General Pathology Associated with Schistosome Infections
• Infection with Schistosoma mansoni
The pathology associated with infection with S. mansoni can be divided into two main areas, acute and chronic schistosomiasis.
Acute Schistosomiasis - Also called 'Katayama' fever. This is associated with the onset of the female parasite laying eggs, (approximately 5 weeks after infection), and granuloma formation around eggs trapped in the liver and intestinal wall. It resembles 'serum sickness' (i.e. acute immune complex disease), with hepatosplenomegaly, and leucocytosis with eosinophilia. This phase of the infection is often asymptomatic, but when symptoms do occur they include fever, nausea, headache, an irritating cough and, in extreme cases diarrhoea accompanied with blood, mucus and necrotic material. These symptoms , if present, last from a few weeks, to several months. It is not as commonly associated with S. mansoni infections compared with those of S. japonicum.
Chronic Intestinal Schistosomiasis - This and the hepatic schistosomiasis detailed below, manifest a number of years after infection. The pathogenic reaction is a cellular, granulomatous inflammation around eggs trapped in the tissues, with subsequent fibrosis. All areas of both the small and large intestine may be involved, with the large intestine showing the most severe lesions, whereas severe pathology in the small intestine is only rarely observed, even though large numbers of eggs may be deposited here. Colonic polyps are also sometimes seen, especially in Egypt (Cheever et al 1978). The reasons for the high prevalence of this aspect of pathology in Egypt are not clear.
Chronic Hepatosplenic Schistosomiasis - Again, this aspect of the disease is only seen a few years after infection. The pathology is similar to that seen in the intestine, with a cellular, granulomatous inflammation around eggs trapped in the liver, leading to fibrosis and hepatosplenic disease. Other organs may more rarely also contain granulomas around eggs, particularly the lungs.
• Infection with other schistosomes
S. haematobium Infection. Adult parasites are found in small venules around the bladder and ureter, with the majority of egg deposition in the tissues of these organs, as eggs pass through the bladder wall, to leave the body in the urine. The disease cause is chronic in nature, with the most frequently affected organ being the urinary bladder, where calcification of eggs trapped in the tissues often occurs. The disease is characterised by blood in the urine (haematuria), hence the infection is often refered to as 'Urinary Schistosomiasis'. Cancer of the bladder is an important complication of infection with S. haematobium. Eggs may be deposited in the liver, often in high numbers, and granuloma formation may occur, but this is much less severe than with S. mansoni.
S. Intercalatum infection. This is diagnosed by the presence of terminally spined eggs in the faeces, (the other terminally spined schistosome eggs, those of S. haematobium , are only found in the urine). Blood may be seen in the faeces, and diarrhoea may occur. However the portal hypertension . seen in S. mansoni infections has not been reported, and infections are often asymptomatic.
S. mekongi infection. This has not been greatly studied, but the pathology appears to be similar to that of S. japonicum.
• Schistosome species where man is NOT a permissive host
Cercarial Dermatitis - This is also called 'Swimmers Itch', and is mainly associated with cercaria of species of schistosome that normally infect birds or small mammals, and which are not capable of fully developing in man (i.e. man is not a permissive host). Cercarial dermatitis occurs shortly after penetration of the hosts skin by these cercaria, and is caused by dead or dying larvae in the skin. It is a hypersensitivity reaction, characterised by a skin rash, the severity of which varies considerably, depending on factors such as the degree of previous exposure to the cercaria.