Fecal samples are taken and analyzed to determine the horse’s parasite load. Routine fecal egg counts are utilized to assess the effectiveness of deworming methods. These tests will not detect all parasites but are a good baseline to work from. Your veterinarian can provide guidance on which horses to sample based on the environment, management and horse groups. Each horse will have individual deworming needs, which are based on their unique parasite load on fecal egg counts and other health needs. High shedders (those who have a large number of eggs on the fecal egg count) will need more frequent treatment compared to horses considered to be low shedders.

Effective deworming or anthelmintic medications should reduce parasite loads by more than 95% two weeks after treatment, which is why it’s important to check fecal egg counts before and after treatment. Drug resistance has become increasingly common among parasites. If egg load levels are still high after treatment, the parasites may have some resistance to the dewormers. In this case, the veterinarian will advise on an alternative class of dewormer and evaluate its effectiveness with additional fecal egg counts.

There are three classes of drugs used to deworm horses. These include benzimidazoles (fenbendazole/oxibendazole), pyrimidines (pyrantel), and macrocyclic lactones (ivermectin/moxidectin). Benzimidazoles such as Panacur and Safe-Guard target large and small strongyles, ascarids, and pinworms. Pyrimidines such as Pyrantel, Exodus, and Strongid target large and small strongyles, tapeworms, and pinworms. Macrocyclic lactones such as Equimax, Zimecterin, and Quest target large and small strongyles, ascarids, pinworms, and bots. Studies have shown that small strongyles are highly resistant to benzimidazoles and pyrimidines, while macrocyclic lactones have shown the early stages of resistance. There is no currently known drug resistance issues among large strongyles. Ascarids have early resistance to benzimidazoles and pyrimidine and high resistance to macrocyclic lactones.

Strongyles are the most common internal parasites found in horses. Small strongyles (cyathostomins) are seen in grazing horses but are rarely the cause of disease. Large strongyles, including Strongylus vulgaris (also known as the bloodworm) can cause blood clots to break off and travel through the bloodstream, blocking other blood vessels and leading to deep vein thrombosis and/or pulmonary embolism. Tissue damage, organ failure, and death can also occur as a result of large strongyle infestation. Deworming medication reduces the level of large strongyles to an undetectable level in the United States.

Ascarids (Parascaris spp.) are also known as roundworms and are common. Cough, nasal discharge, and gastrointestinal impaction are all symptoms of ascarids. This parasite is especially important to control in young horses and is rarely seen in older adult horses that have healthy immune systems.

Tapeworms (Anoplocephala perfoliatta) are associated with colic, as the attachment of the worm occurs in the ileum or cecum and can cause gut impactions. For horses not kept on pasture, tapeworms are likely to not be an issue and do not need to be a major priority in deworming protocols.

Pinworms (Oxyuris equi) affect young and older horses. The signs vary, but include tail rubbing in more severe cases. Horses may be infected and show no signs. Due to the tail rubbing, transmission occurs via infected stalls, tail wraps, and fence posts.

Botflies are common with two dominant species living in North America. These include Gasterophilus nasalis and G. intestinalis. The larvae are rarely cause for concern, but can be found in the stomach and can create oral problems such as periodontitis and chewing issues.

Threadworms (Strongyloides westeri) are common in foals and do not pose a high disease risk, nor do they typically cause diarrhea. These parasites can be transferred from the mare to the foal via milk, or can be acquired if the foal comes into contacted with infected feces.

Stomach worms (Habronema and Draschia spp.) are rarely associated with disease, but can create lesions that are difficult to treat. In more arid parts of the United States, these are more common when compared to tapeworm or strongyle levels.