Overview of parasitism

At a glance

1. Parasitism is one form of symbiosis in its broad sense, a close life together: in mutualism both partners gain, in commensalism one gains and the other is neither helped nor harmed, and in parasitism one gains at the other's expense.
2. The parasites of people are protozoa, single cells that multiply inside us; helminths, worms that as adults do not multiply inside us; and ectoparasites, arthropods that live on the skin.
3. The definitive host carries the adult, sexual stage; the intermediate host carries larval or asexual stages that must develop there; a paratenic host carries a stage that waits without developing; a reservoir host keeps the parasite going in nature; in an accidental host the cycle usually ends.
4. A zoonosis is passed to people from other vertebrates; an anthroponosis has people as its only vertebrate host. A vector carries the parasite between hosts, as a biological vector if it develops there, or a mechanical one.
5. Parasites enter by mouth, through the skin, with a vector's bite, across the placenta or by sex, and cause disease by damage, by robbing the host of food and by the host's own immune response; helminths in the tissues raise the eosinophil count.

01Parasitism and other ways of living together

Organisms that live in close, long contact are said to live in symbiosis, in the word's broad sense; some books use it for mutualism alone. There are three kinds, by who gains:

• Mutualism: both gain, as we and many of the bacteria in our colon do.
• Commensalism: one gains, the other is neither helped nor harmed, as Entamoeba coli and E. dispar live harmlessly in the colon. Trichomonas tenax, long counted a harmless commensal of the mouth, is found more often in periodontitis, though whether it adds to the damage is unsettled.
• Parasitism: one, the parasite, lives on or in the other, its host, and gets its food from or at the expense of it. Most parasites of people do harm slowly, over months or years, since a host that dies quickly takes its parasites with it; some cycles, though, depend on the intermediate host being eaten.

The line between them is not fixed: a commensal can turn harmful when the host's defences fall, as many opportunistic infections show.

02Kinds of parasites

Parasites are also named by how they live:

• By where they live: ectoparasites on the surface, endoparasites inside; among endoparasites, intestinal, blood and tissue parasites, and intracellular ones such as Plasmodium, Leishmania and Toxoplasma.
• By how much they depend on the host: an obligate parasite cannot live without one; a facultative parasite usually lives freely but can become a parasite, as Naegleria and Acanthamoeba amoebae live in warm fresh water and soil. Strongyloides is in between: it can go through one free-living generation in the soil.
• By how long they stay: a permanent parasite lives its whole life on its host, as a louse does; a temporary one comes only to feed, as a bed bug or a mosquito does.
• An accidental or aberrant parasite is in the wrong host or the wrong place, such as the dog's hookworm larvae wandering in a person's skin.

Helminths differ from protozoa in one rule that matters for treatment: an adult worm does not multiply inside a person (Strongyloides and Hymenolepis nana, which can reinfect their own host, are the exceptions; and the larval cysts of Echinococcus do bud inside a person). So the number of worms reflects how many eggs or larvae were taken in, and worms do not build up unless the person is infected again.

03Hosts

Every parasite's life cycle is told by its hosts. The same person can be a different kind of host to different parasites, or even to the same one.

• Definitive host (primary host): the parasite reaches its adult stage and reproduces sexually in it. People are the definitive host of Taenia saginata, Ascaris and Schistosoma; the cat is the definitive host of Toxoplasma; and for Plasmodium, whose gametes fuse in the mosquito, Anopheles is the definitive host.
• Intermediate host (secondary host): it carries the larval or asexual stages, which must develop there before they can go on. Some cycles have two, as the snail and the fish for Opisthorchis. People are the intermediate host of Plasmodium, in whose blood its gametocytes form.

A cycle with one host is direct (Ascaris, Enterobius); one that needs an intermediate host is indirect (Taenia, Schistosoma, Plasmodium).

• Paratenic host (transport host): it carries a stage that waits there without developing, and passes it on when it is eaten. It is not needed for development, but bridges a gap in the food chain.
• Reservoir host: an animal that carries a parasite which also infects people, often with few signs of illness, and keeps it going in nature, as rodents do for many parasites; dogs keep Leishmania infantum going around the Mediterranean, many of them without signs though others fall ill. Controlling the disease in people means dealing with the reservoir too.
• Accidental host (incidental or dead-end host): one the parasite does not normally use, in which its cycle cannot go on. A person with a hydatid cyst is an accidental intermediate host of Echinococcus, since no dog will eat the cyst; a person with cysticercosis is one for Taenia solium.

The figure beside the text shows Echinococcus granulosus: the dog is its usual definitive host, the sheep its usual intermediate host, and the patient, who swallows eggs from a dog, an accidental intermediate host in whom the cycle ends.

Toxoplasma shows the rest: the cat as definitive host, where the sexual stages form; mice, sheep, pigs and people as intermediate hosts; and the fetus, infected across the placenta when its mother is first infected during pregnancy.

04Zoonoses and vectors

• A zoonosis passes to people from other vertebrates: toxoplasmosis, hydatid disease, trichinellosis, fascioliasis, leishmaniasis in the Mediterranean, toxocariasis. About 60% of human infectious diseases, and up to three quarters of newly emerging ones, are zoonoses.
• An anthroponosis has people as its only vertebrate host, though it may pass between them through a vector: pinworm, Trichomonas vaginalis, falciparum malaria (carried by Anopheles), lice and scabies. Malaria from P. knowlesi in South-East Asia, caught from macaques, is a zoonosis.
• A vector is a living carrier, usually an arthropod, that takes a pathogen from one host to another. In a biological vector the parasite develops or multiplies, as Plasmodium does in Anopheles; a mechanical vector only transports it, as the housefly carries cysts and worm eggs on its body.

05How parasites get in

06How parasites harm, and how the host answers

• Mechanical damage: a mass of worms blocks the bowel or the bile duct (Ascaris), a cyst presses on the brain or the liver (Echinococcus, cysticercosis), migrating larvae tear through tissue.
• Destroying cells: Plasmodium bursts red cells, Entamoeba histolytica lyses the bowel wall.
• Robbing the host: hookworms and Trichuris drain blood and cause iron-deficiency anaemia; Diphyllobothrium takes vitamin B12; heavy worm loads stunt children's growth and learning.
• Toxic and allergic effects: the products of parasites, and the host's response to them, cause fever, urticaria and, if a hydatid cyst bursts, anaphylaxis.
• Immune damage: much of the harm is the host's own, as granulomas round schistosome eggs scar the liver and the bladder.

The immune response depends on the parasite. Against intracellular protozoa it is mainly cellular, led by T helper 1 cells and macrophages. Against helminths, too big to engulf, it is a T helper 2 response: IgE, mast cells and eosinophils, which is why eosinophilia points to helminths in the tissues, highest while larvae migrate. Parasites escape in turn: African trypanosomes change their surface coat (antigenic variation), as Plasmodium falciparum does on the red cells it infects; Plasmodium, Leishmania and Toxoplasma hide inside cells, and schistosomes coat themselves in the host's own molecules.

07Diagnosis and prevention, in outline

• Finding the parasite: eggs, cysts or larvae in the stool, adult worms passed, eggs in the urine (Schistosoma haematobium), parasites in a blood film (malaria, trypanosomes, microfilariae), or in a biopsy or aspirate (Leishmania). Pinworm eggs are picked up from the skin round the anus with sticky tape.
• Finding its antigens: rapid tests for malaria, and stool tests for Giardia, Cryptosporidium and E. histolytica.
• Finding the response: antibodies (toxoplasmosis, hydatid disease, schistosomiasis), and eosinophilia.
• Finding its genes: PCR, increasingly used for protozoa.
• Prevention breaks the cycle at its weakest link: safe water, latrines and hand washing against faecal-oral spread; cooking meat and fish, and meat inspection; shoes in soil where hookworms live; nets, repellents and vector control; treating dogs, keeping offal from them; and treating whole communities where worms are common.

Check yourselfAnswer, then say how sure you were: a lucky guess still counts as a weak point.

1In which host does a parasite reach its adult stage and reproduce sexually?

The intermediate hostThe definitive hostThe paratenic hostThe reservoir host

2A shepherd develops a hydatid cyst of Echinococcus granulosus in his liver. What kind of host is he?

Definitive hostAccidental intermediate host, a dead end for the parasiteParatenic hostReservoir host

3Which describes a paratenic host?

A host in which the parasite reproduces sexuallyA host that must be passed through for the parasite to developA host that carries a larval stage which waits there without developing, and passes it on when eatenAn insect that injects the parasite with its bite

4Which of these is a zoonosis?

Enterobius vermicularis infectionTrichomonas vaginalis infectionToxoplasmosisHead lice

5A traveller returns from Africa with a raised eosinophil count. Which kind of parasite does this point to first?

Protozoa living inside red cells, such as PlasmodiumHelminths migrating through the tissuesBacteriaViruses