Equine Health: Let’s Talk about Worms

Oct 4, 2017 / Student Blogs

Dr. Johnson and horse

You may ask yourself, “How do I keep my horse from being negatively impacted by worms?”

Recent research and new strategies have determined that the timing of treatment with an effective anthelmintic (dewormer) is the most effective way to keep your horse from the ill effects of worms. The traditional practice of frequent anthelmintic treatments is no longer the recommended method. This article will discuss new anthelmintic strategies, significant worm pathogens found in horses currently, how and when to deworm, management practices to reduce environmental worm burdens, and goals of parasite control.

Strategic Use of Dewormers

The traditional method of deworming was a rotational deworming schedule of the three different classes of anthelmintics every three months. This method has developed resistance in worm populations to the anthelminitics and is no longer necessary due to the shift of important worm pathogens in horses today. Currently, Cyathostomins (small strongyles) and Anoplocephala perfoliata (tapeworms), are recognized as a primary equine parasite pathogen and Parascaris (roundworms) is the major parasitic pathogen in foals and weanlings. The new strategy to deworming takes into consideration the life cycle of these pathogens, and recommends deworming at appropriate times of the year with an effective anthelmintic that will correspond with parasite cycles of transmission and the relative worm burdens in individual horses.

In order to determine if the anthelminitc is effective, in other words that there is no resistance, a fecal egg count reduction test (FECRT) should be performed. This is a simple test that can be done by your veterinarian. A fecal sample is collected prior to deworming and another one is collected two weeks after the anthelmintic is administered. The number of eggs in each sample is calculated and a percent reduction is determined. A 90% reduction in eggs means there is no resistance to that anthelmintic and it should be efficacious to use.

Another critical part to the strategic use of dewormers is to perform fecal eggs counts on individual horses to determine if they are low, moderate, or high egg shedders. Horses on the same pasture will share the same population of parasites, and resistance should always be evident across that population. However, horses demonstrate significant differences in their level of egg shedding. For instance, within a group of mature horses (>3 years), only 15-30% generally shed approximately 80% of the eggs. These would be considered “high shedders.” High shedders usually have to be dewormed three or four times a year, but an individual deworming schedule should be created for any particular horse. Your veterinarian can help you determine to which category your horse belongs. Identifying horses that are high shedders is important to eliminating contamination of eggs in the environment.

How and When to Deworm

Healthy horses that are low shedders usually need to be dewormed once to twice a year. The optimal times to deworm are in the spring and fall when the environmental conditions are conducive to egg and larval development and survival. All horses should be dewormed in the fall after the first frost utilizing either ivermectin or ivermectin plus praziquantel to kill the bots and tapeworms should they exist. If the horse is a moderate to high shedder, three or four treatments may be needed during the year.

Foals and weanlings must be dewormed differently because their immune system is naive and thus can be infected with worms more easily. The primary pathogen in young horses is ascarids and their eggs are highly resistant to environmental conditions, which allows them to be ubiquitous in the environment. It is recommended by the AAEP to begin deworming at two months of age. Work with your veterinarian to set up a deworming protocol for the younger horses. Once the horse reaches three years of age, the deworming treatments per year may be reduced depending into which egg shedding category any particular horse fits.

Ways to Control Parasites

There are some important management practices that can be undertaken to control parasites. The most effective method in decreasing worm burdens in horses is manure removal from the surrounding environment. Horses will shed the strongyle eggs in the manure, the egg then develops into an infective larvae in the manure, migrates to the pasture, and ultimately the horse ingests the larvae when grazing. Thus, the likelihood of infection can be significantly lessened with prompt removal of manure from the pasture. Additionally, manure should be composted. Extreme cold and heat will kill eggs and larvae, and the composted manure will generate high internal temperatures that may kill the parasite. It is important to remember that non-composted manure should never be spread on pastures. Grazing on infected pastures by ruminants (cows, goats, sheep, and camelids) may also assist in parasite control. Equine strongyle larvae are host specific and cannot infect ruminants.

Why Controlling Parasites Is Important

The primary goal for parasite control is to limit parasite infections so the horse may remain healthy and clinical illness related to these infections does not develop. Horses are going to live with a small population of worms because eradication is not possible and if attempted can lead to accelerated drug resistance. Treatments should be timed to control the level of egg shedding into the environment with an effective deworming medication. Fecal eggs counts should be performed on each horse to categorize their level of egg shedding and fecal egg count reduction tests should be performed to ensure the worms are not resistant to the dewormer medication. Work with your veterinarian to create a deworming protocol for individual horses, including which anthelmintic to use and how many times per year to deworm.

Dr. Katheryn Johnson, equine intern

Resource: AAEP Parasite control guidelines paper