McKenna's Pharmacology for Nursing, 2e

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P A R T 2  Chemotherapeutic agents

Eggs hatch in fresh- water stream or lake and infect snails.

Eggs excreted in human waste.

Within the snails, larvae (cercariae) develop.

Adults move to intestines and urinary bladder. Females lay masses of eggs.

Cercariae are shed into water.

Larvae

Larvae enter the bloodstream and go to lungs and liver, where they mature.

Cercarial larvae burrow into the skin of humans coming in contact with the water.

Hair follicle

FIGURE 13.1  Life cycle of schistosomes.

and certain South American and Caribbean countries that have climates and snails conducive to the life cycle of schistosomes. Eggs that are excreted in the urine and faeces of infected individuals hatch in fresh water into a form that infects a certain snail. In the snail, larvae known as cer- cariae develop. The snail sheds the cercariae back into the freshwater pond or lake. People become infected when they come in contact with the infested water. The larvae attach to the skin and quickly burrow into the blood- stream and lymphatics. Then they move into the lungs, and later to the liver, where they mature into adult worms that mate and migrate to the intestines and urinary bladder. The female worms then lay large numbers of eggs, which are expelled in the faeces and urine, and the cycle begins again. Signs and symptoms may include a pruritic rash, often called swimmer’s itch, where the larva attaches to the skin. About 1 or 2 months later, affected individuals may experience several weeks of fever, chills, headache and other symptoms. Chronic or severe infestation

may lead to abdominal pain and diarrhoea, as well as blockage of blood flow to areas of the liver, lungs and central nervous system (CNS). These blockages can lead to liver and spleen enlargement, as well as signs of CNS and cardiac ischaemia. (See Critical thinking scenario for a case study of a person diagnosed with chronic schistosomiasis.) ■■ Helminths are worms that cause disease by invading the human body. Some helminths invade body tissues and can seriously damage lymphatic tissue, lungs, CNS, heart or liver. ■■ Pinworms are the most frequent cause of helminth infection in Australia and New Zealand, and roundworms called Ascaris are the most frequent cause of helminth infections throughout the world. ■■ Education is important for decreasing the stress and anxiety that may occur when individuals are diagnosed with a worm infestation. KEY POINTS