What is the ratio of serum acclimatization used in USA when PRRS break out? How long would be the interval to vaccinate with a MLV when the PRRS is unstable in the farm?
The producers we work with in the United States use modified-live virus vaccines (MLV) more frequently than they use live virus inoculation (LVI).
For those using LVI, they also often expose the herd to MLV to reduce the clinical consequences associated with LVI.
Regarding the dilution of the LVI material, there is no consensus amongst veterinarians. Some dilute it more, some less. A good ‘average’ will be 1 part of piglet serum to 200-300 parts of saline solution. There is the need for more data on this topic. The key is to do PCRs on the material to ensure you have PRRS virus, and not other key pathogens at risk on your farm.
For the PRRSV, ideally you run genotyping using molecular diagnostics to ensure the strain(s) you are including in the LVI material are representative to the strain(s) circulating in your herd.
For MLV use in unstable breeding herds, we see veterinarians implementing whole-breeding herd vaccination as soon as possible after the outbreak.
This way homogenization of exposure is ascertained and the attenuated MLV virus has the chance to build immunity ahead of the wild-type strain. When the whole-herd MLV exposure is combined with other bio-management practices, the program is repeatedly successful, with farms reaching full stability within about 30 weeks post outbreak.
Waiting a few weeks (or months) to intervene will extend the time to stability since the outbreak, producing more PRRSV-positive pigs in the long run.
One of the key aspects when using MLV is to continuously monitor the herd with PCR-based assays to understand when the virus is under control.
The suckling pig population (and the breeding herd) will recovery clinically several weeks before they clear the virus. When that happens, it is important to maintain strict bio-management practices making it hard for the virus to transmit between pigs, crates, rooms, and therefore week over week persisting in the herd.
If you want to know more about immunization of sows read out chapter: