Not only anal swabs, but also bring "mental pain" to the Japanese

The river rushes into the sea and merges with the salt water in the sea, blending into "semi-brackish water" with a salinity between seawater and fresh water. Animals living in semi-brackish water have long been able to maintain their body balance under the osmotic pressure that changes with the tide. Such waters are full of risks and opportunities. Migratory fish such as salmon and eels gather here in groups to adapt to the changes in water salinity during migration.

Salmon gathering | Totti / Wikimedia Commons

Today's story starts with an egg of the Japanese Sea tapeworm. The egg hatches in the water, and a tiny ciliated larva is born, starting its short journey of free life. The tiny baby worm has great life pressure. Although it has just been born, it needs to make good use of this, the only chance in its life to live freely, to roam in the water and find a place to live and a meal ticket for itself.

A tumultuous life history

Its first meal is copepods living in brackish water, in short, water fleas. Water fleas are the first intermediate host of the worm, and it needs to complete a crucial development in the water flea, evolving from ciliated larvae to procercariae. But it cannot waste its life in water fleas, which is far from the ultimate ideal of passing on the family line.

Various species of Daphnia | Adrei Savitsky / Wikimedia Commons

As the water fleas were eaten by small fish, and the small fish were eaten by ferocious big fish, the worm successfully achieved a leap in the food chain. Its form became larger and stronger, and it developed into the ultimate form of the larval period - the sparganum. The sparganum is about one centimeter long and hides in the muscles of the host (such as salmon). Relying on its retractable head segment, it can even stroll around in the muscle tissue of the salmon. Let's assume that this big fish is unlucky. After successfully fattening, it was hijacked by a passing fishing boat and sent to the vegetable market. This sparganum is luckier. It evaded quarantine and successfully mixed into the sashimi. It was not even discovered by the diners. It went into the stomach in a daze and changed to a bigger house and meal ticket. This meal ticket is the final host, which means that the worm's turbulent life can be put to a semicolon, and it can rest assured to grow up and reproduce.

The body of an adult tapeworm is divided into three parts: the head, the neck, and the segments. The thin part of the body is not the tail, but the head. The segments closer to the neck are immature and small, while the segments further back are mature and wider. The segments are like copy and paste, and can be said to be exactly the same. Each mature segment has a set of reproductive systems for both males and females, which can merge their lives on their own and breed offspring in the segments. When the eggs of the offspring are almost fully grown, the gravid segment also reaches the tail end of the tapeworm, and is discharged with the feces of the final host. After reaching the water, a new life cycle begins.

Proglottid segments of a tapeworm | Wikimedia Commons

Since the worm's final host in this story is a smart human, its life may be relatively short. Shortly after eating the sashimi, the diner noticed mild abdominal pain and diarrhea. He went to the doctor and received an anthelmintic. The adult worm was born with the movement of the sphincter. This is a large white worm several meters long, like an overly long wide noodle, with a thin neck and a thick body, and the body is flat as if it was hit. Combining morphology and molecular identification, people determined that this is a Japanese Sea tapeworm, which looks like the traditional Japanese fabric Sanada button, so it is also known as the "Sanada worm".

8-meter-long parasite

The Meguro Parasitology Museum is located in Tokyo. It is a private museum specializing in parasitology. It was founded in 1953 by Dr. Kametani Ryo (1909-2002). The most precious treasure of the museum is a Japanese Sea tapeworm. Its life may be very similar to the story above. A 40-year-old Japanese man found the body of a tapeworm in his feces after eating raw fish for three months. After receiving anthelmintic treatment, a Japanese Sea tapeworm of 8.8 meters was removed. The museum also thoughtfully prepared a white rope of the same length next to the tapeworm specimen, allowing people to intuitively feel the incredible length of the tapeworm.

The treasure of the Parasite Museum, an 8-meter-long tapeworm | Stan Chow / Wikimedia Commons

March to July every year is the salmon fishing season in neighboring Japan. Large numbers of pink salmon (Oncorhynchus gorbuscha), king salmon (Oncorhynchus keta) and trout (Oncorhynchus masou) are forced ashore. The tapeworms living in the fish bodies move into the human body, bringing about the peak season for tapeworm infection.

Grilled Sakura Trout | Geographer / Wikimedia Commons

Fortunately, compared with the pathogens of tapeworm disease (Taenia solium, Taenia saginata, and Taenia asiatica), and the Anisakis nematode with which it is often compared, the Japanese Sea tapeworm is gentle. Among the 136 patients infected with the Japanese Sea tapeworm reported in Japan between 2012 and 2015, 71.1% of the patients were asymptomatic or had mild diarrhea, 26.9% of the patients had abdominal pain or discomfort, and one patient each had constipation, nausea, and weight loss. Compared with the physical pain, the patients suffered more "mental pain when expelling the worm."

Since most of the symptoms are mild, how did they find out they were infected? A few people found it during a colon exam at the hospital, but most people found parts of the tapeworm in their stool... So, it's good to look back after taking a shit.

We all know that eating raw river fish is prone to various parasites, but in fact, eating raw marine fish also has the risk of parasite infection. Infections are generally divided into two categories, one is Anisakis spp. and the other is Diphyllobothrium spp. The best way to avoid infection is, of course, to cook fish, and low temperatures can also kill this type of worm parasite. It is safe to freeze it at -35°C for 15 hours or at -20°C for 7 days.

However, in Japan, where the Sea of ​​Japan tapeworm is prevalent, cooked fish or frozen fish for a long time seems extremely cruel given the traditional delicacy of raw fish. Therefore, the Meguro Parasite Museum recommends freezing fish at -8°C for 12 hours or -10°C for 6 hours to kill the tapeworms without destroying the taste of the fish.

Parasites are life forms that are very different from humans. When we face such creatures, even if we don't know that they have been dead for decades, we always feel intuitive and strong disgust. This disgust is the human body's way of self-protection in evolution, reminding itself to stay away from these creatures that are not beneficial or harmful to the body.

Today, the Meguro Parasitology Museum is a tourist attraction in Tokyo, famous for its curiosity. Its main research direction is parasite morphology and taxonomy. Scholars who are willing to conduct in-depth research in this direction must not hate parasites. Ryo Kametani, the founder of the Meguro Parasitology Museum, described parasites as "cute and strange" in his popular science book. If we change our perspective and simply regard parasites as a kind of incredible life, we may be able to experience fun other than curiosity when watching the parasite exhibition.