Surveillance program could protect human and swine health

Published on February 15, 2009
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A pilot swine influenza surveillance program could lead to improvements in vaccines for swine and development of vaccine seed strains for humans.

Diverse strains and novel subtypes of swine influenza began emerging in the late 1990s, and there is no centralized monitoring system to determine what viruses are prevalent in the nation's hog populations or what risk they pose to human health.

Investigators examine a pigCollaboration between the Department of Agriculture and the Centers for Disease Control and Prevention led to the two-year pilot program that began in September 2008, and it may 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.

Pork producers have struggled to control swine influenza over the past decade because of the emergence of the diverse strains, particularly viruses that contain genetic material from human-, avian-, and swine-type influenza viruses, said Dr. Amy Vincent, a veterinary medical officer with the USDA Agricultural Research Service.

"Now we have many different variants and at least three different subtypes that cocirculate in the swine population," Dr. Vincent said. "There's been quite a bit more variability, and it has made it very difficult to control influenza with the traditional vaccination methods."

Dr. Vincent took part in a 2006 investigation into a novel subtype that emerged in two Midwestern herds and served as a "wake-up call."

"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," Dr. Vincent said. "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 led to the deaths of about 70,000 people in the U.S, according to the Department of Health and Human Services.

Dr. Carolyn Bridges, associate director for epidemiological science in the influenza division at the CDC, said nobody has died from a swine influenza infection in recent years, but reports of infections have increased.

Prior to 2005, the CDC received a report of a human infection with swine influenza every one or two years, Dr. Bridges said. Eleven have been detected since, and it is unclear whether the rise is related to higher prevalence in humans or improved detection.

Most people who have contracted the virus have been young adults and children, and most reported exposures in the past three years have involved exhibitions such as fairs, Dr. Bridges said.

Identify strains to improve vaccines

Dr. Thomas Burkgren, executive director of the American Association of Swine Veterinarians, said he hopes a swine influenza surveillance program will improve swine health through advancements in vaccination, prevention, and control of the viruses.

"The virus reassorts and mutates quite easily, so the vaccine that worked the previous go-around may not work this time," Dr. Burkgren said.

Dr. John Korslund, a staff epidemiologist with the Veterinary Services' National Surveillance Unit of the USDA Animal and Plant Health Inspection Service, said divergent strains of swine influenza have proliferated since 1998, and their antigenic dissimilarity has eroded the usefulness of existing vaccines.

"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," Dr. Korslund said. He later added that swine influenza infections are increasingly difficult to manage because of "the virus population being more diverse and making changes more rapidly."

Dr. Korslund said the surveillance program could lead to creation of improved SIV diagnostic reagents, more effective SIV outbreak and disease management activities, and improved strain prevalence data to help USDA Center for Veterinary Biologics staff review modified SIV vaccine licensing proposals.

The surveillance program could also lead to creation of vaccine seed strains that would be ready for production and human use in case of a pandemic, Dr. Bridges said.

Dr. Marcus Kehrli, research leader for the Virus and Prion Disease of Livestock Research Unit with the ARS, said data support the fact that swine and humans with occupational exposure to the animals regularly expose one another to influenza viruses, but the overall prevalence of swine influenza infection is low among people with occupational exposure. The surveillance program could lead to improved biosecurity practices that protect both pigs and workers.

The success of such a surveillance program would depend in part on practitioners who submit samples to veterinary diagnostic laboratories, Dr. Vincent said.

"The whole system is going to rely on practitioners doing their field workup at swine farms that are experiencing respiratory disease outbreaks," Dr. Vincent said.