Anthelmintics or antihelminthics are a group of antiparasitic that expel parasitic worms (helminths) and other internal parasites from the body by either stunning or killing them 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, particularly small such as and sheep.
Anthelmintic medication is also used in mass deworming campaigns of school-aged children in many developing countries.
Types
Many early treatments were herbal, such as the oil of herbs of the genus
Chenopodium that were given as anthelmintic treatment for centuries. In 1908 it was found that the active constituent was
ascaridole.
From the 1920s to the 1970s, halogenated hydrocarbons (such as chloroform, carbon tetrachloride, tetrachloroethylene and hexachloroethane) were used in a string of continually more efficacious anthelmintics, until their underlying host toxicity was revealed.
The modern broad-spectrum anthelmintics were developed by pharmaceutical companies that can afford the screening programs and testing systems that modern drug development involves.
Historically, there have been three main classes of broad-spectrum anthelmintics.
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Benzimidazoles disrupt parasitic worms' , a critical part of their cells' .
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Imidathiazoles/tetrahydropyrimidines are nicotinic acetylcholine receptor ,
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Macrocyclic lactones are Glutamic acid-gated chloride channel agonists,
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Drugs not members of these three main categories include:
Anthelmintic resistance
Anthelmintic resistance occurs when a heritable genetic change occurs in the parasite's DNA, rendering it insensitive to a previously effective anthelmintic drug. This is a particularly serious problem in helminth parasites of small ruminant farm animals.
There are many factors that contribute to anthelmintic resistance, such as frequent, mass anthelmintic treatment, underdosing, treating repeatedly with only one anthelmintic, and resistance being transmitted during transfer of animals.
Anthelmintic resistance in parasites is widespread; drug resistance exists in all livestock hosts and to all anthelmintic drug classes.
This is a major threat to the sustainability of modern ruminant livestock production, resulting in reduced productivity, compromised animal health and welfare,
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%,
moxidectin 52%, and
levamisole 48%. 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.
[ ] However, resistance is not seen as often in the parasitic helminths that affect cattle, compared to sheep. Reasons for this include the fact that cattle receive anthelminthic drugs less frequently than sheep, and the different nature of their faecal pats that could leave different numbers of resistant infective larvae on the pasture.
Unlike sheep, cattle can develop sufficient immunoprotection against such parasites.
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 drug resistance parasites survive and pass on their "resistance" genes. Resistant varieties accumulate, and treatment failure finally occurs.
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.
See also
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Dysphania ambrosioides, an herb native to Central and South America
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Santonin, a historical anthelmintic no longer in use
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4-Hexylresorcinol
External links
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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
