A parasite is an organism that lives in another organism, called the host, and often harms it. It depends on its host for survival.
Without a host, a parasite cannot live, grow and multiply. For this reason, it rarely kills the host, but it can spread diseases, and some of these can be fatal.
Parasites, unlike predators, are usually much smaller than their host and they reproduce at a faster rate.
Fast facts on parasites
- Parasites live on or in other organisms and thrive to the detriment of their host.
- Many different parasites can affect humans, and they can pass on diseases such as malaria and trichomoniasis.
- Ensuring that food is fully cooked, using insect repellant, and following good hand hygiene rules can reduce the risk of getting parasites.
A parasite is an organism that lives within or on a host. The host is another organism.
The parasite uses the host’s resources to fuel its life cycle. It uses the host’s resources to maintain itself.
Parasites vary widely. Around
Parasites are not a disease, but they can spread diseases. Different parasites have different effects.
These live inside the host. They include heartworm, tapeworm, and flatworms. An intercellular parasite lives in the spaces within the host’s body, within the host’s cells. They include bacteria and viruses.
Endoparasites rely on a third organism, known as the vector, or carrier. The vector transmits the endoparasite to the host. The mosquito is a vector for many parasites, including the protozoan known as Plasmodium, which causes malaria.
These feed on other parasites in a relationship known as hyperparasitism. A flea lives on a dog, but the flea may have a protozoan in its digestive tract. The protozoan is the hyperparasite.
There are three main types of parasites.
Protozoa: Examples include the single-celled organism known as Plasmodium. A protozoa can only multiply, or divide, within the host.
Helminths: These are worm parasites. Schistosomiasis is caused by a helminth. Other examples include roundworm, pinworm, trichina spiralis, tapeworm, and fluke.
Ectoparasites: These live on, rather than in their hosts. They include lice and fleas.
Symptoms that might occur include:
- skin bumps or rashes
- weight loss, increased appetite, or both
- abdominal pain, diarrhea, and vomiting
- sleeping problems
- aches and pains
- weakness and general feeling unwell
However, parasites can pass on a wide variety of conditions, so symptoms are hard to predict.
Often there are no symptoms, or symptoms appear long after infection, but the parasite can still be transmitted to another person, who may develop symptoms.
Many types of parasites can affect humans. Here are some examples of parasites and the diseases they can cause.
This tiny ameba can affect the eye, the skin, and the brain. It exists all over the world in water and soil. Individuals can become infected if they clean contact lenses with tap water.
This disease that comes from parasites that are spread by ticks. It affects the red blood cells. The
This is passed on by Balatidium coli, a single-cell parasite that usually infects pigs but can, in rare cases, cause intestinal infection in humans. It can be spread through direct contact with pigs or by drinking contaminated water, usually in tropical regions.
This affects the intestines. The blastocystis enters humans through the fecal-oral route. A person can get it by eating food or drink contaminated with human or animal feces where the parasite is present.
This affects the intestines. Coccidia is passed on through the fecal-oral route. It is found around the world. It can also affect dogs and cats, but these are different kinds. Dogs, cats, and humans cannot normally infect each other.
Isosporiasis or cystosporiasis
This is a disease that is
Primary amoebic meningoencephalitis (PAM)
Different types of plasmodium affect the red blood cells. It exists in tropical regions and is transmitted by the Anopheles mosquito.
This is caused by Rhinosporidium seeberi. It mainly affects the mucous of the nose, conjunctiva, and urethra. It is more common in India and Sri Lanka but can occur elsewhere. Polyps result in nasal masses that need to be removed through surgery. Bathing in common ponds can expose the nasal mucous to the parasite.
This is a parasitic pneumonia
A person with a healthy immune system will not usually have symptoms, but it can pose a risk during pregnancy and for those with a weakened immune system.
Also known as “trich” this is a sexually transmitted infection (STI) caused by the parasite Trichomonas vaginalis. It affects the female urogenital tract. It can exist in males, but usually without symptoms.
Trypanomiasis (Sleeping sickness)
This is passed on when the tetse fly
Worms, or helminth organisms, can affect humans and animals.
Anisakiasis: This is caused by worms that can invade the intestines or the stomach wall. The
Roundworm: Ascariasis, or a roundworm infection, does not usually cause symptoms, but the worm may be visible in feces. It enters the body through consuming contaminated food or drink.
Raccoon roundworm: Baylisascaris is passed on through raccoon stools. It can affect the brain, lungs, liver, and intestines. It occurs in North America. People are advised not to keep raccoons as pets for this reason.
Clonorchiasis: Also known as Chinese liver fluke disease,
Dioctophyme renalis infection: The giant kidney worm
Diphyllobothriasis tapeworm: This
Guinea worm: This affects subcutaneous tissues and muscle and causes blisters and ulcers. The worm may be visible in the blister. As the worms are shed or removed, they enter the soil or water, and are passed on from there.
Echinococcosis tapeworm: Cystic echinococcosis
Enterobiasis pinworm: A pinworm, or threadworm, Enterobius vermicularis can
Fasciolosis liver fluke: This
Fasciolopsiasis intestinal fluke: This affects the intestines. It can also transmitted when consuming contaminated water plants or water.
Loa loa filariasis: Also known as loaisis,
Mansonellosis: This is
River blindness: Caused by a worm known as Onchocerca volvulus,
Lung fluke: Also known as paragonimiasis,
Schistosomiasis, bilharzia, or snail fever: There are different types of schistosomiasis. They
Sparganosis: Humans can become infected if they eat foods tainted with dog or cat feces that contains the larvae of a tapeworm of the Spirometra family. It can lead to a migrating abscess under the skin. It is rare.
Strongyloidiasis: This can
Beef and pork tapeworms: Taeniasis is
Toxocariasis: A roundworm
Trichinosis: This is caused by the roundworm of the Trichinella family. Infection can lead to intestinal symptoms, fever, and muscle aches. It is passed on by eating undercooked meat.
Elephantiasis lymphatic filariasis: This is
Ringworm is sometimes mistaken for a worm, but it is not a worm. It is a fungal infection.
These are parasites that live on the outside of the body, such as fleas.
Bedbug: These can affect the skin and vision. They are found all over the world. Sharing clothing and bedding can spread infection. They may be present in newly rented accommodation and hotel rooms.
Body lice: These are common worldwide. Infection can spread through sexual activity, skin-to-skin contact, and sharing bedding or clothing.
Crab lice: These affect the pubic area and eyelashes. They are common all over the world and spread through sexual activity, skin-to-skin contact, and sharing bedding or clothing.
Demodex: These affect the eyebrow and eyelashes. They are common all over the world and can spread through prolonged skin contact.
Scabies: This affects the skin. It is common all over the world and can spread through sexual activity, skin-to-skin contact, and sharing bedding or clothing.
Screwworm: This is transmitted by a fly, and it affects skin and wounds. It is found in Central America and North Africa.
Head lice: These live on the scalp and affect the hair follicles. They are common all over the world and spread through head-to-head contact. A reaction to their saliva causes itching.
Parasites come in many shapes and sizes and can lead to a wide variety of symptoms and health issues. Some parasites are treatable and others are not.
To increase your chance of avoiding parasites:
- find out which kind are prevalent in your area or in locations you may travel
- take precautions, for example, using insect repellant in places where mosquitoes are common
- be careful to eat only well-cooked fish and meat
- when traveling, drink only water from bottles with a sealed top
- take care when bathing in fresh-water lakes or rivers
If you have any symptoms, see a doctor.
According to the Centers for Disease Control and Prevention (CDC), the following
- Chagas disease
- trichomoniasis, or trich
The CDC is working to increase awareness of these diseases and to improve diagnostic testing.