For river blindness (onchocerciasis) and lymphatic filariasis, ivermectin is typically given as part of mass drug administration campaigns that distribute the drug to all members of a community affected by the disease.[32] Adult worms survive in the skin and eventually recover to produce larval worms again; to keep the worms at bay, ivermectin is given at least once per year for the 10–15-year lifespan of the adult worms.[33]
Ivermectin is also used to treat infection with parasitic arthropods. Scabies – infestation with the mite Sarcoptes scabiei – is most commonly treated with topical permethrin or oral ivermectin. A single application of permethrin is more efficacious than a single treatment of ivermectin[citation needed]. For most scabies cases, ivermectin is used in a two dose regimen: a first dose kills the active mites, but not their eggs. Over the next week, the eggs hatch, and a second dose kills the newly hatched mites.[42][43] The two dose regimen of ivermectin has similar efficacy to the single dose permethrin treatment. Ivermectin is, however, more effective than permethrin when used in the mass treatment of endemic scabies.[44]
For severe "crusted scabies", where the parasite burden is orders of magnitude higher than usual, the U.S. Centers for Disease Control and Prevention (CDC) recommends up to seven doses of ivermectin over the course of a month, along with a topical antiparasitic.[43] Both head lice and pubic lice can be treated with oral ivermectin, an ivermectin lotion applied directly to the affected area, or various other insecticides.[45][46] Ivermectin is also used to treat rosacea and blepharitis, both of which can be caused or exacerbated by Demodex folliculorum mites.[47][48]
Contraindications
The only absolute contraindication to the use of ivermectin is hypersensitivity to the active ingredient or any component of the formulation.[49][50] In children under the age of five or those who weigh less than 15 kilograms (33 pounds),[51] there is limited data regarding the efficacy or safety of ivermectin, though the available data demonstrate few adverse effects.[52] However, the American Academy of Pediatrics cautions against use of ivermectin in such patients, as the blood-brain barrier is less developed, and thus there may be an increased risk of particular CNS side effects such as encephalopathy, ataxia, coma, or death.[53] The American Academy of Family Physicians also recommends against use in these patients, given a lack of sufficient data to prove drug safety.[54] Ivermectin is secreted in very low concentration in breast milk.[55] It remains unclear if ivermectin is safe during pregnancy.[56]
Adverse effects
Side effects, although uncommon, include fever, itching, and skin rash when taken by mouth;[11] and red eyes, dry skin, and burning skin when used topically for head lice.[57] It is unclear if the drug is safe for use during pregnancy, but it is probably acceptable for use during breastfeeding.[58]
Ivermectin is considered relatively free of toxicity in standard doses (around 300 μg/kg).[59][60] Based on the data drug safety sheet for ivermectin,[a] side effects are uncommon. However, serious adverse events following ivermectin treatment are more common in people with very high burdens of larval Loa loa worms in their blood.[61] Those who have over 30,000 microfilaria per milliliter of blood risk inflammation and capillary blockage due to the rapid death of the microfilaria following ivermectin treatment.[61]
One concern is neurotoxicity after large overdoses, which in most mammalian species may manifest as central nervous system depression,[62]ataxia, coma, and even death,[63][64] as might be expected from potentiation of inhibitory chloride channels.[65]
To provide context for the dosing and toxicity ranges, the LD50 of ivermectin in mice is 25 mg/kg (oral), and 80 mg/kg in dogs, corresponding to an approximated human-equivalent dose LD50 range of 2.02–43.24 mg/kg,[68] which is far in excess of its FDA-approved usage (a single dose of 0.150–0.200 mg/kg to be used for specific parasitic infections).[3] While ivermectin has also been studied for use in COVID-19, and while it has some ability to inhibit SARS-CoV-2 in vitro, achieving 50% inhibition in vitro was found to require an estimated oral dose of 7.0 mg/kg (or 35x the maximum FDA-approved dosage),[69] high enough to be considered ivermectin poisoning.[68] Despite insufficient data to show any safe and effective dosing regimen for ivermectin in COVID-19, doses have been taken far in excess of FDA-approved dosing, leading the CDC to issue a warning of overdose symptoms including nausea, vomiting, diarrhea, hypotension, decreased level of consciousness, confusion, blurred vision, visual hallucinations, loss of coordination and balance, seizures, coma, and death. The CDC advises against consuming doses intended for livestock or doses intended for external use and warns that increasing misuse of ivermectin-containing products is resulting in an increase in harmful overdoses.[70]
Pharmacology
Mechanism of action
Ivermectin and its related drugs act by interfering with the nerve and muscle functions of helminths and insects.[71] The drug binds to glutamate-gated chloride channels common to invertebrate nerve and muscle cells.[72] The binding pushes the channels open, which increases the flow of chloride ions and hyper-polarizes the cell membranes,[71] paralyzing and killing the invertebrate.[72] Ivermectin is safe for mammals (at the normal therapeutic doses used to cure parasite infections) because mammalian glutamate-gated chloride channels only occur in the brain and spinal cord: the causative avermectins usually do not cross the blood–brain barrier, and are unlikely to bind to other mammalian ligand-gated channels.[72]
Pharmacokinetics
Ivermectin can be given by mouth, topically, or via injection. Oral doses are absorbed into systemic circulation; the alcoholic solution form is more orally available than tablet and capsule forms. Ivermectin is widely distributed in the body.[73]
Ivermectin does not readily cross the blood–brain barrier of mammals due to the presence of P-glycoprotein (the MDR1 gene mutation affects the function of this protein).[74] Crossing may still become significant if ivermectin is given at high doses, in which case brain levels peak 2–5 hours after administration. In contrast to mammals, ivermectin can cross the blood–brain barrier in tortoises, often with fatal consequences.[75]
Ivermectin is metabolized into eight different products by human CYP3A4, two of which (M1, M2) remain toxic to mosquitos. M1 and M2 also have longer elimination half-lives of about 55 hours. CYP3A5 produces a ninth metabolite.[6]
Chemistry
Fermentation of Streptomyces avermitilis yields eight closely related avermectinhomologues, of which B1a and B1b form the bulk of the products isolated. In a separate chemical step, the mixture is hydrogenated to give ivermectin, which is an approximately 80:20 mixture of the two 22,23-dihydroavermectin compounds.[76][77][7]
The avermectin family of compounds was discovered by Satoshi Ōmura of Kitasato University and William Campbell of Merck.[7] In 1970, Ōmura isolated a strain of Streptomyces avermitilis from woodland soil near a golf course along the south east coast of Honshu, Japan.[7] Ōmura sent the bacteria to William Campbell, who showed that the bacterial culture could cure mice infected with the roundworm Heligmosomoides polygyrus.[7] Campbell isolated the active compounds from the bacterial culture, naming them "avermectins" and the bacterium Streptomyces avermitilis for the compounds' ability to clear mice of worms (in Latin: a 'without', vermis 'worms').[7] Of the various avermectins, Campbell's group found the compound "avermectin B1" to be the most potent when taken orally.[7] They synthesized modified forms of avermectin B1 to improve its pharmaceutical properties, eventually choosing a mixture of at least 80% 22,23-dihydroavermectin B1a and up to 20% 22,23-dihydroavermectin B1b, a combination they called "ivermectin".[7][79]
The discovery of ivermectin has been described as a combination of "chance and choice." Merck was looking for a broad-spectrum anthelmintic, which ivermectin is indeed; however, Campbell noted that they "...also found a broad-spectrum agent for the control of ectoparasitic insects and mites."[80]
Merck began marketing ivermectin as a veterinary antiparasitic in 1981.[7] By 1986, ivermectin was registered for use in 46 countries and was administered massively to cattle, sheep and other animals.[81] By the late 1980s, ivermectin was the bestselling veterinary medicine in the world.[7] Following its blockbuster success as a veterinary antiparasitic, another Merck scientist, Mohamed Aziz, collaborated with the World Health Organization to test the safety and efficacy of ivermectin against onchocerciasis in humans.[10] They found it to be highly safe and effective,[82] triggering Merck to register ivermectin for human use as "Mectizan" in France in 1987.[10] A year later, Merck CEO Roy Vagelos agreed that Merck would donate all ivermectin needed to eradicate river blindness.[10] In 1998, that donation would be expanded to include ivermectin used to treat lymphatic filariasis.[10]
Ivermectin earned the title of "wonder drug" for the treatment of nematodes and arthropod parasites.[83] Ivermectin has been used safely by hundreds of millions of people to treat river blindness and lymphatic filariasis.[7]
Half of the 2015 Nobel Prize in Physiology or Medicine was awarded jointly to Campbell and Ōmura for discovering ivermectin, "the derivatives of which have radically lowered the incidence of river blindness and lymphatic filariasis, as well as showing efficacy against an expanding number of other parasitic diseases".[15][84]
Early in the COVID-19 pandemic, laboratory research suggested ivermectin might have a role in preventing or treating COVID-19.[85] Online misinformation campaigns and advocacy boosted the drug's profile among the public. While scientists and physicians largely remained skeptical, some nations adopted ivermectin as part of their pandemic-control efforts. Some people, desperate to use ivermectin without a prescription, took veterinary preparations, which led to shortages of supplies of ivermectin for animal treatment. The FDA responded to this situation by saying "You are not a horse" in a Tweet to draw attention to the issue, for which they were later sued.[86][87]
Subsequent research failed to confirm the utility of ivermectin for COVID-19,[88][89] and in 2021 it emerged that many of the studies demonstrating benefit were faulty, misleading, or fraudulent.[90][91] Nevertheless, misinformation about ivermectin continued to be propagated on social media and the drug remained a cause célèbre for anti-vaccinationists and conspiracy theorists.[92]
Economics
The initial price proposed by Merck in 1987 was US$6 per treatment, which was unaffordable for patients who most needed ivermectin.[93] The company donated hundreds of millions of courses of treatments since 1988 in more than 30 countries.[93] Between 1995 and 2010, using donated ivermectin to prevent river blindness, the program is estimated to have prevented seven million years of disability at a cost of US$257 million.[94]
Ivermectin is considered an inexpensive drug.[95] As of 2019, ivermectin tablets (Stromectol) in the United States were the least expensive treatment option for lice in children at approximately US$9.30, while Sklice, an ivermectin lotion, cost around US$300 for 120 mL (4 US fl oz).[96]
It is sold under the brand names Heartgard, Sklice[99] and Stromectol[3] in the United States, Ivomec worldwide by Merial Animal Health, Mectizan in Canada by Merck, Iver-DT[100] in Nepal by Alive Pharmaceutical and Ivexterm in Mexico by Valeant Pharmaceuticals International. In Southeast Asian countries, it is marketed by Delta Pharma Ltd. under the trade name Scabo 6. The formulation for rosacea treatment is sold under the brand name Soolantra.[4] While in development, it was assigned the code MK-933 by Merck.[101]
Ivermectin is also of interest in the prevention of malaria, as it is toxic to both the malaria plasmodium itself and the mosquitos that carry it.[104][105] A direct effect on malaria parasites could not be shown in an experimental infection of volunteers with Plasmodium falciparum.[106] Use of ivermectin at higher doses necessary to control malaria is probably safe, though large clinical trials have not yet been done to definitively establish the efficacy or safety of ivermectin for prophylaxis or treatment of malaria.[107][59] Mass drug administration of a population with ivermectin to treat and prevent nematode infestation is effective for eliminating malaria-bearing mosquitos and thereby potentially reducing infection with residual malaria parasites.[108] Whilst effective in killing malaria-bearing mosquitos, a 2021 Cochrane review found that, to date, the evidence shows no significant impact on reducing incidence of malaria transmission from the community administration of ivermectin.[107]
One alternative to ivermectin is moxidectin, which has been approved by the Food and Drug Administration for use in people with river blindness.[109] Moxidectin has a longer half-life than ivermectin and may eventually supplant ivermectin as it is a more potent microfilaricide, but there is a need for additional clinical trials, with long-term follow-up, to assess whether moxidectin is safe and effective for treatment of nematode infection in children and women of childbearing potential.[110][111]
During the COVID-19 pandemic, ivermectin was researched for possible utility in preventing and treating COVID-19, but no good evidence of benefit was found.[119][120]
Veterinary use
Ivermectin is routinely used to control parasitic worms in the gastrointestinal tract of ruminant animals. These parasites normally enter the animal when it is grazing, pass the bowel, and set and mature in the intestines, after which they produce eggs that leave the animal via its droppings and can infest new pastures. Ivermectin is only effective in killing some of these parasites, because of an increase in anthelmintic resistance.[121] This resistance has arisen from the persistent use of the same anthelmintic drugs for the past 40 years.[122][123] Additionally, the use of Ivermectin for livestock has a profound impact on dung beetles, such as T. lusitanicus, as it can lead to acute toxicity within these insects.[124]
In dogs, ivermectin is routinely used as prophylaxis against heartworm.[125] Dogs with defects in the P-glycoprotein gene (MDR1), often collie-like herding dogs, can be severely poisoned by ivermectin. The mnemonic "white feet, don't treat" refers to Scotch collies that are vulnerable to ivermectin.[126] Some other dog breeds (especially the Rough Collie, the Smooth Collie, the Shetland Sheepdog, and the Australian Shepherd), also have a high incidence of mutation within the MDR1 gene (coding for P-glycoprotein) and are sensitive to the toxic effects of ivermectin.[127][128] For dogs, the insecticide spinosad may have the effect of increasing the toxicity of ivermectin.[129][130]
A 0.01% ivermectin topical preparation for treating ear mites in cats is available.[131] Clinical evidence suggests 7-week-old kittens are susceptible to ivermectin toxicity.[132]
Ivermectin is sometimes used as an acaricide in reptiles, both by injection and as a diluted spray. While this works well in some cases, care must be taken, as several species of reptiles are very sensitive to ivermectin. Use in turtles is particularly contraindicated.[133]
A characteristic of the antinematodal action of ivermectin is its potency: for instance, to combat Dirofilaria immitis in dogs, ivermectin is effective at 0.001 milligram per kilogram of body weight when administered orally.[79]
Notes
^This recommendation is not universal. The World Health Organization recommends ascariasis be treated with mebendazole or pyrantel pamoate,[38] while the textbook Parasitic Diseases recommends albendazole or mebendazole.[39] A 2020 Cochrane review concluded that the three drugs are equally safe and effective for treating ascariasis.[40]
^Campbell WC (May 2012). "History of avermectin and ivermectin, with notes on the history of other macrocyclic lactone antiparasitic agents". Current Pharmaceutical Biotechnology. 13 (6): 853–865. doi:10.2174/138920112800399095. PMID22039784.
^ abcdeMolyneux DH, Ward SA (December 2015). "Reflections on the Nobel Prize for Medicine 2015—The Public Health Legacy and Impact of Avermectin and Artemisinin". Trends in Parasitology. 31 (12): 605–607. doi:10.1016/j.pt.2015.10.008. PMID26552892.
^ abcde"Ivermectin". The American Society of Health-System Pharmacists. Archived from the original on January 3, 2016. Retrieved January 16, 2016.
^World Health Organization (2019). World Health Organization model list of essential medicines: 21st list 2019. Geneva: World Health Organization. hdl:10665/325771. WHO/MVP/EMP/IAU/2019.06. License: CC BY-NC-SA 3.0 IGO.
^World Health Organization (2021). World Health Organization model list of essential medicines: 22nd list (2021). Geneva: World Health Organization. hdl:10665/345533. WHO/MHP/HPS/EML/2021.02.
^Crump A (May 2017). "Ivermectin: enigmatic multifaceted 'wonder' drug continues to surprise and exceed expectations". The Journal of Antibiotics. 70 (5): 495–505. doi:10.1038/ja.2017.11. PMID28196978. S2CID24474879. Ivermectin was a revelation. It had a broad spectrum of activity, was highly efficacious, acting robustly at low doses against a wide variety of nematode, insect and acarine parasites. It proved to be extremely effective against most common intestinal worms (except tapeworms), could be administered orally, topically or parentally and showed no signs of cross-resistance with other commonly used anti-parasitic compounds.
^Despommier DD, Griffin DO, Gwadz RW, Hotez PJ, Knirsch CA (2019). "26. Other Nematodes of Medical Importance". Parasitic Diseases(PDF) (7 ed.). New York: Parasites Without Borders. p. 294. Archived(PDF) from the original on November 24, 2021. Retrieved January 26, 2021.
^ abDespommier DD, Griffin DO, Gwadz RW, Hotez PJ, Knirsch CA (2019). "27. Aberrant Nematode Infections". Parasitic Diseases(PDF) (7 ed.). New York: Parasites Without Borders. p. 299. Archived(PDF) from the original on November 24, 2021. Retrieved January 26, 2021.
^Despommier DD, Griffin DO, Gwadz RW, Hotez PJ, Knirsch CA (2019). "18. Ascaris lumbricoides". Parasitic Diseases(PDF) (7 ed.). New York: Parasites Without Borders. p. 211. Archived(PDF) from the original on November 24, 2021. Retrieved January 26, 2021.
^Despommier DD, Griffin DO, Gwadz RW, Hotez PJ, Knirsch CA (2019). "17. Trichuris trichiura". Parasitic Diseases(PDF) (7 ed.). New York: Parasites Without Borders. p. 201. Archived(PDF) from the original on November 24, 2021. Retrieved January 26, 2021.
^Wilkins AL, Steer AC, Cranswick N, Gwee A (May 2018). "Question 1: Is it safe to use ivermectin in children less than five years of age and weighing less than 15 kg?". Archives of Disease in Childhood. 103 (5): 514–519. doi:10.1136/archdischild-2017-314505. PMID29463522. S2CID3441595.
^"Ivermectin (topical)". The American Society of Health-System Pharmacists. July 27, 2020. Archived from the original on March 18, 2023. Retrieved January 16, 2021.
^ abNavarro M, Camprubí D, Requena-Méndez A, Buonfrate D, Giorli G, Kamgno J, et al. (April 2020). "Safety of high-dose ivermectin: a systematic review and meta-analysis". The Journal of Antimicrobial Chemotherapy. 75 (4): 827–834. doi:10.1093/jac/dkz524. PMID31960060.
^Office of the Commissioner (March 12, 2021). "Why You Should Not Use Ivermectin to Treat or Prevent COVID-19". U.S. Food and Drug Administration (FDA). Archived from the original on August 6, 2021. Retrieved July 13, 2021. You can also overdose on ivermectin, which can cause nausea, vomiting, diarrhea, hypotension (low blood pressure), allergic reactions (itching and hives), dizziness, ataxia (problems with balance), seizures, coma and even death.
^El-Saber Batiha G, Alqahtani A, Ilesanmi OB, Saati AA, El-Mleeh A, Hetta HF, et al. (August 2020). "Avermectin Derivatives, Pharmacokinetics, Therapeutic and Toxic Dosages, Mechanism of Action, and Their Biological Effects". Pharmaceuticals. 13 (8): 196. doi:10.3390/ph13080196. PMC7464486. PMID32824399. Based on the reported neurotoxicity and metabolic pathway of IVM, caution should be taken to conduct clinical trial on its antiviral potentials. The GABA-gated chloride channels in the human nervous system might be a target for IVM, this is because the BBB in disease-patient might be a weakened as a result of inflammation and other destructive processes, allowing IVM to cross the BBB and gain access to the CNS where it can elicit its neurotoxic effect
^Brunton LL, Lazo JS, Paker KL (2006). Goodman & Gilman's The Pharmacological Basis of Therapeutics (11th ed.). New York: McGraw-Hill. pp. 1084–87. ISBN978-0-07-142280-2. OCLC1037399847.
^"Ivermectin". LiverTox: Clinical and Research Information on Drug-Induced Liver Injury. Bethesda, Maryland: National Institute of Diabetes and Digestive and Kidney Diseases. 2012. PMID31644227. Archived from the original on March 18, 2023. Retrieved May 30, 2021.
^Borst P, Schinkel AH (June 1996). "What have we learnt thus far from mice with disrupted P-glycoprotein genes?". European Journal of Cancer. 32A (6): 985–990. doi:10.1016/0959-8049(96)00063-9. PMID8763339.
^Lasota JA, Dybas RA (1991). "Avermectins, a novel class of compounds: implications for use in arthropod pest control". Annual Review of Entomology. 36: 91–117. doi:10.1146/annurev.en.36.010191.000515. PMID2006872.
^Jansson RK, Dybas RA (1998). "Avermectins: Biochemical Mode of Action, Biological Activity and Agricultural Importance". Insecticides with Novel Modes of Action. Applied Agriculture. Berlin, Heidelberg: Springer. pp. 152–70. doi:10.1007/978-3-662-03565-8_9. ISBN978-3-642-08314-3.
^Geary TG (November 2005). "Ivermectin 20 years on: maturation of a wonder drug". Trends in Parasitology. 21 (11): 530–532. doi:10.1016/j.pt.2005.08.014. PMID16126457.
^Pampiglione S, Majori G, Petrangeli G, Romi R (1985). "Avermectins, MK-933 and MK-936, for mosquito control". Transactions of the Royal Society of Tropical Medicine and Hygiene. 79 (6): 797–799. doi:10.1016/0035-9203(85)90121-X. PMID3832491.
^Klingenberg R (2007). Understanding reptile parasites: from the experts at Advanced Vivarium Systems. Irvine, Calif: Advanced Vivarium Systems. ISBN978-1882770908.
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