Equine Neurologic Disease

Equine encephalitides can be clinically similar, usually cause diffuse encephalomyelitis and meningoencephalomyelitis, and are characterized by signs of CNS dysfunction and moderate to high mortality. Arboviruses are the most common cause of equine encephalitis, but rabies virus, Sarcocystis neurona, Neospora hughesii, equine herpesvirus, and several bacteria and nematodes may also cause encephalitis.

All cases of acute equine neurologic disease must be reported immediately, unless the cause is known toxicity or trauma. The following are reportable equine neurologic diseases in Colorado:


Equine Neurological Disease Case Report Form


AAEP: Infectious Disease Control Guidelines

Equine Disease Communication Center: Disease Outbreak Alerts


CSU Veterinary Diagnostic Laboratory Equine Neurologic Disease Test and Sample Submission Guidance
Agent Option 1

Option 1

Sample Type

Option 2

Option 2

Sample Type


qPCR (NVSL) Brain lgM ELISA (NVSL) Serum (RTT)
WEE qPCR Brain or Whole Blood (PTT) lgM ELISA (NVSL) Serum (RTT)



qPCR Nasal Swab^ AND Whole Blood (PTT) Acute and Convalescent Serum (SN)** Serum (RTT)

Referral Lab


Serum or CSF Necropsy and Histopathology Carcass
Rabies FA Brain, Carcass or Head Histology Brain, Carcass or Head
WNV IgM ELISA Serology Serum (RTT) qPCR Brain or Whole Blood (PTT)


PTT= Purple Top Tube

RTT= Red Top Tube

SN= Serum Neutralization (Antibody Test)

qPCR= Real-time Polymerase Change Reaction (detects DNA or RNA of organism)

FA= Fluorescent Antibody Test (Antigen Test)


All tests conducted at the Colorado State University Veterinary Diagnostic Laboratories, with the following exceptions:

NVSL = National Veterinary Services Laboratory, Ames, IA

CAHFS = California Animal Health and Food Safety Laboratory


Notes about sample submission:

** Submit acute and convalescent serum samples (2-4 weeks apart). Submit blood samples in a red top tube to lab ASAP so sample does not hemolyze, or centrifuge and separate the serum and submit serum only.

^Swabs should be polyester swabs with a plastic shaft, submitted in 1 ml sterile saline in a sterile tube.



For questions about testing at Colorado State University, Veterinary Diagnostic laboratory, please call (970) 297-1281 or email dlab@colostate.edu. Visit the Test Information website for details on general test information, submission, and prices. Visit the Client Services website for details on submission of samples.


Equine Encephalomyelitis (EEE, WEE)

Eastern and Western equine encephalomyelitis are mosquito-borne pathogens that can cause nonspecific illness and encephalitis in equids (horses, mules, burros, donkeys, and zebras) and humans in the Americas. Some of these viruses can also affect birds and occasionally other mammals. No specific treatment is available, and depending on the virus, hose, and form of the disease, the case fatality rate may be as high as 90%.


The Merck Veterinary Manual: Overview of Equine Arboviral Encephalomyelitis

Center for Food Security & Public Health (CFSPH): Eastern Equine Encephalitis

Center for Food Security & Public Health (CFSPH): Western Equine Encephalitis

Center for Disease Control and Prevention (CDC): EEE

Center for Disease Control and Prevention (CDC): WEE



Equine Herpes Myeloencephalopathy (EHM)

Equine Herpes Myeloencephalopathy (EHM) is a neurological disease of horses caused by equine herpesvirus (EHV). Equine herpesvirus is a common DNA virus that occurs in horse populations worldwide. The two most common strains are EHV-1, which causes abortion, respiratory disease and neurological disease; and EHV-4, which usually causes respiratory disease only but can occasionally cause abortion and rarely cause neurological disease. EHM is very contagious and can have a mortality rate as high as 30%.

Resources for Veterinarians

The Merck Veterinary Manual: Equine Herpesvirus Infection

Colorado State University (CSU) paper on EHM

Awakening the Dormant Dragon: Neurological Form of EHV-1

Resources for Horse Owners

A Guide to Understanding the Neurologic Form of EHV Infection

Keeping Your Horses Safe at Weekend Events

Rodeo Horses and Respiratory Disease Prevention

EDCC Fact Sheet on EHM

Resources for Event Organizers and Barn Owners

EHV-1 Biosecurity Guidance for Equine Event Organizers



Equine Protozoal Myeloencephalitis

Equine Protozoal Myeloencephalitis (EPM) is a common neurologic disease of horses in the Americas; it has been reported in most of the contiguous 48 states of the US, southern Canada, Mexico, and several countries in Central and South America. In other countries, EPM is seen sporadically in horses that previously have spent time in the Americas.

Most cases of EPM are caused by a protozoan, Sarcocystis neurona. Horses are infected by ingestion of S neurona sporocysts in contaminated feed or water. The definitive host for S neurona in the US is the opossum. Opossums are infected by eating sarcocyst-containing muscle tissue from an infected intermediate host and, after a brief period (2-4 weeks), infectious sporocysts are passed in the feces.


The Merck Veterinary Manual: Equine Protozoal Myeloencephalitis

Center for Food Security & Public Health (CFSPH): Sarcocystosis




Rabies is caused by a virus of the Rhabdovirus family and causes a severe, rapidly progressive neurological disease. It is transmitted via saliva, most commonly through bite wounds from an infected wild animal bite. Symptoms can appear in as little as two weeks but can take months for clinical signs to appear. On average, symptoms will be seen four to eight weeks after exposure. Death usually occurs within the first two to four days after the horse begins to show clinical signs, although death may not occur until up to two weeks later with supportive care.

The incidence of rabies in skunks increased dramatically in Colorado in 2008. Skunks are the most common species involved in the transmission of rabies virus to horses. Rabies is transmitted from infected animals to other species through a bite or by the introduction of virus-laden saliva into a fresh wound, cut, or mucous membrane. A rabies-infected horse can expose owners, veterinary personnel, and many other people.


Rabies in Horses: Should Horses be Vaccinated in Colorado?

Colorado Department of Public Health and Environment (CDPHE): Rabies

Colorado Department of Public Health and Environment (CDPHE): Rabies in Colorado



West Nile Virus

West Nile Virus (WNV) is a serious and sometimes deadly viral disease that can be carried by mosquitoes. Mosquitoes become infected with the virus after feeding on infected wild birds and then transmit the virus through bites to people, animals, and other birds.


Center for Food Security & Public Health: West Nile Virus

USDA Equine West Nile Virus Case Reporting and Surveillance Information