This term covers a group of conditions associated with a recessive gene. The group includes acute stress and sudden death (malignant hyperthermia), pale soft exudative muscle (PSE), dark firm dry meat, and back muscle necrosis. Heavy muscled pigs are more likely to carry the gene than leaner pigs. The gene is called the halothane gene because of the adverse effect halothane anaesthetic has on pigs carrying it. Each pig is homozygous (i.e. possessing a pair of halothane genes), or heterozygous (i.e. possessing one normal gene and one halothane gene) or two normal genes. Homozygous pigs or their meat may show any of the four conditions.
Homozygous (but not heterozygous) pigs can be identified by their response to the anaesthetic with halothane. Recent developments have produced a gene probe that identifies both the homozygous and heterozygous carriers using only a drop of blood or a single hair. Back muscle necrosis is a more localised form of PSS.
When the homozygous state is present and following a period of muscle activity, there is a change in muscle metabolism from aerobic to anaerobic and biochemical abnormalities develop. The body tissues become acid with a marked rise in temperature 42?C (107?F).
Symptoms
Piglets
- Rarely seen.
- If so as in sows.
Sows
The onset is sudden with:-
- Marked muscle tremors.
- Twitching of the face.
- Rapid respiration.
- The skin becomes red and blotched.
- Death usually occurs within 15-20 minutes.
- Rigor mortis (stiffening of the muscles after death) within 5 minutes is a striking feature.
- Rise in temperature > 41?C (106?F).
Weaners & Growers
- The onset is sudden with muscle tremors.
- Twitching of the face
- Rapid respiration.
- The skin becomes red and blotched.
- Death usually occurs within 15-20 minutes.
- Back muscle necrosis is a more localised form of PSS.
- Whilst the gene produces a leaner carcass, growth rates are slower and the levels of sudden death increase.
Causes / Contributing factors
- Disease is precipitated by sudden muscle activity.
- The carrier pig is genetically susceptible.
Diagnosis
This is based on the sudden onset, symptoms, breed, susceptibility and the known presence or absence of the gene in the pig. In many cases the pig is just found dead and a post-mortem examination is necessary to eliminate other disease. Rigor mortis (stiffening of the muscles after death) within 5 minutes is a striking feature.
This condition has to be differentiated from other causes of sudden death, twisted bowel, internal haemorrhage, mulberry heart disease and pyelonephritis.