The textbook definition of a parasite is an organism that spends at least part of its life cycle in or on another organism, harming it in a way that benefits the parasite. The organism the parasite feeds on is called the host. Most parasites are smaller than the host organism - usually much smaller, as with fleas and ticks - and reproduce more quickly.
While there are many kinds of parasites, all fit into one of two fundamental types of parasitism: endoparasites, which exist inside a host's body; and ectoparasites, which exist outside a host's body. There are many kinds of parasites within each of those categories as well.
Endoparasites in Humans
There is a huge number of human endoparasites. We homo sapiens are a very successful species and therefore popular to chew on. Here follow seven examples of human endoparasites and their effects:
- Fasciola hepatica (Liver fluke): This little sucker does exactly what its name suggests; it latches onto your liver. Actually, it's more a fan of sheep and cattle, but humans will do in a pinch. This parasite helps itself to tissue and bile. Thankfully, it's fairly easily treated.
- Naegleria fowleri (Meningoencephalitis): These amoebas are bad news. There are 47 species of amoeba in the Naegleria genus, but only this one feels the need to start trouble. Most commonly contracted in unsterilized water, N. fowleri causes severe inflammation of the brain and its surrounding membrane and has a fatality rate of over 95%.
- Plasmodium (Malaria): You may have heard that malaria is caused by mosquitoes. That isn't technically true. It's caused by five species of Plasmodium microorganisms, which mosquitoes pick up when feeding on infected blood and subsequently spread around to other victims.
- Schistosoma (Parasitic flatworm): This usually microscopic flatworm is about as bad as pathogens get. Schistosomiasis, contracted from freshwater infected with the worms, causes abdominal pain and swelling, bloody urine and feces, and over time potentially liver and kidney failure. It's considered the second most destructive disease worldwide, just after malaria.
- Taenia solium (Pork tapeworm): This unpleasant critter generally makes its way into the human digestive tract through undercooked pork. T. solium is the most common species of tapeworm in humans, and can live in the human digestive tract for years. Fun fact: tapeworms can be more than three meters long.
- Trichinella (Trichinosis): The parasitic roundworm Trichinella infests the intestines and causes the logically named trichinosis, which leads to cramps, difficulty digesting and other unpleasant things. Thankfully, improved meat handling has made trichinosis exceedingly rare in the developed world. The United States had all of 20 confirmed cases in the 2000s.
Trypanosoma (Sleeping sickness) - This order of protozoa contains multiple human parasites. T. brucei causes sleeping sickness, an infection of the brain that makes people sleep all the time. T. cruzei parasitizes assassin bugs, blood-feeding insects not unlike mosquitoes, and causes the potentially fatal chronic illness Chagas disease.
Ectoparasites in Humans
As noted above, humans are basically a buffet for parasites. That absolutely extends to our outsides. Here are five examples of human ectoparasites:
- Cimex (Bedbugs): Thirty years ago, it seemed bedbugs (Cimex lectularius and Cimex hemipterus) were a solved problem. But life finds a way. In the past few decades, bedbugs have made a spectacular comeback. These near-invisible insects feed on human blood through a sharp proboscis they cheerfully sink into any exposed skin.
- Demodex (Eyelash mites): You've probably got these. Happily, you should go your whole life without noticing. These little mites are way less harmful than most of the other parasites listed. They usually cause no symptoms at all. At worst, people with compromised immune systems or sensitive skin might get a rash when the mites bite.
- Pediculus humanis (Head and body lice): Pediculus is actually two almost identical species of louse that lives on the human body. P. humanis capitis is the human head louse (the bane of school administrators, schoolkids, and parents the world over). P. humanus humanus is its thankfully rarer cousin, the body louse. Both cause infuriating itching and can spread nasty diseases like typhus.
- Pulex irritans (Fleas): Meet the human flea. It's happy to chew on just about anything with warm blood, but humans hold a special place in its horrible little heart. On their own, they're just itchy, but they become a whole new problem when there's a disease going around. P. irritans is infamous as the human vector for the Black Death, the dreaded bubonic plague that hopped happily from rats to humans and back again. The "Black Death" spread through Europe and Asia starting in 1334 AD, but there was an equally destructive epidemic centuries earlier, beginning in 541 AD in the Byzantine Empire (modern Turkey), and another starting in 1880 in China.
- Sarcoptes scabiei (Scabies) - This nasty little arachnid causes scabies, an intensely itchy and unpleasant skin disease. The S. scabiei mite travels through skin-to-skin contact, and it only takes 10 or 15 of them to start a full-scale infection.
Ectoparasites in Humans
Parasitism is a winning evolutionary strategy. As long as you've got the biology for it, it solves the issue of food and shelter in one fell swoop. Unsurprisingly, animals are as plagued with parasites as humans.
- Acari (Mites and ticks): Mites might turn up on human eyelashes, but both they and ticks are much more common on other animals. Most warm-blooded organisms have a species or two of parasitic acari that view them as fine dining. In fact, one of the ways scientists distinguish between similar species is by looking at the mites, ticks and other parasites that feed on them.
- Hirudinea (Leeches): Obviously leeches can parasitize humans as well, but they rely more on other animals to complete their life cycle. Freshwater fish, invertebrates like slugs and snails, and vertebrates from mice to bears are all popular dining choices for leeches.
- Ichneumonoidea (Spider hawk wasps): These parasitic stinging insects come by their name honestly. Ordinarily solitary predators, when a female hawk wasp is ready to lay eggs, it parasitizes a spider, stinging and paralyzing it with venom, then laying eggs in its body. Baby wasps then hatch and eat the still-living spider alive. Scary stuff.
- Siphonaptera (Fleas): As noted above, humans have their very own flea species, but there are all sorts of others. Fleas are among the most widespread insect species, and it's all thanks to their habit of chewing on other organisms.
- Tabanidae (Horseflies): As their name suggests, these big, slow cousins of the housefly are fond of feeding on domesticated animals. Like mosquitoes, only female horseflies bite, because a blood meal is necessary for them to nourish their eggs. Also like mosquitoes, they have a nasty habit of spreading disease, including other parasites like filarial worms and trypanosoma microbes. (The latter cause sleeping sickness, you might remember.) Horseflies mostly spread disease among animals, hence the name, but humans are sometimes affected.
Parasites in the Ocean
The ocean is the biggest ecosystem in the world. Parasites flourish there, taking advantage of the limitless diversity of animals and plants to be found beneath the waves.
Cyamidae (Whale lice): It's not actually a louse, lice are insects, and this is an isopod - a crustacean and oceangoing cousin of the pill bug. It snags onto whales for food and transportation. They feed primarily on algae that collect on the whale's skin, but also attack open wounds and damaged tissue. They are so common among right whales that scientists use the patterns of their infestations to tell one whale from another.
Cymothoa exigua (Tongue-eating lice): Also not technically a louse. We wish we could say it wasn't actually tongue-eating, but no such luck. This isopod, a type of marine crustacean, actually eats and replaces the tongues of fish and other warm-blooded organisms, feeding on the blood that should be flowing to the tongue.
Glochidium (Mussels): Glochidium doesn't refer to a specific species of mussels. Rather, it refers to a stage in the mussel lifecycle. Before mussels settle down and build shells, they have to find an appropriate place. To that end, they evolve barbed hooks and hijack rides on nearby fish, dropping off when they find a likely spot. By themselves, glochidia are harmless, which makes them commensals rather than parasites. However, mussels often grow in enormous groups, and infestation by multiple glochidia at once can injure or even kill fish. That's some proper parasitism.
Pinnotheres pisum (Pea crabs): The pea crab is exactly what it sounds like: a crab the size of a pea. They live inside molluscs, including mussels, oysters, sea urchins, and sand dollars, stealing food and occasionally taking bites of their hosts.
Sacculina (Barnacles): Most barnacles aren't parasitic; they encrust wood, stone, and other solid surfaces and filter the water for food. Not so with Sacculina. This particular breed of barnacle attacks crabs, replacing their genitals and causing them to treat Sacculina larvae as they would their own eggs, helping Sacculina to spread and thrive.
Parasites in Plants
Plants are perfect hosts for parasites. Just to start with, they don't move around much. They're rich in valuable nutrients, and if the parasite is small enough, plants make great shelter too. Hence, it is not surprising that there are all sorts of plant parasites.
Aphidoidea (Aphids): Aphids feed on plant sap, usually too much plant sap for the wellbeing of the plant they infest. Aphids are tiny and soft-bodied, making them the favored prey of predators like ladybugs, but they have protection too. Ants sometimes "farm" aphids for their sugary secretions, protecting them while they feed.
Cynipidae (Gall wasp): These small, non-stinging wasps cause plants to grow galls. Galls are basically plant tumors, big balls of useless tissue. Useless to the plants, that is. They're great for the wasps, which lay their eggs inside, providing food and shelter for their developing young.
Santalales (Mistletoe): This festive holiday plant has some unpleasant feeding habits. Mistletoe attaches to a host plant with a special root called a haustorium, which it uses to drain the host of water and nutrients. Fun fact: mistletoe is not only parasitic, it's also toxic! Mistletoe really wants you to leave it alone.
Unique But Unpleasant Organisms
Parasites may seem unpleasant, but they're a fundamental part of nature. Ecosystems would collapse without the presence of parasites, as populations would grow out of control and predator-prey relationships would change beyond sustainability. We won't tell you to stop swatting mosquitoes, but at the very least, we hope you come away with a deeper understanding of these unique, if unpleasant, organisms.
Want to know more? Check out Examples of Symbiosis for healthier (but just as interesting) relationships between organisms. Happy learning!
Barnacle coated crab at the beach.