Swine influenza virus is a costly, year-round disease, researchers and veterinarians have concluded. It’s true that incidence may spike in the fall and spring, but the virus still circulates and infects sows and pigs during the summer.

In an Iowa State University study, looking at the age of pigs infected with SIV as submitted to the Veterinary Diagnostic Laboratory in Ames, the highest incidence of infection occurred in the fall with a lower peak in the spring.

“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 Bruce Janke, DVM, Iowa State University. In contrast, more cases were observed in older pigs during the other months — particularly in the fall.

“There’s no doubt that young pigs are experiencing flu in the summer months,” he adds.

University of Minnesota diagnostic numbers agree with that assessment.

“We see SIV all year,” agrees Marie Gramer, DVM, Veterinary Diagnostic Laboratory, University of Minnesota. “There is never a month without flu.”

However, more data is needed before the summer SIV spike in young pigs can be labeled a trend. Still, it reinforces the idea that SIV is a year-round disease needing year-round attention.

“Nursery pigs were the most commonly affected and diagnosed group with SIV during 2003,” states Janke.

Janke attributes some of the SIV spike in nursery pigs to the fact that they’re mixed from different sources. With multiple SIV strains occurring in a herd at the same time, it can be difficult to achieve similar immunity levels among sows of different parities.

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 Robyn Fleck, technical-service veterinarian at Schering-Plough Animal Health. “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.”

That’s why year-round vaccination programs in sow herds continue to provide long-term protection.

Some producers are tempted to eliminate SIV vaccinations in their growing pigs during the summer months.

“Producers who choose not to vaccinate grow/finish pigs are trying to perform a balancing act between economic pressures and disease pressures,” says Scanlon Daniels, DVM, Circle H Animal Health, Dalhart, Texas. “The ultimate cost of that plan depends on your risk tolerance along with your herd’s porcine respiratory and reproductive syndrome and Mycoplasma status. That can be a tough balancing act.”