PORK & PORK QUALITY
PIH-26
pork industry handbook
COOPERATIVE EXTENSION SERVICE • PURDUE UNIVERSITY • WEST LAFAYETTE, INDIANA
Porcine Stress Syndrome
Authors
Dennis N. Marple, Auburn University
Max D. Judge, Purdue University
Reviewers
Richard J. Epley, University of Minnesota
David E. Schafer, Kansas State University
Mrs. Wm. J. Heger, Jasper, Missouri
The porcine stress syndrome (PSS) is a non-pathological disorder that has been of major concern to swine producers for the past 8-10 years. The disorder, when present, is usually associated with heavily muscled animals and results in sudden unexplained death losses. Animals having PSS often show signs of nervousness and may have muscle tremors indicated by a slight twitching of the tail. When these animals are exposed to a stressful situation such as a change in surroundings, a sudden change in the weather, vaccination, castration, estrus or mating, they often respond by becoming overly excited and developing blotches on their skin 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 their animals by spraying with water, but the condition progresses so rapidly that it is virtually impossible to cool the animal 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, because these animals are unable to rid themselves of body heat. Research has revealed many characteristics of these animals. Some of these findings will be summarized in this fact sheet.

Genetic Factors
It should first be established that no breed is entirely free of the PSS problem and, likewise, no breed can be categorically termed stress-susceptible. Current theories suggest that the genes controlling the PSS trait are recessive in that both the sire and the dam must be carriers of the gene or genes responsible in order to get stress-susceptible offspring. Therefore, if there is a problem in the herd, the quickest and most economical step is to replace the sire with one you can confidently predict is not a carrier. Although the PSS condition is sometimes found in animals with superior muscling, it is not necessary to sacrifice carcass merit for freedom from the PSS problem. Instead,
one should incorporate meat type animals into the breeding herd that do not appear to be of the PSS type.

Tests for PSS
It is now possible to evaluate candidates objectively for the breeding herd by using one of two tests. The first test involves catching a small drop of blood on a special card and sending the card to a chemical laboratory to be analyzed for the activity of creatine phosphokinase (CPK), a serum enzyme that is abnormally high in PSS swine. The producer may obtain blood for this test by making a small cut on the pig’s ear.

A second test is a bit more involved and requires some specialized equipment. In this test the pig is put to sleep using an anesthetic called Halothane. Animals that are the PSS type respond to the Halothane anesthesia by showing signs of extreme muscle rigidity within 5 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 not often the case. This test provides immediate results, but the equipment involved is expensive and must be used under the direction of a trained technician. The Halothane test is generally regarded safe for only young pigs, since older PSS pigs are likely to die after Halothane exposure.
Both of these tests will give reliable results for animals that are severely prone to the PSS condition. However, animals that are carriers of the trait and are not highly stress-susceptible themselves may not react to either test. Producers should discuss the conditions under which these tests will be conducted with their livestock Extension Agent to assure proper evaluation of the results.

Other tests to identify PSS animals have been proposed but have not been as reliable as the two tests described. These tests include visual appraisal for the degree of muscling and response to stressors, the increase in blood acidity after the animals have been stressed, the steroid hormone binding capacity of a serum protein, and the levels of metabolites in muscle samples taken from live pigs. These tests appeared to hold some promise at
Cooperative Extension Work in Agriculture and Home Economics, State of Indiana, Purdue University and U. S. Department of Agriculture Cooperating. H. G. Diesslin, 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 its programs and facilities without regard to race, color, sex, religion, national origin, age or handicap.