Swine influenza

Swine influenza (also known as Porcine influenza or swine flu ) is an acute infectious disease of the respiratory tract in pigs. This is caused by porcine ( pig related ) influenza viruses that belong to the virus genus influenza virus A, species influenza A virus. By reassortment of segments of the RNA genome of porcine and human influenza viruses can outbreaks of swine influenza to the emergence of new antigenic variants are genetic ( antigenic shift ) that hold new pathogenic properties for the animal or human. However, these emerging subtypes ( reassortants ) are not classical causative agent of swine influenza.

Pathogen

The most common causative agent of swine influenza Influenza A viruses of subtypes H1N1, H1N2 and H3N2 rarely. Very rare (human ) influenza C viruses isolated in pigs. The influenza viruses in pigs were formerly referred to as SIV ( swine influenza virus ). 2006 an H3N1 subtype was isolated in pigs in Italy. In an epidemiological survey in central China from 2004 to 2006, even an otherwise occurring only in horses subtype ( H3N8 ) was detected in swine herds. In addition to influenza viruses that cause a disease in pigs, a variety of human, avian and porcine virus subtypes from pigs were isolated, which can be disseminated throughout with no signs of an influenza virus infection.

The first porcine influenza virus ( A/swine/Iowa/15/30 [ H1N1 ] ) was isolated in 1930. In Europe, these pathogens completely disappeared at the end of the 1950s, until it was re- registered in 1976 probably by imported pigs from the U.S. to the European pig herds where it is endemic since then. The porcine H1N1 subtypes showed a high genetic and antigenic stability in the last 60 years. In 1979 appeared the first time an avian H1N1 subtype in pigs in Europe, which was very similar to the subtypes in ducks. Since then finds a Kozirkulation porcine and avian strains provable.

In the late 1990s and H1N2 reassortants in pigs were isolated in the USA, which were a mixture of porcine H1N1 and human / avian H3N2 reassortants. The susceptibility of pigs against avian and human influenza viruses is supported adjacent to the experimental and natural infection by the fact that the epithelium of the trachea in pigs having the two types of surface receptors, the avian α2, 3 - and the human α2 ,6 -sialic acid receptor. The propagation of avian influenza viruses in pigs, therefore, selected additional variants that also preferred to human receptors bind.

Pathogenesis and disease

The porcine influenza viruses are transmitted by droplet infection via the mucous membranes of the respiratory tract, where they initially multiply in individual cells of the epithelium. Even at this stage, the infected animal infect other animals of the stock. Are given 1 to 4 days as the incubation time. The infection spreads within a few days over the entire respiratory system - and thus the lungs - from. A transition of the virus in the blood ( viremia ) does not take place. A fast rising, high fever (up to 42 ° C ), severe airway inflammation, increased mucus in the nose and tearing feature the full clinical picture. Interstitial pneumonia and acute bronchitis are typical.

The swine influenza is highly contagious and spreads rapidly within a livestock from. The disease is thus a high morbidity, but low mortality; after 5 to 7 days, the symptoms sound very quickly again. Complications often additional secondary infections ( superinfections ) occur with bacterial pathogens, which can then dominate the clinical picture. The porcine pathogenic representatives of the bacterial genera Pasteurella, Mycoplasma and Bordetella are often involved in secondary infection. The porcine influenza viruses can still be excreted up to 5 weeks after healing of the disease from animals.

Swine influenza and human influenza

The classical porcine virus strains cause rare in humans, and then only mild disease. The species barrier of human, avian and porcine strains of virus may be disrupted by the genetic mixing of the genome segments by viral replication in the pig, however. The significance of the pig in the evolution of new pathogenic human strains is often characterized as a "mixing vessel" (mixing vessel ). This crossing between species porcine reassortant emerge in humans and human and avian reassortant in pigs. However, the latter are not causative agent of classical swine influenza.

Porcine reassortant human

That H1N1 subtype, which under the name Spanish flu pandemic led to a great 1918/19, is now regarded as a subtype of porcine originally adopted or at least a common ancestral strain.

A transfer to humans was observed in 1976, when a diseased soldiers of Fort Dix in Burlington County ( New Jersey ), a porcine H1N1 strain was isolated, which was then also found in five other soldiers. Four soldiers fell ill with viral pneumonia, one patient died. The virus strain resembled a subtype of a year earlier isolated from pigs. Extensive serological studies showed that at least 500 people had been infected; However, the source of infection could not be determined in detail. In 1988, in a case of fatal infection in Wisconsin, in which a virus was isolated, the virus endemic in pigs resembled in 7 of the 8 RNA segments. Particular mutations in relation to the increased pathogenicity was not found. In Europe, it came through the Kozirkulation of avian and human H3N2 strains in pigs in 1993 to the appearance of a new subtype, were infected with the two children in the Netherlands.

A newly emerged variant of subtype influenza A H1N1 virus was isolated in April 2009 in Mexico and the United States. This newly occurring reassortant is effectively transmitted from person to person. A disease of pigs was not observed, and does not determine the source of infection. The isolated strain is resistant to the in vitro effective in influenza antivirals amantadine and rimantadine, but in vitro after first clues sensitive to neuraminidase inhibitors oseltamivir and zanamivir. The name of the current outbreak as "swine flu" can be criticized because the isolated virus strains are not pathogens of actual swine influenza. You have only one of eight genome segments, could be the porcine origin and were therefore classified as human reassortants with avian and porcine proportions. However, the current virus variant to have originated years before the actual outbreak by combining different viruses and variants in the pig as a mixing vessel.

Humane reassortants in pigs

Infection of pigs with human subtypes was first detected serologically in 1938. Early 1990s was circulating through comparison of human and porcine strains from Italy and China are reported in the regular localized transfer of human viruses to pigs.

Natural reservoir

In nature, pigs are latently infected, as well as waterfowl and lungworm larvae ( Metastrongylus ) in earthworms as a reservoir of pathogens.

Diagnostics

Due to the rapid course of the disease, a serological test to detect antibody during the illness does not make sense. For the detection of porcine influenza viruses direct pathogen detection by virus isolation in embryonated chicken eggs or by polymerase chain reaction from nose and throat swabs is usually successful. In typical symptoms in an animal population occurrence, it is sufficient in most cases, detect the pathogen in individual animals. An outbreak of infection can be determined in retrospect, when after the antibody titer is determined from a serum sample at the time of the illness and another sample of four weeks. A titer of specific antibody by a factor of 4 is considered to be conclusive.

Prophylaxis and control

Against the most common pathogens of swine influenza A combination vaccine for pigs is available that contains more inactivated subtypes of influenza viruses ( H1N1 and H3N2). Vaccination is carried out in piglets around the 10th week of life, a necessary second dose follows four weeks after the first vaccination. To increase the concentration of antibodies in colostrum and thereby protecting vulnerable newborn piglets, sows are vaccinated again four weeks before farrowing.

The swine influenza is subject in Germany, Austria and Switzerland no legal duty to report.