porcine reproductive and respiratory syndrome

4 th International Symposium on Emerging and Re-emerging Pig Diseases – Rome June 29 th – July 2 nd , 2003
real world experiences where the use of vaccine may appear
to have no benefit in preventing a PRRSV strain from causing
severe losses; and 3) the significance of preventing the
introduction of PRRSV into the herd.
In general, biosecurity protocols can be good at keeping out
infected swine. This risk of importing PRRSV with
replacement animals can be at about zero when the only
introduction of genetic material is semen. The risk of semen
is dependent on how the boar stud operates. Boar studs, like
sow herds, can follow extensive biosecurity protocols, yet it is
still possible for them to become infected with PRRSV.
These PRRSV infections are attributed to an indirect area
spread of virus. The exact route of this virus transmission is
not understood and could be one of several as previously
described. This indirect area spread of virus is a vexing
problem because it is difficult to control the unknown.
Moreover, it is difficult to assess the risk/benefit ratio of the
economic investments required to control potential avian,
insect, and mammalian vectors. These unknowns also make
it difficult to determine the risk/benefit of depopulating and
repopulating a swine herd with PRRS-negative stock,
perhaps the easiest way to gain a PRRS-negative status.
Depopulating and repopulating a swine herd with PRRSnegative
stock is an expensive, but relatively quick option to
gain a PRRS-negative status. A less expensive, but longer
option is to follow test and removal strategies designed to
convert a PRRS-positive stable herd into a PRRS-negative
status (17). These test and removal strategies focus on
removing PRRSV-positive animals using several diagnostic
methods to find the positive animals. Serology may be the
most efficient tool since there is a high probability that the
only animals harboring PRRSV in a PRRS-stable herd would
be the seropositive animals. Perhaps the biggest
consideration in converting a PRRS-positive herd into a
PRRS-negative status is how to keep PRRSV out of the
herd. After all, the virus got into the herd somehow, and if
that route of virus transmission is not stopped, then one
would expect the herd to become infected again.
FUTURE
Although there are many strategies to control and prevent
PRRS, it is clear that none of them by itself or in combination
seem to be the total answer to the PRRS problem. No matter
how good the present biosecurity protocol may be for a herd,
there is always the concern that a PRRS epidemic is just
around the corner. If the PRRS problem is going to be solved,
then something new will have to happen.
At present, there are two "something new" pathways; one
involves the eradication of PRRS, and the second involves
the administration of some agent to swine that would be a
significant improvement over current vaccines. Elimination of
PRRSV from a farm can be achieved, but it will be difficult to
implement regional and national PRRS eradication efforts
without a better understanding of how to prevent indirect
transmission of PRRSV, an apparent random event.
Moreover, for the foreseeable future, the concentration of the
swine industry may plague any eradication efforts. There are
a number of research initiatives around the world
investigating the second path. Scientists at these
laboratories are applying cutting-edge technology towards
developing better PRRSV vaccines. The direction of these
research programs falls into two broad areas, adjuvants
(cytokines [21,61] and immunoactive peptides [41]), and
vaccines (DNA [47], PRRSV-deletion mutant [60], and viraland
bacterial-vectored PRRSV genes [3,4]).
34
For "something new" to happen, the research community will
need to be very clever in applying current technology with
limited resources to gain the most information possible.
Perhaps this meeting will provide a good forum on directing
future PRRS research.
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