The parasitic way of life is one of the most common, if not the most common, way of life on Earth. It is likely that more than half of all species of organisms are parasites, many of them of very great economic and medical importance. Indeed, some of the most devastating diseases of man, such as malaria, are caused by parasites, and the economic loss caused by parasites of plants and livestock reaches billions of US$ every year. – Here we give a brief overview of the kinds of parasites, their adaptations, effects on hosts and economic/medical importance.
Different authors use different definitions for parasitism, depending on their practical or research interests. Thus, a medical parasitologist will stress that a parasite causes certain diseases, and will exclude all those organisms from the definition which have no apparent effect on the host, a zoologist might be more interested in the physiological and morphological adaptations of a parasite to its host. We define parasitism in a very wide sense, i.e., as a close association between two organisms, in which a parasite depends on a host that provides some benefit (usually food) to it; the parasite does not always damage the host.
The domain of parasitology
Many groups of organisms that have a parasitic way of life, such as fungi, bacteria and viruses, are usually included in the domain of microbiology, whereas parasitology is restricted to protozoan and metazoan parasites. (In France, fungi are often studied by parasitologists).
Associations related to parasitism
Some kinds of associations resemble parasitism in various aspects and cannot always be unambiguously distinguished from it, either because we know little about particular species, or because intermediate forms exist. Such associations include predation, commensalism, phoresis, mutualism, and symbiosis (sensu strictu).
In predation, one, the predator (often larger than its prey) attacks, usually kills and eats another, the prey. – In commensalism, an organism uses food found in the internal or external environment of the host, and there is no close relationship with the host. For example, certain barnacles are ectocommensals (= commensals on the surface of a host) on whales. – In phoresis, one organism uses another for transport and/or protection. Barnacles can again serve as an example: some species live attached to the surface of whales, by which they are carried around finding new sources of pelagic (plankton) food.- A mutualistic association is one in which both organisms benefit. The Australian Mistletoe Bird Dicaeum hirundinaceum feedson the seeds of mistletoes, the mistletoe depends on the bird for dispersal.- Symbiosis (sensu stricto) is an extreme form of mutualism, in which the association is compulsory, i.e. both partners (symbionts) benefit and cannot live without each other. Very ancient examples of symbiosis are organelles (specialized cell components) of all protistan (unicellular) and metazoan (multicellular) animals and plants, which are thought to have arisen by the “coming together” of originally free-living organisms. However, the term symbiosis (sensu lato) is also used in a wider sense, including parasitism, commensalism, phoresis and mutualism.
Kinds of parasites
Lice, isopods, monogenean flukes, many crustaceans, among many others, are ectoparasites that live on the surface of hosts. Nematodes (roundworms such as pinworms), tapeworms (cestodes such as fish tapeworms), flukes (trematodes such as liver flukes) and malaria parasites of vertebrates are examples of endoparasites found in the tissues of their hosts. Tapeworms and flukes are obligatory parasites which cannot survive without a host at least for part of their life cycle, whereas maggots usually feeding on decaying organic matter are facultative parasites which infect living hosts only occasionally. A permanent parasite, such as most parasitic worms (helminths, such as flukes or tapeworms), is an organism that is parasitic on or in a host over long time spans, whereas a temporary parasite, such as leeches, is parasitic only intermittently. Some species, e.g. gnathiid idopods, live a parasitic way of life only as larvae, they are so-called larval parasites (Figure 1, larvae live on fish sucking blood, adults live on the seafloor). However, most parasites are adult parasites, i.e., they are associated with a host during at least part of their mature phase. Mosquitoes are periodic parasites which visit a host from time to time. When individuals of the same species parasitize individuals of the same species, they are referred to as intraspecific parasites. Examples are males of some deepsea fish that live on females of the same species absorbing food from them. Hyperparasites (of the 1st, 2nd, etc. degrees) are parasites of other parasites. For example, some protistans infect helminths (worms) in the intestine or tissues of fishes. Latent parasites are parasites which do not have any obvious effects on the host. Kleptoparasites are animals which force others to regurgitate their food and then swallow it, such as frigate birds chasing other birds in flight. Cowbirds and about 50 species of cuckoos are brood parasites, i.e., they lay their eggs into the nests of other birds where they are incubated by these birds. Microparasites include viruses, bacteria, protistans and some small worms (helminths), which reproduce in/on the host inducing immune responses in vertebrate hosts. Macroparasites, large parasites, include most helminths and arthropods; they do not reproduce on or in the host, inducing no or weak immune responses depending on infection intensities; infections often last long and usually are not fatal. Many species of hymenopterans are parasitoids which lay their eggs into insect hosts where larval development occurs; hosts may survive for some time before they are killed by the growing parasitoid. – Among parasitic plants, we distinguish holo- and hemiparasites. The former are entirely dependent on their host, since they lack chlorophyll and cannot synthesize their own food, the latter are only partly dependent on their host, since they can use their own chlorophyll to synthesize some of the necessary organic matter (Figure 2).
Figure 1. A third stage female larva of Gnathia sp., a larval isopod parasite of marine fishes. Note the large thorax which fills with blood after a blood meal of the larva. © Professor Alexandra Grutter, University of Queensland.
Figure 2. A hemiparasitic mistletoe on a gumtree (eucalypt) in NSW, Australia. Note the bulbous connection of the parasite to the host tree (arrow), consisting of the haustorium of the mistletoe and host tissue. The crosses indicate the outline of the mistletoe. In Australia, various bird species, particularly the Mistletoe Bird (Dicaeum hirundinaceum) and some honeyeaters are responsible for dispersal of the parasites’ fruits. The association between birds and mistletoe can be considered a mutualistic one.
Adaptations to parasitism
Special adaptations, body size, sacculinization, dispersal
Each parasite species must have special adaptations that guarantee infection of a host and survival in it. For example, a malaria parasite of a bird cannot survive in humans, the human pinworm (roundworm, nematode) Enterobius vermicularis can survive only in humans. In other words, each of these species possesses characteristics enabling it to complete its life cycle using these particular hosts. Such characteristics (in very few cases analysed in some detail) determine not only the kinds of host(s) used, but also the degree of host specificity, i.e., how many host species a parasite can utilize. – Parasites among the flowering plants possess haustoria for food extraction, connected to the vascular system of host plants, either in the roots or shoots.
In addition to special adaptations, many parasites also share some features that can be considered as general adaptations to a parasitic way of life. Among these are the smaller body size of parasites relative to that of their host, only few parasites reaching a body volume or length exceeding that of the host (Figure 3). However, at the first glance perhaps surprising, many parasites are larger than their free-living relatives for two reasons: parasites usually have an almost unlimited food supply, and they must produce a large number of offspring in order to overcome the hazards of their life cycles which necessitates infecting a host: only few eggs or larvae manage to succeed: the larger the body, the larger the egg producing capacity!
Figure 3. The amphilinid tapeworm Austramphilina elongata in the body cavity of a turtle. The parasite is very large (several cm long), but still much smaller than its host. Its large body size (much larger than that of related free-living flatworms) enables it to produce large number of eggs, necessary to overcome the hazards of the life cycle.
© Klaus Rohde.
Many parasites have undergone a process referred to as sacculinization, i.e., a reduction in the complexity particularly of sensory organs and the nervous system, although some exceptions have been well studied (see the knol on sacculinization: http://knol.google.com/k/klaus-rohde/the-aspidogastrea-a-parasitological/xk923bc3gp4/15).
Like all animal species, parasites must be able to disperse, because a population with small distribution may become extinct under unfavourable conditions, succumb to inbreeding, and (perhaps?) run the risk of overinfecting hosts. In parasites, dispersal is often largely or even entirely passive, i.e., due to the dispersal of the host, but many parasites have elaborate dispersal mechanisms, such as flotation organs of larval flukes (cercariae).
Mechanisms of infection
Particularly impressive and well studied in many species are mechanisms of infection. Hosts can become infected by kissing (flagellated protistan Trichomonas tenax), sexual intercourse (flagellated protistan Trichomonas vaginalis), inhalation (protistan Pneumocystis?), contact transfer (mange mite), penetration through the skin (hookworms), inoculation (malaria transmitted by mosquitoes), faecal contamination of wounds (Chagas disease transmitted by assassination bugs), retrofection (migration of larvae from the anus into the gut, human pinworm), ingestion of cysts (amoebic dysentery), ingestion of eggs (human wipworm Trichuris), ingestion of spores (many protistans) (Figure 4), ingestion of transport hosts (some roundworms of fish), ingestion of intermediate hosts (many flukes), penetration into the nasal passage (protistan Naegleria fowleri), intrauterine infection (malaria).
Figure 4. Mature spore of Vairimorpha cheracis, a microsporidian parasite of the Australian yabby, Cherax destructor, transmission electron micrograph. Note the nucleus, posterior vacuole and sections through 11 coils of polar filament. A new host cell is infected by extrusion of the spore’s content into the cell through the everted filament. © Dr. Elizabeth Moodie.
Many species possess astonishing behavioural adaptations that facilitate entrance into a host. We mention two examples of flukes (trematodes). Adult Dicrocoelium lanceolatum (=D. dendriticum) infect the liver mainly of sheep, they produce eggs which are eaten by land snails, in which various larval stages are produced; the last stage is the tailed larva or cercaria many of which cluster in slime balls; these slime balls leave the snail and are eaten by ants; the first cercaria getting into an ant penetrates into the subesophageal ganglion inducing cramp-like behaviour of the ant, which consequently clings to a plant; this enhances the likelihood to be eaten by sheep.
Figure 5. Life cycle of Dicrocoelium lanceolatum. Explanation in text. Public domain.
Outgrowths of the larval stage (the sporocyst) of another trematode, Leucochloridium macrostomum, can extend into the tentacles of its snail host. By pulsating rhythmically they deceive and attract birds which believe that they are eating worms; they bite off the tentacles and become infected. Infection also makes the snails move to more exposed sites (Figure 6).
Figure 6. The sporocyst of the digenean trematode Leucochloridium. Part of the sporocyst, which is conspicuously coloured resembling a worm, can extend into the tentacle of a snail. It pulsates, thus attracting the attention of the final host, a worm-eating bird. The bird bites the tentacle off and becomes infected by the metacercariae located in the sporocyst.
Some species possess conspicuous morphological adaptations that help in the completion of the life cycle. For example, eggs of some blood flukes (schistosomes causing bilharzia) have spines which contribute (together with enzymes produced by the larva within the egg) to erode the walls of the blood vessels and thus facilitate escape of the eggs from the blood stream into the faeces or urine (Figure 7).
Figure 7. Egg of Schistosoma mansoni, a schistosome fluke from the blood stream of humans. Note large lateral spine. Public domain.
Aggregation, hermaphroditism, parthenogenesis and asexual reproduction
Surveys of the distribution of parasites in host populations always find that not all host individuals are infected to the same degree. The majority of parasites is usually concentrated in a few of the hosts. This is meant when we say that distributions are aggregated. There has been some debate, without much evidence, on whether aggregation has a biological function, such as facilitating the finding of mating partners, or limiting the damage done to the host population. – Common among parasites are hermaphroditism, parthenogenesis and asexual reproduction, and these phenomena almost certainly have a biological function. Because in most species, only single or few parasites will reach a host, it is important that populations can be built up from these few individuals or even from a single individual. And parthenogenesis and asexual reproduction permit just that. In parthenogenesis, gametes (sex cells) develop without fertilization, in asexual reproduction, somatic (body) cells can undergo development. An example of asexual reproduction is malaria parasites developing by schizogony (splitting into many daughter cells) in human red blood cells. Trematode larvae developing in snails probably do this by parthenogenesis. Parasitic Platyhelminthes, which include flukes, tapeworms and monogeneans, with few exceptions are hermaphroditic (i.e., have male as well as female reproductive organs); such worms have a much greater chance of meeting a mating partner.
Parasites are never found on all host species that could possibly be infected. In other words, all parasites show a certain degree of host specificity, although the degree of specificity varies. Thus, the large human roundworm Ascaris lumbricoides occurs only in humans (although a closely related species, Ascaris suis, occurs in pigs and is sometimes considered to be the same species), whereas the protistan Toxoplasma gondii has been shown to occur in a wide range of mammals and birds. – The same refers to site specificity. All parasites prefer certain tissues or organs or even narrow niches within them to others, although they do this to different degrees. For example, larval flukes (metacercariae) of many species infect a variety of tissues of fishes, whereas adult schistosomes (blood flukes) are restricted to the blood vessels.
We distinguish two main kinds of life cycles: many parasites (lice, flea, monogenean flukes, many roundworms, etc.) use a single host which harbours larval as well as adult stages, i.e., they have a direct life cycle; others use a final (= definitive) host which harbours the mature stage, as well as one or several intermediate hosts which harbour the larvae, i.e., they have indirect life cycles (for example all digenean flukes = Trematoda). An example of a trematode with two intermediate hosts is the sheep liver fluke Dicrocoelium lanceolatum (Figure 5). In certain parasite species, alternative life cycles are possible. For example, in the aspidogastrean fluke Aspidogaster conchicola, both a direct and an indirect life cycle are possible: adult worms in the mollusc produce eggs which are inhaled by other molluscs, but fish can also become infected by eating infected molluscs (Figure 8). In other aspidogastreans, and in the amphilinid tapeworm Austramphiina elongata, among many others, the life cycle is always indirect, involving an intermediate and final host. In the amphilinid tapeworm turtles serve as final hosts, eggs escape in an unknown way, larvae hatch in freshwater and penetrate into the intermediate host, a crayfish, which is eaten by a turtle (Figure 9).
Figure 8. Life cycle of Aspidogaster conchicola (Trematoda, Aspidogastrea). Note: the parasite can complete its life cycle in the mollusc, i.e., it has a direct life cycle involving only one host. Alternatively, it can use a fish as a final and a mollusc as an intermediate host, i.e., it has an indirect life cycle. © Klaus Rohde
Figure 9. Life cycle of Austramphilina elongata (Cestoda, Amphilinidea). Note: the parasite has an indirect life cycle, always including an intermediate host, a crayfish, and a final host, a turtle. © Klaus Rohde
Virulence of parasites
Virulence of parasites can be defined as the degree of damage done by the parasite to the host. There are two opposing trends which determine the degree of virulence: 1) sometimes it may not be in the parasite’s interest to severely damage or even kill its host, because this would also affect the fitness of the parasite; 2) on the other hand, parasite transmission to another host may be facilitated by such damage: a weak host may be easier prey for a predatory final host than a strong one. Therefore, evolution will lead to an increase or a decrease in virulence, depending on the circumstances.
A considerable range of behavioural patterns leading to (or thought to lead to) the removal of parasites has been observed. They include preening and bathing of birds in dust and water, passive and active anting (where ants are allowed to passively crawl over the body, or where ants are actively squeezed over the plumage). Also, rubbing of dogs against rough surfaces, jumping of fish out of the water, etc. may have a cleaning function. Best known is cleaning symbiosis, in which one animal (the cleaner) cleans another (the host) from parasites and diseased (necrotic) tissues. For example, cleaning behaviour has been observed in birds which remove ectoparasites from cattle, hippopotamus and large marine fish floating on the ocean surface, in several species of shrimps, and in many species of fish. Hosts are freshwater and marine fishes, whales and dolphins, and invertebrates, among others. Many cleaner fish possess special morphological adaptations which enable them to pick up parasites (the mouth is located terminally to facilitate picking up of parasites, the anterior teeth are fused to form cutting plates, and colour patterns are conspicuous, useful in signaling to hosts: I am a cleaner!). The cleaner fish Labroides dimidiatus (Figures 10 and 11) even performs a cleaning dance to attract host fish. “Invitation postures” of hosts signal, in turn, signal to the cleaner that they are ready to be cleaned.
Figure 10. The cleaner wrasse Labroides dimidiatus cleaning a host, the marine fish Diagramma pictum. Note the conspicuous colour pattern of the cleaner fish and its terminal mouth.
© Professor Alexandra Grutter, University of Queensland.
Figure 11. The cleaner wrasse Labroides dimidiatus cleaning a host, the marine fish Diagramma pictum. Note the conspicuous colour pattern of the cleaner fish and its terminal mouth. © Professor Alexandra Grutter, University of Queensland.
Immune- and tissue reactions, resistance
Hosts are protected against parasite infections by two kinds of mechanisms.
1) Humoral and tissue reactions of hosts use the host’s ability to distinguish its own cells from foreign cells and material. In vertebrates three types of such reactions have been demonstrated: phagocytosis, inflammation and adaptive immunity. The first two are non-specific tissue reactions, i.e. they are not directed against specific agents, the third is specific to a certain type of foreign material. Immune reactions involve parasite antigens which induce the formation of specific antibodies in the host. In microparasites immune responses are more effective than in macroparasites.
2) Hosts show different degrees of “resistance” to infections which are not due to acquired immunity. For example, some sheep are more “resistant” to roundworms than others, even before they ever had any contact with such roundworms. In age resistance, older individuals are more resistant than young ones.
Effects on hosts
As pointed out earlier, some parasites may have little, others very strong and sometimes fatal effects on the hosts. Malaria and especially one species of malaria, Plasmodium falciparum, has very serious effects on humans, particularly children, often leading to death. The human pinworm Enterobius vermicularis, in contrast, often (but not always) rather has nuisance value. Monogenean flukes of fish usually have little effect, but some species lead to mass mortalities, particularly in aquaculture.
Species richness of parasites and distribution of parasites in the animal and plant kingdoms
Arndt (1940) was the first who counted the number of parasites as a proportion of a total fauna. He found 10,000 parasitic species out of a total of 40,000 species in Germany, but did not include insects parasitizing plants (“herbivores”). Price (1977) included such species but excluded temporary parasites (e.g., mosquitoes and leeches) in his survey of the British fauna. He estimated that more than half of all British species are parasitic.
Thirteen large taxa (phyla, subphyla or classes) consist entirely of parasites, and many other groups include a high proportion of parasitic species. Even among the vertebrates several species are parasitic. Among the 250,000 to 400,000 species of flowering plants (angiosperms) there are about 4,100 parasitic species included in 16-19 of the approximately 462 plant families.
Economic and hygienic importance of parasites
Some of the most important tropical diseases are caused by parasites, such as bilharzia (caused by the bloodfluke Schistosoma), filariasis (nematodes), amoebic dysentery (the protozoan Entamoeba histolytica), and in particular malaria (four species of the protozoan Plasmodium). Hundreds of millions of people are infected with malaria, and more than a millions die every year, particularly children in sub-Saharan Africa. There is no effective vaccination, and resistance to the various anti-malaria drugs develops very fast.
The web pages of The World Health Organization, Division of Tropical Diseases, CTD, and of the Center for Disease Control (CDC) contain information about the current status of the important parasitic diseases, which is continually updated. Information on prevalences of infection with various parasites and their geographical distribution are available here.
Reduced immune reactions due to AIDS, the development of resistance to certain drugs used to treat parasite infections (malaria, for example) represent problems in the fight against parasitic disease. The prevalence of intestinal parasites world-wide is increasing, partly due to urbanization. Global warming will lead to a spread of parasitic infections into some countries.
Among parasites of livestock, Ostertagia ostertagi, one of the nematodes infecting cattle, alone is estimated to cause an annual loss of $600 million to the cattle industry in the US. Drench resistance is also a serious problem.
Parasites are of very great importance in the aquaculture industry, where they have repeatedly led to mass mortalities.
Many parasites are among the most important pests of plants. Nematodes are of particular importance. The annual loss to crop production due to nematodes in the US was estimated to be US$8 billion (12%), and $78 billion globally.
On the other hand, parasites can also be used to control insect pests.
Aspidogastrea I. http://knol.google.com/k/klaus-rohde/the-aspidogastrea-a-parasitological/xk923bc3gp4/13#
Aspidogastrea II. http://knol.google.com/k/klaus-rohde/the-aspidogastrea-a-parasitological/xk923bc3gp4/15
Aspidogastrea III. http://knol.google.com/k/klaus-rohde/the-aspidogastrea-a-parasitological/xk923bc3gp4/16
Marine parasites. http://knol.google.com/k/klaus-rohde/meeresparasiten-wirtschaftliche-und/xk923bc3gp4/2
Marine parasites of man: http://knol.google.com/k/klaus-rohde/marine-parasites-of-man-anisakis/xk923bc3gp4/59#edit
I wish to thank Professor Alexandra Grutter, University of Queensland, Brisbane, for the photos of Gnathia and the cleaner fish (Figures 1, 9 and 10), and Dr. Elizabeth Moodie, Townsville, for the transmission electron- micrograph of the microsporidian (Figure 4).
My review article in the Encyclopedia of Biodiversity, Academic Press 2001 (see below) contains a more detailed account of parasitism. Important textbooks dealing with various aspects of parasitology are listed below.
Bogitsh B.J. & Cheng T.C. (1990). Human Parasitology. Saunders College Publishing, New York.
Kinne O. (1980 – 1985). Diseases of Marine Animals; Volume 1 – 4. John Wiley & Sons, Chichester, New York, Brisbane, Toronto, and Biologische Anstalt Helgoland, Hamburg.
Odening K. (1969). Entwicklungswege der Schmarotzerwürmer. Akademische Verlagsgesellschaft Geest & Portig K.G., Leipzig.
Press M.C. & Graves, J.D. Eds. (1995). Parasitic Plants. Chapman & Hall, London.
Rohde K. (1993). Ecology of Marine Parasites 2nd Edition. CAB International, Oxford.
Rohde, K. (2001). Parasitism. Encyclopedia of Biodiversity. Academic Press.
Rohde, K. ed. (2005). Marine Parasitology. CSIRO Melbourne and CABI Wallingford, Oxon.
Schmidt G.D., Roberts L.S. & Janovy, J. (1995). Foundations of Parasitology 5th Edition. McGraw Hill, New York.
Urquhart, G.M. & Jennings, F.W. (1996). Veterinary Parasitology 2nd Edition. Iowa State University Press, Ames, Iowa.