Swine-origin influenza virus: What the clinician needs to know

In April 2009, clusters of severe respiratory illness in Mexico and sporadic cases of more typical in­fluenza illness were identified in California and Texas due to a novel influ­enza A H1N1 (human swine) strain. This virus is composed of a complex reassortment of genes from human, avian, and swine influenza.[1] In the weeks after swine-origin influenza vi­rus (S-OIV) was first recognized, an increasing number of cases were reported throughout North America and subsequently elsewhere around the globe.

Illness due to S-OIV was noted pre­dominantly in children and young adults, with more severe manifestations compared with the elderly.[2,3] In the United States, people over the age of 50 years have made up only 5% of confirmed cases,[3] suggesting the possibility of cross-immunity among older adults from prior influenza exposure.

Although swine are believed to be the original source of the virus, the virus has acquired the ability to spread readily from person to person. Further monitoring is required to fully understand its evolving clinical and transmission parameters, but S-OIV is recognized like all influenza viruses to be primarily transmitted by droplets from coughing and sneezing. Infection may also be transmitted indirectly through contact with contaminated surfaces followed by inoculation of mucous membranes. Infection is not transmitted by eating or preparing pork.

Most patients with S-OIV develop an influenza-like illness (ILI) that in­cludes a spectrum with fever, cough, malaise, headache, and sore throat; vomiting and diarrhea may also accompany these symptoms, particularly in children.[3] Fever may not always be prominent. As with seasonal influenza, there may be an increased risk of influenza-related complications among young children, people over the age of 65, pregnant women, and people with chronic medical conditions.[4]

Proper infection prevention and control measures are essential when dealing with patients with ILI. When possible, they should be triaged to a time of day when the waiting area is less crowded. They should clean their hands with alcohol-based hand-rub on arrival and after coughing. When coughing, cover the nose and mouth with a clean tissue that is discarded immediately after use or, preferably, cough/sneeze into the crook of the elbow sleeve rather than the hands. Symptomatic patients should also wear surgical masks if they are able and should be seated at least 2 m from others; if this is not possible then they should be placed in an exam room immediately to reduce transmission in the waiting area. Clinicians in contact with people with ILI should perform hand hygiene and put on gloves, masks, and eye or face protection. An N95 respirator is only recommended during aerosol-generating medical procedures or in the rare instance when a patient is coughing forcefully and not able to wear a surgical mask or use tissue to cover his or her cough.[5]

Patients with mild ILI can be managed without diagnostic testing and in most cases do not require antiviral therapy. They should stay home from work and school until 7 days after symptom onset and should be instructed to maintain strict hand and respiratory hygiene practices as noted above.

In cases of moderate to severe respiratory illness, diagnostic testing for S-OIV is indicated; in these patients, a nasopharyngeal swab should be submitted for testing.[6] Antiviral therapy is indicated for patients with severe illness and those at high risk of influenza complications. S-OIV is currently susceptible to the neuraminidase inhi­bitors oseltamivir and zanamivir but is resistant to amantadine. Antivirals can reduce the severity of illness and the risk of complications but are most effective when initiated within the first 48 hours after symptom onset.[5]

There is currently no vaccine against S-OIV but development of the seed strain required for vaccine production is underway. The seasonal influenza vaccine is unlikely to provide protection against S-OIV. However, annual seasonal influenza vaccination is still important for health care providers and people in high-risk groups because influenza remains an important cause of morbidity and mortality. Pneumococcal vaccine should also be encouraged in high-risk groups to reduce the risk of bacterial superinfection.

As the novel strain of S-OIV continues to circulate, only time will tell how it may evolve. Activity in the southern hemisphere’s influenza season, occurring during our summer months, may provide an indication of what to expect. Influenza viruses are unpredic­table and S-OIV may undergo further mutation. In BC, ongoing surveillance, including monitoring for severe respiratory illness, should help us detect changes as early as possible. Updated information on S-OIV can be found on the BCCDC web site (www.bccdc.ca).


1. CDC. Update: Swine influenza A (H1N1) infections—California and Texas, April 2009. MMWR Morb Mortal Wkly Rep 2009;58:435-437. Full Text
2. Public Health Agency of Canada. Cases of H1N1 in Canada. www.phac-aspc.gc.ca/alert-alerte/swine-porcine/surveillance-eng.php (accessed 20 May 2009).
3. Novel Swine-Origin Influenza A (H1N1) Virus Investigation Team. Emergence of a novel swine-origin influenza A (H1N1) virus in humans. N Engl J Med 2009;361. Full Text
4. Public Health Agency of Canada. Interim guidance for clinicians in ambulatory care settings. www.phac-aspc.gc.ca/alert-alerte/swine-porcine/hp-ps-info_amb-eng.php (accessed 20 May 2009).
5. Public Health Agency of Canada. Interim guidance: infection prevention and control measures for health care workers in acute care facilities. www.phac-aspc.gc.ca/alert-alerte/swine-porcine/guidance-orientation-ipc-eng.php (accessed 20 May 2009).
6. British Columbia Centre for Disease Control. Lab guidelines for swine influenza. www.bccdc.org/content.php?item=497 (accessed 20 May 2009).


Dr Deans is an infectious diseases fellow at the University of British Columbia. Drs Patrick, Skowronski, and Henry are physician epidemiologists with epidemiology services at the BC Centre for Disease Control and faculty with the School of Population and Public Health at UBC.

Gregory D. Deans, MD, MHSc, FRCPC, David M. Patrick, MD, FRCPC, MHSc, Danuta M. Skowronski, MD, FRCPC, Bonnie Henry, MD, MPH, FRCPC. Swine-origin influenza virus: What the clinician needs to know. BCMJ, Vol. 51, No. 6, July, August, 2009, Page(s) 257 - BC Centre for Disease Control.

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