Porcine stress syndrome

Overview

Porcine stress syndrome (PSS), also known as malignant hyperthermia of swine, is a genetic disorder primarily affecting pigs, particularly certain breeds used in commercial meat production. It is characterized by a hypermetabolic response to stress, anesthesia, or physical exertion, leading to rapid muscle breakdown, high fever, and sudden death if not managed promptly. PSS is of significant concern in both veterinary medicine and the pork industry because it impacts animal welfare and meat quality—resulting in pale, soft, and exudative (PSE) pork that is unsuitable for market.

Although PSS is specific to pigs, it shares pathophysiological similarities with malignant hyperthermia in humans, making it a valuable model for research. The condition is most frequently observed in heavily muscled pigs such as the Pietrain, Hampshire, and Landrace breeds.

Causes

Porcine stress syndrome is caused by a mutation in the RYR1 gene (ryanodine receptor 1), which encodes a calcium channel in skeletal muscle cells. This mutation results in abnormal calcium regulation within muscle cells, making them hyperresponsive to certain triggers. When affected pigs are exposed to stressors, calcium floods the muscle cells uncontrollably, causing intense muscle contraction, increased metabolic activity, and rapid heat production.

PSS is inherited in an autosomal recessive pattern, meaning that pigs must inherit two copies of the mutated gene (one from each parent) to exhibit symptoms. Carriers with one copy of the gene appear normal but can pass the trait to offspring.

Symptoms

Clinical signs of porcine stress syndrome may appear suddenly and escalate rapidly, especially in response to stress such as handling, transport, or exposure to volatile anesthetics like halothane. Common symptoms include:

• Muscle rigidity: Stiff limbs and trunk due to sustained muscle contraction.

• Hyperthermia: A dramatic increase in body temperature, often exceeding 42°C (107.6°F).

• Tachypnea and tachycardia: Rapid breathing and heart rate due to increased metabolic demand.

• Skin discoloration: Red, blotchy skin or cyanosis (bluish skin) due to oxygen depletion.

• Tremors or convulsions: Resulting from muscle instability and extreme metabolic stress.

• Collapse and death: If untreated, pigs may die within minutes due to circulatory collapse or cardiac arrest.

Some pigs may not show full-blown symptoms but still produce inferior meat quality due to subclinical muscle damage, contributing to PSE pork.

Diagnosis

Diagnosis of porcine stress syndrome can be made based on clinical signs, genetic testing, and, in some cases, post-mortem examination. Key diagnostic methods include:

• Genetic testing: Blood or tissue samples can be tested for the RYR1 gene mutation. This is the most reliable method for identifying carriers and affected pigs.

• Halothane challenge test: An older method where pigs are exposed to halothane to provoke a stress response. Due to ethical concerns, this test has largely been replaced by genetic screening.

• Histopathology: Examination of muscle tissue post-mortem can reveal signs of necrosis and calcium accumulation in muscle fibers.

• Clinical observation: Sudden onset of symptoms during stress or anesthesia can suggest PSS, especially in high-risk breeds.

Routine screening and breeding programs are used in many countries to eliminate the defective gene from commercial swine populations.

Treatment

Treatment of an acute PSS episode must be immediate to prevent death. Key interventions include:

• Cooling: Apply cold water or ice packs to reduce body temperature as quickly as possible.

• Supportive care: Ensure proper ventilation, administer intravenous fluids to combat dehydration, and correct acidosis.

• Dantrolene sodium: A muscle relaxant that counteracts calcium release from the sarcoplasmic reticulum, used in both veterinary and human cases of malignant hyperthermia. Its use in pigs is limited by availability and cost.

• Immediate removal of stressors: Cease any handling, transport, or surgical procedures to stabilize the animal.

In commercial settings, pigs showing early signs of PSS are often euthanized to prevent suffering and further economic loss. Preventative measures, such as genetic screening and low-stress handling practices, remain the most effective approach.

Prognosis

The prognosis for pigs affected by porcine stress syndrome is generally poor during an acute episode, with high mortality if not treated rapidly. Even with intervention, many pigs suffer permanent muscle damage or die due to cardiovascular collapse. Subclinical forms of the syndrome may not be fatal but still result in poor meat quality, which affects commercial value.

The long-term strategy to control PSS lies in eliminating the RYR1 mutation from breeding lines. Genetic testing and selective breeding have significantly reduced the incidence of PSS in modern commercial swine herds. With proper genetic management, the condition can be virtually eradicated, improving both animal welfare and meat quality.