Equine grass sickness (EGS), also known as Equine Dysautonomia, is a polyneuropathy affecting the central, peripheral and enteric nervous system of horses with particularly serious consequences for the alimentary function in the acute form of the disease, which is rapidly fatal. The disease affects young horses (2-7 yrs old), stressed horses and those which had recently been moved to a different pasture. Grass sickness is rarely seen in suckled foals below 6 months of age, despite ingesting significant volumes of grass. It is not certain whether this apparent protection is due to maternally derived colostral antibodies, age related differences in physiology, toxin metabolism or due to the way ingesta is processed in the young.
There are five diseases that top the list of infectious neurological conditions that affect horses in the western hemisphere:
Eastern Equine Encephalomylelitis (EEE)
Western Equine Encephalomyelitis (WEE)
Venezuelan Equine Encephalomyelitis (VEE)
West Nile Virus Encephalomyelitis (WNV)
Equine Protozoal Myeloencephalitis (EPM)
The first four are caused by viruses – arboviruses of the family Togaviridae in the case of EEE, WEE and VEE, and a Flavivirus in the case of WNV. All are primarily transmitted by mosquitoes and have zoonotic potential with significant numbers of human WNV cases occurring annually in the United States. A case of EEE was recently confirmed in a horse in New Jersey in the United States.
EPM is caused by the protozoan parasite Sarcocystis neurona, transmitted by ingestion of infectious sporocysts in contaminated feed or water. The organism may be carried by a number of hosts, including opossums, raccoons and cats, which shed sporocysts in the faeces. Horses are aberrant hosts and do not shed organisms, so infected horses are not contagious.
The viral infections are characterized by fever in some cases, altered mental state, visual impairment, head pressing, aimless wandering, circling, swallowing difficulty, incoordination in movement (ataxia), weakness, paralysis which can progress further to seizures and death.
EPM infections can affect any area of the central nervous system, so signs may vary a great deal. Spinal cord infections are more common than brain infections. With spinal cord infections there may be asymmetric neurologic deficits in one or more limbs with accompanied muscle atrophy. There may be localized sweating. Brain infections may result in altered mental state, paralysis of various cranial nerves, head tilt and loss of balance, seizures, abnormal behavior, all of which can progress to recumbency and death if untreated. Even with treatment, there may be permanent neurological damage.
Diagnosis of these nervous system infections may be accomplished by various tests on blood or cerebrospinal fluid. Post mortem diagnostics include histopathology of lesions in the brain or spinal cord.
Treatment of the viral infections is purely supportive as there are no effective anti-viral drugs for these viruses. Anti-inflammatory drugs to reduce fever and inflammation, intravenous fluids and, if needed, anti-convulsants may be used. Mortality will vary, but tens to be highest with EEE.
EPM may be treated using coccidiostat drugs such as sulfadiazine or sulfamethoxazole (with or without trimethoprim) in combination with pyrimethamine. Diclazuril or ponazuril may also be used. Recovery is dependent of the severity of the infection and early initiation of treatment. Relapses may occur.
Prevention of the viral encephalitides is by vaccination. Multivalent vaccines for the three EE’s are available as is one for WNV. Horses in endemic areas should be on a vaccination program. Vector control measures are helpful.
EPM prevention includes attention to feed and water safety, reducing the possibility of contamination by cats or other carrier species.
An outbreak of EEE, affecting both horses and humans, occurred in Jamaica in the early 1960’s in the parish of St. Thomas, but was controlled and eliminated by prompt action by the Veterinary Division, with the notable involvement of the late Dr. K. D. Urquhart, then Parish Veterinary Officer. There have been no cases of EEE, WEE or VEE detected in Jamaica since. There have been no cases of West Nile Virus detected in horses or humans in Jamaica either. The presence of EPM remains unconfirmed.