Anthelmintics
Anthelmintics Clasifications and Treatment of Worm infections
Alok Bains
5/8/20235 min read


Anthelmic drugs
Anthelmic drugs (Anthelmintic drug)
Compiled by Alok Bains
Introduction
Helminths (worms) are parasites that live in the GIT lumen or inside the tissue of the human body or animal. Their harmful effects include
Secretion of toxins,
Blood loss, and
Injury to the intestine and other organs.
There are three types of helminths:
Nematodes (Roundworm),
Trematodes and
Cestodes.
"Anthelmintics eradicate or reduce the number of worms (helminths) in the intestine or they block the development of helminths inside human and animal bodies". Usually, a single anthelmintic drug is effective against specific helminths. Thus, Anthelmintics should be prescribed after proper diagnosis and identification of helminths.
Classification
Anthelmintics are classified on the basis of their mode of action, on the basis of chemical structure or on the basis of their effect on parasites.
Classification on the basis of mode of actions
Benzimidazole: They stop the absorption of glucose into parasites. A deficiency of glucose in parasites leads to a decrease in parasite energy and their death. Examples of Benzimidazole are Thiabendazole, Albendazole, and Mebendazole.
Nicotinic agonists: They activate the nicotinic receptors of parasites' nervous system. This lead to paralysis and death of the parasites. Examples are Pyrantel and Levamisole.
Macrocyclic lactone: They stop the synthesis of cuticles in the parasites. This causes the loss of the protective layer on the parasites' body surface and their death. Examples are Ivermectin and Moxidectin.
Piperazine: They paralyse the parasites' muscles. This leads to the ex[ulsion of parasites from the GIT. Examples are Piperazine citrate and Diethylcarbamazine.
Praziquantel: It disturbs parasites' cell membrane permeability. This leads to muscle contraction and the death of parasites. It is used in the treatment of tapeworm infection.
Nitroimidazoles: They interfere in the DNA synthesis of parasites. Examples are Metronidazole and Tinidazole.
Nematodes (Roundworm) infection
Nematodes are elongated roundworms with a bilaterally symmetrical body. They have a complete digestive system and a large body cavity. Nematodes develop an infection in the intestine, lungs, blood and tissues. Examples of Nematodes are Ascaris, Lumbricoids, Strongyloides and Trichuris.
Symptoms: Symptoms of roundworm infection are abdominal pain diarrhoea cough asthma shortness of breath fiver muscle pain, and even visibility of the worm in the patient's eye, but rarely fatal.
Treatment of Nematodes (roundworms): Benzimidazole is a group of anthelmintics that has broad-spectrum anthelmintic activity. Most commonly used benzemidazole are thibendazole, mebindazole and albendazole. Albendazole and mebendazole are most commonly recommended by physicians for roundworm and hookworm infections.
Thiabendazole: Use of Thiabendazole has decreased due to its systemic side effects like nausea, vomiting, abdominal cramp, dizziness, drowsiness, giddiness and headache. Sometimes it also causes visual disturbance, hypotension and bradycardia. It is rapidly absorbed from GIT, metabolised in the liver and excreted in the urine.
Dosage: The recommended maximum daily dose is 3 grams. The recommended normal oral dose is two to three times a day for 3 days.
It should be administered preferably after the meal. It is available as a chewable tablet and suspension. Its tablet should be chewed before swallowing.
Indications: Angiostrongylosis, Capillariasis, Ascariasis, Hook worm infection, Stronyloidiasis, Whipworminfection, Cutaneous Larva migrans, Visceral Larva migrans, Trichinosis. Contraindications: It should not be used as a prophylactic agent for pinworm infection.
Mebendazole: It is a benzimidazole compound that has completely replaced thiabendazole in the treatment of worm infection due to its no side effects. It is the drug of choice to treat a wide spectrum of worms such as whipworm, hook-worm, pin-worm and roundworm, but less effective against strongyloids. It is very poorly absorbed from GIT. Fatty food increases its absorption from GIT. The absorbed drug is metabolised in the liver and excreted in urine and in bile.
Mechanism of action: Mebendazole binds with the microtubular protein of nematodes and inhibits their polymerisation. This leads to a gradual loss of intracellular microtubules in nematodes. It decreases glucose uptake in nematodes which causes decreases in glycogen storage glucose deficiency in nematodes. It also blocks the secretion of acetylcholine esterase. This all leads to immobilisation of the nematode, inhibition of nematode egg hatching and ultimately death of the parasite. The dead parasite is expelled from the human body in faeces. It may take several days to expel all nematodes from the human body. Mebendazole is insoluble in water.
Side effects: Mebendazole is poorly absorbed from GIT unless administered with a high-fat content diet. It is also rapidly metabolised thus it produces very mild side effects or almost no side effects. It is even well tolerated by even very weak patients. Some side effects are mild diarrhoea, nausea, vomiting and abdominal cramp. A higher dose may cause hypothermia and reversible neutropenia.
It is contraindicated during pregnancy due to its teratogenic and embryotoxic effects and in children below two years. It should be used with caution in liver disease.
Therapeutic uses: In single and mixed infections, Mebendazole is administered 100 mg twice a day for five days for a complete cure of the disease. Complete chewing of the tablet before ingestion gives a better result.
Albendazole: Albendazole is more preferred drug than mebendazole as an anthelmintic. The main disadvantage of mebendazole is multiple dosing for a complete cure while Albendazole single dose is sufficient to cure Helminths disease. It has also a broad spectrum of activity excellent tolerability by the human body and better bioavailability than mebendazole.
It is moderately absorbed from GIT and undergoes first-pass metabolism in the liver by sulfoxidation. Rapid and complete sulfoxidation in the liver convert albendazole to active metabolite Albendazole sulfoxide. Albendazole sulfoxide is widely distributed inside the body. It can cross BBB. It is excreted in the urine. Metabolites of mebendazole and thiabendazole are inactive metabolites. Albendazole sulfoxide's half-life is about 8 to 9 hours.
Mechanism of action: Same as mebendazole.
Therapeutics uses: Albendazole is used to treat intestinal nematodes, hydatid disease and neurocysticercosis.
Intestinal nematode treatment: Single-dose 400 mg for one day is used to treat scaries, hook worm, pin worm, and enterobiasis infection treatment. Some effect is produced by three days of treatment with mebendazole. Heavy infection may require two days to treat with Albendazole. Enterobiasis treatment requires the treatment of all family members at once.
Hydatid disease: Parasitic infection caused by tapeworm is called hydatid disease. Albendazole 400 mg twice daily is administered for 28 days. Treatment is repeated three times sat gap of 14 days between 2 treatments. Neurocysstisercosis: Neurological infection caused by tapeworm larvae is called Neurocysstisercosis. Albendazole is an alternate drug to treat Neurocysstisercosis. The main drug is praziquantel.
Side effects: Albendazole is safe during 3 days of treatment in a few patients dizziness, headache, GIT upset, nausea vomiting and diarrhoea were reported. Long treatment with Albendazole causes a headache, fever, liver toxicity, bone marrow depression and alopecia
Pyrantel Pamoate: Pyrantel Pamoate is a pyrimidine compound that is poorly absorbed from GIT. It exerts its effect on the intestine. It is metabolised in the liver and excreted in faeces. Some parts of the metabolites are also excreted in the urine.
Mechanism of action: PyrantalPamoate activates nicotinic cholinergic receptors in worms and depolarises neuromuscular junction in worms. Thus it acts as a depolarizing neuromuscular blocking agent. It also inhibits the cholinesterase enzyme. This all develops slow contraction in worm muscles and spastic paralysis. The paralysed worm is expelled from the intestine of the host. Pyrantel Pamoate does not act as a depolarizing neuromuscular blocking agent in the human body due to its very little affinity with human nicotinic cholinergic receptors.
Side effects: It is poorly absorbed from GIT. Thus therapeutic dose does not produce systemic side effects. Sometimes it may cause GIT disturbance headache dizziness and drowsiness.
Indications: Pyrantel Pamoate is administered to treat worm infections like hookworm, pinworm and roundworm infection. Its anthelmintic effectiveness increases if it is administered with mebendazole.
Contraindication: Pyrantel Pamoate and piperazine combination should not be used. They are antagonists to each other. Pyrantel Pamoate produces an effect by spastic paralysis while piperazine produces an effect by flaccid paralysis in worms. Pyrantel Pamoate should not be recommended to children under 2 years, persons with hepatic dysfunction and during pregnancy.