Resident Program - Case of the Month

February 2024 – Presented by Dr. Yuden Pemba (Mentored by Dr. Karen Matsukuma)

Discussion

Background: Echinococcosis is a zoonotic larval infection that can infect humans globally. There are several identified species of Echinococcosis:

• Echinococcus granulosus: causes cystic echinococcosis, also known as hydatidosis as seen in our case which will be discussed further
• Echinococcus multilocularis: causes alveolar echinococcosis
• Echinococcus vogeli: causes polycystic echinococcosis
• Echinococcus oligarthrus: causes polycystic echinococcosis

Etiology: Hydatidosis occurs when humans or other intermediate hosts are infected by the larvae of the Echinococcus granulosus tapeworm. This condition is marked by the development of hydatid cysts (metacestode) within the internal organs. Dogs and other carnivores serve as the primary hosts, shedding the infective eggs in their feces, which then are ingested by the intermediate hosts.

Epidemiology: Hydatidosis occurs more frequently in communities that use dogs for protection and to herd livestock. The disease is found all over the world but is especially common in the Mediterranean, Russia, China, North and East Africa, Australia, and South America (endemic areas).

Pathophysiology: When people ingest the tapeworm eggs, a larva known as an oncosphere is released from the egg. This larva can penetrate the mucosa and travel in the bloodstream or lymphatic system to organs such as the liver or lungs. There, the larva develops into a hydatid cyst, which is a form of tapeworm larvae. The cyst consists of an internal growing layer and an external protective layer, with a fibrous shell formed by the host's tissues. From the internal layer, smaller cysts may form. In humans, these cysts expand slowly over time and can reach the size of several liters, containing thousands of future tapeworm heads known as protoscolices. As the cyst grows, it forms compartments and additional cysts, altering the usual single-chamber structure of these echinococcal cysts.

Histopathology:

Under microscopic examination, hydatid cysts can be identified by 3 specific features:

1. A dense, acellular, laminated structure that shows a particular affinity for acidophilic stains, such as hematoxylin and eosin.
2. A layer of cells known as the germinal layer
3. The presence of brood capsules or protoscolices, which are the early forms of the tapeworm.

Additionally, a granulomatous reaction may be observed around the cyst.

Diagnosis: The diagnosis is made by a combination of history of exposure or immigration from endemic areas, followed by imaging and serologic tests.

Treatment: Several treatment options are available, including: (1) surveillance, (2) medical therapy (e.g., anthelmintic drug), (3) surgery, and (4) “PAIR” (puncture, aspiration, injection, re-aspiration) therapy. Choice of therapy is dependent on the size and complexity of cysts, symptoms, and other patient-specific characteristics.

Prognosis: With appropriate treatment the prognosis is favorable; however, cysts in anatomic locations with more restricted surgical access (e.g., heart and spine) have a poorer prognosis. Recurrence can occur in some cases.

References

1. Almulhim AM, John S. "Echinococcus Granulosus". [Updated 2023 Aug 7]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan-.
2. “Echinococcal Cyst.” Pathology Outlines - Echinococcal Cyst, Accessed 29 Feb. 2024.
3. Li T, Ito A, Nakaya K, Qiu J, Nakao M, Zhen R, Xiao N, Chen X, Giraudoux P, Craig PS. "Species identification of human echinococcosis using histopathology and genotyping in northwestern China". Trans R Soc Trop Med Hyg. 2008 Jun;102(6):585-90. doi: 10.1016/j.trstmh.2008.02.019. Epub 2008 Apr 8. PMID: 18396303; PMCID: PMC2517144.