Respiratory problems regularly present a challenge on pig farms and both clinical and sub-clinical disease can have a significant effect on herd performance, especially where more than one infection is present.
Drawing together technical veterinary knowledge from across the pig industry, Zoetis is helping to share the latest knowledge by hosting an annual two-day CPD event for younger pig vets, particularly those new to the industry.
The focus during the last event was on swine influenza, porcine circovirus (PCV2) and porcine reproductive and respiratory syndrome (PRRS), the ‘big three’ viruses commonly affecting pigs.
Here, Adrian Cox of Farmvet, Susanna Williamson of the Animal and Plant Health Agency (APHA) and Laura Hancox from Zoetis take a closer look at swine influenza.
See also: Boosting pig farming efficiency through precision technology
Effects and transmission
Swine influenza virus is a challenge for some pig herds, with an ongoing risk of introduction to a herd.
“Historically, a flu infection would last three to seven days in each animal and an outbreak would have worked its way through a pig population in a couple of weeks. However, flu is now more endemic in pig herds,” Dr Cox explained.
“It is spread mainly via aerosol or nose-to-nose contact, as well as through equipment or clothing and, in outdoor herds, there can be repeated contamination via lungworm larvae, in which the virus can survive.”
Swine influenza virus likes cool temperatures and low light and also survives well in water. However, it is easily killed by handwashing, detergents and disinfectants.
APHA’s Dr Williamson added: “Clinical signs due to swine influenza are very variable, from sub-clinical and unnoticed to signs of respiratory disease including fever, lethargy, reduction in appetite, sneezing, nasal discharges, coughing and reddening of eyes.
“Severe cases can show respiratory distress due to pneumonia and pigs affected in this way may die, although this is usually due to secondary infections and is not common in uncomplicated swine influenza in pigs. Producers can be unaware that a breeding herd is infected until clinical signs appear in weaned pigs.”
If reproductive failure occurs during a swine influenza outbreak, it is most often in pigs in late pregnancy or at farrowing and reflects the effects of disease due to swine influenza on the sows (such as fever, lethargy, inappetence and respiratory disease) rather than a direct effect of the virus on placenta or foetuses.
Swine influenza can act as a stressor like environmental and other challenges and could adversely affect intake of nutrients or inclusions – Regumate, for example – at critical times, which could affect fertility.
Vaccination of gilts prior to arrival, or during their isolation period before entering a herd with endemic swine influenza, can be appropriate to prevent respiratory disease and disruption to their appetite and health around the time of service and conception.
Evolution of swine influenza strains
Influenza is an enveloped RNA virus, making it more prone to mutation, and there can also be development of reassortants, Dr Hancox explained.
“Flu virus genomes are made up of distinct segments and are prone to reassort or ‘swap’ RNA segments if animals are infected with two different
flu viruses, meaning progeny viruses can take segments of RNA from each parent virus,” she said.
There are three main influenza strains circulating in pigs in Europe: H1N2, H1N1 and H3N2. APHA surveillance data shows the swine influenza strains currently identified in UK pigs are H1N2, pandemic H1N1 2009 (pdmH1N109) and avian-like H1N1 and their reassortants. H3N2 has not been detected since 1997.
Pigs also have receptors to avian and human strains of flu. Some human strains can infect pigs and vice versa. To help minimise risk, some pig farming businesses offer seasonal human flu vaccinations to staff.
Diagnosis
To diagnose swine influenza, plain nasal swabs or respiratory tissues from recently affected pigs in a group are tested by PCR. This testing is provided free of charge at APHA for UK pigs.
“There is a short window of opportunity to detect swine influenza virus as there is significant excretion for only five to seven days in an individual pig,” Dr Williamson explained.
“Vets should aim to go and collect swabs as soon as possible after the first clinical signs are noticed and sample the most recently affected pigs.”
When swine influenza is detected, further testing identifies the strain involved, which helps in decisions about control and vaccination and identifying possible sources of infection.
Management
When disease becomes endemic, achieving stability is the priority, using preventative measures to keep it to a level where health, welfare and performance can be maintained.
Management to prevent or control swine influenza infections depends on the individual farm and is likely to include a combination of careful pig sourcing, biosecurity, ensuring ideal environmental conditions, management of pig flow and vaccination.
Good biosecurity practices, carried out consistently, are key to prevent swine influenza – and
other diseases – entering the farm or becoming part of a cycle of reinfection. People, pigs, equipment and vehicles all need to be considered.
People who have flu-like symptoms should avoid contact with pigs, including staff, vets and any other visitors.
Farm vets should check the health status of any bought-in pigs before moving them to the unit and pigs should have a period of isolation before being introduced to the herd.
A mass vaccination to help control an outbreak of swine influenza may be required. It is important to test to find out which particular strain is affecting a herd before vets decide if vaccination is appropriate or which vaccine to use.
Regular sampling might be useful to monitor presence of flu and check for any new strains.
“Vaccination to protect pigs against swine influenza has increased over the past few years, mainly breeding herd vaccination to help stabilise immunity in the piglets and reduce disease in weaners,” said Dr Williamson.
Co-infection with PRRS
An influenza outbreak can be particularly severe if PRRS is also present, due to the synergism between the two viral infections.
A co-infection with PRRS (also known as ‘blue ear’) and influenza is colloquially termed ‘blue flu’. Blue ears are still a sign of PRRS, but producers should take care with diagnosis as APP, salmonella and other systemic infections can also cause blue ears.
“PRRS targets immune cells, causing a massive, non-controlled inflammatory response, so its clinical signs can appear similar to flu,” Dr Cox said.
“There is less seasonality with ‘blue flu’ than in the past, but it is still skewed towards autumn and winter. The most likely combination is PRRS and the H1N1 pandemic strain and its immunosuppressive nature brings an increased risk of secondary bacterial infection.”
Management strategies in the face of co-infection with PRRS and influenza can include a mass vaccination against both infections, and strategic medication for any secondary bacterial infections.
A continued regular mass vaccination programme may also be recommended, an example being PRRS and influenza vaccines given alternately every six weeks so each vaccine is given four times a year.
A regular review of external and internal biosecurity measures, ensuring all staff understand them and carry them out consistently, is vital.
Partial depopulation could be appropriate depending on the specific situation, or a management change to break the reinfection cycle – for example, stopping gilts being brought in or not keeping any weaners on site.
More information
- Find information about how vets can access swine influenza testing online
- A useful and detailed swine influenza code of practice, drawn up through partnership between the government, pig industry and the Pig Veterinary Society
- Pig veterinary professionals can email pigteamUK@zoetis.com to request access to more technical information on swine influenza on Zoetis’ CPD portal or to find out about this year’s Young Pig Vets conference in November
- Producers should seek advice from their vet if they have concerns or to review health management planning
