Table 541

Drug

Mode of Action

Piperazine

Paralyzes helminth muscle

Ivermectin

Paralyzes helminth muscle

Pyrantel

Paralyzes helminth muscle

Niclosamide

Inhibits production of energy

Mebendazole

Inhibits protein function

Diethylcarbamazine

Enhances phagocytosis and killing

Praziquantel

Paralyzes helminth muscle

Thiabendazole

Inhibits energy production, protein function

Bithionol

Inhibits energy production

Suramin

Inhibits energy production

Some of the nematodes (filarial worms and guinea worms) live in blood, lymphatics, and other tissues and are referred to as blood and tissue nematodes. Others are found primarily in the intestinal tract. One group, hookworms, undergoes a developmental cycle in soil. The larvae penetrate the skin of humans, enter the venules, and are carried to the lungs, where they enter the alveoli, sometimes causing pneumonitis. The larvae then migrate up the trachea and are swallowed. In the intestine, they attach to the mucosa, and using the cutting plates and a muscular esophagus, feed on host blood and tissue fluid. This may result in vague abdominal pains, diarrhea and, if many worms are present, anemia.

Strongyloides stercoralis infection is acquired, like hookworm, from filariform larvae in contaminated soil that penetrate the skin. This parasite maintains itself for many decades in the small intestine asymptomatically. Persons treated with immunosuppressive drugs or who are debilitated by chronic illness may be at risk for widespread tissue invasion or hyperinfection syndrome. Prompt treatment may be life saving in disseminated disease.

Other intestinal nematodes are acquired by ingestion of eggs from soil. These groups lack cutting plates and may not cause anemia. Still other nematodes, such as pinworms, migrate from the anus to lay eggs, which are transmitted by fingers or through the air. The eggs are ingested and the adult worm develops in the intestinal tract. In some cases, the appendix may be invaded, resulting in symptoms of appendicitis. In most cases, the symptoms are perianal pruritus and a restlessness associated with the migration of the female worm through the anus to the perianal skin. Other nematodes, such as Ascaris spp., are ingested in egg form but have a migration similar to that of the hookworm.

Specific Effect

Blocks myoneural junction;causes chloride-dependent hyperpo-

larization, flaccid paralysis Blocks transmission of nerve signals by interactions with gluta-

mate-gated chloride channels Depolarization and spastic paralysis

Uncouples anaerobic oxidative phosphorylation in tapeworm mitochondria, causing decreased ATP synthesis Binds to tubulin and inhibits polymerization Sensitizes microfilaria, entraps them in reticuloendothelial system Increases membrane permeability, unmasks surface proteins Inhibits fumarate reductase, ATP synthesis; binds to tubulin Uncouples oxidative phosphorylation

Inhibits muscle enzymes of glycolysis and oxygen consumption

The filarial worms differ from other nematodes in that they are threadlike and are found in blood and tissue. The infective larvae enter following the bite of an infected arthropod (fly or mosquito). They then enter the lymphatics and lymph nodes. Fever, lymphangitis, and lymphadenitis are associated with the early stage of the disease. Chronic infections may be characterized by elephantiasis as a result of lymphatic obstruction. Some species of filarial worms migrate in the subcutaneous tissues and produce nodules and blindness (on-chocerciasis).

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