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Report to Iowa Livestock Health Advisory Council
Title: Searching for the etiology of porcine reproductive and neurologic syndrome
Principle Investigator: Kyoung-Jin Yoon, DVM, PhD, DACVM
Associate Professor, VDPAM
College of Veterinary Medicine
Phone: 294-1083
Co-Investigators:
Kent Schwartz, DVM, MS, Diagnostician
Michael Yaeger, DVM, PhD, DACVP, Associate Professor
Vickie Cooper, DVM, PhD, Diagnostician
Diana Jordan, DVM, PhD, Assistant Professor
Veterinary Diagnostic Laboratory, VDPAM
Yoon, Virus X and PRNS
Project Summary
The swine industry in the U.S. has played a major role in the food supply for the public and is
one of the major income revenues for the State of Iowa and the United States. As swine
production has become more industrialized and larger in size per operation, microbiological
threat becomes a major issue for animal well-being, sustaining business, and securing food
supply and safety. Reproductive failure and sow death due to infectious diseases is a significant
concern related to food security. Recently, an apparently new disease outbreak has been
identified in adult female pigs and growers-early finishers, which is clinically characterized by
the decrease in farrowing rate (early gestation) and/or neurologic disorder frequently leading to
death. Based on clinical presentation, the disease is commonly referred to as “Porcine
Reproductive and Neurologic Syndrome (PRNS)”. Diagnostic investigation and laboratory
examination of PRNS cases to date ruled out the involvement of viruses and bacteria known to
be pathogenic to swine, as well as genetic, nutritional and environmental factors, suggesting that
the disease is likely due to a previous unrecognized agent. Extensive virological examinations
have recently resulted in the isolation of a viral agent (small enveloped RNA virus) with
unknown identity which is tentatively named “Virus X”. As we hypothesize that virus X is
responsible for the recently identified disease problem, the main objective of the proposed work
was to determine if virus X is a necessary component in PRNS using animal inoculation and
case-control studies, while laboratory efforts continue to determine the taxonomical
identification of the virus. The animal inoculation study was only attempted due to the fact that
2nd year funding was not granted. The study was to reproduce the neurological disease
component of PRNS in young CDCD pigs by experimental inoculation of either the virus isolate
or homogenate of tissues collected from a clinical case of PRNS. While no clinical sign of
neurologic disorder was apparent, viremia and seroconversion occurred in all of infected animals.
Encephalitis lesions were also observed in the inoculated pigs, indicating that Virus X is
pathogenic to swine and is highly likely responsible for PRNS (at least neurologic disorder).
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Yoon, Virus X and PRNS
1. Statement of the problem/hypothesis
Reproductive failure is the second most economically significant problem for the swine industry
throughout the world. It is the major concern in terms of securing food supply. In addition, loss
of adult, breeding, female pigs imposes significant economic hardship on producers and is a
serious challenge to sustaining animal production.
Since 1995, pork producers and swine practitioners have reported disease outbreaks
characterized principally by reproductive loss at early stage of gestation. Typically, producers
have observed an acute decline in farrowing rate, followed by a prolonged period of sub-optimal
farrowing performance (up to 2 years). Eventually, some herds returned to pre-outbreak
reproductive performance levels (Yoon and Zimmerman 1998). More recent clinical
presentations of the disease include neurological disorder, such as restlessness, posterior
weakness and paralysis, ataxia, lameness, head pressing or hanging, and aggressive behavior
which often leads to abrasions on the forehead, nose and front legs (Yoon 2003). Elevated body
temperature and dyspnea also occurred in some of the affected animals. The mortality of
affected animals could reach 100% without appropriate symptomatic treatment. Based on
clinical manifestations, the disease was named “Porcine Reproductive and Neurologic Syndrome
(PRNS)”. Although the field-based epidemiological investigation suggested that PRNS appears
to be due to a viral challenge, the Veterinary Diagnostic Laboratory (VDL) of Iowa State
University (ISU) has been presented with such cases for which a conclusive diagnosis could not
be made until recently.
The hypothesis is that a previously unknown viral etiology is responsible for the disease
described above. In support, our laboratory has repeatedly isolated a small-enveloped virus with
RNA genome from animals affected by the disease. Since the isolated virus could not be
recognized by antibodies raised against known swine viral pathogens, it was tentatively
designated “Virus X” due to unknown identity.
2. Objectives
Our long-range goal is to identify the etiology of the recently identified disease referred to as
PRNS. Immediate objectives are to:
1. determine if the recently discovered virus X is a necessary component in the disease,
2. assess the frequency and distribution of Virus X infections in Iowa swine.
3. Materials and Methods
As funding was not continued after the first year, only objective one was carried. To address the
objective one, a challenge study was conducted only in young CDCD pigs due to funding
constraint.
The pathogenicity and causative role of Virus X was assessed through an animal inoculation
study using young CDCD pigs. A total of 30, 3- to 4-week old CDCD pigs were purchased from
a commercial vendor and randomly assigned into 3 treatment groups of 10 each. One group (A)
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Yoon, Virus X and PRNS
was inoculated with a virus X isolate designated YP03 and another group (B) with tissue
homogenate filtrate containing virus X but free of other known viral agents. The remaining
group (C) served as a sham-inoculated negative control. All animals were housed in a BSL-2
large animal holding facility for 4 weeks post inoculation (PI) and monitored for any clinical
abnormlities including neurologic signs. During the first 7 days PI, rectal temperature was
measured daily and blood cell counts (total and differential) done. Three pigs were euthanized
every 7 days PI for pathological and virological assessment (virus isolation, frozen tissue FA
test). Blood and sera were collected from all pigs on 0, 3, 7, 10, 14 and 21 days PI and tested for
CBC, the virus and/or virus-specific antibody.
4. Results and Conclusion
No apparent clinical abnormality was observed in all groups. While neither viremia nor
seroconversion was observed in the control group C, cell-free viremia was detected between 3
and 10 days PI in both groups A and B. IFA antibody was detected between 14 and 21 days PI
in group A whereas seroconversion was evident between 10 and 14 days PI in the group B. A
low level of VN antibody was also detected in some of animals inoculated with either virus
isolate or tissue homogenate at day 21 PI.
Total WBC count was decreased in both of the inoculated groups (A and B), which was
attributed to severe lymphopenia. It was also noted that platelet # was significantly lower during
the first 10 days PI as compared to the control group.
All inoculated pigs showed reactive and edematous spleen and lymph nodes at some points
during the trial. The majority of pigs inoculated with either the virus or tissue homogenate had
microscopic lesions of nonsuppurative meningoencephalitis, nonsuppurative interstitial
pneumonia and/or nonsuppurative periportal hepatitis after 7 days PI.
In conclusion, all these observations strongly indicate that Virus X is pathogenic to swine and
responsible for PRNS (the neurologic disease part).
5. References
Straw B, et al. 1998. Disease of Swine, Iowa State University Press, Ames, Iowa.
Yoon K-J, Zimmerman JJ. 1998. Possible role of infectious agents in early infertility.
Proceedings, 6th ISU Swine Disease Conference, p23.
Yoon K-J. 2003. An unconventional reproductive disease in sows: Clinical and diagnostic
perspectives. Proceedings, ISU Swine Disease Conference, p145.
Yoon K-J, Pogranichniy R, Schwartz K, Frey M. 2004. Clinical, diagnostic and research
perspectives on a “new” mystery swine disease. Proceedings, 35th Annual Meeting of
American Association of Swine Veterinarians, pp. 431-432.
Yoon K-J. 2004. “Porcine reproductive and neurologic syndrome” by a viral agent yet to be
defined. Proceedings, Annual Meeting of Interstate Veterinary Medical Association
(unnumbered).
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