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Managing MLV PRRS vaccines for optimum performance and returns

By Joseph F. Connor, DVM, MS
Carthage Veterinary Service, Ltd.


As the world awaits a vaccine for COVID-19, it’s an opportune time to think about the value of vaccines and the importance of managing immunity — not just in humans but also in our swine herds.

Vaccines are administered to stimulate immunity by triggering multiple branches of the immune system, including antibodies. They contain either killed or modified-live virus (MLV). With a killed vaccine, the antigen of focus does not replicate within the cells of the host animal; whereas, with an MLV vaccine, the virus or viral fragments replicate within the host cell. Generally, immunological response to MLV vaccines is more robust than killed vaccines. However, with new technology these differences may be less significant.

While we use vaccines to manage many common swine diseases, porcine reproductive and respiratory syndrome (PRRS) presents a unique situation because the vaccines we have available, while effective in many herds, do not always provide complete protection against different strains of wild-type PRRS virus. Second, PRRS vaccine gives a more robust and predictable response in the respiratory challenges of growing pigs compared to the reproductive challenges, particularly in late-gestating sows.

Basic considerations

MLV vaccines are most widely used for PRRS, but it’s important to use them correctly and strategically for these vaccines to deliver the most value.  A few areas for improvement include these basic considerations when using MLV vaccines:

    1. Vaccine storage — Improper storage and handling of the vaccine may cause it to be inactivated. MLV vaccines need to be stored in temperatures of 35˚ F to 45˚ F and kept cool until administered.
    2. Mixing vaccines — MLV PRRS vaccines often cannot be mixed with killed vaccines, and when they are, the sequence of mixing may be important so you don’t inactivate the live virus. The vaccine companies have done an excellent job of recognizing this and have provided the sequence of mixing or compatible products for such antigens as Mycoplasma hyopneumoniae or porcine circovirus. Always check with your vaccine supplier for specific directions.
    3. Use the correct dose — The vaccine must enter the pig at the prescribed dose in order to stimulate an immunological response and yield the anticipated return on investment. When producers and vaccine crews use syringes that alert administrators to less than full dosing, it is not uncommon to discover in group-based administration — such as a nursery —that only 70% to 80% of the pigs are receiving a full dose.
      Because MLV PRRS vaccines replicate in the host, cost-conscious producers often think they can reduce the dose while providing the same protection. However, studies have shown that using MLV PRRS vaccines at the full dose is advantageous because it helps to reduce the viremia and duration of shedding with wild-type PRRS. This ultimately lowers the risk of transmission to neighboring populations, which on some farms may be the producer’s own source sow herd.
    4. Controlling shed and spread — The MLV in most PRRS vaccines is frequently shed for a short period of time to their pen and airspace cohorts while it works to stimulate immunity. For this reason, there are some locations that don’t allow its usage. These include multiplication herds, boar studs and commercial herds that are geographically secure. By using polymerase chain reaction (PCR) tests, we can distinguish vaccine virus from wild-type virus, but in these particular herds it complicates and intensifies the diagnostics.

Follow a herd plan

A PRRS vaccination program should therefore follow a herd plan that is in alignment with each producer’s objective. Based on the Morrison Swine Health Monitoring database and the Iowa State University database, we know that over the last 3 years our industry herds experienced approximately 20% prevalence of new PRRS virus infection. That’s a downward trend, which is encouraging, but the economic impact on both the sow herds and the growing pig remains significant.

The ideal frequency and interval between PRRS vaccination in sow herds have not been conclusively identified and depend largely  on the economic tolerance of the owner. It is advantageous to give gilts entering the herd that iswild-type PRRS negative time for the gilt to develop immunity.

A common PRRS vaccination program involves administering the vaccine in gilts pre-breeding and then again in the sow population one to four times per year, depending on the experience and risk tolerance. Immunologically, a higher frequency of vaccine should provide more robust protection, i.e., quarterly administration compared to less-frequent administration (annual).

Vaccinating herds with wild-type PRRS

Those herds that utilize less-frequent administration will often administer an MLV PRRS vaccine as soon as clinical disease or laboratory confirmation indicates that there is PRRS wild-type virus activity. This strategy relies on anamnestic response, which means that since the host’s immunological system has previously seen vaccine, administration of this dose of vaccine will result in a very rapid increase in antibody.

Because PRRS wild-type virus frequently moves relatively slowly, depending on separation of airspaces in barns, that allows sufficient time for the immunological response to occur in previously vaccinated sows.  This premise also is based on the tremendous advancements in diagnostics and more frequent use in surveillance. Therefore, when combining routine record monitoring with sampling of targeted individuals or routine surveillance of either processing fluids or oral fluids post-weaning, we can identify wild-type virus entry much sooner in the population than previously, which in turn allows this strategy to be more robust.

When the herds become infective with wild-type virus, the herd transitions from stability to a program attempting to achieve stability. With a herd that is previously vaccinated with MLV vaccine, we immediately revaccinate that herd unless it has been vaccinated within the last 30 days. If the herd has been previously vaccinated, we would generally repeat that vaccine 30 days after the first vaccine.

Piglets born in the first 4 to 8 weeks after wild-type infection will have high viremia as indicated by low PCR cycle threshold (Ct) values. Vaccinating suckling pigs during the period when they have high viremia is generally contraindicated because the nursery mortality may actually increase. However, as the Ct values increase, indicating lower circulating viremia, then vaccinating piglets at 7 to 10 days of age with a similar vaccine used in the sows can improve nursery and grow-finish performance. When viremia as indicated by Ct values drops lower, timing of the vaccine is often moved from 7 to 10 days of age to weaning. A number of studies have indicated that the most robust immunological response peaks at about 30 days after vaccination.

Lower reproductive losses

When comparing PRRS-vaccinated sow herds to non-PRRS-vaccinated sow herds over the past several years, we’ve found that data from a number of sources indicates that the reproductive losses are less in vaccinated sow herds compared to naive sow herds even though it does not prevent the circulation and infection of a wild-type virus.

We often get caught up in transmission of PRRS virus over long distances, but we must never lose sight that a part of the reduction in the annual infection rates is due to intensification of biosecurity, and often viruses such as PRRS are transferred through a breakdown in biosecurity. We are not as secure in some aspects of biosecurity as we should be.

Even though most of the breed-to-wean sow farms pass workers’ lunches through a UV chamber, and although we take measures to pass supplies through disinfectant fogging, we now recognize through studies that both of these methods are incomplete — not because of the source but because of the inability to contact all of the surfaces for the designated time period. For example:

  • Frequently, workers’ lunches are piled on top of one another, and any shadowing does not allow UV light to reach those surfaces. At the same time, the material that is inside the container itself will block the UV light.
  • For supply entry, fogging of the supplies has been shown not to uniformly settle on all of those surfaces and has almost no coverage on the bottom of boxes, jugs, etc. Inclusion of Danish entry benches prior to showering has dramatically reduced the risk. Managing transport through disinfection and drying is also highly effective.

In summary, we have learned a tremendous amount from usage of PRRS MLV vaccines. The greatest value occurs when incorporated in a specific herd plan and an area plan.


Editor’s note: The opinions and advice presented in this article belong to the author and, as such, are presented here as points of view, not specific recommendations by Pig Health Today.
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The important role of vaccination has been considerably underlined recently, but in the pork industry, the greatest value occurs when vaccination is incorporated in specific herd and area plans.

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Posted on May 28, 2020

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US producers and veterinarians have seen an influx of different types of influenza viruses in the last 10 to 15 years, and that is a major reason why influenza is more difficult to control.

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