It's estimated that around 40 percent of animal species are parasites. Forget elephants, hummingbirds, whales, and tortoises—pick a random animal, and it's far more likely to be a blood-sucker, disease-carrier, host-castrator, or flesh-devourer.
Parasitism is the most popular lifestyle on Earth. Roughly, half of plants and animal species are parasitic at some stage of their life cycle. Few species, if any, lack any parasites and most species have at least one host-specific parasite species. Even parasites have parasites.
The best recent estimates suggest that there are ≈6 million species.
Explanation. Most free-living organisms – including humans – host numerous species of symbionts, including viruses, bacteria, fungi, worms, and arthropods.
A world without parasites
Without parasites keeping them in check, populations of some animals would explode, just as invasive species do when they're transplanted away from natural predators. Other species would likely crash in the ensuing melée. Big, charismatic predators would lose out, too.
About half the world's population (over 3 billion people) are in infected with at least one of the three worms forming what Columbia University parasitologist Dickson Despommier calls the "unholy trinity"—large roundworm, hookworm and whipworm.
One study showed that patients infected with helminths experienced fewer symptoms of multiple sclerosis (MS) than those who were not infected. Other results suggest that certain parasitic species could reduce the symptoms of Irritable Bowel Syndrome (IBS), Crohn's disease, Type 1 diabetes and even arthritis.
Consider that parasites play an important role in regulating the populations of their hosts and the balance of the overall ecosystem. First, they kill off some organisms and make others vulnerable to predators.
Different studies have proposed different proportions ranging from 5% to more than 50%, but parasitic species probably outnumber free-living species [8] [9][10][11] .
Humans are the only known host, and about 209 million persons worldwide are infected. More than 30 percent of children worldwide are infected. Adult worms are quite small; the males measure 2 to 5 mm, and the females measure 8 to 13 mm.
Hundreds of fossilised animals seemingly covered in worm-like creatures are the oldest hard evidence of parasitism, dating from 512 million years ago when complex animals were still new.
Recently, scientists suggested that rich parasite faunae are inevitably needed for healthy ecosystem functioning and also that parasites and mutualists are the most endangered species on Earth.
Nonpathogenic intestinal protozoa are single-celled parasites commonly found in the intestinal tract but never associated with illness. They do not harm the body, even in people with weak immune systems.
The answer likely has to do with the wealth of available opportunities they had to branch out, habitat-wise. For a parasite, every animal on earth represents a potential place to live, and within each of those animated habitats exists a surplus of additional microhabitats to colonize.
Adult worms may live up to 17 years in the human body and can continue to make new microfilariae for much of this time.
Virtually all wild mammals carry some intestinal parasites and they generally manage a modest intestinal parasite load with no evident effects. Parasitic round-worms (helminths) expel ova in faeces, which within a few days (depending upon moisture) hatch into mobile larvae that shortly become infective to new hosts.
Arguably without earthworms in our soils, life could vanish pretty quickly. We would have less food, more pollution, and more flooding. No matter how cute a panda looks, it is Darwin's “lowly” earthworms that are doing dirty, but crucial, work in the soil below.
Parasites have lost the ability to live on their own. If they did not live in or on a host, they would lose on nutrition and become extinct. A totally harmless parasite would cause its own extinction as it will not be able to suddenly adapt and live on its own.
Recent research conducted in collaboration with the Smithsonian's National Museum of Natural History predicts that parasites will lose 5–10% of their biodiversity by 2070 due to habitat loss alone. And 30% of parasitic worms may go extinct following the expected loss of the host species these worms need to survive.
What are parasites? Parasites are any organism that live and feed off of another organism, which means that our bodies are the perfect place for them to live. They eat, lay eggs, secrete toxins, and thrive off of foods such as dairy products, sugar and protein.
Worms in your gut eventually pass through your digestive system and are excreted in your feces. Even if you don't have any symptoms, you may find signs of worms in your stool.
In some cases, parasites clear up on their own, especially in a person with a healthy immune system. If the parasite causes concerning symptoms or may cause complications, doctors will likely order an antiparasitic medication that kills the parasite. Some people choose natural methods to clear their body of a parasite.
Any immune response will eventually deploy mechanisms that are able to control, contain or kill an invader. Thus, a variety of killer cells, phagocytes, reactive oxygen species or antimicrobial peptides are the eventual means by which a parasite is killed by the host.
Parasites harboring viruses with unidentified effects on their host. In spite of recently renewed interest in parasites in connection with their viruses, there still remains a number of cases in which the effect of the virus of a given parasite on the human host remains ambiguous.