Swine Influenza Virus (SIV) Constantly evolving, this disease continues to pose challenges As one of the top three swine respiratory health problems in the U.S., swine influenza virus (SIV) is a key concern for the industry. SIV – referred to as “swine flu” or “flu” – can be caused by a number of different swine influenza strains.Influenza viruses are single-strand RNA viruses, giving them the ability to change their antigenic structure and create new, different strains. SIV has evolved from a seasonal disease caused by a stable genotype of SIV to a year-round, endemic respiratory disease caused by multiple SIV genotypes undergoing continuous change.1This, coupled with the fact that the disease is characterized by nearly 100% morbidity2, makes this one of the most challenging diseases for today’s swine operations.1. Gramer MR. Defining swine influenza virus. J Swine Health Prod. 2005;13(3):157-160. 2. Marie Gramer, DVM. Swine influenza virus: the only constant is change. Allen D. Leman Swine Conference, 2006; 61-63.