Parasitology lecture 8 Trematodes infective in the cercarial stage Schistosomes

What are Schistosomes?

They are trematodes (flukes) that cause schistosomiasis, also known as bilharzia.

Unlike many other trematodes, schistosomes have_______

Distinct male and female individuals

Schistosome eggs possess _______

Spines or a cutting tool, which aids in their migration through host tissues

The majority of pathology associated with schistosomiasis is caused by _______

Eggs as they attempt to exit the host

Infective stage of Schistosomes

The cercaria, a free-swimming larval form with a forked tail. It penetrates the skin of the definitive host directly, without encystment as a metacercaria

Schistosoma mansoni

Adults reside in the inferior mesenteric veins that drain the large intestine. Pathology primarily affects the large intestine and liver

Schistosoma japonicum

Adults found in the superior mesenteric veins that drain the small intestine. Pathology primarily affects the small intestine and liver

Schistosoma hematobium

Adults inhabit the vesical, rectal, and pelvic veins draining the lower large intestine (rectal area) and bladder. Pathology primarily affects the lower large intestine and bladder

The eggs of S. mansoni and S. Japonicum are passed through what

Eggs are released in the feces

The eggs of S. hematobium are passed through what

Urine, which hatches in water, releasing miracidia

After eggs of Schistosoma hatch, they penetrate what?

Miracidia are released from the eggs and they actively penetrate the tissues of a suitable snail intermediate host

Within the snail, miracidia of Schistosomes develop into what?

Sporocysts, which undergo successive generations of reproduction

What stage of the Schistosomes are released into the water?

Cercariae, which with their forked tails, penetrate the skin of the definitive host/

As cercariae penetrate the definitive host, they lose their tails and transform into what?

Schistosomulae

Schistosomulae migrate through the circulation to the __________ where they mature into adult worms.

Portal blood in the liver

Egg deposition of Schistosoma

Adult female worms lay eggs in the venules. These eggs either penetrate the intestinal wall or bladder wall to be shed in feces or urine respectively, or they can become trapped in tissues

Which two Schistosoma species can have animal reservoir hosts?

Schistosoma mansoni and Schistosoma japonicum (e.g., dogs, cats, mice, pigs)

Schistosoma hematobium typically does not have ________

Animal reservoir hosts

Where is Schistosoma mansoni found

Africa, Middle East, South America (including Caribbean islands)

Where is Schistosoma hematobium found

Africa and the Middle East

Where is Schistosoma japonicum found

Endemic to the Far East (China, Southeast Asia

Cercarial penetration of Schistosoma mansoni

Causes Swimmers itch— Cercarial penetration of the skin can cause an itchy rash when in contact with water. Goes from Papule → Vesicle → Lesion

When drinking water containing cercaria, they can _______

penetrate the esophagus

Incubation period of Schistosoma mansoni

Larval development in liver hepatic portal vein leads to fever, diarrhea, cough and abdominal pain often referred to as Katayama fever

Pathology related to egg deposition

The spines on the eggs are crucial for their migration. The female worm deposits eggs in venules, and the spines anchor the egg to the vessel wall. Blood flow and pulsations cause the egg to move, and the spine acts like a cutting tool to penetrate the vessel wall and move into surrounding tissues. Peristalsis in the intestine or pressure in the bladder aids their movement towards the lumen.

Early pathology of intenstinal Schistosomiasis

Inflammation,abscesses,diarrhea, dysentery, and papilloma formation due to egg granulomas

Chronic pathology of intestinal schistosomiasis

Tissue proliferation and repair- over time egg deposition causes fibrous thickening of the gut wall, diminished muscle tone, increased risk of colon cancer

Arteriovenous shunts of Intestinal Schistosomiasis

Long term damage to vessels of large intestines can lead to esophageal varices (due to portal hypertension) and hemorrhoids. Bleeding from esophageal varices is a major cause of death by exsanguination

Egg Granulomas in Liver of Hepatic Schistosomiasis

Eggs that are not shed can be carried to the liver and deposited in it’s blood vessels, triggering granuloma formation around them

Hepatic Fibrosis and Cirrhosis of Hepatic Schistosomiasis

Large numbers of trapped eggs can lead to severe fibrosis (Pipe stem necrosis of hepatic blood vessels) and eventually cirrhosis

Splenomegaly and Hepatomegaly of Hepatic Schistosomiasis

Enlargement of the spleen and liver can occur due to egg infiltration and inflammation