PRRS in the breeding herd What to expect

PRRS virus has evolved many unique features to survive. One of its “secrets” is the slow recognition of the virus by the pig — a pig's immune system does not initially “know” that it is infected. What should you expect when you try to develop immunity in your herd?

Expectations

Different expectations exist between the two disease models of PRRS virus that you are concerned about. The two forms of PRRS disease are:

Respiratory disease — growing and finishing pig pneumonia
Reproductive disease — infection in sows, and in utero or perinatal infection of piglets

Protection is quite different comparing the respiratory to reproductive forms. Respiratory protection is very predictable, and repeatable, when pigs receive adequate time to develop immunity prior to exposure. It is well documented in the literature that a four-week period is required for the pig to recognize the infection, and respond to it.

Pigs vaccinated with Ingelvac^® PRRS MLV or Ingelvac^® PRRS ATP vaccines are extremely well protected when challenged four or more weeks later. Scientific publications support this fact! True for old PRRS, new PRRS, even EuroPRRS! This time frame is critical for a performance improvement. It is also the reason for many perceived vaccine “failures” when pigs were vaccinated too close to exposure…there was too little time for vaccine to help pigs.

For producers, the biggest reward is pigs gain more weight. They can also be less affected by secondary infections like Mycoplasma hyopneumoniae, Circo Virus/PMWS, Haemophilus parasuis and Strep suis. Improvement in growth rates of 20% or more in vaccinated pigs can be seen, depending on the severity of the challenge in non-vaccinated pigs.

Reproductive performance impact

Reproductive protection is not so clear cut. As a result, most cases where PRRS continues to frustrate people center on the inability to completely control PRRS virus circulation in the breeding herd, and the resulting infection of pigs prior to weaning. Pre-weaning infection of pigs makes successful respiratory protection impossible too.

Protection against “old” type isolates is very high. Thousands of pigs from tens of studies have been completely protected by the Ingelvac” PRRS vaccines. This allows pigs to be weaned free of virus. From that point, if pigs will be exposed to virus in the future, they can be vaccinated in time to develop protection. Better yet, they can be moved to negative barns where they can grow free of the effects of PRRS virus.

What about killed vaccines?

Killed vaccines are appealing because they may be simpler to use — they don't need to be remixed, and they cannot shed. Killed PRRS vaccines available in the US today, with their conventional technology, stimulate little or no protective immunity in challenge studies.

When used in sows challenged with old type viruses, killed vaccines may reduce the number of pigs positive for virus as birth. As a result, birth weights can be higher when compared to doing nothing at all. Unfortunately, this prefarrowing or in utero protection is lost rapidly after pigs are born. Within a few days, pigs are infected with PRRS virus. Many of these infected pigs die before weaning.

Current live virus vaccines aren't perfect. As few as 60% of the pigs born to vaccinated, challenged sows may survive to weaning when challenging with atypical PRRS viruses, compared to non-challenged sows. It's no surprise that not having PRRS is better than getting challenged with it!

The big difference in reproductive protection comes when comparing vaccinated and challenged sows with those that do not get vaccine, but do get a PRRS virus challenge…real life for real sows in a PRRS positive herd. Studies demonstrate that sows that were not vaccinated, or receive killed vaccines alone, may not wean any pigs at all with atypical virus challenge. Comparing the number of pigs weaned from sows given killed vaccineto those sows receiving no vaccine shows no performance difference…unless you count the cost of the killed vaccine!

When exposure happens in a positive herd, the risk of not vaccinating is not weaning anything, either!

Application

Identification, please! #1

Tracking the culprits and getting a PRRS isolate

How do you know if you have one virus, or more? Typical or atypical? Getting a virus isolate for sequencing is the starting point. Serum from newborn pigs is one option. Lungs from pigs before or after weaning can harbor virus for an extended time, too. In some cases, sequence evaluation may be done directly from tissues, without a virus isolation first. Sample four or more pigs per litter, from at least eight affected litters.

More information

Characteristics of the immune response of pigs to wildtype PRRS virus or to commercially available vaccines: an unconventional response. Meier WA, et al. Proc Am Assn of Swine Vet, March 2000. Pp. 415-418.

Efficacy and safety studies with a killed PRRS vaccine. Christianson C, et al. Proc Am Assn Swine Pract 1998. Pp. 99-102.