bubble_chart Overview

Cysticercosis refers to the disease caused by the larvae (cysticerci) of Echinococcus granulosus parasitizing in the bones.

bubble_chart Etiology

The adult Echinococcus granulosus is found in the small intestine of definitive hosts such as dogs and wolves in Chinese Taxillus Herb, and the eggs are excreted in feces. Intermediate hosts include humans, cattle, sheep, horses, etc. If the eggs are ingested, they hatch into oncospheres in the digestive fluid of the stomach, penetrate the intestinal wall, and enter the mesenteric veins or lymphatic vessels. Upon reaching the liver, approximately 75% of the oncospheres remain there, while the rest pass through the right heart into the lungs and lodge there. Only a very small portion of the oncospheres enter the systemic circulation, forming lesions in bones, the brain, and other areas, resulting in hydatid cysts (echinococcosis). In bones, it forms osseous cysticercosis.

This disease occurs in pastoral areas, and direct contact with dogs is the primary mode of human infection. Because dogs frequently consume raw beef or mutton, and cattle or sheep often ingest dog feces, coupled with the high frequency of direct contact between humans and dogs in pastoral regions, it is easy for hydatid cysticercosis to become endemic. Osseous cysticercosis accounts for 1–2% of cases. Additionally, because the hydatid cysts develop slowly in the bones, clinical symptoms often appear only 10–20 years after infection, making symptomatic cases extremely rare in childhood. Most cases occur between the ages of 30 and 50. Lesions are commonly found in the pelvis, spine, femur, humerus, tibia, and other areas.

bubble_chart Pathological Changes

Hydatid cysts are divided into two types based on their location: the unilocular type. This type primarily occurs in visceral organs and soft tissues, with approximately 70% in the liver, 20% in the lungs, and occasionally in the kidneys, brain, eyes, or muscles. Due to the absence of external forces, these cysts often assume a spherical shape and can grow very large, with diameters reaching up to 20 cm, while smaller ones may be barely visible to the naked eye. The cyst wall consists of three layers: an outer layer of fibrous tissue and hyalinized collagen fibers, a middle layer of white, translucent collagen membrane, and an inner germinal layer with protrusions of brood capsules into the cyst cavity. These may develop into daughter or granddaughter cysts, which can differentiate into scolices. The cysts may rupture into body cavities or hollow organs, become secondarily infected and suppurate, undergo calcification, or spontaneously degenerate and regress. The other type is called the osseous type, where lesions originate from spongy bone or the medullary cavity, commonly affecting the pelvis and spine. When long bones are involved, the lesions begin at the metaphysis. Due to the hardness of bone, hydatid cysts grow restrictively within the bone, preventing spherical formation and lacking a complete outer wall. Instead, they infiltrate along the medullary cavity and weaker bone areas, forming multilocular cysts of varying sizes. These lack a fibrous outer membrane and a typical germinal layer. The cortical bone may atrophy under pressure, enlarging the medullary cavity, but little new bone forms, increasing the risk of pathological fractures. The cysts may also erode and penetrate the bone, invading surrounding soft tissues or even breaking through the skin, leading to secondary infections. In the spine, this can result in paralysis.

bubble_chart Clinical Manifestations

The clinical symptoms and signs of cysticercosis are determined by the size and location of the cysts. Generally, patients with bone echinococcosis seeking medical attention are often already in the advanced stage, frequently presenting with pathological fractures. In the early stages of the disease, the hexacanth larvae grow slowly in the bone tissue, and patients may experience no discomfort for a long time. Even in the rare cases with mild pain, it often goes unnoticed. As the lesion progresses, symptoms such as bone pain, limping, and localized or limb muscle atrophy may appear. When the cyst infiltrates and destroys a significant amount of bone tissue, the pain intensifies. The affected bone may thicken or deform, and even minor external force can lead to pathological fractures, which is often when patients seek medical help. If the spine is involved, the cyst may compress the spinal cord or nerve roots, resulting in neurological symptoms and signs, or even paralysis. In the advanced stage, if the cyst penetrates the cortical bone and invades surrounding soft tissues, a large mass may become clinically apparent. If the skin is breached, secondary infections may occur, with fistulas discharging pus and fragments of the hydatid cyst over a prolonged period, making healing difficult. Clinically, joint involvement is rare, but when it occurs, it may lead to pathological dislocation.

Patients with bone echinococcosis often have concurrent lesions in other organs such as the liver, lungs, or brain, so a thorough and comprehensive examination is necessary.

bubble_chart Auxiliary Examination

1. Intradermal test with hydatid fluid (Casoni skin allergy test) uses diluted hydatid fluid as an antigen, injected intradermally on the patient's forearm to induce a local allergic reaction. The accuracy rate is as high as 90% or more. This test not only has diagnostic value but can also be used to observe treatment efficacy.

2. Hydatid complement fixation test (Weingerg test) is commonly used in clinical practice to perform serological tests based on the immune response of the human body to hydatid cysts. The complement fixation test has a positive rate of up to 90%, especially higher when the cyst ruptures or after surgical treatment, due to the absorption of more cyst fluid antigens by the body.

3. In blood tests, an increase in eosinophils can serve as an auxiliary reference.

4. X-ray findings: The affected bone shows irregular moth-eaten erosions, with trabecular destruction leading to cystic lucent areas that connect in a "grape-like" pattern, surrounded by remaining trabeculae. There is no clear boundary between normal and diseased bone. In the early stages, the bone's shape remains normal. In the advanced stage, the hydatid bone defect area expands, with uneven cortical thickness, and the shaft may thicken (grade I), but there is no new bone formation or periosteal reaction, which is a characteristic feature. When the spine is involved, a pseudo-paraspinal abscess image may appear due to the hydatid cyst protruding into the paravertebral soft tissues on both sides. Generally, the intervertebral disc is not affected, which helps differentiate it from spinal subcutaneous nodules. When flat bones are involved, the lesion expansion is more obvious. In the early stages, X-ray findings should be carefully differentiated from osteosarcoma, neurofibroma, vertebral hemangioma, giant cell tumor, and bone cysts.

bubble_chart Diagnosis

The diagnosis can be based on a history of residence in an endemic area and frequent contact with dogs or sheep; characteristic clinical and X-ray findings; and positive laboratory test results. However, it should be differentiated from giant cell tumor of bone, fibrocystic osteitis, osteosarcoma, and spinal subcutaneous nodules.

bubble_chart Treatment Measures

First and foremost, priority should be given to the prevention of this disease. Human cysticercosis is often transmitted through dogs, so dog breeding should be controlled. Strengthen livestock management to prevent dogs from eating raw beef, mutton, or horse meat contaminated with cysticercosis. Pay attention to personal hygiene and avoid drinking untreated water or raw milk, especially in pastoral areas.

The primary treatment for bone echinococcosis is the resection of the affected bone. However, when it is difficult to completely remove all the diseased bone, a curettage and bone grafting procedure can be employed. This involves thoroughly clearing the lesion, then applying 20% phenol glycerin to the cavity walls, followed by washing with 90% alcohol after 10 minutes to deactivate the infection. The bone cavity is then repeatedly rinsed with saline before being filled with bone fragments. Special care must be taken during surgery to prevent the spread of daughter cysts and the risk of anaphylactic shock caused by cyst fluid leakage. After curettage, a drainage tube is left in place, and a daily infusion of 20–30% hypertonic sodium chloride solution can be administered to kill the scolex, yielding good results.

For patients with advanced-stage disease and widespread lesions where local resection or curettage is not feasible, joint disarticulation or amputation may be considered. However, if the bone lesion is extensive but the soft tissues are not widely affected, prosthetic replacement can be performed. When the spine is involved, early clearance of the lesion and bone grafting should be performed promptly. In cases complicated by paraplegia, thorough lesion clearance, spinal cord decompression, and internal fixation with instrumentation should be carried out, followed by bone graft fusion.

bubble_chart Prognosis

The prognosis depends on the extent of the lesion and the affected area. Lesions in the pelvis and spine have a poorer prognosis, with death often resulting from involvement of vital organs such as the brain, liver, or lungs due to echinococcosis.