Major Neurological Syndromes

Venezuelan Equine Encephalitis

The clinical signs of VEE are similar to both EEE and VEE with a large variation in mortality ranging from 40-90% depending on the outbreak. In addition to subclinical and overt CNS clinical signs, diarrhea has been observed in VEE horses. Florida, Texas, and Louisiana are the three states ecologically at risk but recent activity in Panama could result in a transported case by air travel. A form of endemic VEE called Everglades virus (EGV) does occur in south Florida. Horse "disease" has only been identified serologically as subclinical infection. Recently, several north Florida horses have been identified during the Florida season with very high serum neutralization titers (SN) without history of vaccination. The interpretation of these titers is difficult and may indicate that EGV is more widespread as a result of recent weather upheavals. Whether or not EGV actually causes some sort of encephalitic syndrome is an open question. Vaccination against VEE is controversial. Currently, the only marketed vaccine is a killed product. Vaccination of horses with what is essentially considered a foreign animal disease confounds testing for that horse should an outbreak occur. In addition, the use of a killed vaccine against a virulent viral variant is of questionable efficacy. During previous outbreaks, a conditionally available modified live vaccine was released to contain the outbreak in Texas. It is likely that this potent vaccine will be released.

Japanese Encephalitis Virus (JEV) is actually the type virus of the Flavivirus in which WNV belongs. Many of the disease features are similar, and this discussion will focus on dissimilarities and expectations should outbreak occur. First, like WNV, JEV is spreading geographically and focal, intense outbreaks are occurring new areas of Eurasia. The Ardeid birds (herons and egrets are believed to be the natural wildlife hosts for JE. Pigs also may be important reservoirs and can amplify JEV. The vector is similar to JEV, which Culex mosquitoes most important arthropod hosts. Cycles of outbreaks occur during rainy seasons and in temperate climates late summer is important. Because there are horses that are not exposed to Flaviviruses, severe sporadic outbreaks do occur in naïve horse populations upon encroachment or transport of horses into endemic areas. Like WNV, horses appear to be dead-end hosts. Horses appear to develop long-term immunity to JE virus infection. When naïve horses are shipped into endemic areas, they generally seroconverted within the year and these antibodies persist for several years. Like WNV, horses and donkeys are equally susceptible to disease. Interestingly case fatality rates are between 5 and 15% in some locales, but in Japan 30-40% has been reported. Diagnosis at time relies on neutralization testing. For JE, there are 3 syndromes described: Transient Type: these horses have moderate to high fevers for 2-3 days. The horses can also have anorexia, slow movement, and congested mucous membranes. Lethargic Type: Horses with these types of signs have higher fevers, lethargy, anorexia, nasal discharge, difficulty swallowing, jaundice and petechial hemorrhages. Neurological signs occur with incoordination, staggering, and spontaneous falling. Neck rigidity, radial paralysis and impaired vision all have been noted. These horses usually recover in 4-5 days. Hyperexcitable Type: In less than 5% of cases JE horses develop hyperexcitability. These horses exhibit bizarre mentation characterized by aimless wandering, shaking, violent reactions, blindness, sweating, teeth grinding, and muscle fasciculations. Although the hyperexcitability can be transient, this group of horses is more likely to die from spontaneous collapse. There are available vaccines in Japan for use in horses. Little is known regarding cross-protective immunity with WNV vaccines.

Borna Disease Virus

Although Borna Disease has been reported only in Germany, seropositive horses have been reported in other European countries, Israel, Japan, Iran, Australia and the U.S. In Germany, Borna is the most important viral cause of encephalitis, and possibly this disease has gone unrecognized elsewhere. In Germany, cases occur in central and southern Germany and have a seasonal occurrence in April, May and June. Many horses are seropositive while actual disease occurrence is relatively rare.

Equine Encephalosis Virus

This is a South African virus that is often confused with African Horse Sickness virus. This is a virus that is transmitted by Culicoides also. After an incubation of 3-6 days, horses develop fairly mild clinical signs which included a high fever, depression, and inappetence. A reddish-brown discoloration of the mucous membranes also occurs. There is occasionally varying degrees of selling of the eyelids and supraorbital fossae. The central nervous system, respiratory distress, acute heart familiar