An In-Depth Look at Ivermectin

Susan Schoenian, Sheep & Goat Specialist, University of Maryland Small Ruminant Extension Program
(Previously published in Wild and Wooly – Spring 2020)

(Image Source: Valley Vet Supply)

Recently, several media outlets reported the success of an in vitro study conducted in Australia regarding the antiviral effects of ivermectin on the virus that causes COVID-19. An in vitro study is performed outside the living organism, such as in a petri dish; where- as, an in vivo study is conducted in a living organism. A treatment may work in vitro but not in vivo or it may work in both or neither.

Ivermectin has shown in vitro activity against the zika virus, but its effect was not repeatable in live animal studies (with mice). Ivermectin has shown antiviral activity against pseudorabies (a viral swine disease) both in vitro and in vivo. There is hope that ivermectin may have an in vivo effect against the virus that causes COVID-19, but there is no PROOF.

After the story about ivermectin broke on social media, some farm stores removed ivermectin from their shelves, in fear of people buying it to self medicate. Hopefully, by the time this newsletter is received, shelves will have been restocked with ivermectin (and related drugs) and people will have regained their senses.

Ivermectin: the “wonder” drug
Ivermectin was first discovered in 1975. It was cultured from the soil. It became commercially available in 1981 and soon became the top-selling veterinary drug in the world, earning the title of “wonder drug.” Besides being a dewormer for animals, ivermectin has been used to successfully treat various diseases in humans, specifically those caused by roundworms (river blindness) or insects (malaria). However, the formulations used for humans are different from those used in animals. What is safe in animals may not be safe in people.

Ivermectin for small ruminants
Ivermectin has been used to de-worm small ruminants for decades. Unfortunately, many of the worms infecting small ruminants have developed relatively high levels of resistance to ivermectin (and other avermectins). This is particularly true on farms in the Mid-Atlantic and southern states, where significant testing has occurred. What this means is that ivermectin (Ivomec®) is seldom effective as the sole treatment for worms (in small ruminants). The expectations would be similar for doramectin (Dectomax®) and eprinomectin (Eprinex®), as they are in the same class (macrocylic lactones) and sub-class of de-wormers (avermectins) as ivermectin.

Ivermectin is generally not included in combination treatment protocols (for small ruminants), since moxidectin (Cydectin®) is the more potent drug in the class; therefore, the better choice. Moxidectin is in the sub-group called milbemycins. However, ivermectin (and other avermectins) is more effective against external parasites (than moxidectin). Specifically, ivermectin is labeled for the removal of nasal bots in sheep. Though not labeled, ivermectin also has efficacy against other ectoparasites, such as ticks, lice, mites, and fleas. Use of ivermectin (and related drugs) to control external parasites (other than nasal bots in sheep) is extra-label and should be used under the supervision of a veterinarian.

Ivermectin is sometimes routinely given to camelids (llamas and alpacas; Rx) to prevent meningeal worm infection, although it is not believed to be able to cross the blood-brain barrier (thus, treat meningeal worm). In fact, a study conducted at Cornell University showed no benefit to including ivermectin (injectable) in treatment protocols for meningeal worm (in sheep/goats). Fenbendazole (SafeGuard®) is the drug of choice for treating meningeal worm.