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
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 na´ve 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 na´ve
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