define anthelmintic

What are Anthelmintics?

Anthelmintics, also known as antihelminthics, are a class of antiparasitic medications that either shock or kill parasitic worms (helminths) and other internal parasites without inflicting considerable harm to the host. They are also known as vermifuges (those that stun) or vermicides (those that kill vermin) (those that kill). Anthelmintics are medications used to treat helminth infections, often known as helminthiasis. These medications are also used to treat sick animals.

In many underdeveloped nations, anthelmintic pills are used in large deworming efforts for school-aged children. Mebendazole and albendazole are the medications of choice for soil-transmitted helminths; praziquantel is the treatment of choice for schistosomiasis and tapeworms.

Several types of Anthelmintics

  • Ascaricides are antiparasitics that particularly target worms of the genus Ascaris.
  • Albendazole is helpful against threadworms, roundworms, whipworms, tapeworms, and hookworms;
  • Mebendazole is effective against a wide range of nematodes;
  • Thiabendazole is effective against a wide range of nematodes;
  • Fenbendazole is an antiparasitic medication;
  • Triclabendazole is an anti-liver fluke medication;
  • Flubendazole is effective against the majority of intestinal parasites;
  • Avermectins (including ivermectin) are effective against the majority of common intestinal worms, with the exception of tapeworms, for which praziquantel is usually used in combination with bulk dewormings;
  • Wuchereria bancrofti, Brugia malayi, Brugia timori, and Loa loa are all susceptible to diethylcarbamazine;
  • Pyrantel pamoate – effective against most gastrointestinal nematode infections;
  • Levamisole;
  • Salicylanilide is a mitochondrial uncoupler that is solely used to treat flatworm infestations;
  • Niclosamide, Oxyclozanide, and Nitazoxanide – destroy Ascaris lumbricoides and have antiprotozoal properties;
  • Praziquantel – effective against flatworms (tapworms and schistosoma); Oxamniquine – effective against flatworms (tapworms and schistosoma);
  • Octadepsipeptides (for example, Emodepside) are effective against a wide range of gastrointestinal helminths;
  • Monepantel (aminoacetonitrile class) Is effective against a wide range of nematodes, including those resistant to other classes of anthelmintics;
  • Spiroindoles (e.g., derquantel) – effective against a wide range of nematodes, including those resistant to other classes of anthelmintics;
  • Artemisinin – has antihelmintic properties.

Helminths are classified into three types: cestodes (tapworms), nematodes (roundworms), and trematodes (flukes). Helminths are distinguished from other pathogenic organisms by their complicated body structure. They are multicellular and have organ systems that are either partial or full (e.g., muscular, nervous, digestive, and reproductive). Several medications used to treat worm infections impact the parasite’s neurological system, causing muscular paralysis. Other medications influence glucose absorption and consequently energy storage. All are chemical agents that are usually taken orally and are used in both human and veterinary medicine. However, no anthelmintic is totally effective, has no adverse impact on the host, and is equally active against all worms.

Anthelmintics for cestodes

Tapeworms enter the digestive tract through a sucker or sucking groove on the head (scolex). Tapworms, unlike nematodes and trematodes, do not penetrate the host tissues. As a result, tapeworm infections are less difficult to treat than infections produced by worms that enter host tissues. Furthermore, because tapeworms are restricted to the intestinal tract, the medicine does not need to kill them, and the drug does not need to be absorbed when administered orally. As a result, cestode anthelmintics often have a broader margin of safety than anthelmintics used to treat worm infections in locations other than the gut. The word vermifuge is frequently used to describe medicines used to eliminate intestinal worms.

Albendazole and praziquantel are the most often used medications to treat cestode infections. Albendazole prevents the helminth from absorbing glucose and so producing energy. It causes the worm to spasm or become paralyzed. Praziquantel also causes tetanus-like spasms of the worm’s muscle. Praziquantel, unlike albendazole, is easily absorbed from the gastrointestinal system. It is a broad-spectrum anthelmintic that kills flukes as well as tapeworms.

Quinacrine, an early synthetic antimalarial that has since been surpassed, is frequently used as an anthelmintic to treat tapeworm infection in dogs, cattle, and other animals.

Anthelmintics for nematodes

Roundworm treatment is complicated by the fact that some dwell in blood, lymphatics, and other tissues (filarial worms), necessitating the use of medications that are absorbed from the GI tract and penetrate into tissues. Others are located mostly or exclusively in the digestive system (intestinal nematodes). The digestive tract absorbs diethylcarbamazine and ivermectin, which are used to treat filarial worm infestations. Blood levels are immediately attained, and action against microfilariae is swift. The use of these medications may result in a severe allergic or feverish reaction owing to the death of the microfilariae.

Mebendazole, like albendazole, inhibits glucose absorption and, as a result, energy generation. Mebendazole accumulates in the gut and is used to treat infections caused by big intestinal roundworms (ascarids), hookworms, and whipworms. It is well tolerated, however in individuals with a severe illness, gastrointestinal pain and diarrhea may ensue.

Piperazine, which was brought into human medicine about 1950 and veterinary medicine shortly after, relaxes the ascarids and pinworms (oxyurids) in people and domesticated animals, allowing them to be removed with the feces. Piperazine has mostly been supplanted by medications like mebendazole and pyrantel pamoate.

Pyrantel pamoate paralyzes helminth muscular spasms. The majority of the medicine does not pass through the digestive system, resulting in high levels in the intestinal lumen. It is the preferred treatment for pinworm and an alternate therapy for Ascaris infection, hookworm, and trichostrongolosis.

Thiabendazole, which is structurally linked to albendazole and mebendazole, is primarily used to treat cattle, horse, and sheep nematodes. Another nematode anthelmintic used in veterinary medicine is dithiazanine, which is effective against heartworms and threadworms. Levamisole is a medication used to treat lungworm infections in cattle. Phenothiazine, which was developed in the 1930s, is still used to treat sheep and cow wireworm (Haemonchus contortus).

Hygromycin is an antibiotic that may also be used as an anthelmintic in the form of a feed additive to eradicate or diminish ascarids, nodular worms (Oesophagostomum), whipworms (Trichuris), big roundworms (Ascaridia), and cecal worms (Heterakis) in pigs and large roundworms (Ascaridia) in poultry.

Anthelmintics for trematodes

Praziquantel is the most effective medicine for treating infections caused by intestinal, liver, and lung flukes, and it is the treatment of choice for schistosomiasis (infections of blood flukes). Praziquantal produces worm contraction and spastic paralysis, as well as damage to the worm’s membranes, which triggers host defensive systems.

Anthelmintic resistance is currently prevalent in parasites. It poses a significant challenge to the long-term viability of contemporary ruminant livestock production, resulting in lower productivity, decreased animal health and wellbeing, and higher greenhouse gas emissions due to increased parasitism and farm inputs. In 2020, a database of published and unpublished European AR research on gastrointestinal nematodes was assembled. A total of 197 papers were available for review, encompassing 535 investigations from 22 countries from 1980 to 2020. Since 2010, sheep and goats have shown an average prevalence of resistance to benzimidazoles of 86%, macrocyclic lactones excluding moxidectin of 52%, levamisole of 48%, and moxidectin of 21%. In several investigations, all main gastrointestinal nematode genera survived therapy. Anthelminthic resistance in cattle ranged from 0-100 percent (benzimidazoles and macrocyclic lactones), 0-17 percent (levamisole), and 0-73 percent (moxidectin), with both Cooperia and Ostertagia surviving treatment.

The capacity of parasites to resist treatments that are normally successful at the prescribed doses poses a significant danger to the future management of worm parasites in small ruminants and horses. This is notably true for worms, and it has aided in the creation of aminoacetonitrile derivatives for the treatment of drug-resistant nematodes, as well as the investigation of doxycycline to kill their endosymbiotic Wolbachia bacteria.

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