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Pharmacology for Nurses

7.6 Antiparasitic and Anthelminthic Drugs

Pharmacology for Nurses7.6 Antiparasitic and Anthelminthic Drugs

Learning Outcomes

By the end of this section, you should be able to:

  • 7.6.1 Describe the pathophysiology of common parasitic and helminthic infections.
  • 7.6.2 Identify clinical manifestations related to common parasitic and helminthic infections.
  • 7.6.3 Identify the etiology and diagnostic studies related to common parasitic and helminthic infections.
  • 7.6.4 Identify the characteristics of drugs used to treat common parasitic and helminthic infections.
  • 7.6.5 Explain the indications, actions, adverse reactions, and interactions of drugs used to treat common parasitic and helminthic infections.
  • 7.6.6 Describe nursing implications of drugs used to treat common parasitic and helminthic infections.
  • 7.6.7 Explain the client education related to drugs used to treat common parasitic and helminthic infections.

Parasitic Infections

In the United States, parasitic infections can affect anyone, but they disproportionately affect immunocompromised individuals, members of racial and ethnic minority groups, and those with low income. Parasitic infections are defined by the relationship been the host (human) and the parasite. Common types of parasitic infections are caused by protozoa and helminths.

  • Protozoa: Protozoa are unicellular organisms that replicate within a human host. Examples of protozoal infections include those caused by Giardia, Plasmodium, and Babesia. Many of these conditions affect clients living in areas with poor sanitation because the spread can be attributed to either fecal-contaminated water or fecal–oral transfer. Signs and symptoms of protozoal infections include diarrhea, low-grade fever, nausea, greasy stools, and depressed appetite. Intestinal protozoal infections are most often diagnosed from microscopic stool samples, although molecular tests have recently been developed that may allow faster detection of infections.
  • Helminths: Helminths, or worms, are large complex organisms that can be consumed while they are in a larval stage and then mature into their adult stage in the host’s gastrointestinal tract. Common signs and symptoms include abdominal pain and diarrhea; however, infections can also be asymptomatic. Similar to protozoal infections, microscopic examination of stool samples can reveal helminth eggs and aid in diagnosis.

Antiparasitic Drugs

This section covers the most frequently used antiparasitic drugs, including their mechanisms, adverse effects, indications, and contraindications. It is important to ensure that an accurate diagnosis has been made regarding the causative organism because this will dictate which medication is most appropriate for the client. Some of the most common antiparasitic drugs include:

  • Metronidazole: For Giardia lamblia, Trichomonas vaginalis, Cryptosporidium parvum, and Toxoplasmosis gondii, metronidazole is the drug of choice. It provides good coverage against Entamoeba, Giardia, and Trichomonas, which are the species that cause infection.
  • Tinidazole: Tinidazole is structurally related to metronidazole and has similar actions, adverse effects, and interactions. It also has a longer half-life than metronidazole, so dosing is more convenient. However, tinidazole is much more expensive than metronidazole. Tinidazole is contraindicated in clients consuming alcohol while using the drug due to risk for causing a severe flushing reaction.
  • Nitazoxanide: Nitazoxanide is an alternative antiparasitic that can be used to treat giardiasis and cryptosporidiosis. Its full mechanism is unknown, but it is thought to disrupt the ability of these microorganisms to undergo anaerobic metabolism, leading to cell death. Nitazoxanide is well tolerated but can cause some gastrointestinal signs and symptoms.
  • Scabicides and pediculicides: Scabicides and pediculicides are designed to treat infestations caused by lice and scabies. Lice, depending on the species, can cause symptoms on the head, body, or genitals. Collectively, this condition is known as pediculosis. Lice can be difficult to treat because if the eggs (nits) are not successfully removed from the hair after treatment, reinfestation can occur. Nit combs can be indispensable along with pediculicides. All agents in this category are applied topically to the affected area.
  • Permethrin: Permethrin has a wide range of activity against a variety of arthropods, which makes it useful for lice infestations. Permethrin works by causing neuronal hyperpolarization and paralysis in the organism, leading to death.
  • Lindane: Lindane has activity against lice and scabies and works by being absorbed into the parasite’s exoskeleton, leading to seizures and death.
  • Malathion: Malathion comes from a group of chemicals known as organophosphates, which have been used as agricultural insecticides for many years. It works by inhibiting the enzyme acetylcholinesterase, which is responsible for metabolizing acetylcholine. This causes a buildup of acetylcholine, leading to overstimulation, paralysis, and death of the parasite.
  • Spinosad: Spinosad is another topical antiparasitic with activity against scabies and lice. It works by inducing neuronal excitation and eventual paralysis and death of the organism. The most common adverse effects seen with spinosad include redness and skin irritation at the application site.

Clinical Tip

Ensure Removal of Nits

Nits are the eggs of lice that attach to the hair, and they are unaffected by pediculicides. To ensure that nits do not go on to mature into adults requiring further pediculicide treatment, it is important to carefully comb through the hair with a fine-tooth nit comb after using a pediculicide. Shaving the head is a viable alternative, but this is not a desirable outcome for many clients.

Anthelminthic Drugs

The available anthelminthic agents are broad spectrum and can treat a variety of worms, including roundworms, pinworms, hookworms, and whipworms. Because worm infections occur in the gastrointestinal tract, these agents are taken orally.

  • Mebendazole: Mebendazole works by inhibiting microtubule formation in susceptible helminths and blocking glucose uptake. This action leads to eventual death of the worm, after which it can be passed through the feces.
  • Ivermectin: Ivermectin is an interesting antiparasitic agent because it has activity as both a pediculicide and an antihelminth. When used topically, it can treat lice and scabies infestations; when taken orally, it can treat a variety of helminth infections. Ivermectin works by inhibiting parasite nerve and muscle tissue, causing paralysis and death of the organism.

Table 7.15 lists antiparasitic and anthelminthic drugs and typical routes and dosing for adult clients.

Drug Routes and Dosage Ranges
Lindane Head lice: Apply 30–60 mL shampoo to dry hair and massage into hair for 4 min; add small quantities of water to hair until lather forms, then rinse hair and comb with fine-tooth comb to remove nits.
Ascariasis: 100 mg orally twice daily for 3 days or 500 mg orally once daily.
Giardiasis: 250 mg orally 3 times daily for 5–7 days.
Trichomoniasis: 500 mg orally twice daily for 7 days.
Amebiasis: 500 mg orally every 8 hours for 7–10 days.
Giardiasis: 500 mg orally every 12 hours for 3 days.
Head lice: Apply enough lotion or cream to saturate hair. Leave on for 10 min and then rinse with warm water.
Head lice: Apply sufficient amount to cover scalp.
Table 7.15 Drug Emphasis Table: Antiparasitic and Anthelminthic Drugs (source:

Adverse Effects and Contraindications

Most oral antiprotozoal and anthelminthic drugs can cause gastrointestinal upset. Any topical scabicides and pediculicides may cause skin irritation to the area where they are applied. Excessive use of lindane is not recommended in high doses, especially in children, because it can cause central nervous system excitation and seizure. Malathion should be kept away from children because ingesting it could cause cholinergic poisoning.

Table 7.16 is a drug prototype table for antiparasitic and anthelminthic drugs featuring metronidazole. It lists drug class, mechanism of action, adult dosage, indications, therapeutic effects, drug and food interactions, adverse effects, and contraindications.

Drug Class

Mechanism of Action
Causes loss of helical DNA structure and strand breakage to cause cell death
Drug Dosage
Giardiasis: 250 mg orally 3 times daily for 5–7 days.
Trichomoniasis: 500 mg orally twice daily for 7 days.
Amebiasis: 500 mg orally every 8 hours for 7–10 days.

Therapeutic Effects
Reduces symptoms of infection
Drug Interactions

Food Interactions
Adverse Effects
Abdominal pain

Hepatic impairment
Renal impairment
Seizure disorder
Table 7.16 Drug Prototype Table: Metronidazole (source:

Nursing Implications

The nurse should do the following for clients who are taking antiparasitic or anthelminthic drugs:

  • Monitor for signs and symptoms of anaphylaxis (e.g., shortness of breath, difficulty breathing, difficulty swallowing).
  • Advise the client to take the entire prescribed course of the medication to ensure adequate treatment and to reduce the development of drug resistance.
  • Instruct the client to maintain adequate hydration; monitor kidney function for renally eliminated medications.
  • Provide client teaching regarding the drug and when to call the health care provider. See below for client teaching guidelines.

Client Teaching Guidelines

The client taking an antiparasitic or anthelminthic drug should:

  • Alert their health care provider about any signs of allergic reactions including throat swelling, severe itching, rash, or chest tightness.
  • Alert their health care provider that they are taking these medications, including the dose and frequency.
  • Take the drug with food if it causes an upset stomach.
  • Take a missed dose as soon as they remember; however, they should not take double doses.
  • Avoid taking tinidazole or metronidazole with alcohol because the interaction can cause a severe flushing reaction.

FDA Black Box Warning

Lindane Lotion

Seizures and deaths have been reported following repeated or prolonged use and in rare cases following a single application.


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