Microsoft word - fao brief on swine influenza re final.doc
FAO brief on swine influenza (swine flu) – 27 April 2009
The influenza virus is in a class of viruses known as Orthomyoxoviruses, with myxo referring to the
fact that they infect mucus membranes. The currently circulating Influenza viruses that cause human
disease are divided into two groups: A and B. Influenza A has 2 subtypes which are important for
humans: A(H3N2) and A(H1N1), of which the former is currently associated with most deaths
(http://respiratory-lung.health-cares.net/influenza-virus.php). Type A Influenza’s are named
according to the H antigens (16 possibilities) and N antigens (9 possibilities). All H’s and N’s have
been isolated from various species of birds. Swine, equids, dogs, and other mammalian species
(including humans) have not this wide occurrence of HxNx exposure. Infection with an Influenza
virus may not necessarily develop into illness or disease. Swine Influenza
Swine influenza (SI) is caused by a number of influenza A viruses which are known for their ability
to change their antigenic structure and create new strains. The three commonly found strains that
affect the pig are H1N1, H1N2 and H3N2. Reassortments (gene segment exchange) between strains
of swine, poultry and human origin are frequent.
SI is widespread and endemic in pig populations worldwide and is responsible for one of the most
prevalent respiratory diseases in pigs. In the US, Studies have shown that 30% of the entire US
swine population have been exposed to H1N1 since 1930.
Antigenic drift variants of these H1N1 viruses were isolated in 1991-1998. Emergence of H3N2
viruses occurred in 1997-1998. In particular, H3N2 viruses with genes derived from human, swine
and avian viruses have become a major cause of swine influenza in North America. In addition,
H1N2 viruses that resulted from reassortment between the triple reassortant H3N2 viruses and
classical H1N1 swine viruses have been isolated subsequently from pigs in at least six states.
Monitoring by the European swine network for influenza in pigs (ESNIP2) has shown that subtypes
H1N1, H3N2 and H1N2 co-circulate in the European pig population, but differences between
countries exist, e.g. currently in the United Kingdom, avian-like swine H1N1 viruses co-circulate
with H1N2, but H3N2 has apparently disappeared since the mid-1990s (Information received by Ian
Brown, VLA). Phylogenetic analyses have shown that the Eurasian and American SI strains are
From : Salient points in the history of swine influenza (adapted from Done and Brown, 1999), from Swine influenza: a zoonosis, Paul Heinen
SI is in pigs causes coughing, sneezing, nasal discharge, fever, breathing difficulty. Reproductive
problems such as abortion can be found. While all swine in a herd might become sick, the mortality
rates are generally low. The virus is transmitted through nasal discharges and aerosols created by
coughing or sneezing.
The disease in swine has a strong economic impact on the pig industry, e.g. in the UK, the cost has
been estimated at up to £7 per pig, accounting for a financial loss to the pig industry of
approximately £65 million each year [Kay, R.M., et al (1994) Vet. Rec., 135, 199-204.].
Control measures to stop the virus spread in the pig population consist mainly in movement control
Vaccination is often practiced in developed countries. Pig vaccines are bivalent, (H1N1 and H3N2).
Changes in the virus might lead to lack of protection by the vaccines.
. Swine flu virus infection in humans:
The symptoms of swine flu in human beings are those of regular human seasonal influenza and include fever, lethargy, lack of appetite and coughing. Some people with swine flu also have reported runny nose, sore throat, nausea, vomiting and diarrhea.
Swine flu viruses do not infect humans frequently but human infections can occur mostly associated with direct contacts with infected pigs. Until the recent H1N1 event, swine influenza viruses infected mainly people with occupational exposure, such as pig and poultry workers, most of which recovered swiftly with few fatalities. There is evidence to suggest that in recent years the incidence of human infections with swine influenza virus is growing in importance, in the US and also perhaps elsewhere. Human influenza virus genes and avian influenza virus genes are known to re-assort with swine influenza viruses and enrich the gene pool of viruses circulating in pigs in the eastern and western hemispheres. The genetic diversity of SIV has increased in time. The exchange of swine influenza virus genes circulating in respective Eurasia and the Americas may no longer be a rare event, even when poorly understood.
Human influenza viruses can also be transmitted from animal caretakers to pigs. Similarly, influenza
virus can also be transmitted from poultry to pigs as well as from pigs to poultry. Pigs may act as an
intermediate host and 'mixing vessel' for genetic reassortment between human and avian viruses.
Transmission of swine influenza viruses between persons with no pig exposure has been described
previously, but that transmission has been limited. Recent H1N1 events in Mexico and USA:
854 cases of respiratory disease in humans including 59 deaths due to rapidly evolving atypical
pneumonia were reported in the States of Oaxaca, Distrito Federal, San Luis Potosi, and Baja
California Norte, Mexico between 17 April and 23 April. Health care workers and their family
members with close contact are among those affected. 17 of of 51 samples submitted to the National
Microbiology Laboratory, Winnipeg, Canada, have tested positive for a novel Influenza A H1N1
strain. This strain is similar to the virus that has been diagnosed as well in the US earlier in April
where 8 confirmed cases of H1N1 Influenza A ( 6 in California and 2 in Texas) and 16 suspect
Influenza-Like Illness (ILI) cases are reported. All 8 confirmed US cases have had mild symptoms
with only one requiring brief hospitalization. No deaths have been reported in the US. In some case,
there is a history of recent travel from Mexico.
According to recent updates received from the CDC the novel H1N1 Influenza virus is a genetic
reassortment of four different Influenza strains including swine influenza from North America and
Eurasia, avian gene segments from North America and human influenza gene segments. This
potential genetic combination of swine influenza virus has not been recognised previously among
swine or human isolates from anywhere in the world. Further laboratory testing is ongoing in CDC
and National Microbiology Laboratory, Winnipeg, Canada.
In the diagnosed human cases in the US and Mexico, links to pigs are weak or absent for most cases,
suggesting extended person-to-person transmission. There is no evidence of illness in swine at this
There are still major question marks surrounding the origin of the different virus’ building blocks,
the disease severity in humans in different locations and countries as well as age groups, and how
easily transmission may be sustained in the various epidemiological and demographic settings
around the world.
FAO considers these latest reports on swine influenza virus (SIV) infections in humans very
worrisome. The novel H1N1 virus can spread around the globe and turn into a pandemic with
devastating consequences. Indications are that the virus has already spread among humans over long
distances affecting several countries. There is no vaccine that specifically protects humans against
this novel swine flu virus. According to the US Center for Disease Control and Prevention (CDC), in
Atlanta, is the seasonal influenza vaccine as routinely applied to humans unlikely to provide
protection. CDC and WHO also report that the virus is susceptible to the newer antivirals oseltamivir
(Tamiflu) and zanamivir (Relenza) but not to the older ones, amantadine and rimantadine.
Despite evidence of increased pig and poultry surveillance activities particularly in Europe and
Northern America, there is as yet not intensive enough global surveillance system in place to provide
precise information on the extend and evolution of virus circulation in pigs and poultry around the
world. Following the spread of the H5N1 highly pathogenic avian influenza (HPAI) in poultry in
Asia, Europe and Africa, efforts have been stepped up considerably through the FAO and OIE
reference laboratory network for influenza viruses (OFFLU), but much work remains in order to be
able to monitor worldwide trends and evidence of increased encroachment of domestic animals and
humans by influenza viruses.
FAO EMPRES and the FAO/OIE Crisis Management Centre-Animal Health are closely working
with the network of country, Sub Regional and Regional FAO offices. FAO staff around the world
is requested to remain alert, gather further epidemiologic information, report unusual influenza-like
events in swine, forward specimen to international influenza reference laboratories and respond to
upcoming urgent needs. Coordination and regular update are ongoing with WHO HQs in Geneva,
OIE, WHO/PAHO, CDC, USDA/APHIS, the World Bank, FAO/OIE reference laboratories and with
many other involved partners.
The FAO OIE CMC-AH has been activated to deploy a mission early next week. The proposed
objectives of the mission will be to work with the Veterinary Services in Mexico as well as with the
pig industry and the public health experts to investigate possible links between swine and human
with regard to the recent confirmed and suspected H1N1 human cases and support appropriate
communication at the intersectoral level.
FAO with its partners will try to assist member countries to obtain a comprehensive global picture of influenza virus circulation in pigs and poultry, including at the human-animal interface. The current situation calls for stepping up global virus surveillance programmes and efforts to understand the epidemiology and ecology of influenza viruses in birds, pigs and humans.
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