The trematode flatworms that infect human beings include the schistosomes, which live in venules of the gastrointestinal or genitourinary tract, and other flukes that inhabit the bile ducts, intestines, or bronchi. The geographic distribution of each species of trematode parallels the distribution of the specific freshwater snail that serves as its intermediate host (Table 195.1). Schistosomes infect as many as 200 million persons worldwide; infections caused by the other flukes are more limited in distribution and number. Trematode infections last for years; most are subclinical, and in general only the small proportion of persons who have heavy worm burdens develop severe disease.

SCHISTOSOMIASIS

Clinical Presentation

A history of contact with possibly infested freshwater in an endemic area should prompt an evaluation for schistosomiasis, even in the absence of symptoms (Figure 195.1). Clinical manifestations that suggest the diagnosis vary according to the stage of infection. Some persons complain of intense pruritus or rash shortly after the infective cercariae penetrate the skin. Previously uninfected visitors to endemic areas may develop acute schistosomiasis, or Katayama fever, 2 to 10 weeks after exposure, as the immune system begins to respond to maturing worms and eggs. Symptoms range from mild malaise to a serum sickness-like syndrome that lasts for weeks and may be life threatening. Common features include fever, headache, abdominal pain, myalgia, dry cough, diarrhea, hepatosplenomegaly, lymphadenopathy, urticaria, and marked eosinophilia.