PARASITISMDepartment of Biology, Medical Faculty,

Medical University of Sofia

where one organism benefits without affecting the other. In mutualism both organisms benefit. In amensalism, one is harmed while the other is unaffected (a product of one organism has a negative effect on another organism, e.g. when antibiotics secreted by fungi kill bacteria). In parasitism, one benefits while the other is harmed.

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Ecological relationships

Commensalism is a relationship between two organisms

Parasitism is a non-mutual relationship between organisms where one organism, the parasite,

feeds on the other, called host, and so benefits at

its expense. Parasitism must be distinguished from predation, where one organism (predator) feeds on another (prey) by ingesting its whole body. Unlike parasites, predators are larger than the prey and invariably kill it. Also, they are usually more complex than the prey.

Parasitoids are organisms whose larval development occurs inside or on the surface of another organism, resulting in the death of the host. They are “midway” between predators and true parasites.

Tobacco horn caterpillar with parasitic wasp cocoons

Main characteristics of parasites:

• reduce host biological fitness by general or specialized pathology and modification of host behaviors;

• survive by exploiting hosts for resources necessary for their survival, e.g. food, water, heat, habitat, and transmission.

• much smaller than their hosts;• high degree of specialization;• faster rate of reproduction than their hosts;• use the host alive and do not “intend” to kill it (at least, not quickly);

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Types of parasites

• Ectoparasites - live on the surface of the host.

• Endoparasites - live inside the host.

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- intracellular endoparasites inhabiting cells in the host’s body.

- extracellular endoparasitesinhabiting spaces in the host’s body.

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Hosts• definitive (primary) host - a

host in which the parasite reaches maturity and, if possible, reproduces sexually.

• intermediate (secondary) host - a host in which the parasite passes its asexual reproduction or larval stages.

• Paratenic host - an animal acting as a substitute intermediate host of a parasite, usually having acquired the parasite by ingestion of the original host.

• Reservoir host – a host alternative to human and usually less severely affected.

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Adaptations for parasitism

Parasitic Protozoa:• unicellular eukaryotes;• have short generation times, high rates of reproduction.• tendency to induce immunity to reinfection in those hosts that survive (some blood parasites – antigen variations for avoiding immunity);• complex life cycles;• specialized ways of entering and maintaining themselves in the hosts• forming environmentally resistant cysts. diverge.hunter.cuny.edu

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• Multicellular parasites have attachment devices like hooks and suckers (preventing them from being washed away).

• Parasites have protective devices like thick cuticles, production of mucus and secretion of anti-enzymes (protection from the host digestive enzymes).

• Most of the internal parasites use a secondary host which enhance their transmission.

• Parasites are generally prolific and lay numerous eggs; this increases chances of survival.

• Some of the internal parasites are hermaphrodites; hence they do not depend on another partner for fertilization to occur (they are self fertilized).

• Endoparasites like tape worms have long and flattened body to increase the surface area for efficient absorption of food in the host digestive tract.

Life cycles of parasitesComplex (indirect)

Infection of more than one host species to complete the life cycles.

Direct

Infection of a single host species to complete the life cycles.

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Protozoan endoparasites:• ingesting of cysts (intestinal protozoan parasites);• vector transmission by biting-insects - blood single-cell parasites (plasmodia, trypanosomes).

Main characteristics of parasitic life cycles

Metazoan endoparasites: • active penetration through the host external cover or ingestion by the host; • releasing offspring into the external environment to infect other hosts (intestinal endoparasites – with host excreta);• ingestion by the next host in the life cycle to survive and reproduce (tissue larval stages);• free-living transmission stages - migration through the host’s tissue into the external environment (active search for or await ingestion by other hosts (flatworms and parasitic roundworms).

Metazoan ectoparasites:• direct host-host contact (lice);• shedding eggs that survive off the host (fleas);• waiting in the external environment for an encounter with a host (ticks).

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Ectoparasites usually do less harm than endoparasites, but they can also be dangerous for the host. See the numerous ticks on this deer. Their feeding will result in life-threatening blood loss.

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Ticks have drawn so much blood from their host that it is exhausted and will die.

Ecological interactions and health of the ecosystem:- Parasitism weakens prey- It sometimes reduces the dominance of superior competitors

and allowing competing species to co-exist

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Modification of host behavior to facilitate transmission to other hosts• Toxoplasma gondii – uses predator-prey interactions to get from one host to another. While uninfected rats  avoid cat odors, infected rats are attracted to this scent. So they are more likely to be eaten by cats and this is good for Toxoplasma, bringing it to its next host (the cat).

Systematics of INVERTEBRATESProtozoa Unicellular motile

eukaryotesPhyllum Sarcomastigophora free living and some parasitic forms

Phylum Apicomplexa all parasites

Phylum Ciliophorafree living and some parasitic forms

(There are many more protozoan phyla.)

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Metazoa

Phylum Porifera - sea sponges

RadiataPhylum CnidariaPhylum Ctenophora

BilateriaPhylum Platyhelminthes

Class Trematoda, Class Cestoda

Phylum NematodaPhylum AnnelidaPhylum MolluscaPhylum Arthropoda Class Crustacea, Class Arachnida, Class Myriapoda, Class InsectaPhylum Echinodermata

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Evolution of INVERTEBRATES

• Body symmetry• Formation of germ layers• Tissues and organs• Cephalization• Formation of coelom

Evolutionary trends

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