General Characteristics

Unlike the other trematodes, adult schistosomes are not flattened, but are rather long, thin, and rounded in shape. There is an oral sucker surrounding the mouth and a ventral sucker located just slightly below the oral sucker. The adult male averages 1.5 cm in length and is wider than the female, having a ventral fold that wraps around the female when they mate (Figure 58-1). The adult female averages 2 cm in length and is very thin. The eggs of each species are distinct, and can be distinguished by size, spine morphology, and sometimes specimen type (Figure 58-2). The size range for eggs of S. haematobium is 110 to 170 µm long by 40 to 70 µm wide, and they have a sharply pointed terminal spine. They are fully embryonated without an operculum. The size range for the eggs of S. japonicum is 70 to 100 µm long by 50 to 65 µm wide, and they have a small lateral spine that is sometimes difficult to detect (Figure 58-3). S. mekongi eggs are smaller than those of S. japonicum, ranging in size from 50 to 65 µm long by 30 to 55 µm wide. They are fully embryonated without an operculum and have a small lateral spine. The size range for eggs of S. mansoni is 115 to 180 µm long by 40 to 75 µm wide, and they have a large lateral spine. S. mansoni eggs are unoperculate, immature when released, and take up to 8 to 10 days to develop a miracidium. S. intercalatum eggs are fully embryonated without an operculum, have a terminal spine, and range in size from 140 to 240 µm long by 50 to 85 µm wide. S. intercalatum eggs resemble those of S. haematobium and can be differentiated by Ziehl-Neelsen acid-fast positivity. In addition, S. intercalatum eggs are only found in feces, not in urine specimens. Table 58-1 provides a comparison of the schistosome eggs.

One of the main differences in the schistosomes from other trematodes is that instead of being hermaphroditic, there are separate male and female adult worms. In human infection, the adult worms live in either the veins that supply the intestine (S. japonicum and S. mansoni) or the veins that supply the urinary bladder (S. haematobium). The eggs are passed from the body in either the feces or the urine. To reach the inside of the intestine or bladder, the eggs must penetrate the tissue from the veins. This is accomplished via a spine that is distinctive among the major species. The embryonated egg will release the miracidium (Figure 58-4) once it reaches freshwater, and will enter the snail host, where it will develop into the infectious cercaria. The free-swimming cercariae are capable of penetrating through the human skin directly and do not encyst on aquatic vegetation or other aquatic wildlife (Figure 58-5). The cercariae penetrate the host tissue until they reach a vein; then they travel to capillaries near the lungs and then to the portal vein of the liver, where they mature. When they are mature, the adult males will pair with the females and then travel to the veins of either the intestine or the bladder, where the eggs are produced (Figure 58-6).

Epidemiology

Schistosomes have a worldwide distribution from Egypt and China to Africa and the Americas. S. haematobium is found in Africa and the Arabian peninsula, and has no important reservoir hosts. S. mansoni is found in Africa, the Arabian peninsula, and Brazil. Reservoir hosts include wild rodents and marsupials. S. japonicum is found in China, Indonesia, and the Philippines. Many domestic animals (cats, dogs, cattle, horses, pigs) serve as reservoir hosts, as do some wild animals as well. S. mekongi primarily exists in the lower Mekong River basin in southern Laos and Cambodia. Reservoir hosts include dogs and domestic pigs. S. intercalatum is primarily found in central and western Africa. Reservoirs include rodents, marsupials, and nonhuman primates. Human schistosome infection is caused by fecal (and urine) contamination of small bodies of water that favor the growth of the snail hosts. Infection with S. japonicum is especially prevalent in areas where humans work in rice paddies.

Pathology and Spectrum of Disease

Infection with only a small number of worms may be asymptomatic. Quite often, penetration of the skin by the cercariae causes localized swelling and itching. The migration of the larvae through the body may cause transient symptoms of fever, malaise, cough (when they migrate in the lungs), or hepatitis (when in the liver). The adults are able to acquire some host antigens on their outer surface, and so may not elicit an immune response, although the eosinophil count may be high. Severe tissue damage, with associated pain, fever, and chills, may occur when the eggs travel through the tissue to reach the intestine or bladder. There may also be bloody diarrhea or blood in the urine (hematuria). Necrosis, lesions, and granulomas may develop, as well as obstruction of the bowel or ureters.

Penetration of human skin by the cercariae of blood flukes that commonly infect other mammals or aquatic birds may cause a schistosomal dermatitis known as “swimmer’s itch.” Erythema, edema, and intense itching may develop that usually disappear within 1 week. The cercariae of these species are not able to complete the life cycle by entering the human bloodstream, and are destroyed by the host immune system.

Laboratory Diagnosis

The standard method of diagnosis is by the detection of characteristic eggs in feces or rectal biopsy, for S. japonicum, S. mekongi, S. mansoni, and S. intercalatum (and perhaps S. haematobium if these worms have migrated to a bladder vein that is close to the intestine); and in urine (usually concentrated before examination) or bladder tissue biopsy for S. haematobium. A wet mount with/without iodine from a sedimentation or concentration method can be examined for eggs. Figure 58-2 provides images of three different schistosome eggs. To optimize recovery of S. haematobium in urine, the specimen should be collected between noon and 2 pm.

There are some antibody-based assays that are available for diagnosis of schistosomal IgG antibody (enzyme immunoassay [EIA], enzyme-linked immunosorbent assay [ELISA], and immunoblot), but these methods cannot distinguish between current and previous infections. This type of assay may, however, be useful for travelers who have returned from endemic areas. These assays are performed at the Division of Parasitic Disease at the Centers for Disease Control and Prevention (CDC), and may be available at some private reference laboratories. Several nucleic acid-based testing methods have been developed that demonstrate high sensitivity and specificity using genomic or mitochondrial sequences. In addition, schistosome DNA has been identified in patients’ plasma using real-time polymerase chain reaction (PCR).

Therapy

The drug of choice for treatment of schistosome infections is praziquantel, given in two or three doses in a single day. Infection with S. mansoni may require a larger dose than that for the other species. An alternative treatment for S. haematobium infections is metrifonate (Bilarcil), an organophosphorus compound, given once every other week in a total of three doses.

Prevention

Because human infection is by direct penetration of the cercariae, prevention of schistosome infection is more difficult to achieve. Educational programs are required to help people in endemic areas understand how to help prevent infection. Sanitary conditions need to be improved with proper disposal not only of human wastes but also that of domestic animals (in areas with S. japonicum and S. mekongi). A safe water supply for bathing and washing clothes is also necessary. Various snail control methods have been tried, but these methods are very costly and would need to be repeated on a regular basis to have the desired effect.