Schistosoma japonicum (Katsurada, 1904) Stiles, 1905

(Figure 2-50)


ETYMOLOGY:Schisto = split and soma = body along with japonicum referring to the geographical region in which it was first found.

SYNONYMS:Schistosomum japonicum Katsurada, 1904; Schistosoma cattoi Blanchard, 1905

HISTORY: This parasite was first discovered by Dr. Katsurada in the portal vein of a cat; a single male worm was recovered. Not long after, 32 male and female worms were found by this worker in veins of another cat that was noticed to have a swollen abdomen. In this same year, a female was found in the portal vein of a human being; the first indication of this parasites medical importance. This was also the first schistosome for which the life cycle was described by Drs. Miyairi and Suzuki.

GEOGRAPHIC DISTRIBUTION: Japan, the Yangtze basin of China, Taiwan, Indonesia, Malaysia, and the Philippines.

LOCATION IN HOST: In the portal veins.

PARASITE IDENTIFICATION: The adults of Schistosoma japonicum, like those of other schistosomatids, occur as separate sexes. Males are about 15 mm long, and females are about 20 mm long. Both sexes are very elongate organisms compared to other trematodes; about 90% to 95% of the total body length is posterior to the ventral sucker, and the worms are no more than 1 mm in width. The male is stouter than the female, and the ventral body surface posterior to the ventral sucker has lateral inflations throughout its length that role towards the midline and form a groove, the gynecophoral canal. The female is held within this groove of the male throughout most of her adult life. The male has seven testes just posterior to the ventral sucker. The female has an elongate uterus that appears as a single chain of eggs that takes up most of the anterior half of her body.

The eggs of this species of Schistosome are nearly spherical. the thin eggshell is light-yellow in color and lacks an operculum. On one end of the shell a small bump or process is often visible. The eggs are 70-100 µm long by 50-65 µm wide, and they contain a fully developed miracidium when passed in the feces.

LIFE CYCLE: Within 15 minutes to an hour after such Schistosoma japonicum eggs enter fresh water, the developed miracidium hatches from the eggshell. The released miracidium then penetrates a snail of the genus Oncomelania. Within the snail, the miracidium develops through several stages to produce cercariae; the cercariae produced by a single miracidium will all be of the same sex as adults. The cercaria is characterized by its possession of a forked tail. The final host is infected by the cercaria penetrating the skin. After going through the skin, the tail is lost, and the so-called schistosomulum makes its way into the vascular system and is carried to the lung. The growing flukes then make their way to the liver and ultimately to the mesenteric veins where they develop. The females begin to lay eggs 5 to 6 weeks after infection of the final host.

CLINICAL PRESENTATION AND PATHOGENESIS: There are no descriptions of clinical illness in cats with this infection which suggests that it may be asymptomatic given the large numbers of cats that are probably infected.

TREATMENT: Praziquantel is probably the drug of choice.

EPIZOOTIOLOGY: Numerous mammals are capable of being infected with Schistosoma japonicum. Other hosts besides the cat include human beings, cattle, horses, dogs, pigs, goats, and rats.

HAZARD TO OTHER ANIMALS: Other animals appear likely to develop infection and disease similar to that seen in the human, but only if the infective cercariae penetrate the skin. Thus, the infected cat is not a direct threat to other uninfected animals.

HAZARD TO HUMANS: Numerous human beings are infected with this parasite and serious disease and perhaps death can result from the infection.

Figure 2-50.Schistosomajaponicum female (the thinner fluke) within the gynecophoral canal of the male. These flukes were recovered from a cat.