ch. 12 Helminths

Question 1

Characteristics of (parasitic) Helminths

Answer 1

• multicellular eukaryotic animals
• generally possess digestive, circulatory, nervous, excretory, and reproductive systems.
• highly specialized to live in hosts
• parasitic helminths differ from other helminths
  
  1. may lack a digestive system–absorb nutrients from host’s food, body fluids, tissues
  2. their nervous system is reduced–they do not need an extensive nervous system because they do not have to search for food/respond much to enviro
  3. Their means of locomotion is occasionally reduced or completely lacking–transferred from host-to-host, dont need to search for a suitable habitat
  4. reproductive system is complex–individuals produce a large number of eggs, by which a suitable host is infected

Question 2

life cycle

Answer 2

• can involve multiple intermediate hosts for completion of each larval stage and a definitive host for the adult parasite
• adults can be
  
  • DIOECIOUS–male repro organs in one adult and female in another (reproduction only occurs when two adults of opposite sex are in same host)
  • MONOECIOUS/HERMAPHRODITIC– one animal has both male and female organs (two hermaphrodites can copulate and fertilize each other; some can fertilize themselves)

Question 3

platyhelminths

Answer 3

• aka flatworms
• dorsoventrally flattened
• classes of flatworms include trematodes and cestodes

Question 4

trematodes

Answer 4

• aka flukes
• have flat, leaf-shaped bodies with a ventral and oral sucker
• obtain food by absorbing it through their outer covering called a cuticle
• asian LIVER fluke–found in liver
• LUNG fluke–in lungs–eggs layed in bronchioles, eggs in sputum are swallowed and passed out in excrement
• BLOOD flukes–in blood–enter blood by burrowing though tissue into blood stream (schistosoma)

ex. paragonimus spp.

Question 5

cestodes

Answer 5

• aka tapeworms
• intestinal parasites
• the head (scolex (like skull)) has suckers for attaching to the intestinal mucosa of the definitive host, some even have hooks
• tapeworms do not have a digestive system so they do not consume the tissue of their hosts
• they absorb nutrients through their cuticle
• body consists of proglottids–segments, which are continually produced by the neck region of scolex as long as the scolex is attached and alive. Each proglottid contains both male and female reproductive organs. The mature ones are furthest away from the scolex and contain eggs.

Ex. Taenia saginata–beef tapeworm
-proglottids are released in feces and find a new host, like a grazing animal. once ingested, the larvae hatch from the eggs and bore through the intestinal wall into the muscle and become cysticerci. Then humans eat the muscle (steak) and the scolex attaches to intestines and starts producing proglottids.

ex. Taenia solium (pork tapeworm
ex. Echinococcus granulosus–definitive host is dogs and coyotes–humans are dead ends but in other animals like deer, the larva develop into hydatid cysts that contain brood capsules that can produce thousands of scoleces

Question 6

Nematodes

Answer 6

• phylum nematoda
• AKA roundworms
• cylindrical shape and tapered at each end
• have complete digestive systems consisting of mouth, an intestine and an anus
• most are dioecious (males have male organs, female have female organs)
• Males are smaller and have hardened spicules on their posterior ends to guide sperm to female genital pore
• some are free living in soil and water, and others are parasites in plants and animals. Some nematodes pass their entire life cycle in a single host.
• intestinal roundworms are the most common cause of chronic infectious diseases.

Ex. Ascaris—hookworms and whipworms

infections can be divided into two categories: worms with infective eggs, or infective larva

Question 7

nematodes with infective eggs

Answer 7

• Ascaris lumbricoides–large nematode (30cm) infects over 1 billion people
  
  • dioecious with sexual dimorphism (males and females look distinctly different)
  • adult lives in human intestines only–feeds on undigested food
  • eggs are excreted in feces and ingested accidentally by another host–hatch in small intestine then burrow into blood stream where they go to lungs to grow. They are then coughed up and re-swallowed and returned to the small intestine where they mature into adults.
• Baylisascaris procyonis–raccoon roundworm; raccoon is the definitive host but dogs can be too–intermediate hosts can be rabbits or humans–once ingested the eggs hatch and the larva migrate through a variety of tissues causing a condition called larva migrans–infection often results in severe neurological symptoms or death.
• Larva migrans can also be caused by Toxocara canis (from dogs) or T. cati (cats)
• Trichuris trichiura–whipworm–spreads from person to person by fecal-oral transmission–often occurs in tropical weather.
• Enterobius vermicularis–pinworms–spend entire life in human host–adults found in large intestine–female migrates to anus to lay eggs on perianal skin. Eggs can then be ingested or or exposed through contaminated clothing or bedding

Question 8

nematodes with infective larvae

Answer 8

• strongyloides–live in soil but can enter human host through skin–can migrate to intestine causing abdo pain, or to lungs, causing a cough
• Necator americanus and Ancylostoma duodenale–adult hookworms–live in small intestine of humans–eggs excreted in feces into soil–they hatch and burrow into host skin and enter blood or lymph vessel which carries it to lungs–coughed up and swallowed–intestines
• trichinellosis–obtained by eating encysted larvae in undercooked meat.
• Dirofilaria immitis is spread from host to host via Aedes mosquitoes. Primarily affects dogs and cats but can infest human skin, conjunctiva, or lungs. Larvae migrate to various organs where they mature.
  ex. The heart worm–adult stage often in heart where it can kill its host through CHF.

-Anisakines–wriggly worms–found in fish and squid. When hosts die, they migrate from mesentery to muscle– freezing or thoroughly cooking the fish will kill the larvae

Question 9

arthropods as vectors

Answer 9

• arthropods–animals with segmented bodies, hard exoskeletons, and jointed legs
  
  • class arachnida (8 legs)- spiders, mites, ticks
  • class crustacea (four antennae): crabs, crayfish
  • class insecta (six legs): bees, flies, lice
• vectors: arthropods that carry pathogenic microorganisms

ex. mosquito carries west nile virus, yellow fever, and malaria
ticks carry lyme disease

not all vectors carry the microorganisms inside them, but sometimes just one them–on the feet of flies from feces for example.