Anthelmintics or antihelminthics are a group of antiparasitic drugs that expel parasitic worms (helminths) and other internal parasites from the body by either stunning or killing them and without causing significant damage to the host. They may also be called vermifuges (those that stun) or vermicides (those that kill). Anthelmintics are used to treat people who are infected by helminths, a condition called helminthiasis. These drugs are also used to treat infected animals.

Pills containing anthelmintics are used in mass deworming campaigns of school-aged children in many developing countries.^[1][2] Anthelmintics are also used for mass deworming of livestock. The drugs of choice for soil-transmitted helminths are mebendazole and albendazole;^[3] for schistosomiasis and tapeworms it is praziquantel.^[4]

Types[edit]

Many early treatments were herbal, such as the oil of herbs of the genus Chenopodium that was given as an anthelmintic for centuries. In 1908 it was found that the active constituent was ascaridole.^[5] From the 1920s to the 1970s, halogenated hydrocarbons were used in a string of continually more efficacious anthelmintics, until their underlying host toxicity was revealed.^[5] The modern broad-spectrum anthelmintics were developed by pharmaceutical companies that can afford the screening programs and testing systems that modern drug development involves.^[5][6]

• Benzimidazoles:
  • Albendazole – effective against threadworms, roundworms, whipworms, tapeworms, hookworms
  • Mebendazole – effective against various nematodes
  • Thiabendazole – effective against various nematodes
  • Fenbendazole – effective against various parasites
  • Triclabendazole – effective against liver flukes
  • Flubendazole – effective against most intestinal parasites
• Avermectins (including ivermectin and moxidectin) - effective against most common intestinal worms, except tapeworms, for which praziquantel is commonly used in conjunction for mass dewormings
• Diethylcarbamazine – effective against Wuchereria bancrofti, Brugia malayi, Brugia timori, and Loa loa
• Pyrantel pamoate – effective against most nematode infections residing within the intestines
• Levamisole
• Salicylanilide – mitochondrial un-couplers (used only for flatworm infections):
  • Niclosamide
  • Oxyclozanide
• Nitazoxanide – readily kills Ascaris lumbricoides,^[7] and also possesses antiprotozoal effects.^[8] This is also an ascaricide.
• Oxamniquine – effective against flatworms (e.g., tapeworms and schistosoma)
• Praziquantel – effective against flatworms (e.g., tapeworms and schistosoma)
• Octadepsipeptides (e.g. Emodepside) – effective against a variety of gastrointestinal helminths
• Monepantel (aminoacetonitrile class) - effective against a variety of nematodes including those resistant to other anthelmintic classes
• Spiroindoles (e.g. derquantel) - effective against a variety of nematodes including those resistant to other anthelmintic classes
• Artemisinin – shows anthelmintic activity^[9]

Anthelmintic resistance[edit]

Anthelmintic resistance is when a parasite is no longer sensitive to an anthelmintic that it once was and it is passed from one generation to the next.^[11] There are many different things that can contribute to anthelmintic resistance such as frequency of treatment, mass anthelmintic treatment, underdosing, treating with only one anthelmintic, and resistance being transmitted during transfer of animals.^[10] Anthelmintic resistance in parasites is widespread; drug resistance exists in all livestock hosts and to all anthelmintic drug classes.^[10] This is a major threat to the sustainability of modern ruminant livestock production, resulting in reduced productivity, compromised animal health and welfare,^[11] and increased greenhouse gas emissions through increased parasitism and farm inputs. A database of published and unpublished European AR research on gastrointestinal nematodes was collated in 2020. A total of 197 publications were available for analysis, representing 535 studies in 22 countries and spanning the period 1980–2020. Results in sheep and goats since 2010 reveal an average prevalence of resistance to benzimidazoles of 86%, macrocyclic lactones except moxidectin 52%, levamisole 48%, and moxidectin 21%. All major gastrointestinal nematodes genera survived treatment in various studies. In cattle, prevalence of anthelminthic resistance varied between anthelmintic classes from 0–100% (benzimidazoles and macrocyclic lactones), 0–17% (levamisole) and 0–73% (moxidectin), and both Cooperia and Ostertagia survived treatment. ^[12]

The ability of parasites to survive treatments that are generally effective at the recommended doses is a major threat to the future control of worm parasites in small ruminants and horses. This is especially true of nematodes, and has helped spur development of aminoacetonitrile derivatives for treatment against drug-resistant nematodes, as well as exploration of doxycycline to kill their endosymbiotic Wolbachia bacteria.^{[citation needed]}

Both in vitro (egg hatch assay, larval development test, larval motility test, polymerase chain reaction and in vivo methods (fecal egg count reduction test) can be used to detect anthelmintic resistance.^[11]  

Treatment with an antihelminthic drug kills worms whose phenotype renders them susceptible to the drug, but resistant parasites survive and pass on their "resistance" genes. Resistant varieties accumulate, and treatment failure finally occurs.^[13]

The ways in which anthelmintics are used have contributed to a major anthelmintic resistance issue worldwide. From the 1950s to the 1980s, new classes of effective and inexpensive anthelmintics were made available every decade, leading to excessive use throughout agriculture and disincentivizing alternative anti-nematodal strategies.^[10] Developing new anthelmintics is time consuming and expensive therefore, it is important to use the ones that currently exist in a way that will minimize or prevent the development of anthelmintic resistance.^[11] Some of these methods are ensuring animals are not being underdosed, rotating the anthelmintics that are being used, and rotation of grazing land to reduce the parasite population.^[14] Other methods include using a combination of multiple different anthelmintics, and the use of refugia based strategies. Refugia refers to the portion of the parasite population not being exposed to anthelmintics. This population is therefore not undergoing selection for resistance. Use of refugia helps to slow down the speed of evolution of resistance to anthelmintic drugs.^[15]

See also[edit]

• Dysphania ambrosioides, an herb native to Central and South America
• Santonin, a historical anthelmintic no longer in use
• 4-Hexylresorcinol

References[edit]

^[1][2]

External links[edit]

• Anthelmintics at the U.S. National Library of Medicine Medical Subject Headings (MeSH)
• Holden-Dye, L. and Walker, R.J.Anthelmintic drugs (November 2, 2007), WormBook, ed. The C. elegans Research Community, WormBook, doi/10.1895/wormbook.1.143.1