Take a look at some invasive parasites! From zombie fish to flesh eating parasites to tongue eating lice and holes in fish heads, these parasites are really dangerous!
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4. Hexamita (hole in head)
Just missing out on being in the top three is a parasite known as ‘hexamita’ which mainly causes the disease ‘Head and Lateral Line Erosion’ or simply ‘Hole in the head’.
As hexamita starts its life inside the fish, it will reside in the intestinal tract and move throughout the host invading vital organs including the kidneys. In order to spread its spores, it needs to escape through lesions made in the head of the fish - which is then known as ‘Hole in the head’.
Starting out with small indents in the head; the parasite causes initially mild conditions - including discolouration and those lesions I mentioned earlier. However, when left untreated, these small holes in the head grow larger and deeper - attracting bacteria and fungi to invade the exposed lesions and holes in the fish’s head that parasite spores escape from. It is these that cause death in the fish as the creature will eventually lose its appetite and succumb to the infection.
3. Roundworms (nematodes)
Hitting in at number three is a special contender - it’s the ‘roundworm’, or more commonly known as a nematode. What makes it so special? Well, it’s simply because of how diverse they are! They can affect everything from fish, to insects, to mammals, and that includes humans like you and I. We call these types of organism ‘zoonotic’, as they can thrive in different species - similar to the fish tapeworms which came in at number 5!
There are thousands of different species of roundworms, but they all work in a similar sort of way. They can cause anaemia which is a loss of red blood cells, they can hemorrhage fish and cause them to lose their balance when swimming. They also tend to fray the fish’s gills meaning that the host can’t take in the required oxygen to survive - often choking it to death or causing it to become slow and confused. They often hook into important body parts and sap energy and nutrients from them, including the liver and digestional tract.
Even though these creatures are ‘zoonotic’, they might not always show the same symptoms from host to host. In humans, symptoms are usually less fatal and can cause us to have rashes, diarrhea, stomach pains and general malaise (that’s a feeling of unwellness). They are also known to cause us to lose our appetite and to lose weight. In 2012, a man from Brazil caught a zoonotic nematode after eating raw sashimi - two weeks later he was diagnosed with a parasite which caused his back to blister into red lesions.

2. Rhizocephala
Rhizocephala are very similar to the shark infesting Anelasama; however, these barnacles serve as parasites to crabs. Female crabs have a ‘brood pouch’, where she keeps her eggs; this is mainly how the rhizocephala attacks its host.
After being infected through a joint, the crab will have the parasite racing through its feeding tubes until they reach the brooding pouch. The rhizocephala then completely destroys the eggs inside the pouch, destroys the pouch itself and replaces it with itself; the crab is now infertile and is manipulated by the parasite to believe that it still has an egg pouch - meaning the female crab goes out of her way to protect the parasite.
Well, that’s lucky for male crabs! Don’t be so sure… this parasite also attacks the brain of male crabs making them produce feminine hormones - causing the crab to believe it has an egg sack itself; and infecting it the same way… yikes! I wonder what can be worse than a parasite that tricks you into caring for it?... wait to you see what we have next...
1.Cymothoa exigua (tongue eating louse)
Taking top place is the tongue eating louse, a completely destructive and terrifying parasite that has a very particular way of working.
After entering the host through its gills, the louse will find the tongue of the fish. While attaching itself, it attempts to sever all the blood vessels in the tongue. The lack of blood coming to the tongue then causes necrosis (this is the scientific for dying tissue). The necrosis of the tongue kills all of the tissue making it fall off! With just a stub left, the louse attaches itself to the base of what was the fish’s tongue and replaces itself forever.
Once the fish dies, the louse will detach itself from the tongue and then just reattach itself to the outside of the fish, gaining more nutrients there. What makes this worse is that this process takes a while, so many people have found fish with no tongues in their shopping! While the fish has the parasite attached, it can still use it as a real tongue with no real detriment to its health.
A spooky parasite, this is the only known one in the animal kingdom to become a fully functioning organ when attached!

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