Outbreak of Acute Gastroenteritis Associated with Norwalk-Like Viruses Among British Military Personnel --- Afghanistan, May 2002

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In the United States, Norwalk-like viruses (NLVs) cause an estimated 23 million episodes of illness, 50,000 hospitalizations, and 300 deaths each year. NLVs can be transmitted by fecally contaminated food and water (1) and by direct person-to-person contact or through droplets of infected persons. Outbreaks of NLV-associated gastrointestinal illness are common in military settings. During May 13--19, 2002, a total of 29 British soldiers and staff of a field hospital in Afghanistan became acutely ill after a short incubation period with vomiting, diarrhea, and fever. This report summarizes the investigation of this outbreak and underscores the importance of the diagnostic capacity for NLVs.

The first three patients presented with severe acute illness characterized by headache, neck stiffness, photophobia, obtundation, and gastrointestinal symptoms, which made the initial diagnosis elusive. The third patient's illness was complicated by disseminated intravascular coagulation. Two of these patients required ventilatory support in the field hospital's intensive care unit. All bacteriologic studies performed at the field hospital's laboratory were negative. Because the cause of the illness was unknown, the field hospital was closed to all but patients with gastrointestinal symptoms. Because of the field conditions at the base and the severity of illness in the initial patients, one patient was evacuated to a U.S. military hospital in Germany, and 10 were evacuated to England. Two medical staff who treated the patients on the flight to England and a third contact at the hospital in England subsequently developed gastroenteritis; two of these persons were hospitalized for several days. All patients recovered rapidly and were discharged. The field hospital has since reopened with enhanced infection-control precautions.

In England, fecal specimens were tested for NLVs by electron microscopy (EM), a new antigen-capture enzyme-linked immunosorbent assay (ELISA), and reverse transcription-polymerase chain reaction (RT-PCR). By EM, clumps of small, round-structured viruses were observed and considered to be consistent with NLVs. This finding was confirmed by ELISA and RT-PCR in specimens from five patients. Partial sequence analysis of the polymerase gene identified the virus as belonging to genogroup II (2), the most common NLV genogroup in the United Kingdom and the United States (3).

Reported by: D Brown, J Gray, Central Public Health Laboratory, Public Health Laboratory Svc; P MacDonald, A Green, Surgeon General's Dept, Ministry of Defense; D Morgan, Communicable Diseases Surveillance Centre, Public Health Laboratory Svc, United Kingdom. G Christopher, Landstuhl Regional Medical Center, Landstuhl-Kirchberg, Germany. R Glass, Div of Viral and Rickettsial Diseases; R Turcios, EIS Officer, CDC.

Editorial Note:

Outbreaks of NLV-associated gastrointestinal illness are common, particularly in military deployments. NLVs were the most common cause of disability among soldiers in Operations Desert Storm and Desert Shield, have caused outbreaks aboard aircraft carriers (4), and have been a common problem in the Israeli military (5). NLVs are extremely contagious because of their low infectious dose (<100 viral particles), prolonged asymptomatic shedding (up to 2 weeks after recovery), ability to resist chlorination (10 ppm chlorine), and stability in the environment (stable with freezing and at 140^º F [60^º C]). Secondary cases and nosocomial spread are common (3), although the risk for NLV infection in the health-care setting can be minimized through the use of appropriate infection-control practices (6,7). NLV gastroenteritis has several distinguishing characteristics, including diarrhea, vomiting, a short duration of illness (1--3 days), and a short incubation period (24--48 hours). The illness is generally mild, but it can cause severe disease with associated dehydration and electrolyte imbalance that might require hospitalization and aggressive treatment with intravenous fluids. Severe illness with NLVs has been associated with group O blood phenotype (8).

The diagnosis of NLVs from stool specimens is difficult and depends on the identification of the viral RNA by RT-PCR, direct visualization of the viral particles by EM, and/or evidence of a specific antibody response in acute- and convalescent-phase serum specimens (3). Further characterization of the NLV into genogroups is possible by sequence analysis at reference laboratories. In the United States, detection by PCR is limited to some state health department and reference laboratories. Health-care providers generally consider the diagnosis on clinical grounds without seeking laboratory confirmation. As a result, many more outbreaks probably occur, but attribution to NLVs has been infrequent because of the difficulty of diagnosis. Simpler, less time- and labor-intensive diagnostic methods are under development. New antigen-capture assays, such as the ELISA used in this outbreak investigation, are being tested in Japan and Europe but have not yet been evaluated fully in the United States.

In this outbreak, the inability to identify an etiologic agent promptly and the unusual severity and atypical presentation of disease in the initial cases resulted in the illness being termed a "mystery infection." This uncertainty led to the air evacuation of ill soldiers, during which secondary spread of the infection to health-care providers aboard one of the military flights occurred. The diagnosis was ultimately made in England, where EM and the new ELISA identified the etiologic agent as an NLV. Confirmation and characterization of the virus as a genogroup II strain was obtained by PCR and sequence analysis. Field laboratory capacity for NLV diagnosis might have given on-site health-care providers information useful for limiting secondary spread of illness more effectively and allayed the fear and anxiety associated with the label of "mystery infection." The same observation can be made for most acute gastroenteritis outbreaks in the United States that elude an etiologic diagnosis.

This outbreak demonstrates that NLV-associated illness occurs commonly and needs to be identified promptly so that patterns of transmission can be identified and interrupted. The development of simple and sensitive detection techniques remains a high priority. When these become available, the true burden of illness can be measured and more effective control measures implemented.

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