CDC Requests Nasopharyngeal Swabs for trH3N2 Testing
Recombinomics Commentary 21:15
September 4, 2011

Clinicians should consider swine-origin influenza A virus infection as well as seasonal influenza virus infections in the differential diagnosis of patients with febrile respiratory illness who have been near pigs. Clinicians who suspect influenza virus infection in humans with recent exposure to swine, should obtain a nasopharyngeal swab from the patient, place the swab in a viral transport medium, contact their state or local health department to facilitate transport and timely diagnosis at a state public health laboratory, and consider empiric neuraminidase inhibitor antiviral treatment (4). CDC requests that state public health laboratories send all suspected swine-origin influenza A specimens to the CDC, Influenza Division, Virus Surveillance and Diagnostics Branch Laboratory.

The above comments from the early MMWR reflect an increase in trH3N2 surveillance, but the request also highlights the heavy bias created by focusing testing on patient with a swine link.  The majority of trH3N2 lab confirmed cases do not have a swine contact link.  Instead most live in the vicinity of swine, which is a factor in selecting samples for trH3N2 testing.  The H and N in trH3N2 samples are human, so routine testing only identifies samples as H3N2 positive, not trH3N2 positive.

Many trH3N2 confirmations are made at this time of year, because seasonal H3N2 levels are low, and exposure to swine is high because of agricultural fairs.  Thus, the first trH3N2 case in the United States, A/Kansas/13/2009, was linked to the Riley County Fair in late July, 2009, which was followed by the Iowa case, A/Iowa/16/2009.  The Wisconsin case, A/Wisconsin/12/2010, was linked to a fair in August, 2010 and one of the Pennsylvania cases, A/Pennsylvania/40/2010, was also collected in early September.  The recent Pennsylvania case was linked to the Washington County Fair in August, 2011, which was just after the identification of the Indiana case, A/Indiana/08/2011 (IN/08/11).

However, most cases are not directly linked to swine contact.  The Indiana case did not have contact with swine.  The caretaker had swine contact, but no data indicating the caretaker was symptomatic or trH3N2 positive has been presented, which is also true for the swine contacted by the caregiver.  

Moreover, the presence of the pandemic H1N1 M gene segment in IN/08/11 raises the possibility that the virus was more easily transmitted and may be transmitting in human populations, which is also increased by the presence of the pandemic H1N1 M gene segment in the most recent Pennsylvania case. Recent data links the the H1N1 M gene segment to a critical requirement for human transmission, which when added to the genetic background linked to recent trH3N2 cases and the familial cluster in Minnesota, may generate much more efficient transmission in humans. 

Therefore, the call for nasopharyngeal swabs from patients with flu-like symptoms at this time of the year should extend to all, especially pediatric patients. Both recent cases in Indiana and Pennsylvania had been vaccinated in 2010, which included seasonal H3N2.

Testing of symptomatic Washington County Fair attendees has already begun.

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