Swine influenza

SWINE INFLUENZA

  • Swine influenza is a specific, highly contagious disease of pigs characterized clinically by fever, and signs of respiratory involvement.
  • It is caused by type A influenza virus of the orthomyxovirus group.
  • The virus acts in combination with a Gram-negative bacterium, Haemophilus influenzae suis.
  • The swine influenza virus is closely related to the virus of human influenza type A.
  • Swine influenza first appeared in the United States immediately following the 1918 pandemic of human influenza virus to pigs.
  • This view has been reinforced by the recent appearance of swine influenza A in the human population, not in pandemic form.
  • Molecular microbiology has now revealed the antigenic diversity of the virus.
  • Several different H and N antigens have been identified, and grouped on the basis of serological tests.
  • The H3 N2 strain, similar to H3N2strain found in the human population, has been isolated from an outbreak in England.
  • The H1 N1 strain of the virus can be found in pig tissues at slaughter.

Transmission

  • The influenza viruses may be transmissible between humans and pigs.
  • Pigs may become infected with related type A human influenza strain during epidemics of human influenza, but show no clinical signs of infection.
  • The human strains have been isolated from pigs in HongKong, and pigs may serve as a reservoir for pandemics in humans as well as a source of genetic information for recombination between human and poricine strains
  • Swine lungworms can act as intermediate host and reservoir for the swine influenza virus during inter-enzootic periods
  • The virus is introduced into susceptible pigs by lungworm larvae.
  • Infection is provoked by the presence of H.influenzae suis . The disease can then spread to other pigs in the herd by direct contact .

Pathogenesis

  • Swine influenza is primarily a disease of the upper respiratory tract, the trachea and bronchi being particularly involved.
  • However, secondary lesions may develop in the lung because of the drainage of copious exudates from the bronchi.
  • These lesions disappear rapidly, leaving no residual damage.
  • This is in contrast to the lesions of enzootic pneumonia in pigs, which persist for very long periods.
  • Secondary pneumomia, usually due to infection with pasteurella muitocida, occurs in some cases and is the causes of most fatalities.

Clinical signs

  • The diseases affects mainly young pigs.
  • After an incubation period of 24-48 hours, animals exhibit fever, rhinitis, cough and inappetence.
  • These symptoms usually subside after 3-5 days, but in some cases, transitory(brief) fever may recur within 3 weeks.
  • Dyspnoea associated with severe pulmonary involvement is observed in some cases, and death occurs after severe pneumonia.
  • The mortality rate is usually not high (about1%), but in some outbreaks may assume serious proportions.
  • The morbidity rate may approach 100%.

Gross lesions

  • The specific lesions of swine influenza are restricted to the trachea, bronchi bronchioles, alveolar ducts, and alveoli.
  • The gross changes in part consist of mucopurulent exudate, which lies over the tracheal and bronchial mucosa and fills smaller branches of the bronchi.
  • Plugging of these bronchi and bronchioles results in sharply demarchted areas of atelectasis.
  • Consolidation of lung parenchyma occurs around the bronchi.

Microscopic lesions

  • Microscopically the virus produced necrosis of the lining cells of alveoli and bronchi, and to a smaller extent those of the lower part of the trachea.
  • Proliferation of the epithelial cells accompanies these necrotic changes.
  • In the bronchi and bronchioles the growth of epithelium progresses to such a degree that it fills the adjacent alveoli.
  • The pneumonia is characterized by necrosis of alveolar walls with formation of hyaline membranes lining the alveolar sacs.
  • It is accompanied by congestion, focal haemorrhages, severe perivascular and intralobular oedema and infiltration with leukocytes, mainly mononuclear cells.
  • Areas of collapsed lung parenchyma appear as a result of obstruction of bronchi by pus, mucus and desquamated cells.
  • Peribronchiolar alveoli are often consolidated as a result of infiltration of mononuclear cells, or the ingrowth of respiratory epithelium.
  • Consolidated alveoli are restored to a functional state by phagocytic removal of the consolidated areas by mononuclear cells.

Diagnosis

  • The symptoms and gross and microscopic lesions provide a basis for the presumptive diagnosis of swine influenza.
  • Definitive diagnosis depends on
  • Demonstration of a significant elevation of virus-neutralizing or anti haemagglutinin antibodies in the sera of pigs during the course of an infection.
  • Isolation and identification of the swine influenza virus type A
  • Demonstration of viral antigens with immunological staining techniques
Last modified: Friday, 23 March 2012, 9:43 AM