Clinical specimens from 79 symptomatic individuals with a recent history of travel to countries with verified transmission of influenza A(H1N1)v (North America) were tested with a multiple real-time PCR targeting a broad range of agents that may cause acute respiratory infection. This analysis revealed that besides four cases of influenza A(H1N1)v, other respiratory viruses were diagnosed in almost 60% of the samples. These observations are a reminder that many different viral transmissions occur simultaneously in countries with ongoing spread of influenza A(H1N1)v. The findings demonstrate that the definition of suspected cases by clinical and epidemiological criteria has only a poor capacity for discriminating influenza A(H1N1)v from other viral infections.


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Background

A new influenza A(H1N1)v variant has spread globally since its first appearance in April 2009 [1,2] and as of 17 June 2009 there were 39,620 cases reported by the World Health Organization (WHO) [3]. On 30 April 2009, the European Commission suggested a case definition [4], which has been adopted and modified by most authorities in the European Union Member States. In agreement with this recommendation, testing for influenza A was recommended in Sweden for cases with a clinical presentation including respiratory symptoms and fever above 38°C, and epidemiological circumstances such as recent travel (within seven days) to areas where the new influenza has been observed [5] or close contact with confirmed cases.

The regular sentinel surveillance for seasonal influenza has been extended and now focuses on identification of imported cases with influenza A(H1N1)v, and on preventing secondary transmission by contact tracing and antiviral medication in an attempt to delay sustained community transmission. In order to provide a better basis for the decision whether or not to initiate preventive measures, expanded testing, targeting a broad range of respiratory agents, has been applied to specimens from all suspected cases in the region V?stra G?taland (1.5 million inhabitants). We report here the results of this expanded testing.

Material and methods

This report includes samples of patients who, during the period from 24 April to 10 June 2009 presented with influenza-like symptoms and a history of recent travel to the United States or Mexico, and therefore were recommended for examination and sampling. This clinical examination was performed by infectious disease clinicians on call at Sahlgrenska University Hostpital/?stra in Gothenburg, and our report is based on their evaluation and laboratory results. In summary, of all 79 patients included with a travel history, 90% presented with respiratory symptoms, 5% without respiratory symptoms, and for the remaining 5% this information is not documented. Sixty-six percent had fever above 38°C, 29% had no fever, information on fever was missing for 5%. Nasopharyngeal swabs were sent to the molecular diagnostic unit at the virological laboratory at Sahlgrenska University Hospital for testing by a multiple real-time PCR targeting 13 viruses and two bacteria, run in six parallel multiplex PCRs on an ABI 7500 instrument [6]. Samples that were reactive for the influenza A component (matrix protein target, [7]) of this PCR were subtyped by an additional real-time PCR targeting the haemagglutinin gene, run in three parallel reactions specific for the H3N2 and H1N1 subtypes that have been circulating for a long time, as well as for the new H1N1v strain (Table 1).