Life cycle of Ascaris spp. from humans and pigs
1. Ingestion of infective eggs
2. Release of larval form in the intestines
3. larvae penetrates duodenal wall and enters the bloodstream
4. larvae is carried to the liver and heart and then enters the pulmonary circulatory
system (note dog/cat Ascaris cannot enter pulmonary system and instead migrates
into deeper tissue)
5. Larvae are found in the aveoli of lungs where they grow and molt in three weeks
6. Mature larvae pass through the respiratory system where they are coughed up,
swallowed and returned to small intestines
7. Male and female worms mature in jejunum (small intestines)
Male fertilizes female—female lays 200,000 eggs per day for up to a year.
8. Eggs can be found in feces 60-75 days after the initial infection.
Life cycle of Enterobius vermicularis from humans
1.
2.
3.
4.
5.
Ingestion of embryonic eggs
Larvae hatch in small intestines and migrate to large intestines
Larvae mature into worms in large intestines in 2-6 weeks
Male fertilize female who lays 20,000 eggs per day in perianal folds
Eggs mature rapidly and are infectious within hours
a. Eggs hatch in perianal folds and larvae migrate back into intestines
b. Eggs can be transmitted from hands to mouth/food/inanimate objects by
children scratching in perianal folds in response to irritation by migrating
female worms
c. Eggs can be transmitted to clothing and bedding
d. Eggs can survive long periods in dust that accumulates under the bed and
over the doors and windowsills of infected people
6. Individuals breath in egg laden dust—swallow eggs—that leads to infection or reinfection
Life cycle of Trichinella spiralis—pig (wild game)
Adult form lives in the small intestines of pigs and other omnivores, infectious,
encysted larval forms are found in the striated muscle of these animals
1.
2.
3.
4.
5.
6.
Meat that contains encysted larvae are ingested
Larvae leave the meat in the small intestines
Within 2 days the larvae develop into adult worms
A single fertilized female produces 1500 larvae in 1-3 months
Larvae move from intestinal mucousa into the bloodstream
Larvae are carried by the circulation into the muscle sites throughout the body
7. The larvae coil in striated muscle fibers and become encysted
a. Muscles around the eye and muscles involving the tongue, chest,
diaphragm and calf are most often infected sites
8. Encysted larvae remain viable for many years—but are only infectious if ingested
by a new animal host—humans are dead end hosts for this worm.
Life cycle of pig (T. solium) and bovine (T. saginata) tapeworms
Pigs and cows are intermediate hosts for these worms but human feces can infect
cows and pigs and other humans.
1. Pigs and cows eat food/water grass contaminated with Taenia eggs.
2. The eggs hatch in the stomach-releasing an embryo
3. The embryo penetrates the intestinal wall and enters the bloodstream
4. The circulatory system carries the embryos to the muscle of the animal
5. In 3-4 months the embryos develop into larvae and become encysted in the
musculature
6. Humans eat the undercooked pork or beef.
7. The encysted larvae in the food enter the stomach where the larvae are released
from the cyst
8. Within 2 days the larvae mature into an adult tapeworm colony—that attaches to
the intestines via the scolex
9. Pregnant proglottids (segments) as well as their eggs can be released into the
environment (go to step 1)
Humans infect other humans if they expose others to gravid proglottids or eggs that
are released into the perianal folds or in the stool.
Life cycle of the Fish tapeworm (P. latum)
1. Eggs are released from humans and found in sewage and freshwater
2. Eggs require 2-4 weeks to develop into pre-larval forms
3. Small crustaceans eat the pre-larval forms—where the immature larvae develop
into mature larvae
4. Fish eat the small crustaceans and the larva migrate to the musculature of the
fish
If a larger fish eats the smaller fish—then the larvae simply transfer to the
musculature of the larger fish.
5. Humans eat raw or undercooked fish and the larvae develop in humans.
Pig tapeworms undergo a complex life cycle in humans that lead to severe disease
Fish and cow tapeworms remain in the G-I tract of humans.
Life cycle of the giant intestinal fluke (F. buski). Other flatworms undergo a similar
life cycle and all flatworms require snails as a primary intermediate host.
1. Eggs from a number of animal sources are released into water (humans, pigs
rabbits, dogs etc.)
2. Eggs hatch to release a FREE SWIMMING larval form
3. The larva swims to snails and produces enzymes that allows it to penetrate the
soft tissue of the snail
4. The free swimming larvae develop in snail and is released as a mature free
swimming larvae.
5. The mature larvae swim to water chestnuts plants, watercress, bamboo etc and
form cysts on these plants at which point it is infectious.
6. Humans eat the raw water vegetables
7. The infectious larvae are released into the stomach and travel to the intestines
where they develop into immature flukes
8. Flukes attach to the intestinal mucosa with its sucker and develop into an adult
form.
9. The flukes undergo self-fertilization and releases eggs approximately 3 months
after the initial infection.
The life cycle of the liver and lung flukes are similar to that of the great intestinal
fluke outside of the human. O. sinesis (Chinese liver fluke) and P. westernmani
(lung fluke) have a second intermediate host, fish and crabs or crayfish,
respectively.
However inside the human: the liver fluke larvae migrate to the bile duct and
eventually to the liver and the lung fluke migrates to the pleural cavity and
eventually to the lungs.