|Guo, Baoqing -|
|Kehrli Jr, Marcus|
|Yang, H -|
Submitted to: Meeting Proceedings
Publication Type: Proceedings
Publication Acceptance Date: April 22, 2011
Publication Date: May 10, 2011
Citation: Lager, K.M., Faaberg, K.S., Guo, B., Brockmeier, S., Henningson, J.N., Schlink, S.N., Miller, L.C., Kappes, M.A., Kehrli, Jr., M.E., Loving, C.L., Nicholson, T.L., Yang, H.C. 2011. HP-PRRSV challenge of 4 and 10-week-old pigs. In: Proceedings of the 5th Asian PRRSpective Symposium, May 10, 2011, Bangkok, Thailand. 6 pages. Technical Abstract: In 2006 a unique syndrome was recognized in growing pigs in China with the predominant clinical signs being high fever, anorexia, listlessness, red discoloration of skin, and respiratory distress. The disease had a very high morbidity and mortality rate and became known as porcine high fever disease. Although there was a great concern this syndrome may have been caused by a new pathogen, extensive diagnostic testing only revealed known pathogens. One consistent finding in affected pigs was the detection of porcine reproductive and respiratory syndrome virus (PRRSV) that had unique genetic mutations in the nsp2 gene. This led to the suspicion that PRRSV was the primary cause of porcine high fever disease, an assumption confirmed later by several experiments. Experimental infection of pigs by Zhou, et al., and Tian., et al, reproduced the clinical disease providing strong evidence for the role of PRRSV as the causal agent of porcine high fever disease. However, there was still a question if there was some unknown agent in the PRRSV preparations that increased the severity of the clinical disease over what was expected for a "routine" PRRSV infection. This question was put to rest by Lv et al., when porcine high fever disease was reproduced with virus derived from an infectious clone of the JX143 PRRSV isolate. These studies demonstrated that PRRSV isolates with a common genetic motif had a causal role in porcine high fever disease leading to this lineage of virus being called highly pathogenic PRRSV (HP-PRRSV). In the United States the swine industry has closely followed the porcine high fever disease story. In our laboratory we developed a collaboration with Dr. Yang, China Agricultural University, Beijing, China to import the plasmid containing a full-length clone of the JXwn06 HP-PRRSV isolate. Infectious virus (rJXwn06) was rescued from the clone and used to inoculate young pigs in a series of studies that were recently completed. The clinical results will be summarized in this proceeding and additional data will be presented at the meeting. The first experiment compared the pathogenicity of the rJXwn06 HP-PRRSV with the North American prototype PRRSV, VR-2332, in 4-week-old pigs. Fifty-five conventionally-raised pigs weaned at 3 weeks of age were randomly divided into one of 5 groups (11 pigs per group) that received the following treatments: Group 1 - a sham intranasal inoculation; Group 2 - a "low" dose intranasal challenge with the rJXwn06 virus; Group 3 - a "high" dose intranasal challenge with the rJXwn06 virus; Group 4 - a low dose intranasal challenge with the VR-2332 virus, and Group 5 - a high dose intranasal challenge with the VR-2332 virus. Both challenge viruses had been propagated in MARC-145 cell cultures and each virus was diluted to 1 x 10** CCID50/mL concentration for the high dose. The low dose challenge virus had a concentration of 1 x 10** CCID50/mL. In experiments 2 and 3, all virus challenges consisted of the "high" dose. All inoculations consisted of a 2 ml volume (either sham or virus) given intranasally. Pigs were bled on days 0, 4, 7, 11, and 14 days-post-inoculation (dpi). Pigs were scheduled for necropsy at 14 dpi. Pigs were weighed -1, 7, 14 dpi. The second experiment compared the pathogenicity of the rJXwn06 and VR-2332 viruses in 10-week-old pigs. Thirty-two pigs were divided into 4 treatment groups: Group 1 was sham inoculated controls (n = 8); Group 2 - high dose challenge rJXwn06 virus (n=12); Group 3 was 4 contact pigs that were co-mingled with the Group 2 pigs at 2 dpi and housed together until 13 dpi, the end of the study; and Group 4 - high dose VR-2332 challenge (n = 8). Pigs were bled on 0, 4, 7, 11 and 13 dpi and weighed on -1, 7, and 13 dpi. The third experiment tested the efficacy of a commercially available modified-live vaccine (Inglevac PRRSV MLV, Boehringer Ingelheim, Vetmedica) against challenge with rJXwn06 virus. Fifty-five pigs were divided into 5 equal treatment groups: Group 1 was sham-vaccinated pigs with no challenge; Group 2 was vaccinated at 4-weeks of age and given an intranasal challenge at 10 weeks of age (42 days post vaccination) with rJXwn06; Group 3 was non-vaccinated and received rJXwn06 virus challenge at 10-weeks-of age; Group 4 was vaccinated at 4 weeks of age and given an intranasal challenge with VR-2332 at 10 weeks of age; Group 5 was non-vaccinated and received VR-2332 challenge at 10 weeks of age; Pigs were bled on 0, 4, 7, 11 and 14 dpi, weighed on -1, 7, and 14 dpi, and necropsy was scheduled at 14 dpi. In general, all non-vaccinated pigs in Experiments 1, 2, and 3 responded similarly to virus challenge, including low and high dose challenge groups in Experiment 1. Pigs given the rJXwn06 virus developed a pronounced fever about 48 hours post inoculation, followed by anorexia and listlessness in most pigs and vomiting in some. From about 5 dpi until the end of each study, some pigs became very sick developing respiratory distress, ataxia, and would become moribund at which time the pigs were euthanized for human purposes. During this time some pigs developed diarrhea and had red-blue discoloration of extremities. Despite frequent monitoring some pigs died unexpectedly (Table 1). There was pronounced weight loss in all inoculated pigs. In contrast, pigs given VR-2332 challenge virus were much less affected with some anorexia noted during the first week of infection. At rest, increased respiration rates were noticed in only a few animals. After walking the pigs around the room for several minutes, additional pigs would have an increased respiration rate, but would quickly return to normal upon stopping the exercise. There was a mild to moderate fever detected beginning 2-3 dpi and lasting about a week. There was no mortality in any of the VR-2332 challenge groups. In all 3 experiments, the control pigs appeared clinically normal. In Experiment 3, pigs that were vaccinated and then received the rJXwn06 challenge were less affected when compared to non-vaccinated challenge controls. There was a reduction in fever response (Fig 1) , vaccinated pigs gained more weight (Fig 2), reduction in gross lung lesions (Fig 3), there was less mortality (Table 1), and at the end of the experiment (14 dpi), the remaining pigs appeared to be recovering while almost all of the non-vaccinated pigs appeared to be getting worse. In the vaccinated pigs that received VR-2332 challenge there were no clinical signs of disease observed (Figs 1-3, Table 1) when compared to control pigs in group 1, and non-vaccinated VR-2332-challenged pigs in group 5. In summary, our results from the studies with rJXwn06 virus are compatible with what has been published by others; i.e., the HP-PRRSV isolate induce a severe disease following intranasal inoculation that resembled what has been reported in the field for porcine high fever disease. Based on our experience with experimental PRRSV infections, the HP-PRRSV isolate has a strong immunomodulating capacity that negatively affects the pig's homeostasis allowing subclinical bacterial infections to manifest as acute clinical disease. This was most striking in Experiment 1 where all 22 pigs that received the rJXwn06 virus died or were euthanized between 5-9 dpi in the high dose group, and 4-11 dpi in the low dose group. All of these pigs had developed severe bacterial pneumonia and many were septic as well. The lower mortality rate in the 10-week-old rJXwn06-infected pigs (Experiments 2 and 3) suggest there may be age differences in susceptibility, a common observation in PRRSV studies. However, based on the deteriorating condition of the older pigs at the time of scheduled necropsy (14 dpi), if the experiments would have continued we anticipate many of the rJXwn06-challenged pigs would have succumbed to the infection. Analysis of much of the data is currently underway and will be available for the oral presentation.