Each year presents new challenges in detection and control of this changing viral respiratory disease.

Swine influenza virus (SIV) used to be an infection in finishing swine in spring and fall during changeable weather patterns that lasted 4-7 days. The pigs were then immune.

We dealt with one subtype, H1N1. New strain variation, pig movement and aerosol transmission, and mixed infections are big factors when we look at how SIV has evolved.

The disease now affects all phases of production. In the sow herd, influenza can cause reproductive problems including abortions, lowered conception rates and farrowing rates. Coughing can be seen with nursing pigs and the symptom can carry over into the nursery as pigs are weaned.

In nurseries and grow-finish, flu primarily causes respiratory signs such as coughing and thumping, resulting in reduced feed intake and slow growth. Signs can last for 2-3 weeks.

Since SIV is a virus, antibiotic therapy is only effective on secondary bacteria.

People, poultry and swine isolates of influenza can and do share genetic material and become new subtypes. We now have subtypes (cH1N1, H3N2, rH1N1, H1N2, and possibly more) in our swine population. The cH1N1 refers to classical SIV and rH1N1 refers to a reassorted form of the classical SIV subtype.

The isolates that are used in the human flu vaccine are changed every year to match predicted strain prevalence. Strain choice is important for pigs, too.

Since “neighborhoods” seem to get infected at the same time, pig movement plays a role with new isolates in a given area.

Respiratory outbreaks can be due to SIV alone, or in combination with porcine reproductive and respiratory syndrome, circovirus or Mycoplasmal pnemonia, or all of the above.

The level of maternal antibody from colostrum in piglets is important both when diagnosing and immunizing pigs against influenza.

In sow herds with low exposure, the pigs will come into the nursery with a very low antibody level (titer). In vaccinated sow herds that have experienced infection, pigs may have antibodies up to 14 weeks of age. Since vaccine cannot overcome maternal antibody, knowing the antibody titer at various ages in your herd is important for vaccine timing.

Case Study No. 1

We were called to a 1,300-sow, farrow-to-finish unit which had a history of flu breaks in the sow herd despite use of a commercial SIV vaccine. This farm was vaccinating sows twice a year and all incoming gilts.

During our herd visit, clinical signs included coughing piglets in the farrowing rooms, increased preweaning mortality, a higher percentage of unthrifty pigs in the crates, longer wean-to-first-service interval and poorer breed back. The sow herd returned to normal, however, we continued to see unthrifty and coughing pigs while nursing.

We bled sows that had experienced clinical signs as well as sows that appeared normal. Most sows had antibody titers of greater than 640. These titers are considered high and should be protective. We requested endpoint titers. We were still seeing clinical signs in farrowing despite endpoint titers of 5,000 to 10,000.

We isolated SIV from nasal swabs in spite of the very high titers in sows.

To deal with the recurring SIV problem in this herd, we made a farm-specific (autogenous) vaccine. The percentage of “select” (normal) pigs has improved and coughing has ceased in preweaning piglets.

Case Study No. 2

A 200-sow, farrow-to-finish, single-site farm had a history of respiratory disease in the finisher at about 18-20 weeks of age. Pigs would develop high temperatures, cough and thump. Mortality would range from 5-9%.

The diagnosis was the H3N2 form of SIV. The sows were vaccinated prior to farrowing with an SIV vaccine. The pigs were receiving two doses of flu vaccine and mycoplasma vaccination.

Serology was done in the nursery and finisher phases. Since the pigs were still carrying maternal antibody titers at 10 weeks of age, the vaccination program was changed to give the flu vaccine at 12 weeks, with a booster shot at 14 weeks of age. Finisher mortality dropped to 3%.


The face of SIV has changed and control measures need to be more creative. The good news is we have more diagnostic tests available today for control. We use “direct antigen” tests, which measure the presence of virus antigen on tissue samples and nasal swab. Serum antibody tests measure antibody levels and virus isolation to obtain the actual infective virus, to genetically characterize the virus, in order to compare it to previous isolations.

Your swine veterinarian can help determine which tests are necessary for your situation.