Trichinella spiralis

Today I'm here to talk about one of the most interesting and insidious little parasites out there.  It's the smallest nematode that invades humans and is one of the world's most clinically important and widespread parasites. Imagine, for a moment, a parasite that not only invades your body to feed off of your yummy insides, but one that also manages to force your own body into being a slave to its decadence. This is the modus operandi of Trichinella spiralis.  But before we get into that, let's check out its taxonomic specs.


Taxonomy

Larval form coiled in a spiral.

T. spiralis is a nematode (a.k.a. "roundworm") and thus belongs in phylum nematoda under kingdom anamalia. It belongs to class enoplea, which is largely comprised of non-parasitic species.  However, two orders withing this class contain parasites: Dioctophymatida and Trichurida. Take a guess which one this little guy belongs to! That's right, order trichurida (you're smart as a whip!...though we aren't talking about whipworms...oh wait! We are talking about whipworms!) This order is home to many worms of veterinary importance as well as to human whipworms like Trichuris trichuria. (*Side Note* Anytime that you see the prefix Trich- it means something in relation to hair. It comes from a Greek word which means "hair". Whipworms took on this prefix because they appeared hair-like morphologically to early biologists.) Like human whip worms, T. spiralis belongs in the family Trichinellidae.

Life Cycle
This parasite has an interesting life cycle. It begins with an uninfected animal eating an infected animal.  This can happen amongst domestic animals such as pigs or amongst wild animals living in various environments. Epidemiologist W. C. Campbell constructed four life cycles based on the involved hosts of this parasite. The first was the domestic cycle, which involved pigs and is the most important cycle that involved accidental human infection. The other three may be due to other species of Trichinella and may also involve accidental human infection, but is far less common than the first cycle. These three cycles are known as sylvatic cycles and involve different animals for different climatic regions. To keep things simple, we will only talk about the domestic cycle today.

Cross section of muscle tissue containing nurse cells.

For the domestic cycle to complete itself, a human has to eat some infected pork that has not been cooked well enough to kill off the parasite. Once inside the human body, the parasite larvae are released from their cysts that were formed in the pig's muscle tissue. The larvae mature into adults in the small intestines and search for mates. When a male and a female find one another, they mate and produce offspring that are deposited in the mucosa. The larvae migrate out of the mucosa and follow bloodstreams until they find a nice, quite piece of skeletal muscle to call home.  The larvae encyst in the muscle tissue and form what are called "nurse cells".  These "nurse cells" actually manipulated the surrounding tissues into bringing it nutrients rather than sounding the alarms and bringing in a brigade of immune cells to fight off the invading parasite.  Inside its cozy little nurse cell, the parasite lives awaiting the day that the human will die and be eaten by another suitable host in order to complete its own life cycle.  Little does it know that humans are more often dead-end hosts for these little guys.

Nurse cells that formed in a pig's diaphragm.

The nurse cells begin by instigating new blood vessel formation. The environment inside muscles is hypoxic, meaning that it lacks an adequate amount of oxygen. This environment stimulates other muscle cells to start secreting angiogenic cytokines, which form the new blood vessels that surround the single muscle cell into which the larva will penetrate. The cell continues to pump out the cytokines at the parasite's demand and the cell maintains a constant state of hypoxia. Some research has shown that these cytokines may also lead to an increase in collagen production, further protecting the cell.

Within pigs, this parasite is transmitted by pigs eating infected meat scraps from other animals or from their common practice of cannibalism. (Trust me, I grew up on a pig farm...YES Wilbur will eat his babies if their mother accidentally lays on them, suffocating one while nursing three. They are stupid, dirty creatures, and they deserve to be eaten. Plus, bacon is delicious!)

Trichinosis
 After becoming infected, a person may start having symptoms in as little as 12 hours or as much as 2 days. As the worms move through the body, they damage parts of the intestine and cause immune responses that result in inflammation.  Such responses can manifest as nausea, vomiting, sweating, and diarrhea. 5-7 days later, some people experience fevers or facial swelling (a.k.a. "facial edema"). After 10 days, people will experience intense muscular pain, difficulty breathing, low blood pressure, and possible nervous disorders.  The disease can cause severe damage to the heart, respiratory issues, or kidney malfunction that can eventually lead to death.

An artist's depiction of a
larva in a nurse cell.

In pigs, the symptoms are often undetectable unless the parasite load is so much that it can cause fatality (uncommon).

Diagnosis, Prevention, and Treatment

For a muscle biopsy, you have to make
an incision in the skin to reveal the
underlying muscle, then a hollow
needle is used to extract a small
amount of muscle tissue for
laboratory analysis.

For humans, Trichinosis is often misdiagnosed as flu due to the similarity of symptoms. To confirm a suspected case of trichonosis, a  doctor may order a muscle biopsy (which is invasive and rather painful from what I hear) or a seriological test.

Pigs are diagnosed following ELISA testing.

To prevent the transmission of this disease, the US has a national surveillance system that tracks reported cases and inspects sources of possible contamination. The pork industry has also made changes in order to reduce pig exposure to this disease and to recognize warning signs of infection in order to treat pigs before being slaughtered for their meat. Laws have also reduced risk of human infection by regulating pork processing procedures, such as providing guidelines for specific cooking and freezing temperatures and times as well as for curing procedures.

I like this picture of the medicine because
it looks like a worm was drawn on one
side of this pill! :p

The best way for you to ensure your own safety is to cook pork using hot enough temperatures for long enough to kill off any potential parasites. Or if you are really paranoid, you could freeze the meat for awhile first, and then cook the hell out of it. (No one likes a rare pork chop anyway, right?!

To treat trichinosis, humans and pigs are both prescribed antihelminthic drugs such as mebendazole or albendazole. In humans, this is not always affective. Humans also receive corticosteroids and painkillers to cope with the pain of the infection as it is being treated.



Moral of the Story

Mmmm...Teriyaki pork loin!
Is it cooked all the way through?...

The moral of the story today is to follow good food safety guidelines when preparing pork. (Or bear, or rats if you are into that sort of thing.) It is less important in this country than it would be in countries with less regulation in the pork industry, but it's still always a good idea to make sure you cook your pork all the way through. Trichinella spiralis is only one of many parasites that you can contract from eating undercooked pork. Just do yourself a favor and make sure there's not any pink left by the time you are ready to chow down on some teriyaki pork loin or mojito lime pork chops. (Don't judge, both of those are delicious!)

Because I can't NOT post a picture of a parasite colored in rainbow...