Swine Influenza

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Swine influenza is caused by Orthomyxoviridae virus family. There is circumstantial evidence of mutation from the human strain of influenza and hence the disease is zoonotic. The pig is susceptible to human and avian strains of the disease. Because of this, if infection with both strains occurs at the same time, the virus can mutate in the environment of the pig and become avian in pathogenicity with human attachment proteins. Hence it is thought that avian influenza can infect humans. Avian-like H1 and human-like H3 strains vary in virulence.

The disease in swine is thought to be predisposed to by migrating ascarids or by lung worms. The reservoir is the earthworm as this is the intermediate host for the porcine lungworm.

The virus will tend to recycle on farms as litters outgrow maternal antibody at around 12 weeks of age.


Pigs of any age, but particularly weanlings and growers. It is also transmissible to humans.

Clinical Signs

Initially upper respiratory signs such as rhinitis, a sneezing, serous nasal discharge and conjunctivitis are seen. This is soon followed by pyrexia, lethargy, skin erythema, anorexia, severe barking coughing, dyspnoea and pneumonia. Tracheobronchitis may cause airway obstruction which may lead to atelectasis. Pregnant sows may abort. The virus is often associated with bacteria such as mycoplasma or immunosuppressive viruses such as PRRS.


If the animal has clinical pneumonia, influenza is always a suspected cause.

Post mortem examination can be diagnostic and will often show the pleura normal or covered with serous or serofibrinous exudate and the pleural cavity filled with excess fluid. Lung lesions will include clear demarcation of lesions in cranial and middle lobes, interstitial pneumonia causing consolidation of the lungs and giving a 'meaty' appearance. There may be necrotising bronchiolitis with catarrhal pus. Histologically, there will be acute inflammation of the mucosa of trachea and bronchi.

Serology should be performed and this will provide a retrospective diagnosis by haemaglutination inhibition (HI).


In an outbreak of influenza there should be an immediate isolation of premises. No stock should be moved from group to group during this time and strict hygiene measures should be put into place as well as sanitisation of hands prior to entering a farrowing house. Secondary bacterial infections should be treated with antibiotics.

Control measures should include 'all in/ all out' systems being put into place.

Vaccination is not licensed in the UK.


Blood, D.C. and Studdert, V. P. (1999) Saunders Comprehensive Veterinary Dictionary (2nd Edition) Elsevier Science

Cowart, R.P. and Casteel, S.W. (2001) An Outline of Swine diseases: a handbook Wiley-Blackwell

Jackson, G.G. and Cockcroft, P.D. (2007) Handbook of Pig Medicine Saunders Elsevier

Merck & Co (2008) The Merck Veterinary Manual (Eighth Edition) Merial

Radostits, O.M, Arundel, J.H, and Gay, C.C. (2000) Veterinary Medicine: a textbook of the diseases of cattle, sheep, pigs, goats and horses Elsevier Health Sciences

Straw, B.E. and Taylor, D.J. (2006) Disease of Swine Wiley-Blackwell

Taylor, D.J. (2006) Pig Diseases (Eighth edition) St Edmunsdbury Press ltd

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