Issue 18 - Epidemiology of SARS Cases Further Defined

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Epidemiology of SARS Cases Further Defined
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One of the newest challenges for syndromic surveillance is how to distinguish SARS from influenza. The November 2003 Annals of Internal Medicine describes the epidemiology of the SARS outbreak in Hong Kong and identifies symptoms most often associated with it. The patients described in this study have not been described in previously published reports. (1)

Kin Wing Choi et al, describes a cohort study of 267 hospitalized patients from February 26 through March 31, 2003 in the Princess Margaret Hospital in Hong Kong.

Princess Margaret Hospital is a tertiary care hospital with 1200 beds, including 32 general wards, 3 isolation wards, and one intensive care unit. It was the sole quarantine hospital for Hong Kong. Over the course of the epidemic, more than 700 patients with suspected or probable SARS were managed there.

In all, 267 consecutive patients admitted between February 26th and March 31st with probable or confirmed SARS were included in the study. All history and physical findings, lab and x-ray studies as well as treatment were reviewed retrospectively. All surviving patients were followed for three months after hospitalization.

The study found that the 267 patients presented with fever (99%), chills (74%), myalgias (50%), cough (44%), rigor (40%), headache (33%), anorexia (23%) and productive sputum (20%). These are all “influenza-like” symptoms. However, this study confirms previous reports that upper respiratory symptoms like sore throat (14%) and runny nose (11%) are uncommon in SARS. This is a key difference from influenza. Another key symptomatic difference was the late appearance of diarrhea. While only 15% of patients presented with diarrhea, the incidence of diarrhea increased to 53% during the hospital stay. Of course, diarrhea could have been caused by the combination of antibiotics, antiviral, and corticosteroids that most patients received, but tests for Clostridium difficile toxin were negative.

Laboratory findings during hospitalization included lymphopenia (73%), thrombocytopenia(50%), hyponatremia (60%), and elevated levels of lactate dehydrogenase (47%) and C-reactive protein (75%).

96% of the patients in this series presented with abnormalities on chest x-ray. Even the 12 patients with “normal” chest x-rays were found to have “patchy ground-glass opacification of the lung parenchyma” on CT exam. Acute renal failure occurred in 15 patients, contributing to the death of 13 patients. Previous studies have not identified renal failure as a complication of SARS.

Patient demographics were also unusual. The median age was 39 years, certainly much lower than the average age of patients with community-acquired pneumonia. Of these, only 6% had one or more underlying medical conditions. Sixty-one percent were female. Thirty-two patients (12%) died within 3 months of admission.

Rapid RT-PCR tests (reverse transcriptase–polymerase chain reaction) currently available for SARS-CoV RNA remain suboptimal. In fact, the CDC requires two RT-PCR positive results (from different sites or on different days) to be confirmed by a reference laboratory before it will consider a case “laboratory-confirmed.” (2) Therefore, early detection of SARS cases still depends on alert clinicians and good syndromic definitions. This study helps to refine the SARS “symptom complex” most useful for syndromic surveillance. It also provides additional symptoms that are different from influenza (e.g., diarrhea, acute renal failure) which may manifest themselves later as the disease progresses.

The authors challenge the WHO and CDC definition of SARS. Both organizations require the presence of respiratory symptoms for diagnosis. In this study, 13% of patients did not present with any respiratory symptoms, although they had fever and contact with a SARS case. Further refinement of SARS case definitions and laboratory tests are needed or may need to be changed so that they do not preclude the possibility of SARS.

1. Choi, Kin Wing, et al. “Outcomes and Prognostic Factors in 267 Patients with Severe Acute Respiratory Syndrome in Hong Kong”. Annals of Internal Medicine. 2003;139:715-723.

2. Centers for Disease Control, “Guidelines for Laboratory Diagnosis of SARS-CoV Infection” at http://www.cdc.gov/ncidod/sars/labdiagnosis.htm Last accessed 12/29/2003.

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