Final Diagnosis -- Chronic Infestation of Rectal Wall by Schistosoma



Due to the advanced calcification of the bodies in the specimen, the primary infection was presumed to have occurred decades ago, and the possibility of active infection was considered unlikely. The patient was a poor historian, and was unable to relate a recent or remote travel history to an area endemic for schistosomiasis.


Schistosomiasis (bilharziasis) is one of the world's most common parasitic infections. The most common species infecting humans include S. haematobium, S. japonicum, S. mansoni, S. mekongi, and S. intercalatum.

The life cycle of the organism has been extensively studied. Eggs from feces, when deposited in a fresh water site, hatch quickly and infect an appropriate snail host. After about 4 weeks, the cercarial form emerges from the snail and swims actively in search of the next host. The cercariae penetrate skin, usually on the feet of susceptible hosts such as humans. After penetration, the organisms enter the bloodstream, pass through the lungs, and lodge as adult forms in the portal-mesenteric system.

Male and female forms of the adult worms mate with subsequent production of eggs which disseminate through the bloodstream; the eggs then tend to deposit out into tissue in areas of sluggish blood flow (perirectal, perivesical veins and liver sinusoids).

Pathology of schistosomiasis includes several different disease states, which are related to different stages in the life cycle of the organism. Sites of cercarial invasion of the skin in swimmers in schistosome-infested waters present as intensely pruritic papules, particularly on the feet and lower legs, referred to as cercarial dermatitis or "swimmer's itch."1 Individuals with large adult worm burdens may exhibit features of serum sickness, including fever, arthralgia, and possibly cardiovascular collapse when the worms begin to release large numbers of schistosome eggs into the circulation. This reaction to the eggs in the bloodstream is also known as Katayama fever.2

The human is not the definitive host for the schistosome, and eggs generally deposit in the soft tissues and are destroyed by the resultant florid inflammatory response which is predominantly granulomatous. Some of the eggs become calcified rather than resorbed, and such schistosome "fossils," which are generally surrounded by dense fibrosis, can be seen even decades following the original infection. (as noted in our case).

The abundant fibrotic reaction to these eggs can give rise to the chronic presentations of schistosomal disease. These include extensive "pipestem" fibrosis of hepatic portal tracts (with liver dysfunction and failure in severe cases)3, granulomatous fibrosing angiitis of the lung (with cor pulmonale in severe cases)4 and mural fibrosis of the urinary bladder with obstructive/reflux uropathy. Granulomatous inflammation with fibrosis has also been noted occasionally in the CNS, female genital tract, and lymph nodes.5,6

The connection between schistosomiasis and neoplasia has intrigued scientists for decades. The best described association is that between S. haematobium infection of the urinary bladder wall (via the large caliber, slow flow venous system draining the bladder base) and squamous metaplasia and carcinoma of the trigonal area.7 Studies in endemic areas (Egypt and the Middle East) demonstrate a marked elevation in incidence of squamous cell carcinoma of the bladder, even in relatively young patients with long standing schistosomiasis (primarily with S. haematobium).8 Moderate success in reduction of the incidence of schistosomiasis-related bladder cancer has been achieved in endemic areas through the expanded use of urine cytology, urinalysis (for hematuria), and bladder ultrasound.9

Although less well established, a pathogenetic link between Schistosomiasis and neoplasia of other organs has been proposed. Deposition of eggs in the distal colon and areas of similar sluggish venous flow (i.e., the hemorrhoidal veins surrounding the distal rectum and anus) has been associated with intense fibroinflammatory reaction and even stricture formation. This reaction is similar to that noted in the urinary bladder, and recent studies have demonstrated a moderately increased incidence of colonic adenocarcinoma in the setting of severe infestation of a chronic nature (as in this case).10,11

Diagnosis of schistosomiasis in feces or urine is most reliably achieved using wet mount microscopy. This is the best chance of accurate speciation, given the often degenerated condition of the characteristic spines in tissue sections. The identification of a large terminal spine (at the end of an oval egg) is consistent with S. haematobium species. A lateral spine of similar size on an oval egg form is indicative of S. mansoni, and a very small, often hooked or curved spine on a slightly more rounded egg form is consistent with S. japonicum

If suspicion of infection is high, but feces and urine are consistently negative by wet mount examination, diagnostic yield can often be increased by the taking of small "snip" biopsies of bladder or rectal mucosa. When examined histologically, the eggs in the mucosa and submucosa are often enveloped in an intense inflammatory reaction; stains such as the modified Ziehl-Neelsen acid fast can help delineate the egg morphology and may allow speciation.

In patients in the chronic phase of disease (who have ceased excreting eggs), the diagnosis can be made by serological analysis. In endemic countries, serodiagnosis is made primarily by the performance of enzyme-linked immunosorbent assays (ELISAs) or radioimmunoassay (RIA) using worm extracts or purified S. mansoni antigen.12,13 Experimental urinalysis kits which detect a secretion from living worms (circulating anodic antigen--CAA) by a rapid ELISA method have shown promise as a means of evaluating worm burden and success of therapy, and may be in use in the field within a few years.14


The photomicrographs of Schistosoma japonicum and Schistosoma mansoni were used by the kind permission of J. Michael Miller, Ph.D., at the Centers for Disease Control (CDC), Atlanta, Georgia, USA.


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  14. van Lieshout L. de Jonge N. Bassily S. Mansour MM. Deelder AM. Assessment of cure in schistosomiasis patients after chemotherapy with praziquantel by quantitation of circulating anodic antigen (CAA) in urine. American Journal of Tropical Medicine & Hygiene. 44(3):323-8, 1991 Mar.

Contributed by Kevin D. Horn, MD, A. William Pasculle, ScD, Uma N. M. Rao, MD


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