Brent Jones from South West Ontario Veterinary Services, PRRSV Recombination Can Result In Increased Virulence

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In July 2019, a PRRSV-negative boar station in Demark was infected with a PRRSV recombinant that involved two PRRSV vaccine strains. At the time of infection, the boar stud PRRS monitoring involved performing PRRS antibody testing every 2 weeks.  The PRRSV that was isolated from the boar station had never been detected in Denmark or elsewhere prior to July 2019. This PRRSV was considered to be a new PRRS1-variant and was a recombination between the vaccine strains included in the Unistrain PRRS vaccine (Hipra, Spain) and the Suvaxzyn PRRSV (Zoetis Animal Health, USA). The ORF 1, ORF 2 and part of ORF 3 was more than 99% identical to the Suvaxyn strain, and the rest of the genome was more than 99% identical to the Unistrain vaccine strain. Recombinants involving two modified live PRRSV vaccines have previously been documented and include a previous small outbreak in Demark in 2013. The PRRSV variant in this case subsequently spread to at least 36 herds that had received semen from the boar station. In the following months, all the infected herds reported reduced productivity. These Danish researchers  wanted to evaluate the impact of the PRRS recombinant outbreak. Production data was collected from 13 of the herds that chose to participate. The average levels of farrowings/week, liveborns/litter, stillborns/litter, pre-weaning mortality and weaned pigs/litter were compared for the five-month period after infection and the preceding 7 months before infection with the new variant of PRRSV-1.

The researchers found the following:

  • Twelve herds experienced a decrease in farrowings/week (0.1-10.8% fewer farrowings/ week). One herd surprisingly had increased farrowings.
  • Thirteen herds experienced
    • fewer liveborns (0.8-4.8 fewer liveborns/litter) avg 2.8 fewer pigs / litter in the 2019 case whereas the decrease in the 2013 recombinant case was 0.7 fewer pigs / litter
    • more stillborns (0.6-2.6 more stillborns/litter
  • Pre-weaning mortality nearly doubled in half of the herds
  • Overall, the 13 herds were missing 2.4-6.5 pigs/litter at weaning during the 5 months after infection compared to the seven months before infection.

Take Home Messages:

  • A novel PRRSV-1 variant was the result of a recombination event involving two commercial vaccine strains (Unistrain PRRS vaccine (Hipra, Spain) and the Suvaxzyn PRRSV (Zoetis Animal Health, USA).)
  • This new PRRSV-1 variant resulted in higher production losses than older variants of PRRSV seen in Denmark.
  • So here’s the thing. Recombination events can involve commercial vaccine strains and the recombination may increase disease causing ability of the resulting virus. It is important to remember however that field virus also has the ability to recombine. The decision to use of commercial vaccine must take into account all of the known pros and cons.
  • And by the way, the boar stud tightened up their PRRS monitoring by switching to routine PRRS PCR testing backed up with biweekly PRRS  antibody testing

Submitted by Dr. Brent Jones

Ref:  C S Kristensen , M G Christiansen , K Pedersen , L E Larsen  Production losses five months after outbreak with a recombinant of two PRRSV vaccine strains in 13 Danish sow herds   Porcine Health Manag  2020 Oct 6;6:26. doi: 10.1186/s40813-020-00165-z. eCollection 2020.