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Sparganosis

Until a few years ago, sparganosis was of no real interest to radiologists. In the first edition of this book (1981) it was discussed in six lines because there were no significant images. However, CT and MRI have altered that by making the worm and its surrounding chronic and granulomatous inflammation visible within the brain. Sparganosis must now be added to the differential diagnosis for patients with severe headaches, seizures, hemiparesis, or other CNS symptoms and signs.

The first human infection by a sparganum was detected by Manson in 1882, a discovery acknowledged by the name given to the responsible tapeworm, Spirometra mansoni (and also to a second species, S. mansonoides, not recognized until 1935). Although the CNS infections are the most dramatic, there are many other presentations which, if not of prime radiological importance, are more than a minor irritation for the unfortunate patients. Prevention is relatively easy, treatment is not; the foibles and tastes of humanity so often ignore caution that this tapeworm must be added to the list of those which, if they live in the patient long enough, can cause death, at least in those unfortunate individuals infected with the rare S. proliferum species.

Synonyms

Cestodiasis (a general term). Spirometrosis. Sparganum infection. Larval diphyllobothriasis (an older term).

Definition

Sparganosis or spirometrosis is infection by a sparganum, the second stage (plerocercoid) larva of a pseudophyllidean tapeworm of the genus Spirometra of the family Diphyllobothriidae. There are at least three species of Spirometra (and about 13 of the genus Diphyllobothrium) which infect humans. The more common nonproliferative form of sparganosis is caused by Spirometra mansoni or S. mansonoides. The proliferative, systemic form of sparganosis is caused by S. proliferum and is very rare. The exact definition of an infection in a patient should specify the causative species.

Geographic Distribution

Human sparganosis is found worldwide, but most frequently in China, Korea, Japan, Taiwan, and elsewhere in Southeast Asia. Infections have been reported in the Southeastern United States, and elsewhere in the Americas, and from East Africa. The adult tapeworms are found in domestic cats and in wild carnivores in tropical and subtropical regions of America, Asia, Australia, and Africa. Frogs, snakes, birds, and small mammals can also be infected.

Spirometra mansoni is the usual cause of infection in China, Japan, and Southeast Asia, whereas S. mansonoides is the more likely cause in the Americas. The rare S. proliferum is found in Japan and Southeast Asia, with at least three cases reported in the Americas.

Epidemiology and Pathology

The life cycle of the Spirometra involves several hosts. The definitive hosts are dogs, cats, and wild carnivores and the Spirometra live in their intestines. Eggs are shed in the feces and hatch in stagnant fresh water: the motile, ciliated coracidia which are released are eaten by the first intermediate host, a cyclops in which they develop into procercoid larvae. When these tiny crustaceans are eaten by fish, snakes, amphibians, or mammals, including humans (the second intermediate hosts), the larvae penetrate the intestinal wall, becoming plerocercoids, and spread throughout the body, especially to muscles and subcutaneous tissues. Here the plerocercoid or sparganum develops and encysts, awaiting completion of its life cycle when a dog, cat, or other carnivore consumes the infected flesh. The sparganum then develops into an adult tapeworm within the small intestine of its final host. Adult worms are variously estimated to survive from 10 to 30 years. Human sparganosis is acquired in three different ways:

1. By drinking contaminated water in which there is an infected cyclops, the most common method of transmission in the United States.

2. By eating infected raw or undercooked flesh containing spargana.

3. By use of a poultice of raw infected flesh applied to skin, conjunctiva, or mucosa. This is a common treatment in parts of Asia where the poultice may be applied to an open wound, allowing the worm in the infected flesh to crawl directly into human tissues, thus making the patient an unwitting intermediate host. Raw pork, frogs, and snakes used in this way, or eaten, are the most likely reservoirs for spargana.

There are two forms of sparganosis, the proliferative and the nonproliferative. In the much more common nonproliferative form, when the worm is alive there is little tissue reaction as the sparganum migrates through the host. There may be eosinophilia or leukocytosis. A live sparganum may be seen as a white glistening and slowly undulating flat, ribbonlike worm, from 2 mm to 10 cm or more in length (Fig. 7.48). Each sparganum varies in width, having some broad areas and other narrow, string-like areas (Fig. 7.49A). The worm, which moves slowly and less often encysts in the abnormal human host, can leave a track of focal necrosis surrounded by chronic inflammation and numerous eosinophils. If the sparganum dies, a granuloma or an abscess surrounds the carcass. There is a fibrous tissue and chronic inflammatory reaction also, including focal areas of eosinophilia and often Charcot-Leyden crystals.

In the CNS, the granulomatous reaction is similar, with adjacent gliosis, which may be dense, and associated acute and chronic inflammation. The dead sparganum is usually found centrally within the lesion. Worms have also been found in the subarachnoid space.

The proliferative form of sparganosis produces similar granulomas, but these are much more widespread; there may be thousands of granulomatous cysts within the infected tissues, including the CNS. These granulomatous cysts match the size of the sparganum they contain. Some descriptions of S. proliferum refer to continuous branching and budding, unorganized and without symmetry. Some authorities believe that S. proliferum is an aberrant type of the nonproliferative sparganum, but recent studies have shown it to be a unique species of sparganum. The adult worm has not been identified.

Laboratory Diagnosis

An accurate diagnosis can be made in two ways: either by identifying the sparganum (Fig. 7.49 B) or by the antisparganum IgG antibody enzyme-linked assay that is over 96% sensitive and specific. However, MRI and CT cerebral scanning have a significant role in establishing the diagnosis. Both can show unilateral or bilateral lesions that vary in size and shape, and often change in position on subsequent scans.

Fig. 7.48. Sparganum, the larval phase of the tapeworm Spirometra, can survive in damp surroundings for several days, which is why contaminated poultices used in wound dressings may lead to human sparganosis. This plerocercoid larva, or sparganum, was removed from the skin of the abdomen of a white tree frog (Litoria caerulea), a native of Australia. It was kept in tap water and tied itself in two knots. (Courtesy of Prof. J. H. Theis, Davis).

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Fig. 7.49. (A) The sparganum from a subcutaneous nodule. The arrow indicates the flattened anterior end. The ribbon-like body varies in thickness and shows pseudosegmentation. x2. (B) High magnification (x50) of a sparganum showing loose stroma with fluid-filled spaces, calcareous corpuscles (arrows), mesenchymal fibers, and longitudinal bundles of smooth muscle. (Courtesy of Marty and Andersen 1995).

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