Chapter 5....associated with the below chapters with respect to diseases

Chapter 18-23 - Eukaryotic Organisms [Fungi, Protozoans, Helminths, & Arthropod Vectors]

 Features that distinguish protozoal & helminthic infections :

1.)    More important in tropical countries than in countries like the U. S. However, parasitic diseases are becoming more prevalent in the U. S. as more infected people move here; also because people with immune deficiencies such as AIDS are more susceptible to certain parasites.

2.)   How the immune system responds to these parasites is a mystery.  An immune response is activated, but the immune system is seldom able to rid the body of them.

3.)   They have more complex life cycles, with multiple hosts involved. 

 

I.  FUNGI  

 

A.  Some General Characteristics:

         eukaryotic cells

         nonmotile

         heterotrophic (use organic compounds a carbon source; they can’t make their own sugars; no photosynthesis)

         prefer more acidic conditions than bacteria

         can tolerate higher osmotic pressure and lower moisture than bacteria

         larger than bacteria and have more cellular and morphologic detail

         cannot tolerate the high temps. that bacteria can (fungal spores aren’t as resistant as bacterial spores)

         most are aerobic; some are facultative anaerobes (ex. yeasts) & some are anaerobes

         important in ecosystems as decomposers (called saprophytes - they obtain nutrients by decomposing dead & decaying matter); some are parasites, causing disease (mycosis; mycoses is plural); some produce toxins that cause disease (mycotoxicosis; mycotoxicoses is plural).

         major cause of plant diseases

         the study of fungi is mycology

 

B.  General Morphology:

         most, with exception of unicellular species, have a vegetative structure called a mycelium (a multinucleate mass of cytoplasm enclosed within a system of rigid, branched, tube-like filaments called hyphae).

         hyphae can be coenocytic (undivided network of branching tubes) or have septa (cross walls).

         cells walls are composed of cellulose, chitin (contains nitrogen - also found in the exoskeletons of insects, crayfish, etc.), or a combination of the two.

         specific morphology will be discussed later for each group of fungi

      

C.  Reproduction - Fungi are classified by how they reproduce (sexually or asexually).

            [functions of spores include dissemination and reproduction]

           

1.       Asexual Reproduction - Occurs by elongation of hyphae, budding, or asexual spore production. 

                       

Asexual spores are specialized cells that are dispersed & germinate in a favorable environment to produce a new fungus; they are products of a type of cell division called mitosis (one cell divides to form 2 daughter cells that are identical to one another and to the original parent cell).  Types:  sporangiospores , conidiospores.

 

2.      Sexual Reproduction - Occurs by producing sexual spores, which form following sexual fusion of gametes (similar to sperm & eggs).  Types:  zygospores, ascospores, & basidiospores.

 

D.  2 General Groups of Fungi - Yeasts vs. Molds

[These are descriptive terms, not taxonomic!  These organisms belong to many groups of fungi.]

 

            1.  Yeasts - characteristics:

         nonfilamentous, unicellular

         reproduce asexually by budding

         reproduce sexually by producing various kinds of spores

         aerobic or facultative anaerobes

         used to prepare bread, wine, beer, etc.  (fermentation of carbohydrates produces ethanol & carbon dioxide)  ex. Saccharomyces cerevisiae  (cervesa means beer in Spanish)

         some are pathogenic; ex. Candida albicans  (causes yeast infections, thrush; see below)

 

            2.  Molds - characteristics:

         filamentous, multicellular

         have a vegetative structure called a mycelium (a multinucleate mass of cytoplasm enclosed within a system of rigid, branched, tube-like filaments called hyphae).

         hyphae can be coenocytic (undivided network of branching tubes) or have septa (cross walls).

         also possess reproductive hyphae which produce different kinds of spores (discussed above and below)

         see below for examples.

  

E.  Classification of Some of the Lower Fungi:

 

            1.  Zygomycetes:

 

a.       Characteristics:  coenocytic hyphae, produce sporangiospores (asexual spores) & zygospores (sexual spores).

b.      Ex.  Rhizopus nigricans  - black mold that develops on stale bread; the tiny black dots on the mold are the sporangia, which hold the sporangiospores; sporangia look like tiny mushroom caps.

c.       can be opportunistic; some are pathogenic in the immnocompromised        

 

F.  Classification of Some of the Higher Fungi:

 

       1.  Ascomycetes (Sac Fungi)

 

a.       Characteristics:  includes molds with septate hyphae and some yeasts; ascospores (sexual spores) develop within sacs called asci (sing. ascus); also produce conidiospores (asexual spores).

 

                        b.  Examples:

1.)    Saccharomyces cerevisiae  - yeast is used to make beer, bread, wine; cervesa means beer in Spanish.

2.)   Trichophyton - causes athlete's foot (tinea pedis); ringworm of the feet; other species infect different parts of the body (dandruff, nail fungus, jock itch)

3.)   Penicillium  spp. - conidiospores form long chains on branching conidiophores, creating a brush-like structure that looks like a broom (penicillus means “brush"); some species produce the antibiotic penicillin.

4.)   Aspergillus  spp. - form long chains on a globelike conidiophore; cause aspergillosis, a pulmonary disease of animals & humans; infection is often secondary to tuberculosis, immunodeficiency, & steroid therapy.

5.)   Histoplasma capsulatum - causes Mississippi Valley fever (histoplasmosis); can get from bird droppings and bat guano; endemic disease in this area; pulmonary disease.

6.)   Candida albicans  - part of our natural flora; opportunistic; becomes a problem when defenses are weakened or balance of microbes is upset (ex. from antibiotic treatment); cause of vaginal & intestinal yeast infections & thrush in the mouth ("cottage cheese patches") - called candidiasis.

             2.  Basidiomycetes (Club Fungi)    

a.       Characteristics:  many form basidiocarps (mushrooms, puffballs, or shelflike bodies on trees); some are molds, a few are yeasts; produce conidiospores; also produce basidiospores (sexual spores); basidiospores form on the "gills" of mushroom basidiocarps.

                   b.   Examples:

1.)    Amanita  - poisonous mushroom; toxin causes a mycotoxicosis

2.)   Cryptococcus  - yeast cells surrounded by a capsule; causes fatal meningitis (cryptococcosis); transmission – inhalation of contaminated dust; found in 8% of AIDS patients.

 

3.  Deuteromycetes (Imperfect Fungi)

a.       Characteristics:  called the imperfect fungi because no sexual stage has been observed; we put them in this group until a sexual stage is observed; these fungi grow as yeasts or molds; identify on basis of shape & arrangement of their conidiospores (asexual spores); some species are pathogenic; many of these fungi have recently been placed in other phyla.

 

G.    Dimorphic Fungi - Some fungi switch between a single-celled yeast phase of growth & a mycelial phase (called dimorphism); discovered by Pasteur; some species will switch if oxygen supply decreases.  Pathogenic dimorphic fungi are mycelial outside of the host & single-celled inside the host.  With pathogenic species, it is usually high body temperature that causes the switch.  Candida changes in response to the higher nutrient concentrations found in the body.  The problem with dimorphism is that single cells are more readily spread in bloodstream, leading to systemic infections.

 

H.  Mycoses (Fungal Diseases)

         Humans usually acquire fungal disease from nature; they are not highly contagious.

         mycotoxicosis vs. opportunistic mycoses:

         See above for diseases

         Some produce toxins that are hallucinogenic; ex. muscarin - produced by a mushroom

         Some produce toxins that are highly poisonous; ex.

1.)    Claviceps (rye mold) - produces ergot; causes death to anyone eating bread made from contaminated rye; LSD is made form fruiting structures (causes hallucinations)

2.)   Aspergillus – produces aflatoxin; which grows in many plant materials; low levels of toxin can be carcinogenic.

3.)   Amanita  - poisonous mushroom

           

I.  Antibiotics:  Penicillins produced by Penicillium ; Cephalosporins produced by Cephalosporium.

 

II.  PROTOZOA

 

A.  General Characteristics:

         Unicellular eukaryotes.

         The protistan lineages continue into the kingdoms of plants, fungi, and animals.

         Limited to a moist environment because they lack a cell wall

         Heterotrophs

         Most reproduce asexually by fission (one cell divides to form 2 identical daughter cells & budding; some (ex. Plasmodium that causes malaria) under go schizogony (multiple fission).  Sexual reproduction occurs by conjugation, the fusion of vegetative cells, or by the fusion of specialized gametes called gametocytes.

         Some have complex life cycles, requiring multiple hosts and changing their morphology (ex. Plasmodium  uses the mosquito as an intermediate host)

         Trophozoite - active, motile, feeding stage of protozoans; parasitic stage that causes the disease in the host.

         Cyst - resistant, inactive stage; how diseases are usually transmitted by the fecal-oral route; usually more useful than trophozoites for lab identification.

 

B.  Classification:  [based on mode of locomotion or motility]

 

       1.  Mastigophora or Zoomastigophora (move by means of flagella)

a.       Trypanosoma gambiense - infects the blood and tissue fluids; causes African sleeping sickness (it leads to the loss of consciousness and death when it invades the  CNS); can also infect cattle; vector is the tsetse fly.

b.      Giardia lamblia  - body has the appearance of a human face (4 “eyes” are nuclei); have 2-6 flagella; form cysts; causes a waterborne dysentery (traveler's diarrhea); one of the “don’t drink the water” diseases; firs sigh is usually an explosive, foul-smelling watery diarrhea followed by copious amounts of campers are a high-risk group because of a sylvatic cycle (parasite is found in mountain streams contaminated with human feces or animal feces, especially beavers).

                        c.  Trichomonas vaginalis  - causes vulvovaginitis; numerous flagella     

  

2.    Sarcodina (move by means of pseudopodia or "false feet" - temporary extensions of the cell body caused by protein filaments of the cytoskeleton pushing on the cell membrane); feed on algae, bacteria, and other protozoans by phagocytosis.

a.       Amoeba proteus - freshwater; not pathogenic

b.      Entamoeba histolytica - causes amoebic dysentery; usually acquired by consuming fecally contaminated water or food; flies and cockroaches can also be mechanical vectors; produce cysts; first protozoan to be shown to be a pathogen (1875); one of the “don’t drink the water” diseases; trophozoites may invade the intestinal mucosa where they can cause ulceration and escape into the blood vessels; they may allow bacteria in fecal material to enter the body cavity and cause peritonitis.

c.       Naegleria fowleri - causes amoebic meningioencephalitis; usually seen in swimmers.

d.      Acanthamoeba polyphaga – accumulates on the water surface of contaminated hot tubs when tubs are covered; cause ulceration of the eyes and skin; can invade the central nervous system and cause meningioencephalitis.

           

       3.   Ciliophora  (move by means of cilia)

a.       Paramecium caudatum  - freshwater; not pathogenic

b.      Balantidium coli  - only ciliophoran that causes disease; produces cysts; causes diarrhea of large intestine; rare except in the Philippines; symptoms are similar to those of amoebic dysentery.

           

4.   Apicomplexa or Sporozoa or Haemosporina - Basically nonmotile.  All have an infectious, sporelike stage (sporozoite) that is often transmitted to new hosts by an insect vector.  All are parasitic (obligate parasites - cannot live apart from the host).  Some have elaborate life cycles, changing body form (trophozoite, sporozoite, merozoite); life cycle includes schizogony (multiple fission).  Examples

a.     Plasmodium vivax  - causes malaria; vector is the mosquito; kills 1-3.5 million people each year; malaria = bad air; used to infect people with malaria to stop the progression of syphilis (fevers would kill the bacteria).

b.    Toxoplasma gondii  - causes toxoplasmosis; humans acquire the disease by consuming cysts in the meat of infected animals or ingesting material contaminated by cat feces containing the parasite (can get it from cleaning the litter box - doctors warn pregnant women not to do this).

c.     Cryptosporidium - form cysts; cause enteritis & diarrhea; can occur in water supplies; can also be transmitted by fecal-oral transmission from kittens/puppies; resistant to chlorine (it can survive full-strength Chlorox!); threat only AIDS patients and those immunocompromised; no effective treatment found.

d.    Pneumocystis carinii  - may be a fungus!!; causes pneumocystis pneumonia; spread in respiratory droplets; common in AIDS patients.

  

III.  HELMINTHS – Flatworms & Roundworms

 

General Characteristics:

       Animals

       Cephalization - concentration of sensory receptors toward the anterior end.

       Organ/system level or organization/

       Sexual reproduction.  Most flatworms are monoecious (male & female reproductive organs in same animal). Roundworms are dioceious (separate sexes).

 

A.   Platyhelminthes (Flatworms = Trematodes + Cestodes) - most are free-living; marine and freshwater; predators, scavengers, or parasitic; some have regenerative capabilities.

 

  1. Trematoda  (Flukes) - all parasitic of vertebrates; have complex life cycles that include sexual and asexual phases; they require at least 2 kinds of organisms to complete the cycle - they reach sexual maturity in a primary or definitive host (always a vertebrate), their larval stages develop or become encysted in an intermediate host (usually an invertebrate).

 

            a. Clonorchis sinensis  (Chinese or Human Liver Fluke)

       Adults live in bile ducts (in the liver) of humans (definitive host)

       Intermediate hosts:  snail (first) and fish (second)

       Life cycle:  a snail ingests the eggs; the eggs hatch & release a larval stage which goes through several transformation before finally forming a tadpole-like cercariae; the cercariae bore through the flesh of the snail, & escape into the water; they swim until they find the appropriate species of fish; they encyst in the muscle tissues of the fish (forming metacercariae); the adult flukes develop in livers of humans who eat raw, infected fish; eggs of the parasite are excreted in the feces; when human feces end up in ponds, etc., snails ingest the eggs & the cycle repeats itself.]

 

b. Schistosoma mansoni  (Schistosomes or Blood Flukes) - adults live in circulatory system; spiny eggs break through the blood vessel wall and through the gut wall to be expelled in feces; eggs hatch into cercaria in water; cercaria then penetrate skin when a person is bathing or swimming; cause spleen and liver enlargement, dysentery, and cirrhosis of the liver

 

  1. Cestoda (Tapeworms) - intestinal parasites of vertebrates; no digestive system like in trematodes & nematodes; they absorb nutrients through their tegument!

 

Morphology:  scolex (head) with suckers and/or hooks (for attachment), proglottids (body units - each one has male and female reproductive organs):  immature proglottids (closest to the scolex), mature proglottids (next closest to the scolex), and gravid proglottids (furthest from the scolex - in these proglottids, the uterus is filled with eggs).

  

General life cycle of tapeworms: the gravid proglottids break off and are passed in the definitive host's feces; larval forms hatch when the eggs are ingested by the intermediate host; larvae then encyst in the intermediate host (called a cysticercus or bladder worm); adult worms usually develop in the definitive host when raw or poorly cooked infected meat is eaten.  Examples:

 

a.     Taenia solium  (pork tapeworm) – reaches a length of 2-7 meters; primary host: humans, etc.; intermediate host: swine

       Humans can be infected with the adults by consuming rare pork containing cysticerci larvae; larvae then develop into adults in digestive tract of the human.

       Humans can also be infected with larval forms when they accidentally ingest eggs (they get them from other infected humans who contaminate food, etc. with the eggs when they don’t use proper hygiene after going to the bathroom).  In this case every organ in the body may harbor cysticerci.  When a cysticercus dies, it releases toxins and usually causes a severe allergic reaction, which is sometimes fatal.

 

b.    Taenia saginata  (beef tapeworm) – reaches a length of 5-25 meters; primary host: humans, etc.; intermediate host:  cattle, sheep, etc.; life cycle similar to that of         T. solium above; beef riddles with encysted larvae is called “measly beef.”

 

c.     Echinococcus granulosus  (dog tapeworm); small - only 3 proglottids long; typical life cycle: 

       dogs are infected by adults when they eat raw butchered livestock containing larvae (ex. raw bones, etc.)

       eggs are passed in feces of dog; livestock eat vegetation with eggs when grazing

       larvae hatch and encyst in the muscle tissue of livestock

 

Humans can get hydatid cysts (larvae) from ingesting the eggs (the eggs are passed in feces of dog, dog licks himself, then dog licks your face).  These cysts develop in the liver, lungs, and brain.  Each fluid-filledcyst, containing many larvae, can reach the size of a grapefruit.

 

d.    Dipylidium caninum  (dog & cat tapeworm) - often seen in children; flea is the intermediate host - it eats the eggs on an animal; larvae develop in flea; if a dog, cat, or human ingests the flea, the adult will develop.  Note:  Larvae are not transmitted through the bite of the flea!!

 

e.     Hymenolepis nana (dwarf tapeworm) - most common tapeworm of humans in the world; intermediate host is a grain beetle; humans can ingest the eggs in cereals and other foods that contain parts of the insects; intermediate host is optional (meaning that if you ingest the eggs of this worm, you get an adult infection).

  

B.    Nematoda (Nematodes)

 

   General characteristics:

       Nematodes are everywhere!!!!  They are freeliving in soil, fresh & salt water, & are parasitic in plants and animals.

       Dioecious (separate sexes).

       Possess a nonliving cuticle, which is secreted by the epidermis and is resistant to the digestive enzymes of the hosts.

       More highly developed than flatworms.

       Adults do not latch onto the host like the tapeworms.

 

1.       Ascaris lumbricoides (intestinal roundworm of pigs, horses, humans)

       largest intestinal nematode in humans.

       disease is called ascariasis

       Eggs can remain viable even in preservative!

       Females are longer; males are shorter & have a hooked posterior end.

 

2.       Enterobius vermicularis  (pinworm)

       parasitizes large intestine of humans (especially children)

       at night the females migrate to the perianal region to lay eggs

       their presence there causes itching

       hosts become infected by accidentally ingesting the eggs.

 

3.       Ancylostoma caninum & Necator americanus  (hook worms)

       larvae can penetrate the skin of a barefoot person

       once inside, the larvae travels through the bloodstream to the lungs, moves up the trachea, is coughed up & swallowed; it matures in the small intestine.

       Ancyclostoma has “fangs”; Necator has cutting plates.

 

4.       Trichinella spiralis  (pork roundworm)

       humans usually become infected by eating insufficiently cooked pork

       larvae are encysted in the muscle tissues of the pig

       causes trichinosis; larval migration can cause death.

 

5.       Wucheria bancrofti 

       adults live in lymph nodes where obstruction of lymph vessels leads to grotesque enlargement of these nodes & to a condition called elephantiasis

       a mosquito is the intermediate host.

 

6.       Dirofilaria immitis  - causes heartworm disease in dogs; a few cases in humans & cats; mosquito is intermediate host.

 

7.       Trichuris trichiura - (whipworm) - adults partially embed in the mucosa of the large intestine; each adult produces 1,000-7,000 eggs/day!

 

Return to Chp. index