Neatorama has a nice article about six sickening parasites.
A female sacculina begins life as a tiny free-floating slug in the sea, drifting around until she encounters a crab. When that fateful day arrives, she finds a chink in the crab's armor (usually an elbow or leg joint) and thrusts a kind of hollow dagger into its body. After that, she (how to put this?) "injects" herself into the crab, sluicing through the dagger and leaving behind a husk. Once inside, the jellylike sacculina starts to take over. She grows "roots" that extend to every part of the crab's body — wrapping around its eyestalks and deep into its legs and arms. The female feeds and grows until eventually she pops out of the top of the crab, and from this knobby protrusion, she will steer the Good Ship Unlucky Crab for the rest of their co-mingled life. Link (Thanks, Frank!)
Reader comment: Michael says:
It was nice to see the screw worm mentioned in the article "Six horrifying parisites". My father works for APHIS helps fight the battle against these pests down in Panama. Most of us in the U.S. haven't heard of them although they were once a quite a problem in the Southwestern United States. The U.S. government has since eradicated them here and down through most of Central America. To get rid of them they irradiate the male flies with very low level radiation which makes them sterile then release them into the wild to mate. The females can only mate once so when they hook up with a sterile male… no more flesh eating maggots. All this without pesticides too.
I took a tour of the plant in Tuxtula Gutierrez, Mexico and it was quite a gruesome experience. It is a quarantine facility, so we had to shower and change clothes to enter. It is five-football fields under roof, divided in to large rooms where the flies were raised in different stages of their lives.
There isn't much to see in the egg room, just flies laying eggs. In the pupae room the action is starting to happen. There was tray after tray (tray= 18"x36"x6") of black stuff. On closer inspection it is all moving, thousands of little screw worms only a few millimeters long. My father's coworker turns to me and says "This is when they are most dangerous, say you get one on your finger then scratch your eye." He left it up to me to imagine what would happen. (I do know that one screw worm isn't going to kill you, but a bunch of them left untreated will.)
In the larvae room the smell almost knocks you down. I almost didn't think I could handle it. The smell comes from their feed. It is a mixture of animal blood and other nutrients. The larvae are about an inch long now and are kept in trays stacked eight high on rolling carts. There were many, many carts. When it is feeding time there is a guy standing in a vat of blood on wheels and using a bucket, he sloughs the blood into the trays. When the maggots are full, they crawl out of the trays and into gutters on the floor. Then they are swept up, literally, and go on to the next room to grow into adult flies. Males get to be irradiated, females get to make more flies to be irradiated, and so on.
"Six horrifing parasites" gives you an idea of what kind of damage these things can do but to give you a real world example… My father once saw a brahma bull that was badly infected. From across the field he could see a large red patch across the bull's shoulders. When they got closer he could see the wound was covered in screw worms. To get rid of them, they had to sedate the bull and douse the screw worms with alcohol. Nasty huh?
