Fall Horse Deworming: Timing is Everything


Connie Fike
M.S. Student, Department of Animal Science
Auburn University

October 19, 2016

As the season begins to shift from summer to fall and the nights get cooler, I find myself engaging in more conversations about the changes in management in preparation for the fall and winter. Finding hay, when to start blanketing, and when to transition turnout schedules are all common topics around my barn. But, of all the management adjustments that are being addressed, deworming protocols have been on everyone’s mind. When to deworm and what to use are the most basic questions and sometimes the most difficult to give a straight answer. There is no “one-size fits all” program.

The goal of any deworming protocol should be to reduce the incidence of re-infection of parasites in the horse. No horse is free of intestinal parasites, and the overuse of rotational deworming has led to parasites that may be resistant to one or more drugs on the market. A better method is to follow strategic deworming protocols established by the American Association of Equine Practitioners’ (AAEP), allowing for targeted treatment and control of small strongyles and ascarids based on a fecal egg count (FEC). Your veterinarian can perform an FEC to determine which parasites are present and the rate at which parasite eggs are being shed into the feces.

The rate or amount of eggs in the feces determines how often a horse should be dewormed. Horses with FECs >500 eggs per gram (epg) of feces are considered high shedders and will typically need to be dewormed four times per year. Moderate shedders are those with FECs 200-500 epg and will typically be dewormed three times per year. Low Shedders with FECs

A major factor in the efficacy of strategic deworming is timing. Well timed administration is based on the life cycle of the parasite and peak transmission periods. Ascarid and small strongyle larvae hatch from eggs in the horse’s manure and then move to the pasture grass where they are picked up by grazing horses. Survivability of both eggs and larvae is poor when the weather is very hot or very cold. A mild climate creates the ideal environment for larvae to thrive and be most active. For this reason, spring and fall are the peak times for re-infection.

It is important to note that not all parasites can be identified using FECs. Bots and tapeworms are prime examples. Bot flies lay their bright yellow eggs on the horse in locations that the horse will most likely lick or bite in order to gain access to the horse’s mouth. Once inside the horse the larvae hatch and spend several months until they are shed in the feces to become flies and complete their lifecycle. Tapeworms are transmitted to horses by ingesting mites that are infected with tapeworm eggs. Segments of the tapeworm will break off and are passed in manure where the mites will ingest the eggs. Bot and tapeworm larvae are best controlled by a single annual treatment in the late fall or early winter.

When choosing a dewormer, the goal is to target the parasite of utmost concern for the time of year. In the fall, the parasites of interest are encysted small strongyles, bots, and tapeworms. Praziquantel will control tapeworms and ivermectin or moxidectin will control encysted small strongyles and bots. To address all three types of parasites, a combination dewormer is the way to go. Ivermectin plus praziquantel or moxidectin plus praziquantel are the two options for controlling fall parasites. Two commonly used brands include Zimectrin Gold (ivermectin/praziquantel) and Quest Plus (moxidectin/praziquantel).

Pharmaceutical companies suggest that for a “fall” deworming the drugs be administered in September, October, or November. But, depending on your region’s climate, these months of the year might not be applicable. In Alabama, a good rule of thumb is to wait until the weather is consistently cold, usually around December or January. If you are concerned about the timing of the initial dose, follow up with a second dose of dewormer before the temperature rises in early spring.

Communication between you and your veterinarian is key to designing a parasite treatment program that is specific to your location and your operation.