Swine influenza
Swine influenza is one of the most important diseases in the swine industry worldwide, with different subtypes of circulating in the field. Swine influenza can not only cause great economic losses to the swine industry but also lead to epidemics or pandemics in the human population.
Clinical signs
Infection in pigs may result in clinical signs of respiratory disease including:
fever
lethargy
reduction in appetite
sneezing
nasal discharges
coughing
reddening of eyes.
Severe cases can show respiratory distress due to bronchopneumonia potentially resulting in mortality, although this is not common. In breeding pigs, signs are more variable than in growing pigs, and range from subclinical to transient lethargy and inappetence, to obvious respiratory disease. Transient infertility, abortions and stillbirths can occur in a breeding herd.
Effects on breeding herds
There have also been reports of reduced milk production and delay in return to fertility after farrowing in affected sows.
Further information can be found at: Swine influenza: how to spot the disease - GOV.UK (www.gov.uk)
Transmission
Routes of transmission
Swine influenza viruses are spread from infected pigs in droplets of respiratory and oral secretions, mostly through direct contact, but also over short distances. Virus can also be transferred on equipment, clothing or other items contaminated by infected secretions, including through watering points and troughs.
The length of time that SwIV circulates in a herd depends on the production sector and various factors including type of pig unit, pig flow and management, and biosecurity measures in place. Influenza can persist in a herd for prolonged periods if there are susceptible pigs (i.e., pigs not previously exposed to virus) regularly present or introduced, for example in breeder-finisher herds or continuous growing units.
It can be difficult to eliminate virus from infected herds, therefore it is ideal to prevent introduction of the virus where possible.
Genotypic features
SwIV lineages and subtypes
SwIVs are split into distinct lineages based on their antigenic and genetic diversity.
H1s are divided into 3 distinct subtypes;
1As: classical swine
1Bs: human-like
1Cs: avian-like
In the late 1990s, the classical swine H1N1s reassorted its internal genes with triple reassortant H3N2s, spurring on a new diversification of the HA genes within the lineage, including the development of the 2009 pandemic H1N1.
The 1B lineage arose from spillover of human seasonal IAVs into pigs, and the 1C lineage resulted from a spillover from wild birds in Europe.
Genetic drift and reassortment
Continual exchange of viral subtypes between swine and humans has added complexity to classification. As with all influenza viruses, genotypic variation can occur due to accumulation of mutations (genetic drift) as well as reassortment events (genetic shift) following co-infection that can result in segment swaps which can lead to substantial genotypic and resultant phenotypic differences.
Epidemiology
Global distribution and disease impact
Swine influenza (SI) is a contagious viral respiratory disease caused by any strain of the influenza family of viruses. Identified strains include the subtypes of influenza A (SwIV) known as:
H1N1
H1N2
H2N1
H3N1
H3N2
H2N3
influenza C viruses
Influenza A viruses are of greater importance due to their ability to exchange gene segments (reassort) with a wide range of human and avian influenza viruses.
SwIV infection is present in pigs (endemic) in most pig-producing countries. The subtypes and lineages circulating globally are highly diverse.
Outbreaks tend to be seasonal, although infections are detected year-round and may be sub clinical. and though they tend to cause high levels of illness in pig herds, fatalities are less common.
Swine influenza in the United Kingdom
In the UK, the 1B lineage is the dominant strain circulating, with frequent detections of the 1A lineage, and less so the 1Cs. H3N2s are not currently circulating within the UK, but this is being continually monitored through passive surveillance to identify any incursions.
One Health
Cross-species transmission
Influenza A viruses of pig- or human-origin are genetically similar and can potentially cause infection across the species barrier termed ‘reverse zoonosis’ (humans transmitting infection to pigs), this is how the pandemic H1N1 2009 strain entered the UK pig population in 2009.
Accordingly, to monitor potential influenza threats, many countries maintain surveillance programmes for swine influenza in pigs.
Pandemic preparedness and vaccine development
SwIV genetic drift as well as reassortment can result in emergence of strains that can escape pre-existing host immunity or limit the therapeutic effectiveness of antiviral agents.
This necessitates continuous reassessment of the likely efficacy of the human seasonal influenza vaccines. Global influenza pandemic preparedness capability is facilitated by OFFLU (Vaccine Composition Meeting - offlu) that produces a bi-yearly report providing up-to-date information.
This has a particular ‘One Health’ focus on antigen selection for vaccines to protect human health, should a new variant virus emerge as occurred with the pandemic 2009 H1N1 strain.
APHA Contribution to OFFLU
We, as the WOAH/FAO IRL contribute whole genome sequencing data and assists with bioinformatic and antigenic analysis for the bi-yearly OFFLU recommendation to the Vaccine Composition Meeting (VCM) of the WHO, regarding SwIV antigens, as part of the influenza pandemic preparedness efforts.
PHA and WOAH and FAO IRL responsibilities
As the WOAH/FAO International Reference Laboratory (IRL) and other groups at APHA, we hold a number of responsibilities regarding the surveillance, and research into SwIVs.
UK surveillance programme
UK surveillance for swine influenza virus in domestic pigs with respiratory disease is provided at APHA to veterinary practices at no charge under a government-funded project.
Changes in the trends in diagnostic rate of swine influenza in pigs and strains identified, or unusual cases are highlighted in quarterly GB pig disease surveillance reports.
European Collaboration: ESFlu
APHA is a founding member of ESFlu, an EU COST action on swine influenza. ESFlu’s overarching goal is to strengthen the management of swine influenza in Europe through the sharing of surveillance data, improving detection capabilities, and improving management and public knowledge on swine influenza.