Swine influenza: the disease in pigs
Since March 2009, an A(H1N1) epidemic in humans, which initially started in Mexico, has been spreading globally. At the time this column is being composed (at the end April 2009), it is not yet clear if the so-called “swine flu” is to become a pandemic. The printed media and televised reports, addressing the unfolding worldwide H1N1 epidemic, persistently include pictures of pigs and piggeries to demonstrate the role journalists and the public relate to these domestic animals as the producers and disseminators of the causative agent. The current review is meant to clarify the issue and, in particular, to describe the disease in pigs.
Stay connected - Sign Up for the E-mail News Wire!Influenza A viruses include avian, swine, equine and canine influenza viruses, as well as the human influenza A viruses. They can cause disease in birds, swine, horses, ferrets, dogs, cats, mink, seals, whales and other species.
Avian influenza viruses mainly infect birds, but some strains can also infect and/or cause disease in mammals, including humans (eg H5N1), without further spread among them.
Equine influenza viruses mainly affect horses, donkeys and mules; they have also been reported in zebra. Antibodies to the equine H3N8 viruses have been reported in humans. A H3N8 equine influenza virus appears to have jumped into dogs.
Canine influenza viruses have been seen only in dogs. To date, there have been no infections reported in other species, including humans.
Swine influenza — an animal disease, in contrast to the so-called “swine flu” A(H1N1) in humans — is the subject of this month’s column.
Swine influenza
“Swine influenza” (also called “hog flu” and “pig flu”) in senso stricto is an animal disease, caused by a specific porcine virus, namely Swine influenza virus (SIV). SIV’s are very contagious, mainly affecting pigs, but can sporadically cause disease in turkeys and humans, without further spread in the affected populations (“dead end” hosts). The virus is present in all pig-producing countries and can have significant economic impact.
Subtypes of SIV that are most frequently identified in pigs include classical and avian H1N1, reassortant (r) H3N2 and rH1N2. Other subtypes have been identified in pigs as well. The H1N1, H1N2 and H3N2 viruses found in Europe are antigenically and genetically different from those found in America. Studies have shown that the SIV H1N1 strain is common throughout pig populations worldwide, with around 25% of animals showing evidence of past exposure to infection.
Pigs have receptors in their respiratory tract that will bind swine, human, and avian influenza viruses. Consequently, pigs have been called “mixing vessels” for the development of new influenza viruses when swine, avian and/or human influenza viruses undergo genetic reassortment in pigs.
Unlike avian influenza, swine influenza is not a notifiable or statutory disease and it is not listed by the World Organization for Animal Health (OIE). Pig keepers and their veterinarians are therefore responsible for dealing with outbreaks of influenza in pigs to safeguard the welfare and productivity of their animals.
The clinical disease
SIV infections cause respiratory disease characterized by coughing, sneezing, nasal discharge, elevated rectal temperatures, lethargy, breathing difficulty and depressed appetite. In some instances, SIV infections are associated with reproductive disorders, such as abortion. Clinical signs and nasal shedding of SIV can occur within 24 hours of infection; shedding typically ceases seven to 10 days after infection. Two forms of the disease occur in swine, epidemic or endemic. In the epidemic form, the virus quickly moves through all phases of a swine unit with rapid recovery, which can exacerbate the clinical signs following infection with SIV. In the endemic form, clinical signs may be less obvious and not all pigs may demonstrate traditional clinical signs of infection. Morbidity rates can reach 100% while mortality rates are generally low. The primary economic impact is related to retarded weight gain resulting in an increase in the number of days to reach market weight. Secondary bacterial infections are common.
Transmission is through contact with SIV-containing secretions such as nasal discharges and aerosols created by coughing or sneezing.
PM lesions
In uncomplicated infections, the gross lesions are mainly those of a viral pneumonia. Affected parts of the lungs are depressed and consolidated, dark red to purple, and sharply demarcated. The lesions may be found throughout the lungs, but are usually more extensive in the ventral regions. Other parts of the lungs may be pale and emphysematous. The airways are often dilated and filled with mucopurulent exudate. The bronchial and mediastinal lymph nodes are typically edematous but not congested. Severe pulmonary edema, as well as serous or serofibrinous pleuritis, may also be seen. Generalized lymphadenopathy, hepatic congestion and pulmonary consolidation were reported in one outbreak of severe disease in swine.
Diagnostic tests
SIV can be diagnosed by virus isolation, detection of viral antigens or nucleic acids, and serology.
Isolation can be achieved in embryonated chicken eggs or cell cultures. The virus can be isolated from lung tissues at necropsy, or nasal or pharyngeal swabs from acutely ill pigs. Isolated viruses are subtyped with hemagglutination inhibition and neuraminidase inhibition tests or RT-PCR.
Immunofluorescent techniques can detect antigens in fresh lung tissue, nasal epithelial cells or bronchoalveolar lavage. Other antigen detection tests include immunohistochemistry on fixed tissue samples, and ELISAs. RT-PCR assays are used to detect viral RNA.
Serology on paired samples can diagnose swine influenza retrospectively. The hemagglutination inhibition test, which is subtype specific, is most often used. It may not detect new viruses. ELISA kits are available. Uncommonly used serological tests in swine include agar gel immunodiffusion, the indirect fluorescent antibody test and virus neutralization.
Vaccines
Inactivated influenza vaccines are available. The vaccines do not always prevent infection or virus shedding, but the disease is usually milder if it occurs.
Influenza vaccines may change periodically to reflect the current subtypes and strains in a geographic area. In general, swine and equine viruses display less antigenic drift than human viruses, and these vaccines are changed less often.
In pigs and horses, influenza is usually introduced into a facility in a new animal. Isolation of newly acquired animals can decrease the risk of transmission to the rest of the herd. Good biosecurity is also important. Once a herd of swine has been infected, the virus usually persists in the herd and causes periodic outbreaks; however, good management can decrease the severity of disease.
Zoonotic aspects
Human infections with SIV can occur and a limited number of deaths have been reported. Precautions should be taken to prevent human infection in the veterinary microbiological laboratory and animal facilities. Conversely, human influenza viruses can be transmitted from animal caretakers to pigs. Similarly, influenza virus can also be transmitted from poultry to pigs as well as from pigs to poultry.
Some comments on the Mexican influenza A(H1N1)
The widely publicized designation of this - apparently novel - influenza A (H1N1) disease in humans as “swine flu” is based upon the fact that initial genotyping results revealed that the involved virus contains gene segments from 4 different influenza types: North American swine, Eurasian swine, North American avian, and human (permitting human to human transmission). More specifically, its neuraminidase and matrix antigens are similar to corresponding genes of swine influenza viruses of the Eurasian lineage, and the hemagglutinin (HA) gene includes components which are similar to those of swine influenza viruses that have been circulating among pigs in the United States since 1999. The reassortment of this novel virus is likely to have taken place in SIV-infected pigs, co-infected by avian and human influenza viruses.
In fact, the current HiN1 virus spreads readily among humans without any further known involvement of, or contact with pigs; when and where its initial reassortment had taken place is under investigation. Such information is, apparently, not relevant anymore to the dynamics of the epidemic: the causative virus can persist among humans independently of further animal involvement.
For more information:
- Anna Rovid Spickler (2009). Influenza. Disease fact sheets – Iowa State University Centre for food safety and public health. http://www.cfsph.iastate.edu/Factsheets/pdfs/influenza.pdf.
- OIE Manual of Diagnostic Tests and Vaccines for Terrestrial Animals, 2008. Chapter 2.8.8 - Swine influenza.
