Anthelmintics -- Antiparasitic Drugs For Intestinal Parasite Infections From Worms
Helminths (worms) can be divided into three groups: cestodes, or tapeworms; nematodes , or roundworms; and trematodes, or flukes . The helminths differ from other infectious organisms in that they have a complex body structure. They are multi-cellular and have partial or complete organ systems (e.g., muscular, nervous, digestive, and reproductive). Several of the drugs used to treat worm infections affect the nervous system of the parasite and result in muscle paralysis, either spastic or flaccid. Other drugs affect the uptake of glucose and thus energy stores. All are chemical agents and are generally administered orally. There are no antibiotics available for the treatment of these infestations.
TAPEWORMS attach to the intestinal tract by a sucker or a sucking groove on the head (scolex). Unlike the nematodes and trematodes, tapeworms do not enter the host tissues. The primary drugs used for these infections are niclosamide and praziquantel. Niclosamide inhibits the uptake of glucose by the helminth and therefore the production of energy. It has a spastic or paralytic effect on the worm. Because it is poorly soluble, high concentrations are obtained in the intestinal lumen. Praziquantel also produces tetanus-like contractions of the musculature of the worm. Unlike niclosamide, praziquantel is readily absorbed from the intestinal tract. It is a broad-spectrum anthelmintic affecting both flukes and tapeworms.
ROUNDWORMS: Treatment of roundworms is complicated by the fact that some live in blood, lymphatics, and other tissues (filarial worms) and thus require use of drugs that are absorbed from the intestinal tract and penetrate into tissues. Others are found primarily or solely in the intestinal tract (intestinal nematodes).
Diethylcarbamazine , used for treating filarial worm infections, is absorbed from the intestinal tract. Blood levels are reached quickly, and it has rapid action against the microfilariae. A severe allergic or febrile reaction due to the death of the microfilariae can follow use of the drug. Piperazine causes a flaccid paralysis of the worm and its expulsion from the intestinal tract. It is used as alternative therapy for treating Ascaris infection.
Thiabendazole and mebendazole interfere with glucose uptake and consequently with the production of energy. Mebendazole accumulates in the intestine and is used for treating Ascaris (roundworm), hookworm, and whip worm infections. It is well tolerated but abdominal discomfort and diarrhea can occur in patients with a strong infestation. Thiabendazole is rapidly absorbed from the gastrointestinal tract, making it effective against organisms found in tissue. It is used in treating cutaneous larva migrans (creeping eruption), visceral larva migrans, trichinosis, and trichostrongylosis. About one-third of patients treated with thiabendazole have anorexia, nausea, vomiting, and vertigo.
Pyrantel pamoate causes spastic paralysis of helminth muscle. Most of the drug is not absorbed from the intestinal tract, resulting in high levels in the lumen. It is a drug of choice in treating pinworm and Ascaris infection and is a recommended alternative therapy for hookworm and trichostrongolosis.
FLUKES: Praziquantel is the most effective drug in treating infections caused by intestinal, liver, and lung flukes. Bithionol is used for treating Fasciola hepatica (sheep liver fluke) and Paragonimus westermani (lung fluke) infections and is absorbed from the intestinal tract. Tetrachloroethylene is an alternative agent for treating Fasciolopsis buski (large intestinal fluke) and Metagonimus yokogawai (small intestinal fluke) infection. Praziquantel is the drug of choice for treating schistosomiasis (infections of blood flukes). Metrifonate, a drug used as an alternative agent for Schistosoma haematobium infections, is metabolized to dichlorvos, an anticholinesterase agent. Dichlorvos acts on cholinesterase in the helminth and can also cause a reversible inhibition of plasma cholinesterase in patients. Oxamniquine is an alternative oral therapy for the treatment of Schistosoma mansoni infestation.