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Chemotherapy of Helminth Infections

This chapter will be most useful after having a basic understanding of the material in Chapter 51, Chemotherapy of Helminth Infections in Goodman & Gilman’s The Pharmacological Basis of Therapeutics, 12th Edition. In addition to the material presented here, the 12th Edition contains:

• A detailed discussion of helminth infections and their treatment including nematodes (roundworms), cestodes (flatworms), and trematodes (flukes)

LEARNING OBJECTIVES

   Understand the common helminth infections, the clinical symptoms, and the mainstays of therapy.

   Describe the therapeutic uses of antihelmintic drugs.

   Understand the mechanisms of actions of antihelmintic drugs.

   Identify the toxicities and contraindications of antihelmintic drugs.

  CASE 37-1  

A 3-year-old boy is brought into your office. His mother is complaining that her child is scratching between his legs which has caused irritation and redness.

a. Although you suspect a pinworm (Enterobius vermicularis) infection, how is the diagnosis made?

Often the worms or their eggs can be seen in the perianal area early in the morning. A “tape test” is used to diagnose pinworm. In the morning before bathing or using the toilet a piece of cellophane tape is pressed in the skin around the anus, and removed. The tape is then examined with a microscope to look for the worms or their eggs.

b. What is the appropriate treatment of this child?

The relative incidence of common helminthic infections in humans worldwide is illustrated in Figure 37-1. Enterobius, pinworm, is one of the most common helminth infections in temperate climates, including the United States. Although this parasite rarely causes serious complications, pruritus in the perianal region can be severe, and scratching may cause secondary infection. Salpingitis or even peritonitis is a rare complication in female patients.

FIGURE 37-1 Relative incidence of helminth infections worldwide.

Pyrantel pamoate, mebendazole, or albendazole as a single dose are highly effective in the treatment of Enterobius infection. A second dose is often recommended to be administered 2 weeks after the first. The infection easily spreads throughout members of a family, a school, or an institution, and may require treatment of all individuals in close contact with an infected person.

  CASE 37-2  

A 53-year-old man comes into your office with a 3-month history of weight loss, abdominal pain, and fatigue. His history is also significant for the consumption of partially cooked fish during a camping trip 6 months previously. A physical examination reveals a blood pressure of 140/60 mm Hg, pulse of 72 beats/min, and a respiratory rate of 16 breaths/min. The remainder of his physical examination is also normal with the exception of pale nail beds and skin. A complete blood count shows the presence of a megaloblastic anemia. An examination of a fresh stool specimen reveals the eggs and segments of a worm.

a. What is the likely diagnosis?

Diphyllobothrium latum, known as fish tapeworm, is found most commonly in rivers and lakes of the Northern Hemisphere. Eating inadequately cooked infected fish introduces the larvae into the human intestine. Although most infected individuals are asymptomatic, the most frequent manifestations include abdominal symptoms and weight loss.

b. What is the cause of this patient’s anemia?

Megaloblastic anemia develops due to a deficiency of vitamin B₁₂ (see Chapter 25), which is taken up by the parasite.

c. What is an appropriate treatment for this patient?

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