Six reasons why you should give parasites a break

The Internet appears to think of parasites as bad, disgusting, or even evil.

Sure, some may make your head itch, leave gaping holes in your skin, or inadvertently transmit flesh-eating diseases, but at the end of the day they are only out to make a living; they just choose a different way to go about the daily grind.

Many parasites are beautiful.

You may think that parasites are nature’s couch potatoes, sitting around sucking on life juices, causing horrific diseases and giving nothing back. But life can be hard for parasites, and they may be helping us more than you think. Below are six reasons parasites should be treated with compassion and respect.

1. Their home is constantly trying to evict them.

The threat of being evicted from home is terrifying, but for parasites, it’s a way of life. Parasites have to constantly avoid being discovered and eliminated by their hosts. Hosts have an armory of cruel defences against parasites, including roasting them alive with a fever, pulling them out with beaks or claws, and poisoning them with toxic plants.

Host defenses are hard to get around: parasites have had to develop resistance to various poisons, and find creative ways of sticking to the slippery insides of intestines, bladders, or genitalia. Parasites have evolved stealthy adaptations that make ninjas look like novices: for example, vampire bats and leeches use anesthetic saliva for some stealthy eating while their hosts remain oblivious. Feather-chewing lice go more Mission: Impossible style, with long, thin bodies that can squeeze between feather barbs on birds.

How would you like to be a feather louse, and contend with this beak several times a day?

Even with these adaptations, life for parasites is far from easy; hosts continue to evolve better defenses, and many parasites cannot survive for long in the outside world. Yet they continue to struggle on, making their living wherever they can. In fact, if someone calls you a parasite, consider that they may mean you cling to your purpose in life despite overwhelming odds.

2. They sometimes get lost and die.

Making a wrong turn can be frustrating for us non-parasitic organisms, but getting lost is deadly for parasites. Parasites are often so well-adapted to particular hosts that if they end up in the wrong species, they die without completing their life cycles.

Humans can become accidental hosts for several parasites, such as Echinococcus (dog tapeworm). Normally, Echinococcus peacefully infects and passes through carnivores and herbivores in a merry dance, completing its lifecycle and having lots of happy little tapeworm children. But if it infects humans, it ends up in all sorts of organs including livers, lungs, and eyes. Since humans aren’t normally eaten by carnivores, the worms will eventually die without completing their lifecycle, either as a result of treating the human for the (deadly and debilitating) infection, or when the human dies.

This is why you should not eat dog poo.

Another example is Angiostrongylus (rat lungworm), which normally infects rats and molluscs. However, if it ends up in a bird or human, it can’t even develop into an adult and dies at a tender young age.

3. They have their own parasites to deal with.

As if the parasitic life isn’t hard enough already, they also have to deal with hyperparasites. Hyperparasites are parasites that parasitise other parasites. Hyperparasites include wasps, flies, beetles, and mites.

A 2.2 lb mallard duck may find a few tiny feather lice annoying, but imagine being a tiny louse and carrying several mites sucking on your sides! An equivalent burden to the average human would be several 20 cm long round bean bags…only attached to your side and sucking your blood. That’s what chewing lice, such as Trinoton querquedulae, have to put up with.

Another hapless parasite, the mother crypt gall wasp Bassettia pallida, keeps her babies safe in little “crypts”. If she has been parasitised by the crypt-keeper wasp Euderus set, however, the gall wasp mother plugs the exithole with her head and dies. She was only trying to be a good mother, but for that she suffers a tragic end as the crypt keeper emerges from her head. There is no honor among thieves or parasites.

Parasite-ception: this bat fly sucks blood from its host, but it is infected with a hyperparasitic fungus. [source]

4. Most are not actually out to kill anyone.

Though there are plenty of parasite horror stories, most parasites cause mere injury or discomfort to their hosts. Just as a good farmer won’t kill his hen if she lays lots of eggs, a parasite needs its host alive in order to continue feeding and breeding. The exceptions are the parasitoids, a group of rogue parasites which exploit their hosts to the point of death.

The “true” parasites, on the other hand, are happy to peacefully coexist with their hosts, often causing no discernible symptoms at all. At most, they may cause a little itching, blood loss, severe pain or moderate illness; only in exceptional circumstances (such as an allergic reaction) can a host die from an infestation of true parasites.

Now, take a moment to acknowledge your eyelash mites. [Image credit: Alan R. Walker]

5. They are useful in science and medicine.

The ability of parasites to infiltrate and suppress immune systems has attracted the attention of the medical research industry. Parasites are used to investigate the immune system, such as how primates have evolved different immune systems between individuals. From a scientific perspective, parasites are not only interesting in themselves, but also open windows into the evolution and ecology of their hosts. For example, we can work out roughly when a particular animal lineage split off from another by studying the corresponding split in that animal’s parasite lineage (with major caveats).

This image [source] shows that pocket gopher family trees closely resemble the family trees of their lice.

In addition to their usefulness in medical research, parasites can be applied directly to patients as medicine. Leeches, once popular blood-letters in mediaeval medicine, have made a comeback due to the proteins in their saliva. Leech proteins, which include anti-coagulating, anti-inflammatory, and vaso-dilating components, are not easily harvested, so the best way to use them is direct application.
Doctors and geneticists are not the only people who find parasites useful and fascinating: invasion biologists and agricultural scientists have found that many parasitoids target pest species, such as the Argentine stem weevil (Listronotus bonariensis), an agricultural pest, which is parasitised by a small wasp (Microctonus hyperodae). The wasp is used throughout New Zealand to control weevil numbers. There are many such examples of parasites and parasitoids being used in pest control, and whole research institutes are dedicated to testing the parasites thoroughly before they are unleashed.

In addition to giving scientists plenty of fodder for research, parasites such as mites and lice have such fun names that they give scientists an opportunity to have pun show-downs. The louse community (or “circle of lice”, as one scientist put it) are particularly creative with lousy puns.

6. They may be helping their hosts in mysterious ways.

The definition of a parasite is an organism that benefits while its host is harmed. If only nature were so simple as to divide organisms into beneficial, mutual, and harmful! In reality, host-parasite relationships are far more complex than that. Parasites can be more helpful to their hosts than they appear, and some hosts may even depend on parasitism. For example, some parasites regulate and suppress the immune systems of their hosts, which may reduce allergic and inflammatory responses such as asthma and hayfever.

Maybe this guy needs more parasites to distract his overactive immune system.

The immune system of animals (including humans) is tied up with parasites, as infections promote the development of immunological memory. In other words, the nasties we are exposed to as children influence the characteristics of our individual adult immune systems. To some extent, parasite presence during childhood reduces our risk of allergic reactions as adults, although more research is needed as the relationship between parasitic infections and allergic responses is still unclear.

Most parasites go unnoticed by their hosts (it’s easier to graze on somebody’s feathers or dead tissues if they don’t know you’re there). Some take so little from you that you don’t notice it’s gone; others may live off your waste or other material you don’t need (for example, your eyelash mites eat dead skin). Such benign critters may out-compete more harmful parasites, or stop new ones from taking hold. Since you have developed resistance to your current parasites, you may prefer to hold onto them than risk being attacked by new parasites. Stick with the devil you know!

Humans aren’t the only animals to benefit from a parasite or two: researchers have also found that parasitic worms in fish accumulate heavymetals at a much faster rate than their hosts, leading the scientists to suggest that parasites protect their hosts from heavy metal poisoning. Future research is likely to reveal other subtle benefits that our unsung parasitic heroes give their hosts.

While you may react with disgust at the idea of one organism parasitising another, parasites are not the layabouts that society seems to think they are.  Take a moment to consider that parasites can be useful, beneficial organisms that lead difficult lives under harsh conditions.

Friends forever! [source]