Influenza viruses that circulate in swine are called swine influenza viruses when isolated from swine, but are called variant viruses when isolated from humans. Since July 2011, sporadic human cases of infection with a variant influenza A(H3N2) virus have been detected in the United States. All cases, investigated so far, were found to be preceded by direct contacts with pigs.
This virus has different virological characteristics from current circulating seasonal influenza viruses in humans, and has a new gene constellation: seven genes from the triple reassortant A(H3N2) viruses known to have been circulating in pigs in North America and the M gene from an A(H1N1)pdm09 virus, a seasonal virus currently circulating in humans.
In one of the foci of human infection at the county fair, mild respiratory illness among swine was reportedly recorded. From both species, collected specimens were found positive for influenza A (H3N2) virus. Preliminary genetic analysis has shown a very high level of similarity between the gene sequences of H3N2v viruses from humans and the H3N2 viruses from swine.
In order to improve communications and avoid confusion, in late 2011 FAO, OIE and WHO established a working group of experts to standardize the terminology for variant influenza viruses. The joint recommendation for the above mentioned A(H3N2) virus was: A (H3N2)v , where ‘v’ stands for ‘variant’.
There are A influenza virus strains which cause a serious swine disease, Swine Influenza. Swine influenza is a highly contagious viral infection of pigs. Swine influenza virus (SIV) infections cause respiratory disease characterized by coughing, sneezing, nasal discharge, elevated rectal temperatures, lethargy, breathing difficulty and depressed appetite. In some instances, SIV infections are associated with reproductive disorders such as abortion. Clinical signs and nasal
shedding of virus can occur within 24 hours of infection. Morbidity rates can reach 100% with SIV infections, while mortality rates are generally low. Secondary bacterial infections can exacerbate the clinical signs following infection with SIV. Transmission is through contact with SIV-containing secretions such as nasal discharges and aerosols created by coughing or sneezing. (H3N2)v, causing sub-clinical or mild disease in swine, seems not to be regarded as the notifiable SIV.
During 2009/2010, when the A(H1N1)pdm09 virus (‘Pandemic influenza A/H1N1 (2009) virus) was reportedly spreading globally in humans becoming a cause for international concern, more than 20 countries notified the OIE about the identification of the virus in swine, as an emerging disease. This virus was regarded to spread through the human-to-swine route.
According to CDC current recommendations, persons who raise swine or come into close contact with swine at fairs or other venues should be aware of the potential risk for influenza transmission between swine and humans. To reduce this risk, preventive measures such as practicing frequent hand hygiene and respiratory etiquette are recommended. Persons also should avoid close contact with animals that look or act ill, when possible, and if experiencing influenza-like illness themselves, should avoid contact with swine. Additional guidelines on prevention of influenza transmission between humans and swine are available at www.cdc.gov/flu/swineflu/industry_guidance.htm.
Arnon Shimshony, DVM
Infectious Disease News Editorial Board member