This resource has been updated as part of the veterinary review process as of July 11, 2022.
Porcine Stress Syndrome (PSS), or malignant hyperthermia (MH), is a serious, but rare, inherited condition that can be triggered by stress and certain medications. Thankfully, many sanctuary caregivers will never find themselves faced with a resident experiencing a PSS episode. In fact, almost all of the long-term pig caregivers we spoke to have never come across a confirmed case. Currently, we are aware of two cases of PSS in sanctuary pig residents, both of which were triggered by anesthesia.
What Causes PSS?
An abnormality in the mechanism that controls the uptake, storage, and release of calcium in the muscle fibers, caused by a mutation in the ryanodine receptor 1 (RYR1) gene is responsible for MH predisposition in both pigs and humans. These skeletal muscle fibers become much more sensitive to a variety of triggers than typical muscle fibers. In pigs, this gene is sometimes known as the Halothane Gene (or “HAL” Gene), due to the association of PSS with halothane sensitivity. Individuals can be categorized into three genotypes: normal/stress-resistant (NN), stress-positive (nn), and stress-carrier (Nn). Carriers do not tend to be PSS-susceptible. Positive and carrier genotypes are found more often in large breedDomesticated animal breeds that have been selectively bred by humans to grow as large as possible, as quickly as possible, to the detriment of their health. pigs than in mini pigsThe term “mini pig” is used to categorize smaller breeds of pigs, such as Potbellies, Julianas, and Kunekunes, and to differentiate between these breeds and large breed pigs, such as Yorkshires, Landrace, or Hampshires, who are sometimes referred to as “farm pigs”. The descriptor “mini” can be a bit misleading- some breeds of mini pigs can weigh over 200 pounds! Terms like “teacup”, “micro”, “pocket”, “nano”, and “micro mini” are deceptive terms that are sometimes used to intentionally mislead people into thinking an individual pig will remain a certain size., and PSS-susceptibility is typically associated with heavily muscled individuals. Though overall rare, incidence of PSS varies by geographical region and occurs most often in Pietrain, Poland China, and Landrace breeds.
It is thought that the gene responsible for PSS spread quickly between the 1930’s and 1950’s when the Pietrain breed was first bred in Belgium to promote leaner pigs for human consumption. It is likely that the breeding of a single pig carrying a mutation has resulted in all subsequent PSS occurrences. The syndrome was not discovered until 1968, and by that time it had spread globally. Certain producers have allegedly continued to intentionally breed pigs with the stress-carrier genotype (Nn) due to a perceived outcome in how this genotype affects their body composition for human consumption. However, overall, there has been a decrease in the number of positive individuals (nn) since the high numbers seen in the late 1970’s and early 1980’s. DNA testing conducted in the 1990’s found that 2% of the pigs tested in the US and 1% of the pigs tested in Canada were stress-positive. We’ll talk more about testing later on, but we want to point out that not all stress-positive individuals will go on to have a PSS episode.
What Can Trigger PSS?
A major challenge of PSS is that susceptible individuals often do not show any signs of concern until faced with a triggering event. Potential triggers for a PSS episode include:
- Inhalational anesthetic (halothane; less commonly isoflurane, sevoflurane, and desflurane)
- Certain muscle relaxants (especially succinylcholine)
- Exercise
- Fighting
- Stress (including that caused by medical procedures)
- Excitement
- Transportation
- Going into heat
- Intercourse
- Giving birth
- Hot weather
What Happens If PSS Is Triggered?
In a PSS episode, a dangerous amount of calcium is released into muscle fibers, causing a vast array of potentially catastrophic symptoms in a short amount of time. As a susceptible individual gets older and larger, their risk for a PSS episode increases. Symptoms of a PSS episode include muscle and tail tremors, labored and irregular breathing, whitening and reddening of their skin, a very rapid rise in body temperature, blood pressure fluctuations, collapse, muscle rigidity, kidney failure, arrhythmiaAn arrhythmia means that your heart beats too quickly, too slowly, or with an irregular pattern., cardiac arrest, and without prompt intervention, often death. Once triggered, individuals will exhibit symptoms very quickly. The time between onset and death can be as little as 10 minutes. Rapid onset rigor mortis is a common telltale sign of a PSS fatality.
Can Anything Be Done If PSS Is Triggered?
As mentioned at the start of this resource, overall, PSS is thankfully rare. Other health challenges could present some similar symptoms, and it’s important to contact your veterinarian immediately if you see any of the signs above. Regardless of the cause, reducing stress will be important, so if a resident develops concerning signs during a stressful event, be sure to take immediate steps to remove or discontinue the stressors and keep them cool.
Most of the available medical information, as well as information from the sanctuary community, involves an episode triggered by anesthesia. This makes PSS different from many other health challenges we cover because it is more likely to be triggered while they are under the care of a veterinarian, and the complex response and treatment required are outside of the scope and capabilities of the vast majority of caregivers. Below, we discuss information regarding how to respond to a PSS episode, but this is only meant to be informative. Given the immediate response necessary and the complex nature of treatment, your veterinarian will determine the best course of action. However, it is worth mentioning that the most effective treatment to combat rapid onset PSS is the intravenous administration of dantrolene sodium.
Dantrolene sodium is a muscle relaxant that affects muscle cells without affecting cardiac or smooth muscle cells. Prompt administration of dantrolene sodium can alleviate PSS symptoms and has a good chance of saving an individual’s life. Dantrolene sodium administration is also used in a similar fashion in humans, horses, and dogs. It is important to note that injectable Dantrolene is very expensive and is therefore not something that most veterinarians will regularly stock. Oral formulations of Dantrolene are available and could be considered prior to any stressful procedures or anesthetic events in predisposed individuals (see testing below). Other treatments will be dictated by the symptoms presented and typically include lowering their body temperature and responding to metabolic changes.
If treatment is successful, talk to your veterinarian about testing the individual for disseminated intravascular coagulation, which can result if their body temperature exceeds 41 degrees Celsius (or 105.8 degrees Fahrenheit). They should also be monitored for signs of kidney failure secondary to muscle damage. Watch the individual very carefully for the next 48-72 hours as up to 25% of pigs get triggered again during this period. Be sure to talk with your veterinarian about preventative measures to prevent future triggers. If an individual has an anesthesia-induced PSS episode, you will need to carefully weigh the risk of future anesthesia. Whether or not this risk is acceptable will depend on many factors, including the reason for the procedure. If the individual has a poor prognosis without a particular procedure, you may decide that the potential benefit outweighs the risk of anesthesia (and can discuss pretreatment options to reduce the likelihood of an episode with your veterinarian – discussed below).
Can I Test My Pig Residents For PSS-Susceptibility?
Testing to determine if a resident is at risk of having a PSS episode is complicated. There is a DNA test available that can identify all three PSS genotypes – normal/stress-resistant (NN), stress-positive (nn), and stress carrier (Nn). However, if your pig resident tests positive, this isn’t a guarantee they are at risk of having a PSS episode. There are 31 different known causative mutations on the RYR1 gene, and it is currently unknown which are definitively problematic in regards to PSS. Your veterinarian will be the best resource when discussing whether to test residents or not.
There are other more invasive tests available, but these may not be necessary or even appropriate in a sanctuary setting.
Can A PSS-Susceptible Pig Be Anesthetized Safely?
If you have reason to be concerned that a resident is PSS-susceptible, or if you have tested them and they are found to be stress-positive, there are ways to reduce the risk of triggering a PSS episode. It’s recommended to avoid treating PSS-susceptible pig residents with inhalational anesthesia (including halothane and isoflurane) if at all possible. Sevoflurane has been especially known to trigger PSS in Poland China pigs. Desflurane has been especially known to trigger PSS in Large White, Pietrain, and Pietrain crosses. Talk to your veterinarian about the use of total IV anesthesia or local/regional anesthesia instead of inhalational anesthesia. Your veterinarian can also look into drugs that have been shown to be safe for PSS-susceptible pigs. You should discuss giving the individual pre-anesthetic medicine to minimize their stress and ask if they have dantrolene sodium available. The Merck Veterinary Manual suggests administering dantrolene sodium as a prophylactic aid before surgery for susceptible individuals. There may be certain medications they may be able to prescribe for you to administer at the sanctuary before the individual travels to the clinic for their procedure to further reduce their stress – for a list of suitable anesthetics, as well as pretreatment recommendations, check out the Merck Veterinary Manual here.
PSS is a challenging condition because, unlike many other conditions, learning about PSS does not necessarily make you better equipped to respond to it due to the complex treatments that are necessary. However, should you need to discuss PSS with your veterinarian, we hope that having a basic understanding of this syndrome can help make this conversation easier and give you tools to advocate for your residents’ needs.
SOURCES:
Overview of Malignant Hyperthermia | The Merck Vet Manual
Dantrolene Dose Response in MHS Pigs | NCBI
Porcine Stress Syndrome | Swine Medical Database
Use Of A DNA-Based Test For PSS | Journal Of The American Veterinary Medical Association
Skeletal Muscle Relaxants | The Merck Vet Manual
Porcine Stress Syndrome | University of Tennessee (Non-Compassionate Source)
Porcine Stress Syndrome | Pig Progress (Non-Compassionate Source)
Porcine Stress Syndrome | Peter James O’Brien (Non-Compassionate Source)
Porcine Stress Syndrome | Zuku Review Veterinary Test Prep Online (Non-Compassionate Source)
Porcine Stress Syndrome | Iowa State University College Of Veterinary Medicine (Non-Compassionate Source)
