Porcine Stress Syndrome

Also Known As – PSS - Malignant Hyperthermia (MH) – Back Muscle Necrosis – Fulminant Hyperthermia Stress Syndrome – Pale Soft Exudative (PSE) pork/musculature – Hyperthermia Syndrome – Transport Myopathy

Introduction

Porcine stress syndrome (PSS) is a congenital, autosomal recessive pharmacogenetic disorder which affects pigs, dogs, cats, horses and humans. It is caused by a fundamental intolerance of stress due to a defective ryanodine receptor which affects closure of calcium channels in the sarcoplasmic reticulum and causing a sudden, sustained rise in intracellular calcium and consequent muscle contracture and upregulation of metabolism.

It can also be induced by exposure to certain anaesthetics, most markedly halothane, which can also generate the disease in people and also intense exercise, coitus, parturition and transport.

PSS leads to an increase in metabolism and intense production of heat, carbon dioxide and lactic acid and contraction of skeletal musculature. The fall in muscle pH following slaughter causes denaturation of muscular proteins, myofibre shrinkage and the PSE appearance of the pork. PSS mainly affects the Type IIB muscle fibres.

PSS is not zoonotic. Although PSS meat is of poor quality and poor aesthetic appeal, it is not a hazard on its own.

Distribution

Worldwide. Prevalence is dictates by emphasis on quality or quantity of meat produced and cultural/traditional differences in pig selection and production.

Signalment

The genetic trait is most commonly manifested in Landrace, Poland-China, Pietrain and associated crossbreeds. Because affected pigs often have more developed musculature and therefore larger carcass weights, the genes translating PSS are often favoured when selecting breeding stock.

Clinical Signs

Affected pigs undergoing an episode of PSS will be hyperthermic, panting, sweating excessively, markedly tachycardic with possible arrhythmias, trembling, hypertonic, stiff or paretic, lame and sometimes cyanotic. Muscle atrophy may be evident, usually of large muscle groups around the back and hindlimbs. The skin often becomes blotchy, erythematous and cyanotic. Haemoglobinuria/myoglobinura is a common consequence of muscle damage/lysis. Pigs may be found dead, particularly after times of stress, e.g. transport.

Early signs of disease may be tail tremors and fine fasciculations.

PSS pigs will develop rigor mortis very rapidly after slaughter and significant economic losses from rejected meat.

Diagnosis

The traditional method of diagnosis is the halothane challenge. Susceptible pigs become rigid within 5 minutes of exposure to the agent. It will however only detect homozygous animals.

A DNA PCR is available to accurately detect heterozygous (carrier) pigs. This can be performed on any material containing DNA including hairs. A complete elimination of the recessive gene is thus possible, but there remains an interest in breeding heterozygous animals for the production of pigs with superior carcasses.

A pig that has died of PSS may have congested viscera on post-mortem and have frothy bronchiolar contents due to terminal pulmonary oedema. The most striking feature is the rapid onset of rigor mortis, as mentioned above. Most pigs develop pale, watery musculature within 15-30 minutes of death. No microscopic lesions or abnormalities are present.

Treatment

If only early signs are present, removal of the animal for the impending stress and allowing rest may prevent the episode without further intervention.

If rigidity and blotchiness has begun, the pig should be sedated with a fast acting agent, and given hydrocortisone and bicarbonate to alleviate the lactic acidosis.

Dantrolene sodium is the most effective method of preventing symptoms from halothane exposure and also reversing them if they occur.

Control

Stress at slaughter is critical as PSS prone pigs will be very vulnerable during transport and progress through the slaughterhouse.

Testing breeding stock or suspect animals and eliminating them from breeding stock is ideal, but often poorly accepted due to the favourable characteristics of PSS animals.

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References

Animal Health & Production Compendium, Porcine Stress Syndrome datasheet, accessed 03/07/2011 @ http://www.cabi.org/ahpc/