Ecology - Mod 12 (Ch 13): Parasitism

parasites

consume parts (tissues/fluids) of a living prey organism (host)

may be external (ectoparasite) or internal (endoparasite)

may be large (macroparasite) or small (microparasite)

pathogens: parasites that cause diseases

represent ~50% of the species on earth because:

A) most parasites are specialized for one or a few host spp.

B) most host spp. are attacked by >1 parasite spp.

C) even parasites have parasites (e.g. Lentille virus on amoeba, lentille virus has Sputnik 2 virus)

examples of ectoparasites

tongue-parasitizing isopod on greater weaver fish

eyelash mites

dust mites on hair

many fungi (e.g. trichophyton rubrum, rust)

parasitic plants (2 classifications: holoparasites and hemiparasites)

holoparasitic plants

ectoparasitic plants that cannot photosynthesize

extract all H2O, minerals, and carbohydrates from host plant xylem and phloem

completely dependent on host (obligate)

e.g. Haustorium, dodder, corpse flower

hemiparasitic plants

ectoparasitic plants that can photosynthesize

extract H2O, minerals, and some but not all carbohydrates from host plant

may be completely dependent (obligate) or not dependent (facultative) on host

e.g. mistletoe

examples of endoparasites

tapeworms

roundworms

nematoda

Plasmodium

tapeworms

intestinal endoparasites of most vertebrates

class Cestoda

highly specialized

specialized attachment organs (hooks and suckers)

no sense organs

no brain or nerve cords

no mouth or gut

absorbs host’s pre-digested nutrients

roundworms

endoparasites in phylum Nematoda

~25k spp. known, likely 20x more in existence

highly abundant (90% of animals on ocean floor, 80% of all individual animals on earth)

ubiquitous (in soil (terrestrial, under fresh and salt water), pole to pole, harsh deserts, deep in the earth, in plant and animal tissues and fluids)

predators, scavengers, and parasites of animals, plants, fungi, bacteria

some are parasites of vertebrates

e.g. Ascaris

Trichinella spp. (trichinosis)

endoparasite in phylum Nematoda

mammalian and reptilian parasites

cysts live in muscle tissue (hatch when eaten by new host, young worms burrow through intestinal wall and muscles. Sometimes enter CNS)

symptoms: cramping, diarrhea, fever

in US, ~12 cases per year (reduced by not allowing the feeding of raw meat to hogs)

guinea worm (Dracunculus medinensis)

endoparasite in phylum Nematoda

1) human ingests copepods with larvae in unfiltered H2O or undercooked fish

2) larvae burrow into stomach lining

3) female emerges from blister into H2O (~10-14 months after ingestion)

4) copepods eat larvae

may become the first non-viral parasite to be eradicated

filaria worms

endoparasites in phylum Nematoda

cause elephantiasis (lymphatic filariasis)

spread mainly by biting insects (e.g. mosquitos and blk flies); infection requires repeated bites over months to years

live in human lymph system usually undetected

some have swelling due to blocked lymph nodes

African eye worm (Loa loa)

endoparasite in phylum Nematoda

causes filariasis in eye

~12-13 million humans infected with larvae

human form is restricted to the rainforest and swamp forest areas of West Africa

contracted through biting insects (deer fly, mango fly)

life cycle of an endoparasite

complex

>1 host

several life stages (may have free-living stages)

e.g. Plasmodium

Plasmodium

organism that causes malaria

parasite of mammals, birds, reptiles (5 spp. infect humans, Avian malaria has decimated Hawaiian birds)

~250 million people currently infected, ~500,000-900,000 people die each year

hard to make vaccine since it hides inside cells and constantly changes surface protein

very complex life cycle (sexual and asexual stages, diploid and haploid stages, two or more hosts needed for reproduction)

Plasmodium life cycle

1. mosquito bites human and injects saliva with Plasmodium in stage 1 (sporozoite)

2. goes inside liver cells, divides asexually, transforms into stage 2 (merozoites) which are released into the blood stream

3. stage 2 form goes inside RBCs, divides asexually, are released into blood stream every 48-72 hrs, causing fever

4. most reinvade RBCs, fever subsides; but some transform into nonpathogenic stage 3 (gametocytes)

5. mosquito bite transfers stage 3 form into mosquito

6. in mosquito, transform into male and female gametes

7. in mosquito stomach, male and female gametes combine (fertilization/sexual reproduction) and form zygote

8. zygote encased in hardened protective capsule (oocyst) divides asexually to produce new stage 1 form

   1. oocyte bursts and releases stage 1 form, which go to mosquito salivary gland

advantages of ectoparasitism

ease of dispersal

safe from host’s immune system

disadvantages of ectoparasitism

vulnerability to natural enemies

exposure to external environment

feeding is more difficult

advantages to endoparasitism

ease of feeding

protected from external environment

safer from natural enemies

disadvantages to endoparasitism

vulnerability to host’s immune system

dispersal is more difficult

parasitoids

organisms gradually consume tissues of hosts, eventually killing host

combine traits of parasites and predators

most are wasps or flies

typically host-specific

adults are free-living; only females search for host

only attack a particular life stage of one or several related spp.

eggs are laid in, on, or near host

immature life stage develops on or within a single insect host (feeds on body fluids and organs, ultimately kills host, leaves host to pupate or emerges as an adult)

can alter behavior of host

e.g. humpbacked flies, phorid flies used to control red imported fire ant populations, horse hair worms, lancet liver fluke, ophiocordyceps

humpbacked flies

parasitoids in family Phoridae, order Diptera

~4000 spp.

most occur in tropical regions worldwide

different spp. have highly varied lifestyles (predators, fungus feeders, dung-dwellers, decomposers of animal flesh, plant parasites living between leaf tissue layers, parasitoids)

red imported fire ant

invasive to US from central Brazil

being controlled using 2 spp. of decapitating parasitoid phorid flies

also trying a protozoan and fungus

horse hair worms

parasites of terrestrial and aquatic insects and freshwater crawfish

~320 spp

live in freshwater, leaflitter, and algal mats near edges of streams and ponds

adults have no mouth or functional digestive tract

causes infected insect to go crazy, jump in water and drown

adult then burrows out and is now in water again to lay eggs

lancet liver fluke

infects cattle and occasionally humans

alters behavior of intermediate host to facilitate spread

ophiocordyceps

fungal parasites of ants and other rainforest insects

ascomycete fungi (sac fungi) that includes ~400 described spp, ~140 of which grow on insects

“zombie ant”

host defenses against parasites

immune system (phagocytosis, antimicrobial peptides, natural killer cells, lymphocytes (memory cells))

biochemical defenses (e.g. transferrin that moves iron from plasma into cells, making parasite growth in blood very difficult; e.g. hemlock in diet of caterpillars targeted by parasitic flies, which increases chance of survival and causes development to adult to be faster)

parasite adaptations to circumvent host defenses

e.g. Plasmodium merozoites infect RBCs, which don’t grow and don’t need nutrients, but plasmodium adds channels to RBC surface to transport nutrients to cell. Plasmodium causes RBCs to stick to other cells, preventing movement to spleen when misshapen

e.g. parasitoid wasps laying eggs in Drosophila inject a virus that kills encapsulating cells that would normally cover and kill parasites/eggs

example of extinction caused by parasites

American chestnut once ranged from Maine to Mississippi, former abundance was ~3 million (~25-30% of all hardwoods in PA)

nuts were very important for wildlife

chestnut blight (fungus) killed nearly all in <30 years

~100 trees remain in original distribution

brood parasites

not really true parasites

but do reduce population growth rate (lambda)

lay egg in nest of another animal (usually bird)

brood parasite egg is larger, kicks out other eggs, takes all care and nutrients from mother bird