Swine influenza

Swine influenza

G R THOMSON AND B C EASTERDAY
Swine influenza

Introduction

Swine influenza (SI) is a highly contagious, acute, respiratory disease of pigs caused by type A orthomyxoviruses. It is generally characterized by the sudden onset of a disease manifested by fever, coughing, dyspnoea, nasal discharge, prostration and high morbidity but low mortality rates. The nature of the disease varies according to the strain of the infecting virus, the age and immune status of the animal, and the presence of intercurrent infections. It has a wide distribution in the world.

The place and date of the first recognition of SI is believed to have been on farms in western Illinois, USA, in the summer of 1918 during the time of the worldwide 1918 to 1919 pandemic of human influenza.34 Dr J.S. Koen is credited with being the first person to recognize the disease as being different from any other previously encountered disease of pigs.9 He was impressed with the similarity of the disease in pigs to the influenza in humans.The record indicates that he was the first to apply the name ‘flu’ to the disease in pigs because of the similarity of its clinical signs to the symptoms of influenza in humans, and because of his belief that the two diseases were the same. With the isolation of the influenza virus from pigs in 193033 and from humans in 1933, it was shown that the viruses were related serologically, which lead to speculation that the porcine virus was related to the virus responsible for the disease in humans in 1918 to 1919. It has subsequently been shown by genetic analysis that the 1930 porcine viruses and the 1918 human viruses are closely related.29, 36 It remains unknown whether humans were infected from pigs in 1918 or vice versa.

Aetiology

Influenza viruses belong to the family Orthomyxoviridae and are classified as type A, B or C, based on the antigenic relatedness of the nucleoprotein (NP) and matrix (M) protein. The viral genome consists of eight single-stranded RNA segments. The antigenic nature of the two surface glycoproteins, haemagglutinin (H) and neuraminidase (N) is the basis for dividing the viruses into subtypes. So far, among the type A influenza viruses, there are 15 distinct haemagglutinins and nine neuraminidases. A brief description of the general characteristics of the influenza viruses is provided in Equine influenza.

Swine influenza virus with the antigenic configuration H1N1 has persisted in the USA since it was first identified in the 1930s. Prior to the mid-1970s there were few reports of SI in places other than the USA and no viruses with antigenic characteristics other than the classical swine H1N1 were isolated. H1N1 virus infections in pigs in the UK in 1941,4 Czechoslovakia in 195013 and Germany in 1959 were recorded. 15 Infection of pigs with the H3N2 virus was reported in Taiwan in 1970.19

In recent years viruses with other antigenic configurations have been identified in pigs in various parts of the world. In addition to the classical swine H1N1 virus, there are H1N1 viruses of avian origin, an H4N6 of avian origin, H3N2 viruses, reassortants of H3N2 of both porcine and human origin, and H1N2 virus, a reassortant of classical swine H1N1 and H3N2 viruses.10, 16, 17, 18

Porcine infections with other than type A influenza viruses appear to be rare. Type C influenza virus infections in pigs have been described in Japan and China.11, 40

In southern Africa, infection of pigs with SI viruses has not been recognized although it has been reported in Kenya.31

Epidemiology

Swine influenza is common in countries in North America and Europe (including the UK) and has been identified in at least 25 other countries on all of the continents in the world except Australia.1, 2, 3, 8, 10, 20, 26, 32, 35

Historically, episodes of acute SI in the USA were most commonly reported in late summer and winter, although it was clear, based on viral isolation and serological studies, that the virus was circulating in pigs throughout the year. As more pigs in the temperate climates are being maintained in confinement housing and are therefore not subjected to climatic fluctuations, the disease has become less seasonal in nature.14, 24 In the USA infections occur throughout the year.10, 24, 27

Transmission of the virus is generally by direct contact between infected pigs and susceptible individuals, although infected fomites may also be a source of infection. The first appearance of SI in a population is commonly associated with the movement of animals, such as the introduction of new breeding animals or fattening stock into a herd or the return of stock from agricultural shows to the home farm. The primary method of transmission is direct pig to pig by the nasopharyngeal route. The virus is abundant in nasal secretions during the acute febrile stages of infection. Infection by exposure to small particle aerosols of virus suspensions is readily demonstrated under experimental conditions. Airborne transmission may be responsible for sudden and expanding outbreaks in densely populated areas especially if the populations are immunologically naive.10, 37 Under experimental conditions it was determined that the duration of virus excretion may continue intermittently for as long as four months,5 but the results of another study suggested that the duration of excretion is not longer than four weeks.38 The results obtained in other...

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