Preventing Swine Influenza in the Breeding Herd

By DuPont Animal Health Solutions - Swine influenza is a common problem throughout pig production worldwide. It is caused by a number of closely related influenza type A viruses. These are identified from each other by surface proteins. The most common ones are H1N1, H1N1 variant 195852, H1N2 and H3N2. Distribution of these vary but currently in the USA it is estimated that 50% of outbreaks are H1N1 and 50% H3N2.
calendar icon 7 May 2005
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One important characteristic of these viruses is their ability to change (via antigenic drift and antigenic shift) and so produce new strains or subtypes, which differ in pathogenicity and antigenicity from previous ones. Cross protection by immunity to different strains and even subtypes may not exist, or do so only partially.

Major Clinical Signs

The clinical signs will vary depending on the type of pig and the immunity of the population affected. It is usually impossible to differentiate between the different virus types based on clinical signs. Spread is usually very rapid.

Acute infection in a naïve herd leads to a variety of signs including inappetence, coughing, nasal and/or ocular discharge, dyspnoea and elevated temperature (to over 105°F/40.5°C).

Following elevated temperatures sows can abort, return, show as not in pig or have reduced viability piglets. Lactating sows can exhibit agalactia leading to weaker piglets. Boar fertility can be reduced for 5 weeks or more due to elevated temperatures affecting semen production.

Endemic infection in a breeding herd results in sporadic reproductive effects and some respiratory disease, mainly as natural immunity falls (the duration of this can be as short as 6 months) or susceptible stock are brought in.

In the growing herd sporadic influenza outbreaks can be seen, but more commonly swine influenza is a significant part of Porcine Respiratory Disease Complex associated with PRRS, EP, PRCV, PCV2 and bacterial pathogens such as Actinobacillus pleuropneumonia, Bordetella bronchiseptica, Haemophilus parasuis, Salmonella and Streptococcal species.

Transmission of Swine Influenza

The major form of transmission is via very short distance pig-to-pig aerosol spread or introduction of affected or carrier animals (30 days) into the herd.

Local aerosol spread has been reported over 1 mile or more (2-3 km)¹ and during new outbreaks in pig dense areas spread can be rapid between farms.

Influenza A viruses can interchange between pigs, humans and avian species, so spread can be by visitors or birds, especially waterfowl. Vehicles visiting the unit also pose a risk.

Preventing Infection Entering a Unit

If a herd is free from Swine Influenza it is obvious we would want to retain this status. However, even if the unit is already infected we still need to try to prevent new strains or subtypes from entering as these could easily cause further major problems.

The main factor in preventing infection is good external biosecurity. Achieving this coupled with good geographical location gives the best chance of success.

Access of people, animals, birds, and transport must be controlled. Incoming stock should be from tested free herds. It should be isolated, preferably over 1 mile away from the unit and other pigs. Ideally test stock in isolation for freedom from Swine Influenza.

All visitors should be controlled and free from flu symptoms. They should shower and change prior to entry.

Routine use of hand hygiene systems and boot dips will help prevent entry of the virus as personnel move about the unit.

All vehicles should be thoroughly cleaned and disinfected (DuPont Animal Health Solutions (DAHS) Vehicle Biosecurity Programme) using a broad spectrum proven disinfectant, such as DAHS Virkon® S. Bird control must be high, as should rodent control. Here a DuPont Animal Health Solutions IPM (Integrated Pest Management) programme could be used.

Aerial disinfection practised in the pig houses several times daily using DAHS Virkon® S can reduce the chances of introduction of Swine Influenza viruses into those houses.

Control of Spread

The majority of pig units will be infected and so we need to try to reduce spread of the virus around the unit to minimise virus hot spots and disease. We are not seeking to eradicate the virus, but rather to prevent excessive challenge that can overcome immunity, be it natural or vaccinal.

At the same time we are aiming to reduce secondary and concurrent infections, which can increase the severity of clinical disease and economic losses. Finally we aim to prevent the introduction of new strains.

The first tool for achieving this is by adopting All-in, All-out techniques with good pig flow and age segregation. A terminal biosecurity programme must be implemented between each batch, with good cleaning using a detergent such as DAHS Biosolve™ or Universal Barn Cleaner™ followed by a broad spectrum disinfectant, such as DAHS Virkon® S capable of killing not only Swine Influenza virus but the secondaries and concurrent infections that are present.

Comprehensive lists of the pathogens and product activities are shown in Table 1.

Table 1 - The Activity of Virkon® S against
the Pathogens Discussed in this Article

To ensure regulatory compliance in your country please read the label carefully.

For more information about vehicle disinfection programmes using world-leading products, such as DAHS Virkon® S, visit the DuPont Animal Health Solutions website, the largest biosecurity website resource in the world at

A major method of reducing the spread of Swine Influenza virus and other pathogens into and around the pig house is by aerial disinfection. This is well demonstrated for PRRS virus by Sala et al.2 and similar activity will occur for Swine Influenza virus.

Buildings need to be aerially disinfected several times daily and this is best done using a computerised system.

The agent used must be safe for pigs and staff but effective at reducing pathogen challenge.

DAHS Virkon® S was used in these studies with good results. Aerial disinfection and routine cleaning and disinfection of the facilities will also reduce the levels of noxious gasses, dust and respirable particles, which have been shown to irritate the respiratory tract and increase the effects of respiratory disease³.

Avoid moving in pigs from a source farm where Swine Influenza is active.


Swine Influenza is an important respiratory pathogen that can also lead to significant reproductive losses. It causes primary disease but also is important in combination with other viruses and bacteria in complex respiratory syndromes such as PRDC.

Good biosecurity is a major part of controlling the disease. It is essential in preventing infection entering virus-free herds.

It is also important in preventing new strains and subtypes of influenza virus entering herds that have existing infections. A complete programme, such as the DAHS Swine Biosecurity Programme, is needed to achieve this.

Aerial disinfection using DAHS Virkon® S has been shown to play an important part in prevention of infection and prevention of spread and increased challenge in infected units.


1 Milzholl, C., and Guerin, L. - Influenza A of Human Swine and Avian Origin: Comparison of Survival in Aerosol Forms. Canadian Journal of Comparative Medicine 1972, 36, 9.
2 Sala V., Terreni, M., Montesion, D., and Beghian M. - Role and Benefits of a Complete Biosecurity Programme in Intensive Pig Production. Proc IPVS 1998 2 110.
3 Backbo, P. - Effects of Noxious Gasses, Dust and Micro-organisms on the Incidence and Severity of Respiratory Disease in the Pig. Proc IPVS 1998 1 135.

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Source: DuPont Animal Health Solutions - Updated May 2005
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