11/05/2024
Moxidectin Toxicosis in Horses – Should you be concerned?
The changing of seasons, from fall to winter, marks the end of the true grazing season for most of us. Since parasite transmission mainly occurs while horses are grazing green grass, this change corresponds with the need to deworm our horses to reduce the parasitic burden during the winter. Macrocyclic lactones, including ivermectin and moxidectin, are commonly used in conjunction with praziquantel to combat small strongyles, bots, and tapeworms in the fall (depending on your geographical location). As I have discussed these recommendations with a few clients lately, I have been met with concern over the use of moxidectin due to the potential for toxicity with this product. As a result, I want to discuss some of the research and physiology related to this toxicity so we can understand it better and determine when this would be an issue - I think you may be surprised what you learn!
Macrocyclic lactones are a class of dewormer that work by blocking the transmission of neural signals in parasites leading to paralysis. This, in turn, prevents the parasite from feeding and moving, ultimately resulting in parasitic death. The two most common macrocyclic lactones available for use in horses are ivermectin and moxidectin.
When the two are compared, moxidectin is 100 times more lipophilic than ivermectin, meaning it has a greater ability to dissolve in fat (Alvinerie et al., 1998). This is important because cell membranes are comprised of phospholipids, a type of fat molecule. As a result, when moxidectin is given above recommended rates and is present in high quantities in circulation, it can cross the blood-brain barrier of the horse. Once moxidectin is in the central nervous system of the horse, it can be very detrimental through the disruption of neuronal signaling and result in clinical signs such as shortness of breath, depression, muscle tremors, lack of coordination, and seizures. Toxicity has been noticed when moxidectin was administered greater than 2.5 to 12 times (1.0 to 5.1 mg/kg bodyweight) the recommended dose of 0.4 mg/kg bodyweight (Khan et al., 2002). However, every horse is different as other horses receiving 2.25 to 4.2 times (0.9 to 1.7 mg/kg bodyweight) greater the recommended dose did not exhibit clinical signs (Khan et al., 2002).
So, what can be done to reduce the risk of moxidectin toxicosis? (Schumacher et al., 2008)
⭐️ Know your horse’s weight to prevent overdosing. If you don’t have access to a scale, I recommend looking into the Healthy Horse App developed by the University of Minnesota - I was lucky enough to take part in the research used to develop this app during graduate school!
⭐️ Be careful during administration to ensure the syringe locking mechanism is used properly
⭐️ Avoid using this dewormer for foals, especially those less than 6 months old
⭐️ Do not administer to underweight horses until they reach a healthy weight and condition
⭐️ Caution should be used when administering this dewormer to miniature ponies and donkeys due to their size
But keep in mind, this specific product (Quest Plus) is designed to treat a horse up to 1,500 lbs. So, to administer a dose 3 times greater than the recommended amount with one tube of dewormer, a horse would need to weigh less than 500 lbs or be significantly underweight. On the flip side, you would have to administer 2 full tubes of Quest Plus dewormer to a healthy, adult horse to reach overdose levels. This is why foals and miniatures are at the greatest risk of a toxicity. So, while it is good to be aware of moxidectin overdose and the clinical signs presented, the average horse owner does not need to be scared of using this product when it is administered according to manufacturer instructions!
I also want to note moxidectin has the ability to combat encysted small strongyles and moxidectin and ivermectin have been shown to have contrasting efficacy against different strongyles species (Nielsen et al., 2022). As a result, using both within a deworming protocol can be effective when done properly. You can reach out to your veterinarian to determine the best protocol for your herd and geographical region.
Disclaimer: I am a Ph.D. equine scientist and not a veterinarian. This post is designed to educate horse owners on the research and science of moxidectin but owners should follow veterinarian and manufacturer recommendations for their specific horse(s).
Dr. DeBoer
Khan SA, Kuster DA, Hansen SR. A review of moxidectin overdose cases in equines from 1998 through 2000. Vet Hum Toxicol. 2002 Aug;44(4):232-5. PMID: 12136974.
Alvinerie M, Escudero E, Sutra JF, Eeckhoutte C, Galtier P. The pharmacokinetics of moxidectin after oral and subcutaneous administration to sheep. Veterinary research. 1998;29(2):113-8.
Schumacher J, Taintor J. A review of the use of moxidectin in horses. Equine Veterinary Education. 2008 Oct;20(10):546-51.
Nielsen MK, Steuer AE, Anderson HP, Gavriliuc S, Carpenter AB, Redman EM, Gilleard JS, Reinemeyer CR, Poissant J. Shortened egg reappearance periods of equine cyathostomins following ivermectin or moxidectin treatment: morphological and molecular investigation of efficacy and species composition. International journal for parasitology. 2022 Nov 1;52(12):787-98.
