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Trichinella is a master of innovation in biology: it is a multicellular animal, but it can live in a single cell.
The nematode hatches from the egg in the host's intestine and then burrows through the intestinal wall and travels through the circulatory system.
It follows the bloodstream into the capillaries, where it leaves the bloodstream and burrows into the muscles. It crawls along the long fibers of the muscle, and then burrows into one of the long spindle-shaped cells that make up the muscle.
In the 40s of the 19th century, when scientists first discovered the trichinella cyst parasitic in the muscle, they thought that the muscle tissue had degenerated and the parasite was sleeping in it, waiting to enter its final host.
At first, the invaded muscle cells do appear to atrophy. The proteins that make up the cytoskeleton and give it stiffness gradually disappear. The muscle's own DNA loses its ability to make new proteins, and within a few days of Trichinella entering the cell, the muscle goes from thin and powerful to smooth out of order.
But Trichinella destroys the cell just to be able to rebuild it. Trichinella does not disable the genes of its host cells; in fact, genes begin to replicate themselves until they grow to a factor of 4. But the growing genes now follow the orders of Trichinella to make proteins that turn cells into cozy nests for Trichinella.
Scientists once thought that only viruses had the ability to control genes, using their host's DNA to replicate themselves. They later realized that Trichinella is a viral animal.
Trichinella turns muscle cells into parasitic placenta. It makes muscle cells loose and soft, making room on the cell surface for new receptors to ingest food.
Trichinella also forces the cell's DNA to make collagen, forming a tough capsule around the cell. It causes cells to release signaling molecules called vascular endothelial growth factor. The normal function of this molecule is to send signals to blood vessels to grow new branches, help wounds heal or deliver nutrients to growing tissue.
Trichinella uses this signal to achieve its own goal: to use the collagen capsule as a mold and weave a network of capillaries around it.
Blood carries nutrients through these vessels to muscle cells, allowing the parasite to grow and expand inside the cells; Trichinella sways back and forth inside its cells, exploring its little world, and muscle cells bulge and moan.
Source: Carl Zimmer, Planet of Parasites, Chapter 2