Blood fluke, Schistosoma mansoni

Blood flukes (Schistosoma mansoni) are internal parasitic flatworms that belong to Phylum Platyhelminthes, class Trematoda. They infect human blood and organs.

Adult blood flukes have separate sexes and are less than 1 cm long. The adult male is about 6-10 mm long and has a groove running the length of its body. The female, which is longer and thinner than the male, lives within this groove.

They are found primarily in North Africa, Southeast Asia, and other tropical areas. The World Health Organization (WHO) considers blood fluke infection to be the second most important parasitic disease, next only to malaria, with hundreds of millions infected worldwide.

Blood flukes have complicated life cycles that involve at least two different host animals.

Life cycle:

In developed countries with proper sewage systems, fluke eggs are usually destroyed in the sewage treatment process. However, in many undeveloped parts of the world, blood fluke eggs flourish due to unsanitary conditions and lack of proper sewage treatment.

Adult fluke in blood vessels surrounding human

intestine

Many eggs cause blood vessels to burst and

release eggs/blood into intestine

Eggs come out in human feces and get

into waterEggs hatch; larvae burrow

into snail

Larvae multiply (asexually) inside

snail

Swimming larvae released into

water

Larva burrows into human

skin

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Intermediate host: snail

Primary host: human

Beef Tapeworm, Taenia saginata

Taenia species are parasitic tapeworms that belong to phylum Platyhelminthes, class Cestoda. The species Taenia saginata is known as the beef tapeworm. These parasitic worms infect humans who eat contaminated beef.

Adult tapeworms are hermaphrodites. They are long (usually range from 3-5m but can grow up to 12m!), flat, whitish in colour, and consist of many proglottids (sections that make up the body). Tapeworms also have a head called a scolex on which there are several suckers and a ring of hooks; this helps them attach to the intestinal wall of the human host.

A single tapeworm proglottid may contain up to 100,000 eggs and a single worm can produce more than half a billion eggs each year!

Tapeworms can be found anywhere in the world where beef is consumed, but are most common in Africa, some parts of Eastern Europe, Southeast Asia, and Latin America.

Life cycle:

Tapeworms occur where cattle are raised by infected humans with poor hygiene, where human feces are improperly disposed of, where meat inspection programs are poor, and where beef is eaten without proper cooking.

Adult tapeworm in human intestine

Proglottids (with eggs) in human

feces fall to ground

Cow eats grass contaminated by

eggs

Human eats infected cow muscle (beef)

Immature worms hook onto intestine

and grow

Eggs hatch and larvae encyst in

cow muscle

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Primary host: human

Intermediate host: cow

Adult ascarid in human intestine

Female lays eggs which come out in feces

Another human consumes water/food contaminated with human waste swallows eggsEggs hatch in intestine and burrow up to lungs

Coughed out of lungs, climb to mouth

Larvae are re-swallowed

Human Ascarid, Ascaris lumbricoides

Ascaris is the largest human intestinal nematode. Species that are closely related to human Ascaris affect horses, cattle, pigs, chickens, dogs, cats, and many other animals. Ascaris and its relatives are collectively known as ascarids.

Human adult ascarids have separate sexes (male and female) and live in human intestines. Female ascarids produce many eggs (up to 300,000 per day!) that leave the host’s body in the feces. They are pale pink in colour with a bright red stripe down each side of their bodies and can grow up to 40 cm long and be as thick as a pencil (0.5 cm).

The World Health Organization (WHO) estimates that roughly 1/4th of the world's population is infected by Ascaris.

Human ascarids can be found all over the world but are not very common in Canada due to the cold weather and sanitary habits. They flourish in tropical regions as the warm and humid conditions facilitate their development.

Life Cycle:

Larvae can cause considerable damage to their hosts by entering blood vessels and burrowing through organs to the lungs. There may be up to 5000 adult worms in a human host and even 100 can block the intestine and cause host death. A few worms in the liver can also kill a host.

Hookworm, Ancylostoma duodenale + Necator americanus

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Hookworms are internal parasitic nematodes. They are more dangerous than Ascaris worms and infect human blood and burrow through the lungs of their hosts.

Adult hookworms have separate sexes and are grayish white or pinkish in colour with a slightly bent head. The adult male is about 1 cm long, while the female is often longer and stouter than the male.

Hookworms live in tropical regions where eggs can develop in warm, moist soil; Ancylostoma duodenale flourishes in the Middle East, North Africa, and India, while Necator americanus flourishes in the Americas (particularly the southern US), Sub-Saharan Africa, Southeast Asia, China, and Indonesia.

They are thought to infect more than 600 million people worldwide. They can devour enough blood to cause iron-deficiency anemia, weakness, and poor growth.

Life cycle:

In developed countries with proper sewage systems, hookworm eggs are usually destroyed in the sewage treatment process. However, in many undeveloped parts of the world, eggs flourish due to unsanitary conditions and lack of proper sewage treatment.

Filarial worms (including Eye Worms and Guinea Worms)

Larvae reach small intestine, mature, and feed by hooking

onto intestinal wall and sucking blood

Eggs come out in human feces and get into soil

Eggs hatch into larvae in the

soil

Larvae burrow into the skin of a human host

Larvae in bloodstream carried

to the heart and lungs

Coughed up larvae are swallowed

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Filarial worms are internal parasitic nematodes that live in humans. There are 8 different types of filarial worms that can infect humans and are categorized by whether they infect 1) human tissue/skin, 2) human lymphatic system, or 3) human stomach, lungs, and heart.

Adult filarial worms have separate sexes and are small and threadlike. They are found primarily in tropical and sub-tropical regions of Asia, Africa, Central and South America, and Pacific Island nations.

In severe infections, large numbers of filarial worms may block the passage of fluids within lymph vessels, causing elephantiasis (a condition where the body swells enormously).

Filarial worms are transmitted from one primary host to an intermediate host through biting insects, particularly mosquitoes.

Life cycle:

More than 120 million people in the world are infected with filarial worms , and 1 billion are at risk of infection. Fortunately, filarial worm infections can be treated with various drugs, although not everyone may be able to receive treatment.

Female worm gives birth to live

microfilarial worms

Mosquito sucks up microfilarial worm

during a blood meal from human

Develop into larvae in mosquito

Mosquito injects larvae into another

human during a blood meal

Mature into adult

worms

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Intermediate host: mosquito

Primary host: human