Swine Flu - Occurrence

Swine Flu - Occurrence
Classical swine influenza is caused by influenza A virus subtypes H1N1, H1N2 and H3N2.

Swine influenza first appeared in the United States immediately following the 1918 pandemic of human influenza, and it was generally believed that it was caused by adaptation of the human influenza virus to swine.

Nucleotide sequencing of the gens coding for the internal virus proteins indicates that the human pandemics H1N1 strain and the classic swine strain H1N1 have a common avian ancestor.

It is suggested that a virulent avian strain H1N1 entered the human population in 1918 causing the pandemic.

The pandemic virus was then introduced into swine population where it has persisted unchanged. In contrast, influenza was not observed in swine in Europe until 1979, caused by a virus antigenetically related to contemporary avian H1N1 strains found in ducks.

In 1992, many outbreaks of classical swine influenza occurred in England, caused by a group of H1N1 viruses that were distinguishable from classical swine viruses, the European viruses, and human H1N1 viruses, all of which are known to be circulating in pigs.

Influenza A virus subtypes H1N1 and H3N2 are endemic in pigs in Great Britain.

Two distinct antigenic variants of H1N1 viruses have been associated with the outbreaks of swine influenza, one of which was probably transmitted from birds to pigs in the early 1990s.

The disease usually affects young pigs but all ages may be affected. Typically, sudden onset epidemics occur with a high morbidity rate but with a low case fatality rate of less than 5%. Loss body condition in marked, which is usually the important cause of financial loss, although occasionally death losses may be extensive if the pigs are kept under inadequate conditions or if secondary bacterial infection occur.
Swine Flu - Occurrence