The longest parasite in the world

Many tapeworm species reach impressive sizes, but T. calyptocephalus’s 40-meter length makes it the longest parasite on the planet, according to IFL Science. The most common tapeworms affecting humans are usually found in the tissues of pigs, cows, and salmon. In humans, the longest tapeworms cannot match the size of T. calyptocephalus, but some species still possess impressive lengths.

A prominent example is the Meguro Parasitology Museum in Tokyo, which displays specimens of fish tapeworms (Dibothriocephalus nihonkaiensis) is 8.8 m long. This is a rare specimen of a giant tapeworm taken intact from the human intestine. However, they can grow much larger. According to a 2009 article, fish tapeworms can be 2 – 15 m long. The maximum size ever recorded for this species is 25 m.

Researchers say tapeworms have been parasitic in the intestines for at least 99 million years from the unique amber fossil described in March 2024. It was the first time humans found a part of their body in fossil form. Previously, all that was collected were fluke eggs from Permian shark feces.

“Fossil data for tapeworms is extremely scarce because they have soft tissues and a living environment in the body,” said Bo Wang, leader of the research team from the Nanjing Institute of Geology and Paleontology. “That limits our understanding of their early evolution.”

Tapeworms are extremely successful parasites, infecting everything from birds to bears. However, in terms of length, it is difficult for any species to surpass tapeworms. They do not possess great size in the first place. T. calyptocephalus fluke heads firmly attach to the whale’s intestines, then they gradually lengthen thanks to new segments that appear. The largest individual has up to 45,000 segments with full testicles and ovaries.