Flatworms and Tapeworms Notes

Part of the phylum Platyhelminthes (flatworms).
Approximately 5,000 species exist worldwide.
Size ranges from 1 mm (0.04 inch) to over 15 m (50 feet).
Internal parasites affecting invertebrates and the liver or digestive tracts of vertebrates, including humans and domestic animals.
Some use a single host, while others require one or two intermediate hosts and a final (definitive) host.
The disease caused by tapeworms is called cestodiasis.
Bilaterally symmetrical.
Some have a single long segment; others have a head (scolex) followed by segments called proglottids.
Scolex has suckers and hooks for attachment.
Tough cuticle for food absorption; lacks a mouth and digestive tract.
No circulatory system or specialized gas exchange organs.
Most are hermaphroditic and self-fertilizing; both male and female gonads are present in a proglottid.
Complex life cycle.

Scientific name: Taenia solium or Taeniarhynchus solium.
Found where raw pork is consumed.
Adult stage lives in the human intestine.
Each proglottid can contain up to 40,000 embryos in capsules after fertilization.
Embryos are passed in feces and can be ingested by mammals like dogs, camels, pigs, or humans.
Larva emerges in the digestive tract and bores into the intestinal wall, entering a blood vessel.
Carried to muscle tissue, where it forms a protective capsule (encysts) called a cysticercus or bladder worm.
If the cysticercus is eaten alive in raw meat, it attaches to the host's intestine and develops into an adult.

Scientific name: Dibothriocephalus latus or Diphyllobothrium latum.
Common in waters of the Northern Hemisphere.
Infests humans and other mammals that eat fish (bears, dogs).
Fertilized eggs pass from the host in feces.
In water, eggs develop into a hairlike larva and are eaten by tiny crustaceans.
Crustaceans are eaten by fish.
The tapeworm larva encysts in the muscle tissue of the fish.
When a mammal eats the fish, the larva attaches to the intestine and develops into an adult.
Fish hosts include trout, salmon, pike, and perch.

Soft-bodied, usually flattened invertebrates.
80% are parasitic.
Bilaterally symmetrical.
Lack specialized respiratory, skeletal, and circulatory systems.
No body cavity (coelom).
Body is not segmented; spongy connective tissue (mesenchyme) fills the space between organs.
Generally hermaphroditic.
Three embryonic layers: endoderm, mesoderm, and ectoderm.
Head region with concentrated sense organs and nervous tissue (brain).
Primitive compared to other invertebrates.
Consists of four classes: Trematoda (flukes), Cestoda (tapeworms), Turbellaria (planarians), and Monogenea.
Monogenea is sometimes considered a subclass within Trematoda.
All classes except Turbellaria are parasitic.
Over 20,000 species described.

Many species parasitize humans, domestic animals, or both.
Meat inspection has reduced tapeworm infestations in some regions.
Where sanitation is poor and meat is undercooked, tapeworm infestations are common.
The broad tapeworm (Diphyllobothrium latum) is prevalent in Baltic countries.
The dwarf tapeworm (Hymenolepis nana) is found in parts of the southern United States.
The beef tapeworm (Taenia saginata) is common in Europe and the United States due to eating undercooked beef.
Parasitic larvae can cause serious damage to the host.
The gid parasite of sheep (Multiceps multiceps) lodges in the sheep's brain.
Cysts of Echinococcus may occur in the body of sheep and can be fatal if found in the liver, brain, or lung of humans.

Thirty-six or more fluke species are parasitic in humans.
Infections are widespread in the Far East, Africa, and tropical America.
Many species are ingested as cysts (metacercariae) in uncooked food.
Examples:
- Lung fluke (Paragonimus westermani) in crayfish and crabs.
- Intestinal flukes (Heterophyes heterophyes and Metagonimus yokogawai) and liver fluke (Opisthorchis sinensis) in fish.
- Intestinal fluke (Fasciolopsis buski) on plants.
Blood flukes (Schistosoma species) cause schistosomiasis (bilharziasis).
S. haematobium is found in Africa and western Asia.
S. mansoni is found in Africa, western Asia, the West Indies, and South America.
S. japonicum is important in the Far East.
The sheep liver fluke (Fasciola hepatica) causes epidemics (liver rot) in sheep and animals become infected by eating metacercariae encysted on grass.
Monogenea are common pests on fish in hatcheries and home aquariums.

Turbellarians: Mostly less than 5 mm (0.2 inch) long; planarians may reach 0.5 metre (20 inches).
Trematodes: Mostly between 1 and 10 mm (0.04 to 0.4 inch) long; some grow to several centimetres.
Cestodes: Smallest are about 1 mm (0.04 inch) long; some species exceed 15 metres (50 feet).
Monogenea: Range in length from 0.5 to 30 mm (0.02 to 1.2 inches).
Aspidogastrea: From a few millimetres to 100 millimetres in length.

Free-living flatworms (turbellarians) occur wherever there is moisture.
Except for temnocephalids, flatworms are cosmopolitan.
Found in fresh water, saltwater, and moist terrestrial habitats, especially in tropical and subtropical regions.
Temnocephalids are parasitic on freshwater crustaceans and are found primarily in Central and South America, Madagascar, New Zealand, Australia, and islands of the South Pacific.
Gyratrix hermaphroditus (turbellarian) is cosmopolitan and tolerant of different ecological conditions, found in fresh water and saltwater pools.
Adult forms of parasitic flatworms are confined to specific vertebrate hosts; larval forms occur in vertebrates and invertebrates such as mollusks, arthropods (crabs), and annelids (marine polychaetes).