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Pandemic H1N1-2009 Influenza - Background


Influenza H1N1-2009 virus is an influenza A virus who belongs to the Orthomyxoviridae familiy, RNA viruses. Influenza A viruses can be classified into subtypes based on two surface proteins: hemagglutinin (H) and neuraminidase (N). At this time, 16 distinct haemagglutinins and 9 neuraminidases have been identified. Influenza A viruses can infect swine, equids, dogs, and other mammalian species, including humans, however, only birds have been found to host all of the hemagglutinin and neuraminidase combinations. The two subtypes which are most important causes of human influenza are A(H3N2) and A(H1N1). Influenza viruses are known for their ability to change their antigenic structure. Reassortments (gene segment exchange) between strains are frequent, and can mix viral RNA originating in different species. New viral strains, possibly with differents biological characteristics such virulence, infectivity or host range, are created.


A new strain, the influenza virus H1N1/2009, never before reported among swine or human isolates from anywhere in the world, has been detected for the first time in humans in Mexico in April 2009. On 28 April 2009, WHO raised the pandemic alert from phase 4 to phase 5, indicating that the new virus had spread to several WHO regions around the world, and on 11 June 2009, from phase 5 to phase 6 indicating a global pandemic. Pandemic H1N1/2009 influenza virus has been circulating in people in Mexico since mid-March 2009 and it has now spread worldwide. The transmission is human-to-human by air. Many thousands of cases have been confirmed in over 100 countries, with occasional mortalities on most continents.


Cases are then detected elsewhere in swine, turkey and cats, dogs and ferrets too. So far, no evidence has suggested that animals play any particular role in the epidemiology or the spread of the pandemic H1N1 2009 virus among humans. Instead, investigations led by competent national authorities point to possible human–to–animal transmission in most cases (OIE, article of 4 November 2009). See the map on the website EMPRES.


Swine influenza


“Classical Influenza viruses” in pigs

Swine influenza (SI) is caused by a number of influenza A viruses. The three subtypes that most commonly affect pigs are H1N1, H1N2 and H3N2. SI viruses are endemic in pig populations worldwide and are one of the most prevalent respiratory diseases in pigs. Influenza vaccination is often part of herd health management systems in USA.
Acute swine influenza (SI) is characterized by a short incubation period (1-3 days), fever, coughing, sneezing, nasal discharge, and respiratory difficulty. Reproductive problems including abortion can also be found. Morbidity rapidly reaches 100% of the pigs but mortality is low and usually does not exceed 1%. Generally animals recover after 5 to 7 days after onset. There is no clear evidence to support or reject the existence of long-term carriers. In a study, it has been shown after 30-45 days and 60 days postinfection, the disease failed to be transmitted to susceptible contact pigs. The infection is often present without clear clinical signs.  


H1N1-2009 in pigs

Preliminary results of a study of the cross-reactivity of serum samples from US pigs against the Pandemic H1N1-2009 virus by the United States Department of Agriculture (USDA), Agricultural Research Service (ARS), indicate that pre-existing immunity induced by swine influenza viruses circulating in the US may not protect pigs against the Pandemic H1N1-2009 virus presently circulating in people (see the website of USDA). Consequently, if pigs are infected with this virus the disease will be manifested clinically. During experimental infections of swine, 2009 A/H1N1 influenza virus was only detected in the respiratory tract of infected pigs and the virus did not appear to spread and replicate in other tissues (see the website of USDA).


First cases in pigs are detected in Canada and in April 2009, then in Argentina and Mexico. Then, cases were detected around the world: Norway, USA, England, Ireland, China, Finland, Indonesia, Iceland, Japan, Singapore and Australia (see the map in the website of EMPRES). In the most of cases, ill humans would transmit viruses to pigs.


H1N1/2009 in turkey

In August 2009, pandemic influenza virus was identified in two turkey breeder holdings in Chile. The clinical symptoms had started in mid July with a sudden drop in egg laying and altered egg shells. No increased mortality was observed. Egg production resumed about 20 days after infection and reached normal levels weeks after. The symptoms were very much alike an infection with an LPAI virus. This is the first detection of the pandemic influenza virus in a non mammalian species. The pandemic influenza virus has also been isolated in mid October from a turkey farm in Canada, then in USA (in Virginia November, 30 and in California, January, 6). In the most of cases, ill humans would transmit viruses to turkeys.


Genetic sequencing of the HA gene from the pandemic influenza virus isolated from the Turkeys in Chile showed 99.5% similarity to the Californian human strain and a 100% match to the human strain currently circulating in Chile. Mutations that might explain an increased capability of the virus to infect turkeys have not been detected, but work to further characterise the virus is needed.


Few studies of experimental infection of chickens, turkeys and quails were conducted in Europe and the United States. No signs of clinical disease were observed, only in one study a decline in egg production in the older turkeys were noted. In other studies, virus has found only in low level of infection in quails. So Preliminary results of these studies have shown a certain susceptibility of quails to the pandemic influenza virus. Further research and thorough investigations of events of natural infection of poultry with the pandemic influenza strain, in particular as regards the route of infection, the age of the birds, their immune status, concurrent infections and other external factors will contribute to a better understanding of the epidemiology of the infection.


For more information, see reports and articles:

- USDA studies on 2009 novel H1N1 influenza and turkeys November 30, 2009

- presentation of studies conducted by USDA : 2009 Emergent H1N1 Influenza A Virus: N1 RRT-PCR differentiation test, and infectivity and transmissibility in poultry

- Working document on Surveillance, monitoring and control measures For the pandemic (H1N1) 2009 influenza virus in poultry (EUROPA)

- Terregino, C., De Nardi, R., Nisi, R., Cilloni, F., Salviato, F., Fasolato, M., Capua, I. (2009) Resistance of turkeys to experimental infection with an early 2009 Italian human influenza A(H1N1) virus isolate. Eurosurveillance. 14 (41), 15 October 2009.


Influenza is not a food borne zoonoses and it is not transmitted to humans by meat or meat products. Poultry meat, poultry meat products and eggs, handled in accordance with good hygienic practices jointly recommended by the WHO, FAO, Codex Alimentarius Commission and the OIE, are not a source of infection from the virus.


Recommendations by FAO in case of suspicion

- Movement restrictions should be implemented for all farms or holdings with swine showing signs of clinical respiratory illness until diagnosis of the illness have been made. Where influenza A/H1N1 is confirmed, these restrictions should be in force until seven days after the last animal has recovered.

- Animals suffering from swine influenza can be separated from healthy herd-mates and allowed to recover; there is no need to cull affected animals.

- Animal handlers and veterinarians should wear protective gear to minimize risk of being infected by zoonotic agents, including influenza.

- Persons who work directly with swine should be urged not to go to work if they have any signs respiratory disease, fever or any influenza-like illness.


To more information, look at the website of FAO.  


Recommendations by OIE related to the import of pigs and pig products

- The imposition of ban measures related to the import of pigs and pig products from countries with human or animal cases are pointless and do not comply with international standards published by the OIE and all other competent standard setting international bodies for animal health and food safety;

- Pork and pork products, handled in accordance with good hygienic practices jointly recommended by the WHO, FAO, Codex Alimentarius Commission and the OIE, are not a source of infection from the virus;

To more information, look at the article of 13 July 2009 of OIE

Potentiel risks of pandemic H1N1 2009 influenza virus at the human-animal interface

To assess the potential risks of pandemic (H1N1) 2009 influenza virus at the human-animal interface, the WHO, the FAO and OIE hosted a scientific consultation via teleconference on 3 June 2009. Swine influenza viruses do not constitute a food safety problem. See the complete report and the summary. 


Summary of all cases

See the site of AVMA (American veterinary medical association)-2009 H1N1 flu outbreak 



- EMPRES Watch April 2009

- EMPRES Website

- FAO website

- FAO Guidelines for surveillance of pandemic H1N1/2009 and other influenza viruses in swine populations (FAO, August 2009).

- OIE. Article of 4 November 2009. Evolution of pandemic H1N1 2009 in animals. Recent identification of the virus in different animal species is no additional cause for alarm.

- OIE (2009). Article of 13 July 2009. Pandemic (H1N1) 2009 : the OIE reiterates its recommendations to animal health authorities worldwide. While the virus continues to spread among humans worldwide, the role of animals has not yet been demonstrated in the epidemiology or spread of the pandemic H1N1 2009.

- USDA Website- 2009 H1N1 influenza virus


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