Schistosoma Blood Flukes

Causative agents of schistosomosis or bilharziosis

■ Schistosomosis (bilharziosis) is one of the most frequent tropical diseases with about 200 million infected persons. The occurrence of schistosomosis depends on the presence of suitable intermediate hosts (freshwater snails). Human infections result from contact with standing or slow-moving bodies of water (freshwater) when Schistosoma cercariae penetrate the skin. Schistosoma hematobium causes urinary schistosomosis; S. mansoni, S.japo-nicum, S. intercalatum, and S. mekongi are the causative agents of intestinal schistosomosis and other forms of the disease. Diagnosis can be made by detection of either Schistosoma eggs in stool or urine or of specific antibodies in serum. ■

Parasite species and occurrence. Schistosomosis is also known as bilharziosis after the German physician Th. Bilharz, who discovered Schistosoma hematobium in human blood vessels in 1851. Schistosomosis occurs endemi-cally in 74 tropical and subtropical countries of Africa, South America, and Asia (Fig. 10.2). The number of persons infected with schistosomes is estimated at 200 million (WHO, 2004).

The most important species pathogenic to humans are Schistosoma hema-tobium (Africa, the Near East, and questionable occurrence in India), S. man-soni (Africa, the Caribbean, the north-east of South America), and S. japoni-

Platyhelmintha (syn. Platyhelminthes) 547 — Distribution of Schistosomes -

Platyhelmintha (syn. Platyhelminthes) 547 — Distribution of Schistosomes -

Mekongi Distribution
Fig. 10.2 a Schistosoma hematobium, S. japonicum, and S. mekongi; b S. mansoni and S. intercalatum (according to WHO Technical Report Series No. 830. Geneva: World Health Organization; 1993).

cum (Southeast Asia and the western Pacific, especially China, Indonesia, the Philippines, but no longer in Japan). S. intercalatum occurs focally in central and western Africa, S. mekongi in Laos and Cambodia.

Morphology and life cycle. The various Schistosoma species can be differentiated morphologically (Table 10.2).

The relatively thick male forms a tegumental fold, the ventral groove (or canalis gynaecophorus) in which the threadlike female is enclosed. The male thus appears to be slit longitudinally (schizein: to split, soma: body) (Fig. 10.3).

Table 10.2 Schistosoma Species that Commonly Infect Humans1

Schistosoma species and length (mm)

Main location of adult stages2

Eggs: characteristics, dimensions, and excretion (E)

I: Intermediate hosts (snails), R: Animal reservoir hosts3

S. haematobium

FF: 7-15 99: 9-20

Venous plexus in minor pelvis (draining urinary bladder, etc.)

Ovoid, with terminal spine,

83-187 x 60-70 |im E: urine, rarely stool

I: Bulinus species R: (Monkeys)

S. intercalatum

FF: 11-14 99: 13-24

Mesenteric veins (draining colon)

Ovoid, with terminal spine,

140-240 x 50-85 |im E: stool3

I: Bulinus species R: (sheep, goats)

S. mansoni

FF: 6-10 99: 7-15

Mesenteric veins (draining colon)

Ovoid, with lateral spine, 112-175 x 45-70 |im E: stool

I: Biomphalaria species.

R: (monkeys, dog, rodents)

S. japonicum

FF: 7-20 99: 10-26

Mesenteric veins (draining intestine)

Elliptical, lateral spine tiny or lacking 70-100 x 50-65 im E: stool

I: Oncomelania species.

R: Cattle, buffalo, pig, dog, rodents, etc.

S. mekongi

FF: 10-18 99 : 14-20

Mesenteric veins (draining small intestine)

Elliptical, lateral spine tiny or lacking 50-65 x 30-55 |im E: stool

I: Neotricula species. R: dog

1 Location not strictly specific; adult stages also found in vessels of liver, lungs, and, less frequently, in other organs.

2 In parentheses: of secondary or local significance only.

3 Stainable with Ziehl-Neelsen stain, in contrast to S. hematobium.

— Schistosoma mansoni Pair

— Schistosoma mansoni Pair

Schistosoma Hematobium
Fig. 10.3 The threadlike female is enclosed in a groove in the body of the male.

The adult parasites live in the lumen of veins. Table 10.2 summarizes data on various Schistosoma species. Fig. 10.4 (p. 550) shows their life cycle.

Sexually mature Schistosoma females lay about 100-3500 eggs a day, depending on the species, each containing an immature miracidium (= ciliate larva), which matures in the host within six to 10 days and remains viable for about three weeks (Fig. 10.4).

At the site of their deposition, the eggs lie in chainlike rows within small veins. Some penetrate through the vascular wall and surrounding tissue to reach the lumen of the urinary bladder or intestine (regarding the eggs that remain in the body see section on pathogenesis). Enzymes produced by the miracidium and secreted through micropores in the eggshell and granuloma formation (see below) contribute to the penetration process. The eggs are shed by the definitive host in stool or urine within a few weeks post infection (p.i.) (see below). If the eggs are deposited into freshwater, the mira-cidia hatch from the eggshell and begin their search for a suitable intermediate host (Fig. 10.4).

Various genera and species of freshwater snails serve as intermediate hosts (Table 10.2) in which the invading miracidia reproduce asexually, producing mother and daughter sporocysts, and finally numerous cercariae, which begin to swarm into the water three to six weeks p.i. at the earliest. A characteristic feature of the approximately 340-520 im long cercariae is their forked tail. The cercariae swim freely about or cling to the surface of the water. Upon contact with a human host, enzyme secretion and vigorous movements enable them to penetrate the skin within a few minutes, or less frequently the mucosa when ingested with drinking water. During the infection process, the cercaria loses its tail, sheds the surface glycocalyx, forms a new tegument, and transforms into the schistosomulum.

— Schistosoma mansoni: Life Cycle

— Schistosoma mansoni: Life Cycle

Schistosoma Blood
Fig.10.4 1 Male and female; 2 egg with miracidium; 3 miracidium; 4 intermediate host (Biomphalaria glabrata); 5 sporocyst; 6 daughter sporocyst with cer-cariae; 7 fork-tailed cercaria (modified after Piekarski G, Medizinische Parasitologie in Tafeln. 2nd printing. Berlin: Springer; 1973).

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