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ECOLOGY OF PARASITES

Part 2: Problems and obstacles

Parasite adaptations

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A PARASITE’S ECOLOGICAL NICHE A parasite’s ecological niche includes resources provided by the living body

of another species as well as abiotic conditions encountered by transmission stages such as eggs, cysts, spores, and juveniles.

The digestive tract thus providing numerous microenvironments. - A trip through the gut could be described also in terms of different

symbionts encountered along the way, - from Entamoeba gingivalis in the mouth, - to fourth-stage juvenile Ascaris lumbricoides in the stomach, - to Taenia saginata (or many other helminths) in the small intestine, - to Dientamoeba fragilis, Entamoeba coli, Endolimax nana, and Trichuris

trichiura in the large intestine, and - finally to pinworms (Enterobius vermicularis) crawling around the anal

orifice The blood system Coelom – body cavity In special cells – e.g microphage Organ – e.g lungs, liver, brain etc

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IN THE ALIMENTARY CANAL

1) Total darkness2) pH: 1.5 to 8.43) Many enzymes – digestive enzymes are also

capable of digesting and destroying the parasites.

4) Physiological, chemical and mechanical changes

5) Low level of oxygen

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TOTAL DARKNESS

No light inside the host.

Can be problematic to parasites.

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PH PROBLEM

Mouth: pH 6.7 (5.6 – 7.5) Stomach: - pH 1.49 – 8.38 (human) - pH 3.26 – 6.24 (mice) - pH 2.0 – 4.1 (cattle) - pH 1.05 – 3.6 (sheep) Duodenum: - pH 6.7 (human) – acidic - pH 8.2 – 8.9 (cat, goat) – alkaline Implication from mouth stomach duodenum

small intestine CHANGES IN PH.

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ENZYMES AND CHEMICAL PROBLEMS

Food processing occurs in distinct phases, from- chewing and salivary amylase action of the mouth, - to the acid pH and proteolytic enzyme reactions of

the stomach, - to more neutral pH and numerous amylases,

proteases, lipases, and nucleases working in the small intestine,

- to reclamation of water in the large intestine and- subsequent elimination of solid wastes. Chemical – Different subtracts ingested by the

host can be problematic to the parasites.

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PHYSIOLOGICAL AND MECHANICAL CHANGES All these changes – fast and continuous can be problematic

to parasites Physical

- Change in the habitat/ hosts

E.g filarial worms – mosquitoes human

Mechanical

– Peristalsis

continuous and expansion of the alimentary tract – pushes food – esophagus – stomach – small intestine – large intestine

- Food and water flow

can be problematic to parasites

will sweep away the parasites present in the alimentary tract

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LOW LEVEL OF OXYGEN

The low level of oxygen in the alimentary tract can be problematic to parasites.

Low oxygen level for survival in the hosts.

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PARASITE ADAPTATIONS

Physiological Adaptations Behavioral Adaptations Structural and Functional Adaptations

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PHYSIOLOGICAL ADAPTATIONS

1) Parasite reproduction

Among animals, parental care is one factor that tends to increase the chance of an offspring surviving.

Parasites, on the other hand, exhibit little parental care, although viviparity, or live birth, such as occurs in some nematodes and monogeneans, can be considered a more “caring” approach than indiscriminate scattering of eggs.

Parasites exhibit a variety of mechanisms that function to increase the reproductive potential of those individuals that do succeed at finding a host.

These mechanisms often take the form of asexual reproduction and hermaphroditism.

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Asexual reproduction often occurs in the larval or sexually immature stages as either polyembryony or internal budding.

Hermaphroditism is the occurrence of both male and female sex organs in a single individual.

It sometimes eliminates the necessity of finding an individual of the opposite sex for fertilization if gonads of both sexes function simultaneously and self fertilization is mechanically possible.

Reproductive encounters result in two fertilized female systems.

The specific manifestations of asexual reproduction and hermaphroditism, however, differ depending on the group of parasites.

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Schizogony, or multiple fission, is asexual reproduction characteristic of some parasitic protozoa

In schizogony the nucleus divides numerous times before cytokinesis (cytoplasmic division) occurs, resulting in simultaneous production of many daughter cells.

Simple binary fission is also asexual reproduction.

It is common among familiar free-living protozoa such as Paramecium species as well as some amebas, including parasitic ones.

As with any process in which numbers double regularly, rapid fission can result easily in millions of offspring after only a few days.

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Simple binary fission

Schizogony, or multiple fission

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Trematodes and some tapeworms reproduce asexually during immature stages.

The juveniles (metacestodes) of several tapeworm species are capable of external or internal budding of more metacestodes.

The cysticercus juvenile of Taenia crassiceps, for instance, can bud off as many as a hundred small bladder worms while in the abdominal cavity of a mouse intermediate host.

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Each new metacestode develops a scolex and neck, and when the mouse is eaten by a carnivore, each scolex develops into an adult tapeworm.

The hydatid metacestode of Echinococcus granulosus is capable of budding off hundreds of thousands of new scolices within a fluid-filled bladder.

When such a packet of immature worms is eaten by a dog, vast numbers of adult cestodes are produced.

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Perhaps the most remarkable asexual reproduction in all zoology is found among trematodes, a large and successful group of parasites commonly called flukes.

These animals produce a series of embryo generations, each within the body of the prior generation.

This is an example of polyembryony, in which many embryos develop from a single zygote.

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Trematode eggs hatch into miracidia, which enter a first intermediate host, always a mollusc, and become sac like sporocysts.

Sporocysts may give rise to daughter sporocysts, which, in turn, may each produce a generation of rediae.

These then become filled with daughter rediae, which finally produce cercariae.

And many flukes give birth to thousands of eggs each day.

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(1) Diplostomum flexicaudum(2) Trichobilharzia physellae(3) Alaria mustelae(4) Fasciola hepatica(5) Metorchis conjunctus(6) Proterometra dickermani(7) Stichorchis subtriquetrus(8) Caecincola parvulus

Some life cycles of digenetic trematodes.

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With hermaphroditism, a parasite evidently solves the problem of finding a mate.

Many tapeworms and trematodes can fertilize their own eggs

This method, although not likely to produce many unusual genetic recombinations, guarantees offspring.

Tapeworms also undergo continuous asexual production of segments (strobilization) from an undifferentiated region immediately behind the scolex, or attachment organ.

These segments, called proglottids, are each the reproductive equivalent of a hermaphroditic worm, at least in the vast majority of tapeworm species, because each contains both male and female reproductive organs.

Each fertilized female system in each proglottid eventually becomes filled with eggs containing larvae.

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The result of this combination of asexual reproduction, hermaphroditism, and self-fertilization is a true tapeworm egg factory.

Whale tapeworms of the genus Hexagonoporus, for example, are 100-foot reproductive monsters consisting of about 45,000 proglottids, each with 5 to 14 sets of male and female systems.

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Parasites often increase reproductive potential through production of vast numbers of eggs.

A common rat tapeworm, Hymenolepis diminuta, for example, produces up to 250,000 eggs a day

During a period of slightly over a year, a single tapeworm can thus generate a hundred million eggs.

If all these eggs reached maturity in new hosts, they would represent more than 20 tons of tapeworm tissue.

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Female nematodes are also sometimes prodigious egg layers;

- E.g - A single Ascaris lumbricoides can produce more than 200,000 eggs a day for several months, and over the course of their lifetimes

- Members of the filarial genus Wuchereria bancrofti may release several million young into their host’s blood.

Such high reproductive potential, of course, ensures that such parasites will become medical and veterinary problems when host populations are crowded and transmission conditions are favorable.

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2) Secretion of certain enzymes Parasite secrete pepsin if the environment gets too

acidic to neutralize the acidity environment E.g – Hymenolepis diminuta, Taenia taeniaformis Secrete anti-enyzmes E.g – Ascaris spp. - 2 anti enzymes – Anti-trypsin - Anti-chemotrypsin

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3) Can undergo anaerobic metabolism - In the absence of oxygen, certain species of

parasite can undergoes an anaerobic metabolism.

- E.g – In tapeworms

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4) Thermoregulation - Induction of certain protein to enhance the

transmission - E.g Fillarial worms – Brugia pahangi - Mf – the most abundant protein small heat

shock proteins - The synthesis of small Hsps by Mf may be an

adaptive response to the potentially hostile environment of the mammalian blood stream. 

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BEHAVIORAL ADAPTATIONS

Behavior is an important tool for animal survival this is also true for parasites

Behavior can be used to enhance their chances for success

There are numerous examples of parasite attributes that presumably increase a species’ chances of encountering new hosts.

These attributes often influence an intermediate host in some way, making it more susceptible to predation by a definitive host.

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Simple host finding behaviors Periodic Behaviors Host Modifying Behaviors Use of intermediate larval stages on

intermediate hosts

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SIMPLE HOST FINDING BEHAVIORS

eg. Entobdella (Monogenea) - skin parasite of a stingray - eggs are released and settle to bottom - larvae emerge from eggs within 3 seconds of

sudden darkness - then swim vertically upwards

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PERIODIC BEHAVIORS

Parasite keys in on cyclic stimulus E.g Filarial Worms - live in blood - transmitted by mosquito or fly - larvae (microfilariae) move to peripheral blood on periodic basis - corresponds to “biting hours” of local vector (flies

& mosquitoes)

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E.g Guinea worm (nematode: Dracunculus medinensis)

- occur in tropical areas; lots of rice fields - eggs must be laid in water to be able to get to its

intermediate host - female may contain up to 1 Million eggs each with a

developing larva inside - larvae must be released in water to complete life cycle - to do this female moves to part of body likely to be

immersed in water lower legs - creates an ulcer - discharges 1000’s of infective larvae

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HOST MODIFYING BEHAVIORS

an alternative to modifying the parasites own behavior is to alter the hosts behavior to make it more likely to complete parasites life cycle

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E.g Trematodes of the genus Dicrocoelium, - Which infect large herbivores such as sheep - The second intermediate host of Dicrocoelium

dendriticum is an ant. - A metacercaria lodges in the ant’s brain, making

the insect move to the top of a grass blade, where its likelihood of being accidentally ingested by a definitive host is greatly increased.

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E.g The immature stages of some thorny-headed worms (phylum Acanthocephala)

- Infect freshwater crustaceans of order Amphipoda (side-swimmers).

- Some acanthocephalan juveniles appear as conspicuous white or orange spots in the hemocoel of the translucent amphipods

- Making infected ones stand out from the uninfected.

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E.g Fluke (Leucochloridium) - Adult in birds; larva in snail - When infected, snails tend to crawl to tips of

vegetation instead of hiding like normal in snail, larvae migrate to tentacles of snail

- Larvae are brightly colored with red and green bands

- makes snails very conspicuous in daytime - At night the larvae withdraw into the snails

body

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USE OF INTERMEDIATE LARVAL STAGES ONINTERMEDIATE HOSTS

To enhance chances of getting to final host

simplest life cycle: - adult parasite eggs ingestion by new host

more complex life cycle: - adult parasite eggs intermediate host

definitive host

most complex life cycle: - flukes have several intermediate states that

reproduce

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STRUCTURAL AND FUNCTIONAL ADAPTATIONS

Modification of body structures/ functions Reduction in “unnecessary” structures and

enhancement of reproductive capacity Usually have a resistant stage in life cycle

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MODIFICATION OF BODY STRUCTURES/ FUNCTIONS

1) Structures for penetration and attachment to host - Attaching itself to the host using special organs – suckers, hooks, extended lips/labium, bothrium

40Note the two broad cutting plates in the ventrolateral margins (top).

The mouth of Necator americanus. Ancylostoma duodenale,

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Haemonchus contortus, ventral view of male.

Scanning electron micrographs of Leptorhynchoides thecatusNote some of the major anatomical features of acanthocephalans. P, proboscis; H, hook; N, neck; T, trunk

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Scolex of Taenia solium

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2) Body thin and long - E.g tapeworms - Body can curve according to the current of the

food flow - No resistance - Prevents being broken up by the food flow/

peristalsis

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Taenia solium

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3) Cell membrane - E.g Entamoeba histolyca - The cell membrane becomes turgid that will prevents

the entry of enzymes into its cyctoplasm

4) Bury itself deep in the mucosa - Prevention method from being swept away especially

in the intestine - E.g Entamoeba histolytica

5) Having a thick layer of body wall - Prevent the entry of enzymes into the body - E.g cuticle/ tegument – in many intestinal nematodes

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REDUCTION IN “UNNECESSARY” STRUCTURES

1) Reduced sense organs2) Reduced nervous system3) Reduced locomotion4) Reduced digestive system - Some endoparasites have lost gut entirely - Some ectoparasites use gut mainly for food

storage (eg. leeches, ticks)5) Enhancement of reproductive capacity - Reproductive organs are often the largest, most

apparent organ systems present compare to other organs

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USUALLY HAVE A RESISTANT STAGE IN LIFE CYCLE

1) For getting from one host to another which is often in a different kind of environment

2) If endoparasite - needs to survive trip through digestive system

3) Formation of cysts -Numerous parasites, such as juvenile tapeworms

(cestodes) in various tissues, achieve protection from the host response by envelopment with cystic membrane.

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ENCYSTMENT IN PROTOZOA

Many protozoa can secrete a resistant covering and enter a resting stage, or cyst

Cyst formation is particularly common among parasitic protozoa as well as among free-living protozoa found in temporary bodies of water that are subject to drying or other harsh conditions.

During encystment a cyst wall is secreted, and some food reserves, such as starch or glycogen, are stored

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In coccidians the cystic form is an oocyst, which is formed after gamete union and in which multiple fission (sporogony) occurs to produce sporozoites.

In eimerian coccidians, oocysts containing sporozoites serve as resistant stages for transmission to new hosts,

In haemosporidians (including the causative agents of malaria, Plasmodium spp.) oocysts serve as developmental capsules for sporozoites within their insect host.

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