Answer to Case 682

 Answer to the Parasite Case of the Week 682: Schistosoma mansoni egg.

The following post is from our first ever guest author, Hadel Go. I think you will all agree that Hadel did an outstanding job writing up the answer to the case of the week, and that this is truly one of the best case answers we have had on this blog.

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Hadel Go, Medical Student, Guest Author

This is a Schistosoma mansoni egg in lung tissue as many of you correctly identified in the comments. The large lateral spine is a dead giveaway and creates the “quote bubble” morphology mentioned by Jacob @eternalstudying on Twitter. See more images here: Schistosoma mansoni, eggs, tissue (parasitewonders.com).

Here is an image of a S. mansoni egg in an unstained stool wet preparation.

The burning question in this case is: what is a Schistosoma mansoni egg doing in the lung? Typically, S. mansoni eggs are found in the stool, intestinal tract, and liver of infected humans. Paired adults (male and female) anchor themselves in a venule of the intestinal wall (often in the inferior mesenteric veins) and release eggs. Some of these eventually make their way through the intestinal wall into the lumen and out of the body with the stool, thus allowing the parasite's life cycle to continue. However, many of the eggs remain in the venule and travel via the portal circulation to the liver, where they become lodged and cause granulomatous inflammation.

Chronic infections and high parasite burden can induce portal hypertension due to obstruction by granulomas. This results in shunting and embolization of the eggs in ectopic sites outside the liver and intestines. 

Eggs can also occasionally be seen in the lungs during the acute stage of infection, during which larval migration, maturation and initial egg production occurs. Symptoms of acute pulmonary schistosomiasis occur within 2-8 weeks after exposure and patients present with a dry cough, dyspnea, wheeze, night sweats, or chest pain (as seen in this patient). Chest x-ray and CT imaging of the lungs will usually show small bilateral ground glass nodules measuring 2-15mm. Complications can include pulmonary arteritis and cor pulmonale. 

DID YOU KNOW?

Schistosomiasis is also known as bilharzia, named after the German physician and parasitologist Theodor Bilharz. It is a systemic hypersensitivity reaction to the parasite eggs that occur weeks after the initial infection. Acute symptoms include fever (Katayama fever), cough, abdominal pain, diarrhea, hepatosplenomegaly and eosinophilia. Madagascar has the fifth highest rate of schistosomiasis in the world.

LIFE CYCLE

1. Eggs in stool from infected humans are shed in water and hatch miracidia

2. Miracidia enter Biomphalaria snail tissue and develop sporocysts

3. Free swimming cercariae from snails are released in water

4. Cercariae penetrate human skin, lose their tails, and become schistosomulae

5. Schistosomulae spend a few days in the skin before entering the lymphatics and venules

6. On day 5-7, they migrate to the lungs and into the left side of the heart

7. After >15 days, they reach the portal blood in the liver

8. In the hepatoportal circulation, they meet their partner (they’re monogamous!)

9. They mature into adults and the pair migrate to the inferior mesenteric venules and lay eggs

FUN FACTS

Egg Size: 115–175 µm long and 45–47 µm wide, 150 µm diameter on average

Adult Male: 0.6-1.1cm long, 0.1cm wide; white

Adult Female: 1.2-1.6cm long, 0.016 wide (thinner and longer); darker in color due to hemozoin pigment

Distribution: sub-Saharan Africa, Brazil, Venezuela, Suriname, Caribbean, Arabian peninsula

Intermediate host: Biomphalaria snails

Treatment: Praziquantel (reinfection after treatment is common)

TAXONOMY

Kingdom: Animalia

Phylum: Platyhelminthes

Order: Diplostomida

Family: Schistosomatidae

More information:

1. CDC - DPDx - Schistosomiasis Infection
2. Pulmonary Schistosomiasis – Imaging Features - PMC (nih.gov)
3. Schistosoma mansoni Eggs in Spleen and Lungs, Mimicking Other Diseases - PMC (nih.gov)
4. Frontiers | Schistosomes in the Lung: Immunobiology and Opportunity | Immunology (frontiersin.org)
5. How IVI is confronting schistosomiasis in Madagascar - IVI
6. Madagascar.pdf (stanford.edu)
7. Schistosomiasis (1990): Entire title - YouTube