USA - Summertime Flus: Swine influenza virus never takes vacation

Research has proven it and experts have warned swine influenza virus (SIV) is a costly, year-round disease in US swine herds. True, incidence may spike in fall and spring, but the virus still circulates and infects sows and pigs in summer. Backing off vaccination could actually cost producers more money in the long run. In a recent study, the age of pigs infected with SIV cases submitted to the Iowa State University Veterinary Diagnostic Laboratory, Ames, were evaluated for the months of April through October, 2003. As in most years, the highest incidence of infection occurred in the fall with a lower peak in the spring. "Interestingly, in the summer months of June, July and August, during which the total numbers of SIV cases were lower, the greatest number of cases observed is in 4- to 8-week-old nursery pigs," reports Dr. Bruce Janke, Veterinary Diagnostic Laboratory, Iowa State University. In contrast, more cases were observed in older pigs during the other months, particularly in the fall. "There is no doubt that young pigs are experiencing flu in the summer months," he adds. Numbers from the University of Minnesota show a constant number of flu cases throughout the summer months as well. "We see SIV all year," agrees Dr. Marie Gramer, Veterinary Diagnostic Laboratory, University of Minnesota. "There is never a month without flu." Additional data will be needed before the summer SIV spike in young pigs can be labeled a trend. It does, however, reinforce the message that SIV is a year-round disease demanding year-round attention. "Nursery pigs were the most commonly affected and diagnosed group with SIV during 2003," states Janke. He attributes some of the spike to the mixing of pigs from different parities. With numerous SIV strains operating simultaneously in swine herds, it becomes difficult to achieve similar immunity level in sows of varying parity. For instance, parity 1 sows have lower antibody levels than parity 4 sows on the same vaccination program due to lack of herd exposure to circulating SIV strains. "When we see 4- to 8-week-old pigs with SIV, it reflects a sub-optimal antibody transfer," says Janke. Maternal antibodies for SIV degrade at a standardized rate, notes Dr. Robyn Fleck, a technical service veterinarian at Schering-Plough Animal Health Corporation. "However, pigs from parity 1 sows start with lower maternal antibody titers and, through normal degradation, become susceptible to SIV sooner than pigs from older sows that start with higher titers at birth," she explains. In other words, everybody runs the race at the same speed but parity 1 pigs start out further down the track and reach the end of the race before everybody else. Year-round vaccination programs in sow herds help protect the pork producer's investment in his breeding herd. "Swine producers understand and appreciate the value they receive in maintaining their SIV vaccination program in their breeding herds," notes Fleck. "SIV remains a disease challenge throughout the year." Even so, some producers are still tempted to eliminate SIV vaccinations in their growing pigs during the summer months. "Producers who choose not to vaccinate their grow-finish pigs are trying to perform a balancing act between economic pressures and disease pressures," says Dr. Scanlon Daniels, Circle H Animal Health, LLC, Dalhart, Tex. "The ultimate cost of that plan depends on your tolerance of risk along with the PRRS and mycoplasma status of your swine herd." Not an easy balancing act when you consider the cost of SIV.