paired serum samples collected at least 14 days apart and is reserved for epidemiological surveillance, seroprevalence studies, and retrospective confirmation of past infection.

A key testing principle for exams: in a vignette with a returned traveler from the Arabian Peninsula presenting with severe pneumonia, the next best step is rRT-PCR from a lower respiratory specimen, not serology.

04Management and Treatment

Despite past efforts, there are still no licensed therapies of proven efficacy for MERS-CoV. Supportive measures remain the mainstay of treatment and include respiratory and circulatory support, preservation of renal, hepatic, and neurological function, and prevention of secondary infections.

Acute stabilization follows the principles of critical care for severe community-acquired pneumonia and ARDS. Oxygen therapy should be escalated as needed, moving from nasal cannula to high-flow oxygen to non-invasive ventilation and ultimately to intubation with lung-protective mechanical ventilation. Prone positioning may be considered for refractory hypoxemia, consistent with ARDS management protocols. Hemodynamic instability should be treated with fluid resuscitation and vasopressors.

Regarding investigational antivirals, the most studied combination is ribavirin plus interferon. In one retrospective cohort of patients with severe MERS-CoV infection, ribavirin and interferon alfa-2a therapy was associated with improved survival at 14 days, but not at 28 days. The ribavirin dosing is adjusted based on creatinine clearance and is typically given for 8 to 10 days. However, patients can experience significant toxicities including hemolytic anemia and metabolic abnormalities, making it difficult to assess whether the benefit outweighs the harm without randomized controlled trial data.

The MIRACLE trial investigated lopinavir/ritonavir combined with interferon-beta1b, with outcomes still limited in published evidence. This combination showed promising results in animal and in vitro studies. No definitive RCT has established an approved antiviral regimen for MERS.

Corticosteroids deserve special mention. Certain strategies have been shown to worsen clinical outcomes in the setting of coronavirus infections. Systemic corticosteroids are generally avoided in MERS because they may delay viral clearance and are associated with worse outcomes unless indicated for another condition (e.g., refractory septic shock per surviving sepsis guidelines). This is a frequent "trap" answer on exams.

Infection prevention is absolutely critical and commonly tested. MERS-CoV is transmitted via respiratory droplets and contact. In healthcare settings, airborne precautions (negative-pressure room, N95 respirator), contact precautions (gown, gloves), and standard precautions (hand hygiene, eye protection) are all required, especially during aerosol-generating procedures (intubation, suctioning, bronchoscopy, nebulization).

05Differential Diagnosis and Distractors

06Traps and High-Yield Pearls

The single most common way students miss MERS questions is by failing to connect the geographic/exposure history to the diagnosis. A vignette will nearly always include a sentence about travel to the Arabian Peninsula or contact with dromedary camels. If you read "recently returned from Saudi Arabia" or "works at a camel farm" alongside severe pneumonia, MERS should immediately rise to the top of your differential, regardless of how generic the respiratory symptoms may appear.

A second frequent error is ordering serology as the initial diagnostic test. Serology is for epidemiological surveillance and requires paired sera weeks apart. The correct initial step is always rRT-PCR from a lower respiratory tract specimen. Additionally, students sometimes choose nasopharyngeal swabs over lower tract specimens. While NP swabs are acceptable when lower tract samples are unavailable, the vignette may test your understanding that lower tract specimens have higher sensitivity.

The acute kidney injury component is the clinical discriminator that separates MERS from other severe viral pneumonias. When a vignette describes concurrent pneumonia and rising creatinine early in the disease course, think MERS before other viral pneumonias. This is directly tied to the DPP4 receptor distribution on renal tubular epithelium.

Do not fall for the trap of selecting corticosteroids as part of the treatment regimen. Unlike some other conditions causing ARDS (such as COVID-19 where dexamethasone showed benefit), corticosteroids are generally not recommended for MERS and may worsen outcomes by prolonging viral shedding.

Finally, infection control is a commonly tested management step. In any question asking for the "next best step" after diagnosing or suspecting MERS in a hospitalized patient, placing the patient in airborne isolation with full personal protective equipment is a valid and sometimes the best answer, particularly if the question stem focuses on preventing nosocomial spread.

In short, the core competency being tested with MERS questions is your ability to recognize an epidemiologically linked severe pneumonia with extrapulmonary features (especially renal), confirm it with the correct molecular test from the correct specimen type, manage it supportively without harmful interventions, and implement appropriate infection control measures.