Swine Influenza



Other names

Swine Flu, Flu, Influenza

Causal Agent

Swine Influenza Virus. This belongs to the Influenza type A virus group. In the USA H1N1, H3N2 and reassortant H1N2 are problematic. The US has also developed a novel H1N1 with avian internal genes. Currently common varieties within the UK are H1N1, H1N1 (195852), H3N2 and H1N2. There are 16 H and 9 N types providing a lot of potential. There are at least 7 different pig adapted influenza viruses.

The genome is divided into 8 segments.

Age group

All ages can be affected

Clinical signs

Swine influenza eyes2

Swine Influenza running no copy

Naive herds

Explosive outbreaks with all or many animals becoming ill at the same time.

Disease much more common in the Spring and Autumn

Animals present with inactivity, depression, huddling/pilling. They are anorexic. The animals often are mouth breathing and breathing is laboured. When the animals are moved many cough, some uncontrollably (paroxysm coughing). They often have a nasal discharge and the eyes are puffy. Their rectal temperature increases to 40.5-41.5C. As the disease progresses loss of weight may be seen. Mortality is generally low.

The high rectal temperature in breeding stock can result in abortions, infertility (a boar can become sub-fertile for 6 weeks), production of small weak litters and increased stillbirths.

Recovery generally starts 5 to 7 days after the first clinical signs

Established Herds

Annual re-infection appears, possibly from carrier pigs or the natural spread to younger naive pigs who present few signs in the summer months.


The disease is much more predominant in the spring and autumn months, however antibody investigations reveal little seasonal trend implying pigs get sick during the summer months without presenting with many signs

Spread is mainly from pig to pig via the movement of animals, introduction of breeding stock

Cross-Infectivity between human, pig, duck and turkey strains can occur

Humans can transmit the disease to pigs and vice versa

Virus can survive in the environment for a very short period of time.

Stress factors

Moving pigs

Mixing pigs

Poor isolation facilities

Marked diurnal (day and night) temperature fluctuations


Incubation period

1 to 3 days. Can be as short as 4 hours


Post-mortem Lesions

Swine Influenza lungs

There may be few lesions seen in uncomplicated cases. There may be firm lobular lesions with interlobular oedema. Associated lymph nodes may be enlarged. The trachea can be filled with froth.

The Swine Influenza causes problems because it damages the lining of the trachea destroying the mucociliary escalator.


In combination with pasteurella the gross appearance of the lung is extremely similar to Mycoplasma hyopneumoniae infections.


Clinical signs

Use of histology from fresh lungs and stained with antibody stains

For example, the picture shows the microscope view, the brown areas represent swine influenza

Swine influenza IHC copy

Paired blood samples checked for antibody concentrations (21 days apart), note maternal antibodies may persist for 2-4 months


During outbreak

Cover all services with AI from a third party source

No specific treatments available, all treatment regimes supportive

Careful nursing in the farrowing house essential. Must ensure the farrowing house is draught free.

All-in/All-out will limit the spread of the disease

Provide fresh clean drinking water


Avoid ducks and turkey contamination's contact including staff

Use disinfectants when cleaning buildings

Vaccines. Note vaccines do not provide lifelong immunity 6 to 9 months

Do not allow clinically affected people to work with pigs or on AI stations


Elimination is worth while on breeding stock farms or in previously negative regions.

Common differentials

Enzootic pneumonia. Other causes of reproductive problems. PRRSv

Zoonotic implications

Swine Influenza may rarely affect human beings Influenza A (H1N1)2009 for example.

There is no risk to eating or handling pork or pork products


Treatment advice for pigs with Swine Influenza


Further detail of the Influenza A (H1N1) virus virus evolution tree and discussion


Elimination strategies for swine influenza


Swine Influenza A moving genetic target


Basic make up of the Influenza Virus

Swine influenza belong to the family Orthomyxoviridae group A. The geneome is segmented into 8 negatively sensed RNA pieces. The virus is enveloped.


The picture shows an electronmicroscope picture of an influenza virus and a pig with swine influenza

Swine influenza virus particlesSwine Influenza running nose

There are two major surface antigens

H There are 16 possibilities

N - There are 9 possibilities

Major species with influenza viruses: Bird (most species) Mammals Man, Horse, Pig.

Swine Influenza can change by two major methods:

Genetic Drift

Here parts of the genetic code changes with replication errors. This is typical of an RNA virus


Swine influenza drawing b2

Genetic Shift

Here recombination of different viruses occupying the same cell at the same time make a new type of virus.

image015.jpg Swine influenza drawing 3Swine influenza drawing combined


Current Swine Influenza viruses of importance are H1N1, H3N2, H1N2 . There are some 7 different major types of Swine Influenza virus recognised. Note that there are differences between European and American strains, even with the same type of H or N they may originate from difference species ie avian or mammal.


Further detail of the Influenza A (H1N1) virus virus evolution tree and discussion