Merck Manual

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Pet Owner Version

Central Nervous System Disorders Caused by Parasites in Horses


Charles M. Hendrix

, DVM, PhD, Department of Pathobiology, College of Veterinary Medicine, Auburn University

Reviewed/Revised May 2019 | Modified Oct 2022

A number of parasites (including worms and insects) are associated with central nervous system disease. Diagnosis requires eliminating other possible causes of illness and identifying the specific parasite responsible.


Several types of roundworms are found in domestic animals.

Setaria digitata is found in Asia and is a common parasite of cattle. Mosquitoes are intermediate hosts. In horses, the developing worms invade the central nervous system and cause weakness, lack of coordination, lameness, drooping eyelids or ears, and paralysis.

Migrating larvae of strongyles (Strongylus vulgaris and Draschia megastoma), rhabditids (Halicephalobus gingivales) and the lung worm (Paralaphostrongylus tenuis) have been reported in the central nervous system of horses.

Disease Caused by Insects

Myiasis is the development of larval dipteran flies (bots and warbles) within the body’s tissues or organs. Hypoderma species have been found migrating in the brain of horses. Organophosphate drugs can eliminate certain dipteran larvae from the nervous system, but they can also cause nervous system damage. Corticosteroid drugs are often recommended to prevent additional inflammatory damage and pressure on the brain during treatment.

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