H1N1 Virus in Norway's Pigs and Humans 'Virtually Identical'

Researchers at the Norwegian Veterinary Institute have found evidence that humans were the most likely source of the H1N1 virus, which caused the outbreak of influenza in pigs in the country that began in October 2009. Jackie Linden summarises the findings for ThePigSite.
calendar icon 27 November 2009
clock icon 7 minute read

An investigation of the recent outbreak of influenza in the country's pig herds was carried out by Dr Hofshagen of the Norwegian Veterinary Institute in Norway and co-authors. Their paper was published just one month later in Eurosurveillance.

They explain that serological surveillance programme running since 1997 had shown Norwegian pig herds to be free of swine influenza (the classical strain H1N1 and H3N2) until the present outbreak, which started in October 2009. The pig industry in Norway is relatively small with around 2,700 herds and just inder 1.5 million animals slaughtered in 2008.

With the emergence of a novel influenza A(H1N1) strain affecting humans in April 2009, the surveillance of pandemic influenza in humans was initiated in Norway in late April, explain Hofshagen and co-authors. Within days, the pandemic influenza virus was detected, and sporadic infections, mostly in travellers from abroad, increased gradually through the summer. After a peak in late July, the numbers declined but then a new increase was seen during October and the cumulative number of laboratory-verified cases by 26 October exceeded 3,300.

In their paper, Hofshagen and co-authors describes an ongoing outbreak in pigs of infections with the pandemic influenza virus in Norway, which they say provides insights into the source of infection and on the control strategies put into force for its control.

Course of the Outbreak

The researchers began by reviewing the course of the outbreak. They say that, on 9 October 2009, the Norwegian Food Safety Authority (NFSA) was contacted by a local veterinarian who reported a possible outbreak of influenza in a pig herd of 85 sows and 850 growers and fattening pigs in Nord-Trøndelag county. Over the previous few days, one sow in the farrowing unit had been observed coughing. No other clinical signs of infection were observed in the rest of the herd and no animals died. The NFSA was told that a member of the farm staff had been ill with flu-like symptoms since 1 October, and tested positive for pandemic influenza virus on 8 October. The NFSA took nasal swabs from 20 pigs in the herd, and the samples were sent to the National Veterinary Institute for analysis. On 10 October, a total of 18 of the sampled pigs tested positive for influenza A and for 12 of these, pandemic influenza viruses was confirmed.

The following day, NFSA began an epidemiological investigation on samples collected from six other nearby herds and those with close contact between people and pigs. One of these herds tested positive for pandemic influenza virus; it was owned by the infected animal handler of the index herd. This second herd positive for pandemic influenza was situated in an area of intensive pig farming. Because of concerns about further spread of the virus, the second herd was destroyed and a slaughterhouse nearby.

Originally, the index herd was scheduled for slaughter that week but four more herds tested positive for the virus in the following days. They had all been in contact with people with flu symptoms or those whose infections had been verified. There was no evidence of contact (pigs, staff, vehicles, etc.) between the new positive herds, and the possibility of airborne transmission was also ruled out due to long distances between the positive herds. The sampling strategy was revised to include pig herds throughout the country, where member of staff had flu symptoms or confirmed pandemic influenza.

Materials and Methods

The researchers examined samples from 51 herds in total. Most of these were nasal swabs, and they were examined by real-time RT-PCR at the Norwegian Veterinary Institute to detect influenza A. Samples that were positive were further tested for the pandemic influenza A(H1N1)v virus sub-type. Blood samples were taken from the other two herds and subjected to ELISA (ID Screen® Influenza A Antibody Competition test, IDVET) and for sub-type A(H1N1)v by haemagglutination inhibition test.


Virus testing of samples

The researchers tested 39 herds in Nord-Trøndelag County between 10 October and 26 October. Of these, 18 were positive for pandemic influenza, and 15 of these herds had been in contact with people diagnosed with pandemic influenza (n=10) or with people with flu symptoms (n=5). For the three remaining herds, no information was available on such contact.

Among these 18 positive herds, moderate clinical signs of influenza (coughing, fever) were recorded in four herds, while signs were mild to absent in two herds. In five of these six herds, the clinical signs in the pigs occurred after humans in contact with the pigs became ill.

Between 12 and 26 October, 12 herds from six other counties were tested. Five herds from three counties were positive for pandemic influenza virus. Also in these counties, the majority of positive herds are suspected of having contracted the virus from infected people.

Virus sequence testing

The influenza virus in specimens taken from the index herd in Nord-Trøndelag was sequenced at the Norwegian Institute of Public Health and compared to human strains from Norway and elsewhere, including the virus from the initial human case associated to the outbreak on this farm.

It was found that the virus from two individual animals were identical to the two genome segments analysed for both pigs (full length H1 and 727 nt partial N1). They were also identical to the 1,744 nt H1 gene of the virus from the farm staff member. Very high similarity was also observed to some of the viruses isolated from other humans in Norway, in particular to a virus found in the same geographical region. Within the entire 1,744 nt H1 and 727 nt N1 sequences compared, a difference in only one nucleotide in H1 was observed (99.9 and 100 per cent identity, respectively).

The researchers say that full genome sequencing of the virus from one of the swine specimens confirms a very high similarity throughout the viral genome to the pandemic virus circulating in humans.


Hofshagen and co-authors say that humans infected with the pandemic influenza virus seem to be the most likely source for the spread of the infection to the pigs. However, they add that other routes, like airborne transmission or transmission by vehicles, could not be ruled out. They found no evidence to suggest that animals spread the pandemic influenza to humans.

Norway's pig population had been free of classical swine influenza until the outbreak, and both the pig industry and the NFSA are now faced with the challenge of possible long-term implications for both the industry and for the public in terms of zoonotic potential.

Transmission from humans to pigs and – and possibly vice versa – is especially worrying, say the researchers. Pigs could play host to virus multiplication or act as a reservoir when it is not the season for human flu or offer the opportunity for re-assortment to produce a more virulent strain.

As a result, the Norwegian authorities are taking all possible measures to control the outbreak such as monitoring the situation and the affected farms closely and restricting movements of animals from affected farms. Norway follows the European Union protocol, which recommends not slaughtering animals before at least seven days after the termination of clinical signs.


Hofshagen M., Gjerset B., Er C., Tarpai A., Brun E., Dannevig B., Bruheim T., Fostad I.G., Iversen B., Hungnes O. and Lium B. Pandemic influenza A(H1N1)v: Human to pig transmission in Norway? Euro Surveillance 14(45): pii=19406.

Further Reading

- You can view the full report by clicking here.

November 2009
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