The Life Cycle of Schistosoma
Schistosomes are a form of parasitic worm that infect both an intermediate and final host. They are 7 to 20 mm in length and live in veins and venules within the circulatory system. The life cycle of schistosomes is complex, requiring an intermediate host and a fresh-water source. Humans and animals including dogs, cats and livestock can contract Schistosomiasis.
The Egg Stage
Eggs are carried through the intestinal or urinary tract and expelled from the body in feces or urine into fresh-water ponds, lakes or rivers. The eggs then hatch into an intermediate, motile form known as a miracidium.
Miracidium
Miracidia then seek out a fesh-water snail as an intermediate host and penetrate its tissues. Within the snail, the miracidium grows into a sporocyst.
Sporocyst
Still within the snail, the sporocyst will release free-swimming, fork-tailed cercariae into the water. This will occur in separate successive generations.
Cercariae
The cercariae then swim along, seeking a human host. Once a human host is found, the cercariae penetrate the skin, shedding their tails in the process. At this point the cercariae become schistosomulae.
Schistosomulae
Once in the host body, the schistosomulae migrate to the circulatory system where they grow into adult worms, living within veins and venules.
Adulthood
The worms then mate. Females lay eggs in small venules within the portal and perivesical systems. In some species, the eggs move toward the intestine. In another species, the eggs move to the bladder. They are then passed with stool or urine. If the infected material falls into fresh water, and the life cycle begins again.
Diagnosis and Treatment
The best treatment for Schistosomiasis is prevention by avoiding contaminated water sources, improving water sanitation and controlling snail populations.
Diagnosis is made by identifying acute symptoms such as an itchy dermatitis at the infection site, fever, headache and cough. Chronic symptoms include fatigue, abdominal pain and intermittent diarrhea. The liver and spleen may also be enlarged. Intestinal granulomas may also be present. Stool and urine samples should be examined for the presence of eggs.
The infection is treated with a chemotherapy drug called Praziquantel, which will eradicate all five species of schistosomes.
Tags: circulatory system, Cycle Schistosoma, eggs move, human host, intermediate host