Hydatid Disease of Bone

Hydatid disease of bone is a very rare and is often misdiagnosed because of non-specific presentation and radiology.

As its management is quite different from  other conditions involving the bone, it is important to consider the differential of Hydatid disease of the bone in luscent bone lesions especially where the echinococcosis is prevalent.

Hydatid Disease or Echinococcosis

Echinococcosis results from infestation with Echinococcus granulosus and Echinococcus multilocularis [commonly known as hydatid disease].

Hydatid disease involves the liver in approximately 75% of cases, the lung in 15%, and other anatomic locations in 10%.

Life cycle of hydatid worm involves two hosts.

Definitive host –  usually a dog

The adult worm of the parasite lives in the proximal small bowel of the definitive host. Eggs are released into the host’s intestine and excreted in the faeces.

Intermediate hosts – sheep

They ingest the ovum while grazing on contaminated ground. The ovum loses its protective chitinous layer as it is digested in the duodenum.

The released hexacanth embryo, or oncosphere, comes to  the portal circulation after passing through intestinal wall.

Humans may become intermediate hosts through contact with a definitive host, for example, a domesticated dog, or ingestion of contaminated water or vegetables.

Structure of Hydatid Cyst

The hydatid cyst has three layers:

 Outer pericyst

Composed of modified host cells that form a dense and fibrous protective zone;

 Middle laminated membrane

Acellular and allows the passage of nutrients

Inner germinal layer

Produces the scolices (the larval stage of the parasite) and the laminated membrane.

Cyst fluid is clear or pale yellow. The fluid is antigenic and may also contain scolices and hooklets besides other contents. When vesicles rupture within the cyst, scolices pass into the cyst fluid and form a white sediment known as hydatid sand.

Outer pericyst is not present in bone lesions.

Pathology of Hydatid Disease of Bone

The frequency of bony  involvement in hydatid disease is 0.5%–4% .

Spine and pelvis are most commonly involved structures. Femur, tibia, humerus, skull, and ribs are other commonly affected bones.

The lesion is initially located in metaphyses or epiphyses. Later it extends to the diaphysis.

Because pericyst formation does not occur in bone lesions, the lesions proliferate aggressively resulting in irregular branching fashion along the line of least resistance, especially the bone canals. With time, the parasite reaches and destroys the cortex, with subsequent spread of the disease to surrounding tissues.

It is important to note that extraosseous cysts may calcify but intraosseous cyst rarely shows calcification.

Articular cartilage and intervertebral discs offer little resistance to growth.

Clinical Presentation

Pain and swelling are usual presenting complaints. In spinal involvement, neural deficit could also be a presenting feature [weakness of limb, bladder disturbances].

Diagnostic Work Up

The definitive diagnosis can usually be made by histopathological examination.

Complete blood count

May show eosinophilia.

Serology

Serological tests are based on the immunoreaction of the human body to hydatid cyst.

Immunoreaction depends on location, aggression and integrity of the cyst. Immunoreaction is heavier in ruptured hydatid cysts and lower when intact.

There are two types of serological examinations.

Detection of antigen from the hydatid fluid and protoscoleces

• ELISA
• Gold?labelled method

Detection of the antibody in the blood serum of patients

• Casoni test
• Indirect haemagglutination
• Counterimmunoelectrophoresis
• Enzyme linked immunosorbent assay [High specificity and sensitivity]
• Gold?labelled antibody test

The Casoni test, Indirect haemagglutination and counterimmunoelectrophoresis

are have high false positive results and poor sensitivity. Enzyme linked immunosorbent assay [ELISA] is mainly used as a confirmatory test because of its high specificity and sensitivity.

Imaging

X-ray

A lucent expansile lesion with cortical thinning is most common finding. Cystic or irregular destruction of bone may be seen. This is a kind of non-specific and seen in many other conditions.

CT

Most CT findings  are non?specific and typically show of a round or ovoid space?occupying lesion with a “double layered arcuate calcification.

MRI

MRI is the most helpful technique for diagnosing hydatid cyst disease where the lesion can be seen as multilocular character.

Differential Diagnoses

• Tuberculous spondylitis
• Chronic osteomyelitis
• Aneurysmal bone cysts
• Giant?cell tumours
• Ssolitary cyst
• Nneurofibromatosis
• Fibrocystic disease
• Chondrosarcoma
• Fibrous dysplasia

Treatment

Modern  treatment of osseous hydatid disease is surgical. Surgery aims at

• Removal of the cyst and surrounding involved bone
• Filling of the defect with graft
• Avoidance of secondary infection

Disease is not easy to eradicate  though and require chemotherapy also.

Albendazole, a type of benzimidazoles is now  drug of choice. Alone, a total success rate of approximately 25–30% of is achieved.

are the most commonly used postoperative adjuvant chemotherapy drug.

Praziquantel is only useful in the early young cyst, but has no effect against the mature cyst.

Combination of both the drugs is also used.