Dear Brianna,

A parasite is an organism that steals resources from another organism in order to survive. Our planet is home to all kinds of parasites and organisms that host them.

My friends Kevin Zobrist and Lisa Shipley, scientists at Washington State University, told me about a few holiday-inspired parasites. After all, ‘tis the season.

The first parasite is a type of plant that people often smooch under around the holidays: mistletoe. There are a lot of species of mistletoe, explains Zobrist, a forester at WSU.

An example in the Pacific Northwest is hemlock dwarf mistletoe, which explosively releases sticky seeds during the summer. The seeds can fly up to 50 feet and stick to tree branches they fall on. When the seeds land on trees like western hemlock (the state tree of Washington), the mistletoe starts to grow.

Some kinds of mistletoe have leaves they can use to take in sunlight and help make food. But they still aren’t able to get enough food on their own. They have to feed off trees. Dwarf mistletoes don’t have any leaves. They get everything they need from their host.

In the process, this little mistletoe parasite causes trees to form weird clumps called “witch’s brooms” that can ultimately end up killing them. While the trees might die and become snags of dead wood, this can actually be a good thing for the forest ecosystems.

Zobrist explained that some animals, including some endangered species, will use witch’s broom branches or the insides of dead trees to make their habitat or nest. Even though the parasite takes life from the tree, it’s not all bad for life in the forest.

While some parasites live off plants, other parasites need animals. Lisa Shipley, a WSU professor who works with animals in the deer family, said some reindeers are host to a parasite that is so small we’d need a microscope to see it. It’s a kind of nematode more commonly called a brain worm.

Before the nematode finds the reindeer host, it lives in a different animal. When it’s young, it will go into the slimy bottom part of a snail, called its foot.

As snails slide along leaves of plants, reindeer that are munching on leaves will sometimes eat a snail, too. When they eat the snail, they eat the young nematode. The young nematodes move through the body and are eventually pooped out. But along the way, they can lay eggs and cause damage to the reindeer’s brain.

“The worm can be treated with parasite medications, so if you have your own reindeer—like some people in the North Pole do—you can give them medicine,” Shipley said.

Mistletoe and nematodes are just two of many parasites. Other parasites like ticks or fleas rely on hosts like us cats to get their food. Parasites can be inconvenient and even deadly, but to them, it’s all about survival.

Dr. Universe