PRRSV-1 recombinant found to be less virulent than dominant European field strain in naïve pregnant sows and growing pigs


15 seconds

  • A recombinant of two porcine reproductive and respiratory syndrome virus type 1 (PRRSV-1) vaccine strains, known as Horsens strain, was linked to clinical disease outbreaks in previously PRRSV-free Danish pig herds.
  • A study found that the recombinant was less virulent and had a lower impact on reproductive performance compared to a typical subtype 1 European field strain, findings that were confirmed in a second study in growing pigs.
  • A second study confirmed that modified-live PRRSV-1 vaccines are effective against the Horsens strain.
  • Although modified-live virus vaccines are the best option for controlling PRRSV-1, proper use is essential to minimize the inherent risk of recombination.


A recombinant of two porcine reproductive and respiratory syndrome virus type 1 (PRRSV-1) vaccine strains was found to be less virulent and have a lower impact on reproductive performance than a typical subtype 1 European field strain in a recent study conducted in PRRSV-free pregnant sows.1

First identified in 2019, the Horsens strain may have resulted from the recombination of two modified-live virus (MLV) vaccines against PRRSV-1 (Unistrain® PRRS and Suvaxyn® PRRS MLV) on a pig farm in Denmark. Recombination is the process whereby related organisms exchange genetic material and form hybrid “chimeric” organisms.

Although no adverse effects or clinical signs were observed on the farm where the Horsens strain originated, the recombinant strain subsequently spread to a nearby boar station, via an unknown route. The infected semen then was sent to a large number of pig herds in Denmark, leading to the appearance of clinical signs in herds that had previously been PRRSV-free.

According to Monica Balasch, DVM, PhD, director of research and development at Zoetis and one of the sow study’s authors, recombination among PRRS viruses is common and can occur any time two strains — whether of field or vaccine origin — infect the same cell.

“Recombination does not necessarily lead to increased virulence — in fact, chimeric strains are usually less fit than their parent strains and have little or no impact on the health and productivity of the herd of origin,” she said.

“In rare cases, however, recombinants can be more virulent. Meanwhile, less virulent viruses that have no impact on previously exposed herds can still cause clinical disease in naïve animals,” she continued.

“Following subjective reports that the Horsens strain appeared to be more transmissible and virulent than what is usually observed with PRRSV-1 infection, we conducted a study to assess the recombinant’s impact on reproductive performance in naïve pregnant sows compared to that of a typical European field strain.”

Sow study: Design and results

The objective of the sow study, conducted by Zoetis in collaboration with the University of Copenhagen and Technical University of Denmark, was to evaluate the impact of the Horsens strain on the reproductive performance of naïve pregnant sows in the final third of gestation.

A total of 15 PRRSV-naïve pregnant sows in the final third of gestation were enrolled in the study. Five sows were infected with the PRRSV-1 subtype 1 field strain, Olot/91 (T1), and six sows were infected with the recombinant Horsens strain (T2). The other four served as negative reproductive controls (not-infected group, or NTX).

Reproductive performance was the primary variable. In sows, viremia and nasal shedding (T1 and T2 groups) and, in piglets, viral load in blood and in lungs, as well as macroscopic lung lesions (T1 and T2 groups), were also evaluated.

Results indicated a numerical difference between the two challenged groups (Table 1), with the Horsens strain demonstrating a lower overall impact on reproductive performance.


Table 1. Back-transformed mean (±SD) and range of different reproductive parameters by treatment

Mean ± SD Range Mean ± SD Range Mean ± SD Range
At Farrowing, %
Abortion 0 0 0  
Born alive 89.7 ± 14.03 73.91 to 100 36.0 ± 13.50 21.05 to 55.56 49.9 ± 18.55 27.27 to 71.43
Born healthy 78.0 ± 12.20 69.57 to 98.28 27.1 ± 28.81 0 to 55.56 41.7 ± 18.39 27.27 to 71.43
Low viability 8.8 ± 5.37 4.35 to 17.65 0.9 ± 4.04 0 to 21.05 2.9 ± 8.97 0 to 28.57
Stillborn 10.3 ± 14.03 0 to 26.09 64.0 ± 13.50 44.44 to 78.95 50.1 ± 18.55 28.57 to 72.73
Mummies 0 0 0
Weaned, % 89.8 ± 14.16 73.33 to 100 49.0 ± 47.97 0 to 80 53.2 ± 72.28 0 to 100

NTX: not infected; T1; Olot/91; T2: Horsens strain; SD: standard error


NTX sows farrowed 89.8% live piglets, whereas 36% of piglets born from T1 sows and 49.9% of piglets born from T2 (Horsens strain) sows were born alive. Of those, 78% of piglets from NTX, 27.1% of T1 and 41.7% of T2 were healthy, whereas 8.8% of NTX, 0.9% of T1 and 2.9% of T2 piglets were low viability. At farrowing, NTX sows had 10.3% stillborn piglets compared to 64% of T1 and 50.1% of T2 groups. Of piglets born to NTX sows, 89.9% were weaned compared to 49% of piglets born from T1 and 53.2% of piglets born from T2. No mummies or abortions were recorded in any treatment group.

The virologic data further supported the reduced virulence of the Horsens strain compared to the field strain. Blood samples from T2 piglets showed a viremia level of 1,000- to 5,000-fold lower than the T1 piglets, 1000-fold lower in bronchoalveolar lavages and 31-fold lower in lung exudates. Meanwhile, 68.8% of T1 piglets presented mild lung lesions, compared to 38.1% of T2 piglets.

Based on these findings, the investigators concluded that the Horsens strain was less virulent than the Olot/91 field strain under the study’s experimental conditions. From a reproductive standpoint, the Horsens strain behaved similarly to the field strain, and reports of exacerbated virulence and viral load reported by some Danish farmers were not confirmed.

Furthermore, a subsequent study in vaccinated and naïve growing pigs found that the Horsens strain did not cause any notable clinical signs or significant macroscopic changes, confirming that it is less virulent than previously characterized, highly virulent PRRSV-1 strains (see sidebar).1

Proper MLV use key to limiting recombination risk

According to the investigators, MLV vaccines are the most effective option available to control PRRSV-1 and must be used properly to avoid the inherent risk of recombination, they stressed.

“To limit the occurrence of recombination between vaccine strains, the use of multiple live PRRS vaccines in the same pig flow, either simultaneously or in rapid succession, should be avoided,” they wrote.

“When changing from one MLV to another, it is important to allow the first vaccine time to reduce its circulation and prevalence before introducing the second vaccine. Care should be taken to minimize opportunities for pigs to become infected by both vaccine viruses at the same time.”

To view the full peer-reviewed study in Pathogens, click here.


 Challenge study in growing pigs

A follow-up study in naïve and vaccinated growing pigs also found the Horsens strain to be less virulent than prevalent PRRSV-1 field strains.2

The study, conducted by the University of Copenhagen, the Danish Pig Research Center and France’s food-safety agency ANSES, assessed the pathogenicity of the Horsens strain compared to a PRRSV-1 field strain in 36 young, specific-pathogen-free pigs. It also evaluated the efficacy of three different PRRSV-1 MLV vaccines against challenge with the Horsens strain.

The pigs were divided into six groups. Groups 1, 2 and 3 were vaccinated intramuscularly with either Suvaxyn PRRS MLV, Unistrain PRRS or Porcilis® PRRS, according to label instructions. The pigs in groups 4, 5 and 6 were left unvaccinated. Four weeks after vaccination, all pigs in groups 1 through 4 were challenged intranasally with the Horsens strain, while the pigs in group 5 were challenged with the reference PRRSV-1 strain Finistere. The pigs in group 6 were left unchallenged.

Following challenge, the unvaccinated pigs had significant, increased viral load in serum compared to the vaccinated pigs. No macroscopic changes were observed at necropsy, but tissue from the lungs and tonsils from almost all pigs were PRRSV positive. The viral load in serum was lower in all vaccinated groups compared to the unvaccinated group, and only small differences were seen among the vaccinated groups.

The findings indicate that the Horsens strain indeed is capable of inducing infections in growing pigs as well as in pregnant sows, the researchers explained. However, the absence of notable clinical signs and lack of significant macroscopic changes confirm that this strain is less virulent than previously characterized, highly virulent PRRSV-1 strains such as Olot/91, they added.

1 Genís S, et al. Assessment of the Impact of the Recombinant Porcine Reproductive and Respiratory Syndrome Virus Horsens Strain on the Reproductive Performance in Pregnant Sows. Pathogens. 2020;9(9):772.
2 Kvisgaard L, et al. Challenge of Naïve and Vaccinated Pigs with a Vaccine-Derived Recombinant Porcine Reproductive and Respiratory Syndrome Virus 1 Strain (Horsens Strain). Vaccines. 2021;9:417.