Birds are believed to be a global reservoir of influenza virus genes that make their way to humans and because pigs can be infected with both avian and human strains, swine may also be a “mixing vessel” in the transmission process. USDA photo.
The shortage of human influenza vaccine this season has focused closer attention on the flu virus in birds and swine and the importance of ongoing influenza research projects in both of these species.
The Centers for Disease Control and Prevention (CDC) reported last fall that approximately half the expected supply of flu vaccine supply in the United States for the 2004‑05 influenza season will not be available over concerns about standards at the manufacturing plant.
The flu virus goes through mutations, so the flu vaccine has to be reformulated each year based on the strains of flu that previously appeared.
Humans can be infected with influenza types A, B and C. Influenza types A and B are the most important strains. Type A affects mammals (such as humans and pigs), as well as birds, but types B and C affect only humans. Government scientists continue to learn more about how animals, including pigs, can be hosts for influenza viruses.
As you know, birds are believed to be a global reservoir of influenza virus genes that make their way to humans.
And because pigs can be infected with both avian and human strains, they may also be a “mixing vessel” in the transmission process, scientists say. When bird influenza viruses infect pigs that already contain a swine or human influenza virus, gene segments from each virus can mix and a new influenza subtype virus can emerge.
Oftentimes, the losses caused by flu virus for swine and poultry producers can be devastating. When Virginia experienced an outbreak of avian influenza (H7N2) in 2002, for example, 4.7 million turkeys and chickens were destroyed.
When an avian influenza type A virus (H5N1) infected both chickens and humans in 1997, the Hong Kong incident marked the first time an avian influenza virus had ever been found to transmit directly from birds to humans. USDA Agricultural Research Service (ARS) scientists already had a working H5 influenza vaccine for birds at the time. The ARS Southeast Poultry Research Laboratory (SEPRL) in Athens, Ga., was studying a viral strain called H5N2. This strain attacks only poultry but is related to H5N1, the Hong Kong bird flu strain. Their research allowed them, and industry cooperators, to provide the Hong Kong government with the poultry vaccine.
Researchers at SEPRL want to understand why and how mild viruses become highly pathogenic and are developing better diagnostic tests with improved vaccines. The molecular research includes identifying and characterizing genes using a rapid polymerase chain reaction) procedure, sequencing of the entire virus genome of representative isolates and creating molecular models.
Athens researchers are studying the behavior of the viruses in living hosts and hope to better understand how certain strains evolve to be so pathogenic and to develop ways to predict this potential virulence.
The H5N1 influenza strain turned up again in early 2004 in a few people who died from bird flu in Vietnam and Thailand. The virus killed 30 people in Asia earlier this year. Other nations in Southeast Asia have recently reported fresh cases of bird flu in poultry, months after a mass culling campaign in which tens of millions of poultry were destroyed. The strain has spread widely among poultry and other birds in Southeast Asia and infected people there.
H5N1 also is known to have infected pigs as early as last year. H5N1 has since crossed another species barrier to infect cats. Dutch scientists have found that the virus can also spread among cats. It’s not known whether cats can transmit the virus strain to humans.
Historically, only one type of swine flu virus, H1N1, was primarily found in pigs in the United States. That changed in 1998 when the H3N2 strain showed up here in swine, and most recently, the H1N2 strain.
At the National Animal Disease Center in Ames, Iowa, scientists are working on ways to control swine flu. Part of their research includes developing immunogens that provoke a protective immune response when introduced into a body. They also want to develop diagnostic reagents that can be used to detect the presence of swine influenza in pigs.
Ames researchers in the Virus and Prion Diseases of Livestock Research Unit and their collaborators use a process known as reverse genetics to create swine flu types that could one day help develop a vaccine and stop the rapid spread of the virus.
Editor’s note: Information in this article was excerpted from the Oct. 19, 2004,issue of Healthy Animals, an online newsletter published by the USDA Agricultural Research Service.