A curious case of colitis

Correct answer: a.

Discussion

The mucosal patches shown on the photograph have been referred to as ‘sandy patches’ and when seen are pathognomonic of chronic schistosomal colitis! The arrow on the histology slide shows a Schistosoma egg in the colonic lamina propria.

Among human parasitic diseases, schistosomiasis ranks second to malaria in terms of socioeconomic and public health importance. The three main species of schistosome that infect humans are Schistosoma haematobium and Schistosoma mansoni, which are found in Africa and the Middle East, and Schistosoma japonicum, which is localised to Asia, particularly the Philippines and China.¹

As you may already know, schistosome eggs are passed in the faeces or urine (S. haematobium) of infected people and hatch into larvae in fresh water. These larvae penetrate freshwater snails and multiply. Four to six weeks later free-living cercariae are released from the snail and are able to penetrate human skin within 20 seconds. Immature schistosomes migrate via the lungs to reach mesenteric veins (or the vesical plexus in the case of S. haematobium infestation). Here, adult worms mature and produce 1,500–3,000 eggs per day for the next 5 years or so. Eggs are released through venular walls to reach the colon or bladder and are excreted in the faeces or urine.

Surprisingly, it is the schistosome eggs (rather than the worms) that induce the morbidity of schistosomiasis. This is because many eggs are not excreted but become permanently lodged in the intestines, liver or urogenital system. Here they induce a chronic granulomatous reaction, and fibrosis.

Chronic schistosomal colitis is a nonspecific condition that presents with intermittent abdominal pain, diarrhoea and rectal bleeding.² Colonoscopy in patients with acute schistosomal colitis may reveal an oedematous, congested mucosa and petechial haemorrhages. In those who have chronic schistosomal colitis, the most characteristic finding is the grey-yellow or yellowish white schistosomal nodules (shown in the images). Although these findings are typical, there were only found in 16% of cases in a review of 46 patients published by Cao and colleagues.³

Three distinct syndromes are associated with schistosomiasis. “Swimmers itch” may occur when cercariae penetrate the skin (although itching is more frequently associated with cercariae of nonhuman schistosomes). “Katayama fever” occurs after egg production commences and is probably attributable to the formation of immune complexes in response to the antigenic stimulus provoked by the eggs. Pyrexia, lymphadenopathy, splenomegaly and eosinophilia are common. Finally, liver cirrhosis with portal hypertension is caused by eggs carried to the liver from the intestine; fibrosis of the bladder wall and urogenital system can result in hydronephrosis and renal failure.

The diagnosis of chronic schistosomal colitis is made by identifying eggs in faeces or colonic biopsy samples. Serum antibody detection is of value in travellers returning from endemic areas, whereas antigen detection techniques are useful in distinguishing between past and current infection in indigenous populations.

Praziquantel is effective against all five schistosome species and is the drug of choice. Up to 80% of adults are cured after a day's treatment. However, cure rates are far lower among preschool children. This is perhaps because praziquantel tablets are large and bitter tasting and often cause transient abdominal pain, headache and dizziness. As praziquantel is not active against juvenile worms, people who are not living in endemic areas are offered a repeat course 3–6 weeks after the first dose to kill the initially resistant immature worms. In endemic regions, praziquantel is often offered once a year or so to suppress morbidity.