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Swine Flu Virus Turns Endemic

Frequent changes make this virus difficult to control. As fall fast approaches, cool nights and warm days will challenge producers to keep pigs comfortable and healthy. Likewise, swine influenza virus (SIV)

Frequent changes make this virus difficult to control.

As fall fast approaches, cool nights and warm days will challenge producers to keep pigs comfortable and healthy.

Likewise, swine influenza virus (SIV) is often regarded as being a problem linked with the change of seasons.

However, SIV has changed and adapted to be a part of a pig's respiratory health challenge year-round. As with the human strain of influenza, SIV changes frequently, making control and stabilization of herd health difficult. Control strategies may need to change as the herd health situation dictates.

As we look at genetic differences between the various swine flu virus strains isolated in our practice area, and within farms, we will see the virus change over time. This change is referred to as “viral drift.” If a human influenza virus that is being used for a vaccine changes or drifts 1%, the vaccine is updated to hopefully give better immunity against a challenge.

With the swine influenza virus, we often see changes of 6% or greater. If a virus is isolated from pigs showing clinical signs, and we detect a significant level of drift, we will often reformulate the vaccine to contain the virus that has changed.

Case Study No. 1

Approximately three years ago, a 3,000-sow farm weaning 1,200 pigs/week developed a swine influenza infection in the middle to later stages of the finisher. Clinical signs included a significant decrease in pig activity accompanied by a decrease in feed and water consumption. Death loss during the outbreak would sometimes approach 2-3%.

We were able to isolate an H1N1 swine influenza virus. We began a two-dose vaccine regimen in the nursery using a commercial influenza vaccine that contained an H1N1 strain.

In subsequent groups, we saw a significant reduction in the severity of clinical signs and death loss. The pigs would still develop some influenza symptoms, but the severity was reduced.

About 18 months later, death loss in late finishing began to increase, and we again isolated SIV. We sequenced the virus and saw a significant drift from the virus isolated nearly two years earlier. We decided to formulate an autogenous vaccine from the latest virus isolated. We again saw a nice response to the vaccine as noted by a decreased severity of clinical signs.

Case Study No. 2

A 3,400-sow, farrow-to-finish operation with three-site production was experiencing significant respiratory challenges throughout all stages of production. Different laboratory submissions identified multiple issues, with swine influenza being a consistent finding.

We also did a serological profile across the different ages of pigs from nursery through finishing. Serological titers for both H1N1 and H3N2 strains of influenza indicate significant instability across all ages.

From this experience, it was evident that we needed to formulate a total farm approach. The farm was already using a commercial flu vaccine in the sow herd. It was decided to compare all isolates from the farm and select the most prevalent strains to formulate a farm-specific or autogenous vaccine.

Then we instituted a vaccine protocol to inoculate all incoming gilts twice before introducing them to the breeding herd. We also started vaccinating sows prior to farrowing. Pigs were vaccinated at four and six weeks after weaning.

A serological profile six and 12 months later indicated a more stable herd across all ages. This was also supported by a decrease in clinical signs throughout the entire flow.


There's no doubt that swine influenza has become more and more of an endemic problem within pig flows. Antigenic drift occurs within the different strains, resulting in inconsistent responses to traditional vaccines. Thorough diagnostic investigations are needed to identify the strain of swine influenza causing the problems. Over time, the different strains isolated can be sequenced and compared to see if drift or change has occurred.

If a significant change is discovered, vaccine selection should be based on comparison of the virus identified with the field isolate that most closely matches it. Commercial vaccines can be effective in controlling the problem when the virus strains match enough to have significant cross-protection.

However, if a poor response to a vaccine is experienced, a comparison of isolates may indicate the need to use a vaccine that matches the virus isolated. Autogenous vaccines are made from the specific virus isolated. Continual evaluation of strains will indicate if a change may need to be made. Working closely with your veterinarian is essential to formulate an effective control and monitoring program for control of swine influenza.