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An Inquiry Into the Ectopic Lesions in Schistosomiasis

1. Ectopic lesions in schistosomiasis are definied as those produced by immature or mature stages of schistosomes outside the portal-caval venous blood channels with their extension into the pulmonary arterioles. The lesions occasioned by the migration of young worms have been studied critically in experimental hosts but are not histologically described for man.

2. Interpretation of the lesions produced by schistosomes in man or other definitive hosts requires background information on the usual route of migration of the larvae from the site of their entry into the body to their arrival in the intrahepatic portal vessel, the growth of the worms in this location, their subsequent passage against the venous blood current to the mesenteric venous radicles or vesical plexus where they mature and oviposition takes place, and finally the local and systemic tissue reactions to the presence of the worms in the body.

3. Tabulation of published and other case histories of ectopic schistosomiasis, based on autopsy, biopsy, surgical intervention and substantial clinical data indicates that there are not less than 82 known cases with 86 separate sites where these lesions have been located. Twenty-one cases with 23 lesion sites are attributed to Schistosoma haematobium; 12 cases with 12 lesion sites, to S. mansoni, and 49 cases with 51 lesion sites, to S. japonicum. A majority of reported ectopic lesions in S. haematobium infection have occurred outside the brain and its blood vessels; a significant preponderance of those in S. japonicum infection have been in the brain, while those in S. mansoni infection are too few to show any significant anatomical predilection.

4. The tissue reaction to schistosome eggs which escape from blood vessels nto perivascular tissues is an acute inflammatory one in which histiocytes, epithelioid cells, giant cells, eosinophils, plasma cells and fibrocytes attempt to wall off the invading foreign body, with the eventual production of a pseudotubercle around each egg as a center. Nest of eggs were typically found within relatively circumscribed areas, so that each lesion consists of an aggregate of pseudo-tubercles forming a granuloma that varies in size from a pinhead to an orange. The smallest ectopic lesions have been found in the conjunctivae, the largest ones have occurred in the brain.

5. From the time of entry of the metacercariae of the human schistosomes into the cutaneous venules, following exposure to infection, the worms are characteristically intravascular in their location. There are several records of ectopic location of the adult worms, one in the middle cerebral vein, one in the ophthalmic vein, one in a coronary artery, and larger numbers from gastric, splenic, esophageal, hepatic and renal veins of heavily infected experimental animals. In no instance is there any evidence of local tissue reaction to the presence of the worms.

6. Five separate theories have been adduced to account for ectopic lesions in schistosomiasis: (1) Metacercariae develop to adult worms, with subsequent oviposition, at or near the sites of penetration into the skin or mucous membrane; (2) a patent forament ovale would provide a direct route from the inferior caval veins into the systemic circulation; (3) eggs may escape through the pulmonary capillaries and be deposited in distant arterioles; (4) adult worms may travel against venous blood flow into collateral vessels and on reaching the end venules deposit their eggs, and (5) the vertebral venous system provides a natural, valveless intercommunicating channel from portal and caval veins to all parts of the body, without need of embolic filtration of eggs or the migration of adult worms against blood flow or valves.
The first theory is contrary to all critical studies on the development of schistosomes in the body of the definitive host. The second theory predicates a highly improbable combination of circumstances but might possibly explain the presence of a male worm discovered in a branch of the coronary artery. The third theory is discounted by the characteristic disposition of eggs in nests or aggregates, and tissue reaction around venules rather than arterioles. The fourth theory provides a rational explanation in case the worms migrate against venous blood flow into valveless veins. The fifth theory provides an adequate basis for all but one of the ectopic lesions which have been described.

7. Ectopic lesions in schistosomiasis are not rare, although the cases described are few compared with the millions of cases of intestinal and vesical schistosomiasis. It is significant that during the period 1942–1947 more ectopic cases were reported than from 1889 through 1941, and that autopsy diagnosis has declined while diagnosis by biopsy and surgical removal of the lesion has increased.

8. Ectopic lesions in schistosomiasis, particularly those involving the central nervous system, may be anticipated even several years after intestinal or vesical symptoms have disappeared; moreover, these lesions may develop without a previous clinical history or diagnosis of the disease.

9. Chemotherapy is essential for all cases of schistosomiasis diagnosed by ectopic lesions, to prevent additional ectopic complications, even though there may have been no history or diagnosis of abdominal schistosomiasis. The present drug of choice is potassium antiomony tartrate, administered intravenously in a one-half per cent solution on alternate days for approximately four weeks.