A federal pilot swine influenza virus (SIV) surveillance program could result in improvements in vaccines for swine and development of vaccine seed strains for humans.
Diverse strains and novel subtypes of SIV began emerging in the late ‘90s – when there was no centralized monitoring system to determine what viruses were prevalent in the nation’s hogs or what risk they posed to human health.
To fill that knowledge gap, the Agriculture Department (USDA) and the Centers for Disease Control and Prevention (CDC) began a two-year pilot program in September 2008. It could result in a permanent USDA-funded swine influenza surveillance program, according to the USDA. The program would include sharing of information on human and swine infections.
Efforts to control swine influenza over the last decade have been stymied by the emergence of the diverse strains, particularly viruses that contain genetic material from human-, avian- and swine-type influenza viruses, says Amy Vincent, DVM, a veterinary medical officer with the USDA Agricultural Research Service (ARS) in Ames, IA.
“Now we have many different variants and at least three different subtypes that co-circulate in the swine population,” she says. “There’s been quite a bit more variability, and it has made it very difficult to control influenza with the traditional vaccination methods.”
Vincent says her involvement in a 2006 investigation into a novel subtype identified in two Midwestern herds served as a “wake-up call” to the research community.
“This virus was an H2N3, and we showed that it wasn’t wholly avian – that the HA and NA genes came from an avian virus, but the internal part of the virus was similar to what currently circulates in the swine population,” Vincent says. “So this virus caused some alarm because the H2 viruses don’t circulate in human populations currently, but an H2 virus did cause the 1957 pandemic.”
That Asian flu pandemic caused the deaths of about 70,000 people in the United States, according to the Department of Health and Human Services.
Carolyn Bridges, associate director for epidemiological science in the influenza division at the CDC, indicates there have been no human deaths from swine flu infections in recent years – but reports of human infections have increased.
Prior to 2005, the CDC received a report of a human infection with swine influenza every year or two, compared with 11 cases since that time. It is unclear whether the increase is due to higher prevalence in humans or improved detection.
Most of the human cases have been young adults and children, and most reported exposures in the past three years have taken place at shows such as fairs, Bridges notes.
The hope for a swine influenza surveillance program is that it will improve swine health through advancements in vaccination, prevention and control of the viruses, says Thomas Burkgren, DVM, executive director of the American Association of Swine Veterinarians.
“The virus reassorts and mutates quite easily, so the vaccine that worked the previous go-round may not work this time,” he remarks.
Divergent strains of swine flu have increased since 1998, and their antigenic (part of an agent that induces antibody by the host) uniqueness has limited the usefulness of existing vaccines, says John Korslund, DVM, staff epidemiologist with the Veterinary Services’ National Surveillance Unit of USDA’s Animal and Plant Health Inspection Service.
“As the swine influenza viruses infecting herds have become more diverse, the existing licensed killed vaccines have lost widespread efficacy because they aren’t similar enough to the field strains that have evolved in the swine population,” he explains. He adds that swine flu infections are increasingly tough to manage because of “the virus population being more diverse and making changes more rapidly.”
In Korslund’s view, the surveillance program could lead to creation of improved SIV diagnostic reagents, more effective SIV outbreak and disease control activities, and improved strain prevalence data to help USDA Center for Veterinary Biologics staff review modified SIV vaccine licensing requests.
The surveillance program could also lead to creation of vaccine seed strains to be ready for production and human use in case of a pandemic, Bridges says.
Data support the fact that swine and humans with exposure to hogs frequently expose one another to influenza viruses, but the overall prevalence of swine influenza infection in these people is low, says Marcus Kehrli, research leader for the Virus and Prion Disease of Livestock Research Unit with the ARS. The surveillance program could lead to improved biosecurity practices that protect both pigs and workers.
Vincent explains that the success of such a surveillance program would depend in part on practitioners who submit samples to veterinary diagnostic laboratories.
“The whole system is going to rely on practitioners doing their field workup at swine farms that are experiencing respiratory disease outbreaks,” Vincent says
