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"SI causes severe pneumonia on its own but when it is combined with other infections
such as App and PRRS a chronic respiratory disease syndrome can develop."
Influenza viruses are the cause of outbreaks of acute respiratory disease, known as influenza or 'flu', which affects man and animals. Swine influenza is caused by a number of closely related influenza A viruses. The virus can be introduced to pigs by infected people and other carrier pigs. Water birds are also important reservoirs of infection. Influenza in pigs was first diagnosed around the same time as the human influenza pandemic of 1918.
About Swine Influenza (SI)
The influenza virus is a member of the Orthomyxoviridae family and has a segmented, single-stranded RNA genome. Influenza A viruses infect a wide range of avian and mammalian species, with the latter group including man, pigs, horses, and aquatic mammals. Type A viruses are known for their ability to change their antigenic structure and create new strains. The RNA of this virus is separated into eight segments. If infection with two different influenza strains occurs in the same host cell, the segmented nature of the genome allows for reassortment of the genomes of both virus, leading to increased variability and ability to create new strains.
The type A viruses are further divided into serotypes based on the antigenic nature of their surface glycoproteins, hemagglutinin (HA) and neuraminidase (NA). There are 16 different HA molecules possible, and 9 different NA molecules, leading to many different possible combinations, resulting in different strains of the influenza virus. The three common strains that infect the pig are described as H1N1, H1N2, and H3N2. There are also different strains within these serotypes that vary in pathogenicity.
Swine influenza is endemic worldwide. It is virtually impossible to maintain a population of pigs that is influenza virus free. Swine influenza virus in large herds may become endemic with intermittent bouts of disease and infertility. Different influenza strains may sequentially infect the herd. Immunity to influenza viruses is often short lived (6 months), and the immunity profile in the breeding herd varies considerably with time. Outbreaks occur throughout the year but peak in the winter months. Transmission of the virus between pigs occurs via nose to nose contact, while aerosol transmission is also possible in swine dense areas. Virus replication occurs in the epithelial cells of the respiratory tract and it is unlikely that the virus will spread to other areas of the body. The further the virus spreads down the respiratory tract, the greater the severity of clinical signs. The inflammation of the lungs and other clinical signs are mainly caused by immune system defenses of the pig against the virus, such as inflammatory cytokines, e.g. TNF-alpha, that are produced to help clear virus from the lung tissue.
Swine influenza is endemic worldwide. It is virtually impossible to maintain a population of pigs that is influenza virus free.
SI in large herds may become endemic with intermittent bouts of disease and infertility. Different
influenza strains may sequentially infect the herd. Immunity to influenza viruses is often short
lived (6 months) and the immunity profile in the breeding herd varies considerably with time.
Outbreaks are seen throughout the year but peak in the winter months.
The incubation period is very short: 1 - 3 days.
Severity of the disease depends on:
- The strain of virus
- Age of the pigs
- Immune status of the pigs
- Environmental conditions
- Concurrent infections
High morbidity (up to 100%) with low mortality (less than 1%) usually occurs in any age of pig but usually most severely affects pigs early after weaning due to the decreasing protection from maternal antibodies.
- High fever (105 - 106 deg. F)
- Reluctance to move or get up
- Dyspnea (labored breathing)
- Tachypnea (rapid breathing)
- Conjunctivitis (inflammation of tissues surrounding the eyes)
- Oculonasal discharge
- Coughing after a few days
- Possible abortions due to acute illness and high fever (not likely due to transmission across placenta)
- Quick recovery 5 - 7 days after onset of clinical signs
Concurrent infection with other pathogens can increase the severity and complications of swine influenza virus (SIV) in the individual pig and the entire herd. The following pathogens are commonly found in concurrent infection with SIV:
- Porcine pleuropneumonia
- Pasteurella multocida
- Swine Enzootic Pneumonia
- Glasser Disease
- Streptococcus suis type 2
- Porcine respiratory coronavirus (PRCV)
- PRRS virus
Due to the similarities of the clinical signs of swine influenza virus and other respiratory pathogens, definitive diagnosis of SIV can only be accomplished through isolation of the virus from respiratory tissues, detection of viral RNA via PCR, and detection of antibodies with ELISA tests. Samples for testing can be taken from live pigs at various stages of illness via nasal or pharyngeal swabs. Tissue samples may also be taken from pigs that either died from the disease or were euthanized. These samples may then be tested for the presence of virus through the fluorescent antibody technique applied to the cut sections of the lung or immunohistochemistry to confirm microscopical viral infection on formalin-fixed tissue. Clinical signs during a post-mortem examination can include the following:
- Purple and firm diseased lung tissue (usually in the apical and cardiac lobes)
- Edema between lung lobes
- Fibrinous, blood-tinged fluid in the airways
- Enlarged bronchial and mediastinal lymph nodes
Detection of antibodies via serological tests is generally confirm herd immunity, degree of protection provided to young piglets via maternal antibody levels, antibody titers after vaccination, and infection in pigs that are to be moved to other locations.
Public health implications
Pigs might be responsible for the transmission of influenza viruses to people in the following ways:
- Act as an intermediate host and "mixing pot" for genetic reassortment between human and avian viruses (New strains of virus may emerge that are transmissible to humans.).
- Directly transfer avian influenza viruses to humans
- Be responsible for the re-emergence of a virus involved in a previous epidemic (Pigs can act as a reservoir for old human influenza viruses.)
Treatment and Prevention
Antibiotic treatment may be necessary to prevent or treat secondary bacterial infections, but not for treatment of the swine influenza virus itself.
Antibiotic cover may be necessary to prevent or treat secondary bacterial infections.
- General management procedures to prevent respiratory diseases are effective, especially maintenance of proper environmental conditions for pigs of all stages and all-in/all-out practices.
- SIV is highly susceptible to most disinfectants that protect against viruses.
- Inactivated vaccines are available against swine influenza and are the primary method to prevent disease. Vaccine protocols for SIV consists of two injections 2 - 4 weeks apart. Boosters are recommended for sows twice a year in order to protect both the sow and piglets through the nursery phase. Vaccination of feeder pigs is not common unless influenza is a large problem in the grower/finisher stage of an operation.