Swine Influenza Fact Sheet

This is an excerpt of a Swine Influenza Fact Sheet I wrote for the former Pennsylvania state veterinarian in May 2007. It's a good starting point for a discussion on the current H1N1 swine flu outbreak.

Influenza A:
· Ducks/waterfowl are natural hosts
· All 15 HA and 9 NA virus types circulate among these avian species
· Virus targets the GI tract
· Infections are almost always subclinical in natural hosts

Antigenic Shift: a sudden emergence of antigenically different strains of influenza virus in a species. Can occur through 1 of 3 mechanisms:
· Direct transfer: transfer of a whole virus from one species to another
· Genetic reassortment: 2 different viruses of different species infect the same host at the same time. Gene segments can be exchanged between viruses creating a new virus strain.
· Re-emergence: a virus that may have died down re-emerges, the reasons for the disappearance and subsequent re-emergence are usually unknown
Antigenic Drift: gradual changes in antigenicity of a circulating virus after successive point mutations in the HA molecule until the virus is sufficiently different from earlier strains so that a large proportion of the population is susceptible and cases reach an epidemic level.

Swine Influenza: Clinical signs
· Acute febrile, respiratory disease characterized by apathy, anorexia, labored breathing (+/- coughing in later stages, also sneezing, nasal discharge and conjunctivitis may be seen less frequently).
· Morbidity can be up to 100% in a herd outbreak, mortality is low.
· Recovery is rapid – usually 5 -7 days after onset of clinical signs.
· Secondary bacterial infections can result in pneumonia.
· Gross lung lesions include lesions on apical and cardiac lobes; areas are dark to red-purple, depressed and consolidated. Airways are dilated and filled with blood-tinged fibrinous exudate. Bronchial and mediastinal lymph nodes are usually enlarged.

Swine Influenza: Epizootiology
· Swine influenza is widespread and endemic throughout the world.
· H1N1 and H3N2 infections are responsible for 50% of the cases of acute respiratory disease in pigs.
· SI is related to movement of infected pigs to susceptible herds and clinical disease appears with introduction of new pigs in to a herd.
· In an infected herd the disease persists through production of young susceptible pigs and introduction of new stock.
· Outbreaks peak in colder months.
· Infection is typically subclinical and clinical signs may be seen in 25 – 30% of the herd.
· Disease transmission is direct and occurs via the nasopharyngeal route (aerosols that are formed during sneezing or coughing) or indirectly after physical contact.
· Nasal secretions are laden with virus during the febrile (acute) state. This will last for 5 days.
· Severity of disease is dependent on: maternal immunity, virus strain, route of inoculation and secondary bacterial infections.

Vaccination and Control
Human vaccines:
· Inactivated whole or split or purified preps without adjuvant.
· Increases the chances of antigenic shift.
· Live-attenuated virus vaccine can mimic natural infection and produce local IgA and IgG with intranasal administration. There is a potential risk of reversion to wild-type virus*.
· Anti-virals are an effective means of control/treatment.
Swine vaccines:
· Based on inactivated and disrupted (split) virus suspended in oil adjuvant.
· Chances of antigenic shift are limited due to a shorter lifespan of individual pigs.
· Only injection is practical (intranasal administration is not practical therefore production of IgA is limited).
· No anti-virals are licensed for use in swine.
*Note on vaccination:
· The segmented genome of the virus means that if an individual is vaccinated while infected with an influenza virus, the attenuated virus (vaccine strain) can acquire wild-type gene segments of the other (natural virus strain) and new reassortant viruses can arise.
· This is only a risk related to human vaccination.

Virology: swine viruses currently circulating in North American swine – H1N1, H3N2, H1N2, and recently H3N1.

H1N1: classical swine influenza virus circulating since “Spanish flu” pandemic of 1918; isolated in 1930; endemic in swine populations.
· H1N1 (classical) is prevalent throughout the world, 25-30% of commercial pigs have evidence of disease
o In the US: the virus is antigenically conserved
o In Europe: virus disappeared and reappeared in 1976; in 1979 avian-like swine H1N1 replaced classical swine H1N1.
H3N2: new virus subtype emerged in swine populations in 1998; triple reassortment containing genes from swine, human, and avian viruses; endemic in swine populations.
· In Europe: in 1970 human H3N2 virus transferred to pigs; human-like swine H3N2 circulated in Europe and Asia with no clinical signs; in 1984 H3N2 (reassortment with avian-like swine H1N1) started causing disease in swine; this reassortant human-like swine H3N2 has replaced the original H3N2 in Europe.
o Recently: human-like swine H3N2 has started to circulate in the US causing serious disease and reproductive losses in sows.
H1N2: emerged in 1999; reassortment between H3N2 and H1N1; caused respiratory disease in swine and continues to circulate in swine populations.

H3N1: Lekcharoensuk et. al. characterized a new SIV subtype that may have arisen from reassortment of H3N2 turkey isolate and human H1N1 isolate and currently circulating swine viruses.

New strains: Recently, wholly avian viruses H4N6, H3N3, H1N1 from waterfowl were isolated from swine in Canada. However, there is no evidence that these viruses can be maintained successfully in swine populations

Public Health Implications:
· Human population has little immunity against animal influenza A viruses.
o If infections do arise the source of infection must be investigated as well as the extent of the spread and evidence of human to human infection.
o Only rare human to human transmission of animal influenza A viruses has been documented.
· Human H1N1 viruses are antigenically very different than swine H1N1 viruses.
o Human infections with human H1N1 viruses are rare.
o Influenza A viruses that are new to the human population (like H1N1) that are able to efficiently transmit from person to person and cause illness may represent a pandemic threat.
o H1N1 virus detection in swine workers may not be very important as antibody evidence of exposure is relatively common in this high risk group.
o H1N1 antibody detection in non-occupationally exposed persons is a possible pandemic threat.
· Most persons have some antibody to H3N2 viruses since they occur commonly in humans, swine H3N2 virus infections in people would not represent a possible pandemic threat.
o H3N2 infections can transmit from humans to pigs.
o Seasonal Flu vaccination will likely help provide partial protection against swine H3N2 virus subtype but not swine H1N1.
o Swine H3N2 viruses typically do not infect humans but persons who work closely with pigs are the majority of cases.

Health Implications - Agricultural Workers:
· There is scientific evidence that persons that are occupationally exposed to swine (farmers>veterinarians>meat processors) have an increased risk of infection with influenza viruses that infect pigs (Meyers et.al.).
o Therefore, there is a need to protect agricultural workers in order to prevent transmission among humans and domestic animals during a pandemic.
· It is likely that swine influenza infections occur in humans, especially in high risk groups, at an unspecified rate. However, due to the fact that symptoms are not only mild but also no different from “human” influenza infections, it is likely that non-fatal cases of swine influenza continue to occur but are not identified.
· Persons who work with swine could be considered for sentinel influenza surveillance and are an important group to include in pandemic planning.
· A policy of vaccinating swine workers annually with human influenza vaccine may decrease the risk of reassortment events.

References:

Lekcharoensuk P, Lager KM, Vemulapalli R, Woodruff M, Vincent AL, Richt JA. Novel swine influenza virus subtype H3N1, United States. Emerging Infectious Diseases [serial on the Internet]. 2006 May. Available from http://www.cdc.gov/ncidod/EID/vol12no05/05-1060.htm

Meyer KP, Olsen CW, Setterquist SF, Capuano AW, Donham KJ, Thacker EL, Merchant JA, and Gray GC. Are swine workers in the United States at increased risk of infection with zoonotic influenza virus? Clinical Infectious Diseases, 42 (2006), 14–20.

http://www.cdc.gov/flu/swine/. Accessed April 30, 2007.

http://www3.niaid.nih.gov/news/newsreleases/2005/swineflu.htm.%20%20Accessed%20April%2030, 2007.

http://www.vetscite.org/publish/articles/000041/print.html. Accessed May 1, 2007.

http://mark.asci.ncsu.edu/HealthyHogs/book1994/woodlief.htm. Accessed May 1, 2007.