Horse owners and veterinarians are familiar with ivermectin as a commonly used equine dewormer, one that was introduced in the mid-1980s and is now widely available over the counter as well as through online resources. It's a broad-spectrum anti-parasitic medication used in equine parasite control programs, but did you know it's also used for cats, dogs, birds, and even humans?

How are we able to use ivermectin for so many different conditions and species? It's because while highly toxic to most types of intestinal parasites (except tapeworms), and some arthropods such as ants, mites, and lice, ivermectin typically doesn't cross the blood-brain barrier in mammals and so is considered generally safe.

Ivermectin is related to avarmectin, a common pesticide ingredient in home-use ant baits, and both are derived from the bacterium Streptomyces avermitilis. Pure avarmectin has been shown to be toxic to mammals and fish as well as insects and parasites, but according to the Web site StateMaster.com, in General Use Pesticide (GUP) formulations ivermectin's toxicity is considered very low.

Ivermectin works by interfering with a parasite's nervous system and muscular functions; with the blood-brain barrier protection in mammals, we're typically sheltered from ivermectin's toxic actions on the central nervous system.

An exception would be individuals or breeds that are sensitive to ivermectin and show nervous system toxicity at much lower thresholds, even at levels considered safe for most.

While ivermectin is considered to be safe for equine deworming, under certain circumstances a horse can develop ivermectin toxicosis, a potentially deadly condition where ivermectin does cross the blood-brain barrier and causes acute neurologic symptoms.

One avenue is through ingestion of silver nightshade in close proximity to the ivermectin deworming. Silver nightshade is a poisonous plant found in the Southwest and Pacific Northwest regions of the U.S., and can sometimes be found in hay grown in these regions.

An article on TheHorse.com on Ivermectin Toxicosis illustrates a case presented at the 2008 American Association of Equine Practitioners (AAEP) about a trio of Quarter Horses that showed signs of ivermectin toxicosis within 18 hours of deworming. Although two of the three recovered, one horse was euthanized; an autopsy showed toxic levels of ivermectin in its brain. While the ultimate source of the toxicity was not found, silver nightshade was suspected.

It's a good idea to watch your horse for acute neurologic symptoms for several days following deworming with ivermectin; these signs can include pupil dilation; ataxia or lack of coordination; mental dullness or agitation; and flaccid lips.

TheHorse.com is an excellent resource for learning more about ivermectin sensitivity and toxicity.

If you're interested in reducing your reliance upon ivermectin dewormers in your horses, we're available to discuss your equine worm control program and preventive measures, or to answer any questions you might have.