This section is intended to be an exact copy of what the survivor Helena Walker, the author of the dossiers, has written. There may be some discrepancies between this text and the in-game creature.
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Species
Haementeria laetus
Time
Holocene
Diet
Sanguinivore
Temperament
Aggressive
Wild
It is difficult to be angry with Haementeria laetus, despite its rather terrible nature. The creature has practically no intelligence, and just attaches to any nearby living flesh in an attempt to drain that creature's blood.
Removing Haementeria requires precision bladework, or access to an open flame. Both of these can be nearly as dangerous to the host as to the leech itself.
Once Haementeria attaches to a host, it drains blood at a rapid pace, and the host experiences hunger, loss of health, and lowered stamina. Some Haementeria are also carriers for a dangerous disease I've dubbed "Swamp Fever," which persists beyond the leech's own lifespan. Few creatures on the Island are transmitters for this disease, and it can ultimately be cured with a rare medicine.
Domesticated
While Haementeria is not intelligent enough to be tamed, it is always useful to keep a few around for antidotes and fishing lures. When processed properly, Haementeria venom can be made into a powerful antivenom!
Leeches lurk in the water, waiting for tames or survivors to pass by whereupon they leap gleefully and attach themselves to their victim.
Appearance
Normal Leech
Diseased Leech
One of the most revolting creatures that inhabits the island is Haementeria laetus, a leech that grows as long as a man's arm. A blood parasite, the soft-bodied creature actively seeks out hosts in the swamps it calls home, propelling itself with fluttering fins and latching on with a mouth full of sharp rasping teeth.
In the wild, leeches are the primary vector for a debilitating disease commonly known as swamp fever.
Color Scheme and Regions
The Leech always spawns with the same color scheme, but the Diseased Leech has different colors from the Leech.
The Leech always spawns with the same color scheme and has no color regions.
This means it is currently impossible to make alterations to the Leech's colors.
Variants
Diseased Leech
Diseased Leech is a variant of the Leech distinguished by its red and orange color scheme as well as its name in the HUD. It transmits Swamp Fever, which can be cured with Lesser Antidote. The carcass of the Diseased Leech can also be harvested for significantly more Leech Blood than its normal species.
After the Leech bites it attaches to the target and start sucking their victim's Health over the time.
Can be used every 0.67s by both a rider and the AI.
Combat
This section describes how to fight against the Leech.
General
To get rid of a leech that is already hanging on your body you need fire, e.g. a Campfire or a Cooking Pot. Jump into the burning fire for a short time, the leech will then disappear. Dying and removing the leech off your corpse also works. Keep in mind, however, that sometimes the leech you see on your screen actually lives on your mount. Go back to your mount and remove it manually.
If you see a leech approaching and wish to kill it, a few long ranged attacks should do the trick. Even simple stone arrows will stop it dead in its tracks. Be aware that it can still jump towards you from a distance larger than you may expect, so keeping a slight distance is advised. Keeping it at a distance with a spear or similar weapons could work as well, but a longer ranged weapon is more highly advised, as you are not required to get as close and risk the leech attatching on to you.
To remove a leech from a creature, press and hold the Use key (E by default) and select remove leech from the menu that appears.
There is a diseased variant of this creature (Diseased Leech). You can cure the disease by consuming Lesser Antidote.
Strategy
Bug Repellant will make you invisible to all leeches even if you touch them. If you attack them, however, they will still be aggressive.
Travel well-mounted; you need something tall enough to keep you out of the water and large enough that you stay standing on its back when dismounted. This way you can remove leeches from your mount without having to expose yourself to them.
If your mount needs an escort in the swamps, use a flock of flyers; ground-bound escorts are likely to be leeched without you noticing, and if you do notice, you will have to step entirely off your mount to remove them, potentially exposing yourself to other leeches.
Keep your eyes peeled; use your mount's attacks to 'plow the road' in front of you as you navigate the swamps in the hopes of killing leeches before they latch on.
If you see a leech from far enough away, attack it with ranged weapons to kill it before it has a chance to latch on; once they do, it will not be possible to harvest them even if you remove them.
At present, all mounts are immune to Swamp Fever; only humans and Dodos can catch it.
Leeches don't attack or attach themselves to tamed Beelzebufos and any other mounts native to the swamp, making them great for hunting Leeches.
Weaponry
The leech does not require any serious weapons to kill, although killing it before it reaches melee range is valuable; try to finish it with ranged weapons or, failing that, hold it at a distance with spear or pike.
Dangers
Make sure to bring a friend with you, or stay out of its attack range with a tall mount. If the leeches latch onto you, another player is the fastest way to remove it; if it's attached to your mount, you can remove it yourself.
When they attach onto you, you wont regenerate stamina, and you will lose health slowly. Make sure to use that remaining stamina to get to a safe place fast.
Leeches are often found in areas that contain other dangerous enemies. Beware they don't weaken you at a fatal moment.
You can use campfire to remove the leech by standing on the fire.
Weakness
As it is harmless unless entering melee range, ranged weapons are ideal.
Taming
Leeches can be 'tamed' using Fish Basket. However, as they lack the required intelligence, they will not return once they have latched onto someone or something.
Utility
Roles
Pet: Leeches can add life to your base with a pond or an aquarium
War: Due to the ability to carry diseases one of these can affect a dodo then the dodo could be dropped into an enemy base to infect the base with disease assisting with a raid.
For information pertaining specifically to the real-world Leech, see the relevant Wikipedia article.
Dino Dossier was revealed on 02/08/16 on Steam.[1]
On The Center, Leeches are more commonly found in deep water that the player can submerge him or herself in.
When going into certain waters while raining a swarm of Leeches will come after you. Which will allow you to obtain large amounts of Meat, Chitin, Leech blood and Silica Pearls.
Despite Lesser Antidote being the go-to cure for Swamp Fever, there are a few other ways to cure it. Consuming Nameless Venom will cure you, but it will also take 50 health away. Transferring from any server to Scorched Earth will also cure you.
A maximum of 6 leeches can latch onto you and your dinos. The rest will bite you and your dinos to death.
Harvesting leeches with the Megatherium will get you hundreds of chitin.
This creature was originally proposed by /u/Coloumbia in 2015 on Reddit.com/r/playark
Tamed leeches do not have an inventory or any way for the player to feed them.
The Leech's genus, Haementeria, refers to a modern genus of leeches which includes the Amazon Giant Leech, which can grow up to 17 inches.
Real Leeches lay eggs, which is not the case for ARK's Leeches
When real Leeches give birth they put themselves and their eggs into cocoon.
Real leeches have multiple tiny eyespots to help them see.
Unlike most creatures, real leeches don't have gills or lungs, instead, they breathe by absorbing oxygen through their skin. They can breathe on land and in water.
See also: 
Yes
Fish Basket.
Diseased Leech is a variant of the Leech distinguished by its red and orange color scheme as well as its name in the HUD. It transmits 
Swamp Fever, which can be cured with 
Lesser Antidote. The carcass of the Diseased Leech can also be harvested for significantly more 
Leech Blood than its normal species.
Health
Stamina
Oxygen
Food
Weight
Melee Damage
Movement Speed
Torpidity
Campfire or a 
Cooking Pot. Jump into the burning fire for a short time, the leech will then disappear. Dying and removing the leech off your corpse also works. Keep in mind, however, that sometimes the leech you see on your screen actually lives on your mount. Go back to your mount and remove it manually.
Bug Repellant will make you invisible to all leeches even if you touch them. If you attack them, however, they will still be aggressive.
Swamp Fever, there are a few other ways to cure it. Consuming 
Nameless Venom will cure you, but it will also take 50 health away. Transferring from any server to Scorched Earth will also cure you.
Megatherium will get you hundreds of chitin.
Feces
Coelacanth, 
Piranha, 
Sabertooth Salmon, and 
Trilobite, all became tamable with the
ARK: Survival Evolved Changes
ARK: Survival Ascended Changes
