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LAMS 5160
Large Animal Infectious Disease
Discussion Section 2
Spring 2006
Problem 1
A company that owns a large number of swine production facilities is faced with the
problem that PRRS virus crops up among their sites with a high enough frequency to
reduce their ability to make their predicted profit, but not on a chronic basis. They
typically have attempted to develop closed, confinement herds at all their operations with
all in, all out management practiced. They do their breeding by AI using semen that is
checked for PRRS by PCR, but the detection limit of PCR is about 20 virus particles per
ml. Infection during breeding has been demonstrated with 5 virus per ml; so semen could
still be a major source of infection for the herd.
They are considering developing a vaccine program for their herds. They have the choice
of the conventional MLV vaccine that has been available for about 10 years and a new
intranasal avian pox virus vectored vaccine containing protein expression vectors for
ORF 5 and 6 and portions of several non-structural gene products.
Given what you know about the routes of transmission, ecology and biology of the
relationship between PRRS and swine answer the following:
1) Which vaccine would be more likely to prevent significant disease due to PRRS
in the nursery of these operations?
2) Which vaccine would be more likely to prevent transmission of PRRS during AI
of the sows?
3) Which vaccine would be more likely to minimize abortion problems?
4) Could either be used to address the development of persistently infected piglets?
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If the cost per dose of the new vaccine was three times the cost of the conventional
vaccine, what would this company have to achieve to make using it worthwhile?
(Consider how PRRS vaccination impacts the cost of production and what economic
benefit there would be if the new vaccine reduced nursery disease by 80%, reduced
abortions by 50% and blocked the development of persistently infected pigs).
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