Editor’s note: The following three reports are based on presentations at the World Pork Expo in Des Moines, Iowa, USA, in early June.

Thus far, the new H1N1 virus has not been discovered in any U.S. pig, but that doesn’t mean hogs are safe from it “That is the question, isn’t it?” Dr. Gregory Stevenson, Iowa State University professor and researcher, said at the World Pork Expo in early June.

“We have to ask at risk in what way,” Stevenson said.

“I think the industry’s at risk potentially in two ways. H1N1 is not considered by the swine industry to be one of the major diseases as compared to PCVC2 or PRRS virus. In regard to the economic question, I think we all realize that if the virus gets in our herds, there are potentially a lot of repercussions.”

At this point, however, there was a study conducted where Iowa State University Veterinary Diagnostic Laboratory scientists took 125 H1N1 viruses of the last six months that represented all the geographic regions of state. They took the M and NA genes, which are the unique genes in this new virus, and sequenced them in their entirety. The scientists compared them to the new virus as well as standard endemic viruses in swine and found that all of the viruses found and were isolated “were those that we’ve had for many years,” Stevenson said.

The unique virus, the H1N1 virus that has been in people recently is not “in any of our Iowa swine and this has been the case with laboratories that have done similar studies, so as far as we know to this date, it is not in our swine population.”

People pose largest risk

The biggest risk to pigs of the H1N1 virus outbreak that emerged in Mexico on April 24 is people, Stevenson said. One of the biggest factors potentially increasing the risk to pigs is whether the new human form of H1N1 becomes well established in the human population this fall. If it does, the risk posed to pigs would be workers who come to work sick, he said.

North American swine are endemically infected with a different H1N1 than the recent human form, as well as H1N2 and H3N2, Stevenson said.

“Of these, testing at the ISUVDL has consistently demonstrated H1N1 to be the most common.” Yet, he said, in testing at the lab through the first quarter of 2009, the overall proportion of swine respiratory disease submissions in which swine influenza virus (SIV) has been demonstrated has decreased (see chart).

Stevenson said that while an H1N1 virus was isolated from diseased people in Mexico and rapidly spread around the world, its source remains unknown. This H1N1 flu outbreak virus (H1N1FOV), as it is now know, is composed of six genes that are like those in the common endemic North American swine H1N1 virus. However, the remaining two genes are like those in endemic influenza viruses infecting European and Asian swine. These genes are the NA gene and the matrix (M) gene, Stevenson said.

This virus likely emerged by an event termed “antigenic shift.” In antigenic shift, two different influenza viruses infect the same host at the same time. During viral replication in the host’s cells, the genes from each virus can be mixed such that a progeny virus can derive some of its eight genes from each parent. “Like each of us, this progeny virus then expresses characteristics derived from the genetic material of each parent. In reality, these progeny viruses do not often survive and become established in populations,” he said.

Antigenic shift

In the case of H1N1FOV, it was immediately assumed that this antigenic shift occurred in pigs, because all genetic components of the virus are of swine origin. It was also assumed that pigs transmitted the virus to people in Mexico. However, to date, the H1N1FOV virus has not been discovered endemically infecting any pigs in Mexico or anywhere else in North America. There has been one discovery of H1N1 infection in pigs in Canada where H1N1-infected people from Mexico likely infected the pigs.

Recently, a new test based on a unique region of the M protein in the H1N1FOV has been developed by ISUVDL. “Just this past week we got a test that we don’t believe will give false positives,” Stevenson said. This test will detect and identify only H1N1FOV and will test negative for all know endemic North American swine influenza viruses.

Stevenson noted that the U.S. Department of Agriculture will soon launch a pilot surveillance program in cooperation with officials in individual states for swine influenza virus in pigs in the United States. Officials in Iowa are finalizing details for the program, including the involvement of the ISUVDL where samples will be tested.

The programs will be composed of three surveillance streams, 1) swine that are epidemiologically linked to a human case of the H1N1FOV, 2) swine observed with influenza-like illness at first points of concentration, i.e., exhibitions, fairs and sale-barns, and 3) client authorized (voluntary) influenza-like illness submissions to the veterinary diagnostic laboratory. For the first two surveillance streams, state officials will collect samples and deliver them to the ISUVDL for standard influenza testing as well as for specific H1N1FOV testing. The third surveillance steam will be strictly voluntary. The specific H1N1FOV test will be only be on tissues from submissions in which the submitter has volunteered to participate by completing a written authorization prior to testing.

There has not yet been a statement by USDA on what the agency will do if H1N1FOV is found in pigs, Stevenson said. “I’ve heard rumours, but I’m not going to share those with you.”