Equine encephalitides caused by alphaviruses in the Western Hemisphere

GENERAL INTRODUCTION: TOGAVIRIDAE

Introduction

Several mosquito-borne alphaviruses, endemic to North, Central and South America, cause disease in horses, humans, and, on occasion, other domestic and wild animal species, particularly those that are not native to the New World. Prior to 1999, the principal viruses causing epidemics of encephalitis in horses and humans were eastern, western and Venezuelan equine encephalitis (EEE, WEE, VEE, respectively).  The viruses of EEE, WEE and VEE are closely related and classified as alphaviruses within the Togaviridae family.  In 1999, West Nile (WN) virus was introduced to North America, possibly from Israel by an unknown route.   The virus has since spread throughout the Americas.  It is considered endemic in North America and is recognized as a significant cause of encephalitis in horses, humans and other species. Since WN virus is classified within the Flaviridae family, it is discussed in a separate chapter (See West Nile virus infection).

Annual summaries of the incidence of disease caused by EEE,WEE and WN viruses in the USA are available on the web https://diseasemaps.usgs.gov/mapviewer/ Further information on infections caused by each of these viruses, particularly as it relates to human infection, can be found on the website of the Centers for Disease Control in the USA www.cdc.gov. The World Organization for Animal Health (also known as OIE) has information on the occurrence and diagnosis of these diseases worldwide.  EEE, VEE and WN fever are “listed” diseases (notifiable). www.oie.int/wahis_2/public/wahid.php/Diseaseinformation/Diseaseoutbreakmaps

Before the recognition of EEE, WEE and VEE as antigenically distinct viruses, several epidemics of equine encephalitis were recorded in the western hemisphere. The first epidemic in horses was recognized in Massachusetts, USA, in 1831 and, in retrospect, was probably EEE. Once it had been established in 1933 that epidemics of encephalitis in horses in North America were mostly caused by either EEE or WEE viruses and were largely partitioned by geography, more accurate information became available. The largest recorded epidemic of EEE occurred in southern Louisiana and adjacent Texas in 1947. Over 14 000 horses* (*The term “horse” will be used to denote horses, mules, and donkeys except where qualified) were reported to have been affected, of which nearly 12 000 died. The case fatality rate of 83 per cent in this epidemic is typical of EEE in horses in North America.

The epidemic of encephalitis in horses in California that led to the recognition of WEE was estimated to have caused disease in 6 000 horses with a case fatality of 50 per cent. In another large epidemic in the 1940s in western Canada, the case fatality was lower, around 12 per cent, which is considered more typical of WEE.

Perhaps the best known of more recent epidemics of encephalitis in horses was the one caused, not by EEE or WEE viruses, but by VEE virus. This epidemic began in northern South America in 1969 and, by 1972, had caused the death of hundreds of thousands of horses in Central America, Mexico and Texas, the cost being in excess of US $30 million. Many human cases were also recorded.

With the widespread availability of vaccines to protect horses, epidemics of these magnitudes have not occurred in recent years, but many horses still die annually in the USA of viral encephalitis.  Further details of the history of EEE, WEE, and VEE are available in several comprehensive reviews.^{1, 10, 19, 23, 27, 29, 30, 41, 46}

As mentioned above, West Nile virus was introduced to North America in 1999 and is now endemic in North America. Several thousand horses in the Americas, but mostly in North America, have died of encephalitis caused by this virus.  Most reviews of the arboviruses that can cause encephalitis in horses now include West Nile virus.¹⁹

In contrast with WN virus, the life cycles of EEE, WEE and VEE viruses indicate that these alphaviruses have evolved in the western hemisphere and have established a balanced ecological relationship with indigenous vertebrate species. Those vertebrate species that develop clinical disease are, in terms of evolution, recent introductions to the continents of North and South America.  While humans may have arrived from Asia as early as 30 000 years ago, it is believed that continental dispersion did not occur until approximately 13 000 years ago with the opening of the Canadian ice corridor. The modern horse arrived only after Columbus.

Recent molecular studies on the genetic conservation and evolution of individual alphaviruses have significantly advanced our knowledge of the epidemiology of alphaviruses in the western hemisphere.⁴⁴ Contrary to the evolution of most RNA viruses, the alphaviruses of North America have evolved slowly. This slow evolution may be due to several factors, but it is most likely a result of the adaptation of the virus to specific species of mosquito vectors for their long-term perpetuation. While the situation with VEE, EEE and WEE in Central and South America is possibly...