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  • Writer's pictureJacqui Panozzo

Ivermectin - an antiparasitic worthy of a Noble Prize

Ivermectin: As a veterinary parasitologist I feel the need to tell you all the story of ivermectin, a drug that unfortunately has been in the news lately for the wrong reasons. Since its discovery in 1975 and approval for use in animals and humans in the 1980’s, it has revolutionised the treatment of parasitic infections, being the first drug to target both parasitic worms and insects. It is not only a ‘horse-wormer’ as the news describes it, but one of the world’s essential medicines (as labelled by the WHO) for treatment and control of parasitic diseases in humans and many domestic animals. In 2015, the scientists who discovered ivermectin were awarded the Nobel Prize in Physiology and Medicine. Ivermectin was derived from a soil-bacteria found, in all places, on a golf-course. It works against nematodes and arthropods (worms and insects) by interrupting chlorine channels in the cell membranes of nerve and muscle cells, resulting in paralysis and death. Luckily, for most vertebrates (e.g. mammals), these types of chlorine channels are only present in our brain and spinal cord, which is protected by the blood-brain barrier that ivermectin cannot cross (unless taken in extreme doses). What makes ivermectin such a useful drug for treating parasitic infections is that it usually only takes a single dose to treat the patient. Following a single dose, it accumulates in tissue and fat before slowly being excreted, allowing for the anti-parasitic effects to persist for up to several weeks. It also has a wide safety margin when using the appropriate dosages, and can be administered via the mouth, skin or injection in animals. But multiple doses over a period of days will lead to over-dose type effects, as it is fat soluble and will accumlate within the body.

My favourite story regarding Ivermectin its use to treat Onchocerciasis (or river blindness) in humans. In Onchocerciasis, worm larvae from *Onchocerca volvulus invade the eyes of patients and causes vision impairment or loss, with this parasite being one of the leading causes of blindness worldwide. In the 1970’s it was endemic in many countries around the world, with the WHO later estimating that 17.7 million people were infected worldwide, of whom some 270,000 were blind, and another 500,000 severely visually disabled. In some towns, the burden was so bad that the majority of adults were blind and could not work, leading to more extreme poverty. In the 1980’s, following the approval of ivermectin for use in humans, Merck partnered with the WHO to donate ivermectin to treat river blindness throughout Africa, South America and Yemen. Since 1987, the program has delivered 4 billion treatments, prevented up to an equivalent of 7 million years of disability, and achieved eradication in several countries. It is still recommended for use to treat other neglected tropical diseases, such as strongyloidiasis, scabies and lymphatic filariasis, all diseases that still cause severe disease burdens throughout poorer countries and regions.

I’m not going to mention to C word here – but there’s so much more the ivermectin than the “Your horse knows you took his wormer” memes. Many people are not aware of how certain drugs can have such profound effects on our lives. We all know how important antibiotics were to human health, and compared to them, safe, effective anti-parasitics were late to the party. I have an old family-health book from the 1950’s, and it recommended the use of mercury to treat pin-worms in children (Mercury!). Now, debilitating and disfiguring diseases can be cured with a single pill. In veterinary care, prior to these drugs, the economic loss to farmers from worm infections was incredible, and the top cause of colic in horses was worm infections. But just remember – anti-parasitics are so named because they work against parasites. Just like how antibacterials work against bacteria and antivirals work against viruses. If something’s toxic to all three, then it’s probably toxic to all forms of life, us included.

Extra note: Ivermectin toxicity in some collie-dogs is due to a mutation in their chlorine channels, which results in them being susceptible to the ivermectin. This mutation can be tested for genetically, and is found in Rough and Smooth Collies, Shetland Sheepdogs and Australian Shepherds.

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