PORK & PORK QUALITY                               PIH-26


                      Porcine Stress Syndrome

Max D. Judge, Purdue University
Lauren L. Christian, Iowa State University
Gijs Eikelenboom, Research Inst. for Animal Production,
  Zeist, The Netherlands
Dennis N. Marple, Auburn University

Richard Epley,University of Minnesota
Charimonde Heger, Jasper, Missouri
James and Rita Hogue, Yazoo City, Mississippi
  David E. Schafer, Kansas State University

     The porcine stress syndrome (PSS) is a disorder that  was  a
major  concern  to  the pork industry in the 1960s and the 1970s,
and has again become a significant problem.  The  disorder,  when
present,  is  usually associated with heavily muscled animals and
results in sudden and unexplained death  losses.  Animals  having
PSS  often  show signs of nervousness and may have muscle tremors
indicated by a rapid tremor of the tail.  When exposed to stress-
ful  situations such as a change in surroundings, a sudden change
in the weather, vaccination, castration, estrus  or  mating,  the
pigs often respond by becoming overly excited and developing red-
dish blotches on their skin and by experiencing  muscle  rigidity
followed by rapid, labored breathing. Their body temperature also
begins to rise and they begin to show signs of heat  stress  even
in  cold weather. At this point, many producers have attempted to
save them by spraying with water, but the condition progresses so
rapidly  that  it  is  virtually impossible to cool the pigs fast

      Death losses from PSS usually occur during the  process  of
sorting  and delivering animals for slaughter. In addition, death
losses are higher in the  summer  months  when  temperatures  are
higher  and  pigs  are  unable  to  rid  themselves of body heat.
Research has revealed many characteristics of PSS  animals.  Some
of these findings are summarized in this fact sheet.

What Makes Some Pigs Stress-Susceptible?

     Although the metabolic basis of PSS is not completely under-
stood, researchers have learned many facts about the problem. PSS
pigs cannot cope well with stressful situations.  When exposed to
a  stress, they undergo several reactions, including a very rapid
depletion of their muscle energy stores. As their  muscle  energy
stores are being depleted, there is also a corresponding increase
in lactic acid in both the muscle  and  blood.  Normal  pigs  can
remove  the  lactic acid from the muscle and blood fast enough to
prevent excessive build-up.  However, PSS pigs  have  such  great
quantities  of  lactic  acid  produced that they cannot remove it
from the muscle. Therefore, following a stressful situation,  the
levels  of lactic acid increase in PSS pigs which increases blood
acidity creating a condition known as metabolic acidosis.  Accom-
panying the acidosis is a build-up of heat due to a wasteful pro-
cess of utilizing the muscle glycogen for energy.

      The primary defect of PSS pigs is likely to be in the  mus-
cle  structure itself. Certain muscle organelles lack the ability
to bind calcium.  Higher levels of unbound calcium trigger muscle
contraction  and  the  breakdown  of energy-rich phosphates. This
initiates the series of reactions outlined  above  that  produces
excessive amounts of lactic acid.

Genetic Factors

     No breed is entirely free of the PSS problem and,  likewise,
no  breed can be termed categorically stress-susceptible. In some
European breeds the incidence is extremely  high  and  in  others
extremely  low.   The  trait  is  inherited in a simple recessive
manner meaning that both the sire and the dam must  be  at  least
carriers  of  the gene responsible for stress-susceptible offspr-
ing. On average, one of four offspring of carrier parents will be
PSS,  two  of four will be carriers, and one of four will be free
of the condition. Therefore, if there is a problem in  the  herd,
the quickest and most economical step is to replace the sire with
one  that  can  be  confidently  predicted  not  to  be   stress-
susceptible or a carrier of the disorder.

     Although the PSS condition is  sometimes  found  in  animals
with  superior muscling, it is not necessary to sacrifice carcass
leanness for freedom from the PSS problem.  Instead,  one  should
incorporate  meat-type  animals  into the breeding herd that have
been tested free of the problem or that do not appear  to  be  of
the  PSS type. PSS pigs normally have a high muscle-to-bone ratio
but they have several disadvantages including smaller frame size,
lower feed intake, lower daily gain, and smaller litters born and

Tests for PSS

     It is now possible to objectively  evaluate  candidates  for
the  breeding  herd  by using one of two tests. The most accurate
test requires the pig to  be  anesthetized  with  halothane.  PSS
animals  respond  to  halothane  anesthesia  by  showing signs of
extreme muscle rigidity within 3 minutes from the  start  of  the
treatment.   Occasionally, an animal that does not respond within
this brief period will respond to a longer treatment, but this is
rare.  This  test  provides  immediate results, but the equipment
involved  is  expensive  and  the  operator  requires   training.
Halothane  levels  of  3%  to  6% and oxygen flow rates of 1 to 2
liters per minute administered by a semi-closed system, rebreath-
ing  anesthetic machine have produced successful results. The gas
is delivered to the pig via a large-animal face  mask.  The  rear
limbs  are  monitored  carefully, and the mask is removed immedi-
ately upon observing  muscle  rigidity.  The  test  is  generally
regarded safe for young pigs; however, results are not repeatable
in pigs under 7 weeks of age. Although the test  is  accurate  in
older  pigs, the risk of death due to overexposure increases with
the age of the animal.  Various European countries have used  the
halothane test successfully as a selection criterion but the test
alone cannot distinguish between carriers  and  non-carriers,  so
the gene will never be eradicated using this method.

     Blood typing used in conjunction  with  the  halothane  test
offers great promise in identifying both PSS animals and the car-
riers of the gene.  Researchers have discovered  that  two  blood
group  locations  or  loci, called H and S, and three other loci,
called Phi, Pgd, and Po-2, are contained on the  same  chromosome
that  carries the halothane response gene.  Two or more different
genes (alleles) are known for each location.  Since  these  genes
are  all closely linked to the halothane gene, all of the genetic
factors on  a  single  chromosome  are  likely  to  be  inherited
together  as  a  single  block.  Knowing which of these genes are
linked to the stress gene in positive-testing animals permits one
to  discern  which  littermates  carry  only one undesirable gene
(carrier) and which contain none. This testing procedure  is  now
in widespread use in Sweden. Researchers in the United States are
currently modifying the procedure for field use.  .P  The  second
test involves analyzing blood for creatine phosphokinase (CPK), a
serum enzyme that is abnormally high in PSS pigs. The serum  test
requires  submitting the blood sample to a hospital or laboratory
with CPK testing capability.  It  is  important  that  the  blood
obtained for these tests be drawn at least 2 hours and preferably
8 to 12 hours following a physical stress such as a 100-yard  run
or  a 5-mile truck ride. The blood must be taken from an ear vein
or some other superficial vein so the blood sample is not contam-
inated  with  muscle  tissue. It is important to exercise care in
handling the pigs since the test results will be inflated if  the
pig  has  sustained muscle bruising from fighting or harsh treat-
ment  prior  to  sampling.   .P  A  recent  finding  by  Canadian
researchers  promises  to revolutionize stress testing. A single-
point mutation has been detected in the  gene  for  the  skeletal
muscle  receptor that binds a drug called ryanodine.  This recep-
tor, called the sarcoplasmic reticulum calcium  channel  protein,
controls  movement of the calcium from the sarcoplasmic reticulum
into the muscle cytoplasm. The defect in the PSS pig  appears  to
be  a hypersensitive gating of this channel resulting in it being
more easily opened than normal and preventing or making difficult
its  closure.  The result is muscle contraction, hypermetabolism,
and hyperthermia characteristic of this syndrome.  This  mutation
at  nucleotide position 1843 is a single-base pair change of C to
T and creates an amino acid change of arginine to cysteine.  This
change  can  be detected by electrophoresis of the amplified pro-
duct of this DNA segment, after it has been cleaved with  a  res-
triction  enzyme  (Hin  PI).   This mutation is consistent across
five breeds and hence is likely to have had a  common  origin.  A
noninvasive  diagnostic  test  should  soon be released that will
identify the individual genotypes and provide the basis for elim-
ination  of  the  gene  from  the  breeding population or for its
planned use in breeding programs.

Relation of the PSS to Meat Quality

      Much has been said about the use of pork quality  estimates
when  selecting  breeding stock. It is true that most PSS animals
will yield pale, soft, and exudative (PSE) muscle.  However,  not
all  pigs  that produce normal quality carcasses are free of PSS.
The quality of fresh pork is the result of the genetic make-up of
the   animal  and  the  conditions  under  which  the  animal  is
slaughtered.  .P It has been demonstrated that much of  the  low-
quality PSE pork is the end result of PSS, but research shows the
genetic and  environmental  influences  to  be  closely  related.
Those  animals  that  are  stress-susceptible  may die enroute to
market or, if  they  survive  until  slaughter,  produce  a  high
incidence  of  PSE muscle. In populations with a low incidence of
PSS, preslaughter and slaughter conditions  are  relatively  more
influential in producing PSE pork.

     High-quality, uncured pork is reddish pink in color, firm in
texture,  relatively free of surface juices, and (for some consu-
mers) contains modest amounts of marbling. These  characteristics
result  in  a  juicy,  tender, flavorful, nutritious product when
properly cooked. In addition, high-quality pork will retain  most
of  its  juices  during cutting, packaging, freezing, and cooking
and also during curing, smoking, and emulsifying in the making of
manufactured products.

     On the other hand, PSE pork is low in quality for  the  fol-
lowing reasons:

     o    It is soft, mushy, loose-textured,  floppy,  pale,  and

     o    The muscles  become  acidic,  especially  during  early
          stages  after  slaughter, and consequently the proteins
          lose their ability to retain juices.

     o    The condition appears more frequently in the  loin  and
          outer  ham  muscles,  giving  a two-toned appearance in
          many pork cuts.

     o    Affected muscles appear to have little or no marbling.

     o    In the unprocessed fresh condition, it releases  juices
          during  cutting  and  handling  (shrinkage is sometimes
          greater than 7%) as well  as  in  the  retail  package,
          turns  gray  in color and is unattractive to consumers,
          and has a shorter shelf-life than normal pork.

     o    When  used  for  manufactured  products  (smoked  cuts,
          sausage  products),  it  shrinks  excessively (3 to 10%
          above normal for  fully  cooked  hams),  lacks  uniform
          cured  color,  shows  separation of individual muscles,
          and may be difficult to slice.

     o    Frozen cuts lose excessive amounts of juice upon  thaw-

     In some instances  PSS  pigs  do  not  produce  PSE  muscle.
Several  factors  may  interfere with the usually close relation-
ship. For  example,  the  particular  stage  of  stress  response
developed  by the pig at the moment of slaughter will dictate the
conditions within the muscles. If an animal is stress-susceptible
but  survives  a stress that occurs well in advance of slaughter,
the muscles may be depleted of their  energy  reserves.  In  this
instance  the  meat  may appear dark, firm, and dry (DFD) because
very little acid is produced after death. The  DFD  condition  is
undesirable  in appearance and is more subject to spoilage due to
higher ultimate pH (less acidity), but it does not have the other
disadvantages  of  PSE  muscle.  If  preslaughter  conditions are
right, PSS pigs can yield normal appearing muscle. These  compli-
cating  factors  suggest  that it is more reliable to base animal
selection on direct  measurements  on  the  animals  in  question
rather than on meat quality characteristics of their littermates'

Preslaughter Handling Practices
and Prevention of PSE Pork

     Some  environmental  conditions  may  be  comfortable  to  a
stress-resistant  animal but stressful to the pig with PSS.  Con-
sequently, it may be impossible to handle  pigs  under  practical
conditions without imposing some stress.

     Some of the undesirable meat characteristics can  be  minim-
ized  by observing simple management practices at marketing time.
The following are suggestions for reducing losses associated with
handling market hogs:

     o    Avoid crowding in holding pens and  trucks.  Make  sure
          loading  and  unloading facilities are well-designed to
          minimize excitement. Train handling personnel in animal
          behavior, and be patient.

     o    Eliminate the opportunity for fighting. Do not mix pigs
          that have not been reared together. When handling pigs,
          treat them quietly at all times and refrain from use of
          an electric prod.

     o    Avoid extremes in temperature and  other  environmental
          conditions. Do not move pigs during the hottest part of
          the day.

     o    Use general precautions in all phases of the  marketing
          process.  Do  not  require pigs to walk long distances;
          avoid driving pigs over slippery surfaces; do not  feed
          pigs  12 to 24 hours prior to marketing; and spread the
          stress over long periods and allow time for adjustment.

     o    Include a 2- to 4-hour resting period  in  preslaughter
          handling.  Avoid slaughter immediately after arrival at
          the plant. Use showering for  cooling  if  temperatures
          are  high.  Move pigs from holding pens to the stunning
          location as carefully as possible to minimize  crowding
          and exciting the pigs.

REV 6/92 (7M)

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