PORK & PORK QUALITY PIH-26
PURDUE UNIVERSITY. COOPERATIVE EXTENSION SERVICE.
WEST LAFAYETTE, INDIANA
Porcine Stress Syndrome
Authors:
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
Reviewers:
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
enough.
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
reared.
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
unappealing.
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-
ing.
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'
carcasses.
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)
______________________________________________
Cooperative Extension Work in Agriculture and Home Economics,
State of Indiana, Purdue University and U.S. Department of Agri-
culture Cooperating. H.A. Wadsworth, Director, West Lafayette,
IN. Issued in furtherance of the Acts of May 8 and June 30, 1914.
It is the policy of the Cooperative Extension Service of Purdue
University that all persons shall have equal opportunity and
access to our programs and facilities.
.