EPM vs West Nile Virus
Equine Protozoal Myeloencephalitis (EPM) is a neurologic disorder of the horse caused by the protozoa (microbe) called Sarcocystis neurona. S. neurona is spread to horses by opossum feces. The protozoa then migrates from the intestinal tract, into the bloodstream, and finally to the spinal cord or brain where it begins attacking the cells.
Since the protozoa can attack anywhere along the central nervous system (brain and spinal cord), horses can have presentations. The most common clinical signs include the three A’s – Ataxia (incoordination), Asymmetry (meaning one side is worse than the other), and muscle Atrophy (most noticeable in the large hindquarter muscles). Other signs of EPM include weakness (especially in the hindquarters), abnormal gait or lameness, drooping of the eyelids, ears, or lips, difficulty swallowing, head tilt, abnormal sweating, or loss of sensation along the face, neck, or body. EPM does look similar to many other diseases, which can cause difficulty in diagnosis.
Although a lot of horses are exposed to the S. neurona, it is relatively uncommon (approximately 1%) for horses to have clinical evidence of the disease. Due to the high number of horses exposed, diagnosis can be difficult. Your veterinarian may pull blood or perform a cerebrospinal fluid analysis to test for EPM. Treatment options should be discussed with your veterinarian, but treatment if started early in the course of the disease has a fair to good prognosis for complete recovery.
What can you do to protect your horse? Good housekeeping is key! Keeping opossums out of the barn and especially out of feed sources such as hay and grain are important. You can start by keeping feed rooms and containers closed and sealed, cleaning up any dropped grain promptly, not feeding on the ground, and disposing of any carcasses you see on the property promptly are just a few helpful tips.
West Nile Virus (WNV) also causes neurologic clinical signs in the horses. Mosquitos pick up the virus from birds and transmit it to horses and humans. It is important to note that horses cannot spread the virus to other horses, but multiple horses on the same property may become infected at once due to the presence of WNV infected mosquitos on the property. Once in the horse, the virus can cross the blood brain barrier and start attacking the cells in the central nervous system resulting in West Nile Virus Encephalomyelitis.
Much like EPM, West Nile Virus can have varying clinical signs, depending on where in the central nervous system the virus attacks. Most commonly we see ataxia (incoordination), twitching of the muzzle or lip, muscle twitching over the neck, shoulders, and pectoral muscles, fever, depression, toe dragging, and recumbency. Once horses become recumbent in either disease, prognosis for full recovery is poor.
Because WNV can look like many other neurologic diseases including EPM, rabies, eastern equine encephalitis, equine herpesvirus-1, and botulism, your veterinarian may recommend a blood test to determine which of these diseases your horse is suffering from. Because this is a virus, treatment options are limited to supportive care such as anti-inflammatory drugs, intravenous fluids, pain control, sedatives, and nutritional support.
Since there are no specific treatment options for WNV, it is important to ensure your horse is protected against the virus. Conley and Koontz Equine Hospital recommends vaccinating your horse against West Nile Virus in the spring and boostering the vaccine in the late summer when mosquitos are the most prevalent. Mosquito control including removing standing water is strongly recommended.
EPM and WNV can be debilitating and sometimes fatal neurologic diseases of the horse. It is important to work closely with your equine veterinarian to help prevent these diseases and learn how to identify clinical signs early.
Maci Burns, DVM