Helminths

Biology 342

Helminths facts Globally 2 billion humans are infected with the

intestinal nematode, Ascaris lumbricoides which reaches 35 cm in length.

Hookworms infect people by boring through the skin of bare feet and live in the intestines of over a billion people.

Whipworms reside in 1 billion humans that were infected through feces contaminated food.

The tapeworm, Taenia solium living in the human intestine can reach a length of over 9 feet.

Schistosome trematodes infect 200 million people world-wide by boring through their skin when swimming in feces contaminated water.

Parasitic Helminths Groups

Platyhelminths – tapeworms

Nematoda – roundworms

Trematoda – flatworms

Acanthocephala

– thorny-headed worms

Helminth Eggs

Modes of Infection

Ingestion of contaminated vegetables, water, raw or undercooked meat, e.g., eggs of nematodes, cysts of cestodes and trematodes.

Direct penetration of skin, e.g., Schistosomes and hookworms.

Transmission by vector insects, e.g., Wuchereria, Onchocerca, and Dracunculus.

Geophagy - purposely or accidentally eating contaminated soil

Platyhelminths

Tapeworms

Hymenolepis is the most common tapeworm in humans with a prevalence of 5-20% but up to 50% in some parts of Australia. They are commonly ingested with insects in dry grain or cereal products.

Echinoccocus are dog tapeworms and people become infected by contact with contaminated dog feces. In humans a cyst develops in the liver.

Taenia tapeworms form cysts in pigs or beef and humans become infected by eating raw or undercooked meat.

Trematoda

Schistosoma mansoni (blood fluke), 160 to 200 million infections per year, 12,000 to 150,000 deaths per year, all tropical and subtropical regions.

eggs

cercaria

Adults (M & F)

Nematodes

Ascaris lumbricoides (roundworms), 800 to 100 million infections per year, 20,000 deaths per year, Africa, Asia, Central and South America.

Spreading Helminthiasis

There are many newly emerged zoonotic Helminth infections along with older existing types that have increased in incidence or geographic range.

1) Changes in dietary practices, e.g., dramatic increases in the ingestion of raw fish or fish products has introduced many new trematodes to the human population.

2) Increases in exotic companion pets such as fish, ferrets, rabbits, birds and reptiles have introduced new cestodes.

3) Most dramatically, changes in climate such as temperature and rainfall patterns is providing increased access for mosquitoes that vector dog, raccoon, and monkey nematodes.

Notable Helminth Diseases

Schistosomiasis

Onchocerciasis

Elephantiasis

Dracunculiasis

Enterobiasis

Baylisascaris procyonis

(raccoon roundworm) Widespread in raccoons in North America infecting 70 to 90%

of raccoons and ~90 species of wild and domestic animals.

The nematode lives in the intestines of raccoons (definitive host) and is transmitted via an oral/fecal route between them.

The nematode produces ~ 150,000 eggs per day deposited in raccoon latrines where they can be ingested by other raccoons.

Small mammals such as rabbits and mice, and birds (intermediate hosts) also ingest eggs and become infected.

If the nematode larva finds itself in an intermediate host it migrates out of the gut, travels in the blood stream to organs and eventually the brain where it alters the behavior of its hosts making them vulnerable to predation by raccoons.

Raccoon Roundworm

Eggs remain viable in the soil for years to decades resisting heat and cold.

The larva hatches inside the mouth or digestive tract where it grows to maturity in a raccoon or migrates to the brain in other hosts.

Human Infection with Baylisascaris

procyonis Primarily children that eat soil or play near raccoon

latrines and later put their fingers in their mouth.

The larva don’t recognize the human as a definitive host so they quickly migrate to the brain (3 days post infection).

Antihelminthic medicines are only effective before the larva enters the brain.

The larva travel in blood to organs (visceral larval migrans) into the eye (ocular larval migrans) and brain where they cause inflammation and brain damage.

After the larva enter the brain the prognosis is very poor for the infected person.

Enterobiasis: Human pinworm

Enterobiasis is caused by the human pinworm, a nematode, Enterobius vermicularis.

This nematode has a world wide distribution – and is the most common helminthic infection in the US.

Infection is most common in children but occurs in all ages running around 11% in US but is much higher in the rest of the world – e.g., 60% incidence in India.

The entire life cycle occurs in the gastrointestinal tract of humans – eggs are passed between humans via ingestion.

Symptoms include itching of anus and perineum at night, insomnia, irritability, weight loss and loss of control of urination (30% asymptomatic).

Enterobius vermicularis

Eggs are viable for 3 weeks in the environment

Eggs are sticky and easily adhere to objects such as fingers, finger nails, hands, clothing, linens, furniture, fixtures, and food – and survive in water.

Can be attached to dust particles and inhaled and swallowed.

Pinworm eggs have been found on fossil coprolites 10,000 years old in a cave in western Utah

Dracunculiasis The nematode is Dracunculus

medenisis occurs in Africa and Asia

It is transmitted by drinking water with larval infected micro-crustaceans – cladocera and copepods.

The worms grow (up to 1 meter long) and mature (1 year) in the abdomens of people and surprisingly also dogs.

The mature female worm burrows to surface forming a blister and emerges to lay eggs.

The emerging worm causes such a burning pain and terrible itching that people seek relief in water where the eggs are released.

Guinea Worm Disease

In 1986 there were 3.5 million human cases.

In 2014 there were only 126 cases.

The Carter Center and WHO is on track to completely eliminate the disease via Education programs Filtering drinking water Preventing infected

people from entering water systems