Influenza A (H3N2)v Virus in Humans and Swine in the United States – an updateArchived

ECDC comment

A report in the US Centers for Disease Control and Prevention (CDC) MMWR this week describes the investigation of mild respiratory illness among swine and people at a county fair  in Indiana, USA between 8 and 14 July 2012. This is an important report as it is very clearly documents simultaneous detection of H3N2v influenza viruses in humans and swine in close contact.

Outbreak of Influenza A (H3N2) Virus Among Persons and Swine at a County Fair – Indiana, July 2012 CDC Morbidity and Mortality Weekly Report, July 27, 2012 / 61(29); 561

A report in the US Centers for Disease Control and Prevention (CDC) MMWR this week describes the investigation of mild respiratory illness among swine and people at a county fair that took place in Indiana, USA between 8 and 14 July 2012.

Respiratory specimens were collected from four people with a respiratory illness on 16 July. Two had become unwell on 12 July and sought care at a hospital emergency department, and the other two were identified as part of the subsequent public health investigation. All four people were either swine exhibitors or their family members and had close contact with the animals.  On 18 July the Indiana Department of Health identified suspected influenza A (H3N2) variant (H3N2v) virus in all four specimens. On 21 July, partial genome sequencing at CDC confirmed this was the previously observed H3N2v virus with the influenza A (H1N1)pdm09 virus M gene, very similar to the H3N2v viruses detected in humans in 2011 onwards in the USA.  All four cases recovered from the infection and none were admitted to hospital. No evidence of human to human transmission is reported in this case.  Respiratory specimens were collected from swine at the fair. All of 12 specimens from pigs were positive for influenza H3N2v viruses. 

Preliminary genetic analysis undertaken on these by the US Department of Agriculture showed a very high level of similarity between the sequences of H3N2v viruses from humans and the H3N2v viruses from swine.(1) The human cases were the subject of an alert under the International Health Regulations.

ECDC Comment (1 August 2012):This is an important report as it is very clearly documents simultaneous detection of H3N2v influenza viruses in humans and swine in close contact. Though this is not the first such report, it is one of the most convincing.(1,2)

The A(H3N2)v viruses are more concerning than most other true swine influenzas because there has been some well documented human to human transmissions. On that basis, following a virological risk assessment preliminary work has been undertaken to develop an A(H3N2)v vaccine.(3,4)  Conversely there are no reports of severe illness in humans.

Reports of swine-origin viruses in humans in North America occur most years and there is active case ascertainment for atypical viruses in humans encouraged by CDC (5). In addition there is veterinary surveillance in pigs encouraged by the US Department of Agriculture (USDA), though as in Europe this is constrained by the fact that swine influenza in pigs is not a serious illness and hence is not a legally reportable condition. Consequently what reporting there is to the USDA Animal and Plant Health Inspection Service (APHIS) is voluntary and anonymised and the results therefore difficult to interpret.(6) More may be known on the geographical distribution of these viruses in pigs in the USA through surveillance in humans than the surveillance in swine themselves. Awareness of these infections in humans is rising. Hence though there are increasing numbers of such reports coming to CDC from more and more parts of the USA such reports and hence more reports through the IHR should not be interpreted as necessarily meaning that A(H3N2)v is spreading or that human to human transmission is increasing.(7) Though the viruses are certainly widely distributed in pigs in the USA and given the occasional limited chains of human to human transmission it demonstrates the wisdom of starting to prepare seed vaccines.(4).

It is important to note that though surveillance for influenza in swine is not optimal in either Europe or North America the viruses circulating in pigs seem to be quite different in the two continents at present. A(H3N2) viruses predominate in North America as triple reassortant viruses while in Europe pigs are more commonly infected by Eurasian A(H1N1) swine viruses.(8) The emergence of A(H3N2)v including the M gene from A(H1N1)pdm09 viruses is a relatively recent development following the emergence of the latter in 2009.(5,7)

ECDC and the Community Network of Reference Laboratories (CNRL) have worked to assess and strengthen laboratory capacity in Europe for detecting A(H3N2)v should it appear in persons in Europe.

In assessing the threat posed by A(H3N2)v viruses to humans European genetic research has suggested that many older people in Europe would probably have immunity to an A(H3N2)v influenza that adapted to humans (9). This research was made possible through use of the publicly accessible GISAID database. Given the seeming emergence of the 2009 pandemic from pigs in Mexico there are strong public health arguments for more virological surveillance of pig herds in Europe (and North America) including active surveillance of infections in humans that are in direct or indirect contact with pigs. Cases have occurred in Europe but they are rare in the literature.(10-11) What is unclear is whether these infections are truly rare in Europe or if this represents a weakness in animal and human influenza surveillance.(12) When one of these animal influenza viruses emerges there is always a flurry of concern on the pandemic risk. More formal approaches to assessing emerging influenza viruses for their pandemic potential and such virological risk assessments are continuing to be developed with the lead taken by CDC though its Influenza Risk Assessment Tool (IRAT) and the European Food Standards Authority supported Flurisk Project which is coordinated by the Istituto Zooprofilattico Sperimentale delle Venezie (13.14).  ECDC is collaborating with both projects.

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