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Our Six Favorite Parasites Discovered in 2015

The only end-of-year list you need to read.

Parasites, organisms that live in or on others, receive their nutrients from and often at the expense of their host. Though many are profoundly treacherous to humans, some might be able to protect us from disease. Ranging from microscopic in size to many meters long, parasites are estimated to make up around 40 percent of known species—and many were discovered in 2015.

From parasites that could be complicit in mass amphibian die-offs to parasites hundreds of millions of years old, 2015 uncovered some truly spectacular specimens. That many of them don't harm humans is just bully for us. Here, we've narrowed it down to the best discoveries of 2015.


Honorable Mention: Perkinsea

Our honorable mention is a new parasite on the scene, described in a study published in August of this year.

Related to Perkinsus, a parasite known for causing mass die-offs in shellfish, this newcomer is infecting tadpoles and its reach is wide. Scientists have found it in tadpoles from three continents, six countries, and in various climates. Setting up camp in the tadpoles' livers, the parasite reproduces its numbers into the thousands, causing the livers to swell and take on a yellowish color. Affecting many types of frogs, this pest could be a contributor to the shrinking amphibian population. As of 2008, 42 percent of amphibian species were in decline and 32 percent were considered threatened or extinct.

Fifth Place: Adikeshavus

Long hair-like structures extend from the wing of these newly discovered wasps. Image: Kamalanathan et al./Deutsche Entomologische Zeitschrift

India is the proud new owner of a whopping five different types of parasitic wasps.

Discovered this year, these bugs have long hair-like structures on their wings, a feature researchers plan to name them after. The wasps of the newly proposed group adikeshavus, meaning "first one to have long hairs" in Sanskrit, all infect spider eggs exclusively and their small size (0.7-2mm in length) and little wings allow them to slip between the silk strands that protect the spider egg sacs where the wasps lay their eggs. The spider eggs then act as host, and sacrifice, to the growing wasp larvae.

Fourth Place: Myxozoans

Though these microscopic parasites have been studied since the 1880s, it wasn't until this year that researchers really figured out what they were—super weird, tiny jellyfish.


Known as myxozoans (Greek for "mucus animals"), these parasites are a bane to commercial fish industries, attacking fish brains and causing the fish to swim in circles – a syndrome called whirling disease. The parasites, only a few cells large with no mouth or gut, have structures similar to jellyfish stingers, which ultimately led to their classification as close relatives of jellyfish. Compared to their jellyfish cousins, myxozoan bodies are tiny and simple, as are their genomes, which are 20-40 times smaller than the average jellyfish's. Researchers believe the parasites shed their genome once they degenerated their bodies sometime during their evolutionary history.

Third Place: Invavita piratica

The fossilized parasite attached to its host. Image: Siveter et al./Current Biology

Finding very old parasites is difficult. Finding them with their host is near impossible. But researchers in Britain did just that. Embedded in a 425 million-year-old fossil, scientists found an ancient organism belonging to the tongue worm group of parasites. Named Invavita piratica, meaning "ancient intruder" and "piracy," the parasite was found still attached to its host, an invertebrate marine animal. The two were preserved by a nearby volcanic eruption that coated them with protective ash. The parasite has a worm-like body and four limbs and is the first reported fossilized adult tongue worm. Modern day tongue worms are known to infect vertebrates not invertebrates meaning at some point in the last 425 million years the parasite moved from a water-based environment to a land one, changing its preferred host as it moved.


Second Place: Tongue worms

Microscopic image of the tongue worm found in wild dogs across Australia. Image: Sara Baker

Speaking of tongue worms, earlier this year researchers in Australia found one that was nearly six inches long inside the nose of a wild dog.

Until now, Australia had had only 10 reports of tongue worms infecting animals in the past 200 years, but scientists are finding them in the noses of wild dogs all over the country. The tongue worms live off of nasal secretions and hold on to the inner nostril with hooks. Able to live for up to 15 months, the tongue worms reproduce inside of the dogs and release their eggs, which are then eaten by other animals. The eggs hatch inside the other animal and larvae are thought to settle in the lymphnodes. For the dogs to become infected they must eat an animal with a lymphal infestation though it is unclear what animal this might be. Whatever it is, it's a regular staple of the dogs' diets because the parasites have been found in 70% of the wild dogs tested. There are reports of human infections, too. Those infected describe being able to feel the tongue worms crawling up the throat into the nasal cavity.

First Place: Trematodes

And the number one parasite of 2015 is another new arrival. Presented in a study published this August, this parasite was observed hacking into shrimp brains off of the coast of Portsmouth, England. Reproducing in birds and spread through their feces, the parasite has developed a remarkable way of getting back to its host. Researchers found that the parasite was able to infect shrimp in the ocean and steer them into the light where they were much more likely to become bird food. Scientists tested this in the lab and found that uninfected shrimp preferred to stay in the dark while those infected with the parasite tended to swim into the light. The altered behavior appears to be due to a release of serotonin in the shrimp though researchers aren't sure if the serotonin comes from the parasite or if the parasite is stimulating serotonin already in the shrimp. Either way, these parasites join a kick-ass problem solving group with likes of toxoplasma, lancet flukes, and nematodes.

So there you have it, the best parasites of 2015 from the gross to the brilliant. Here's to you and your New Year's resolutions. May you pursue them with the tenacity of a parasite.