Eastern and Western Equine Encephalitis in 2001

Eastern equine encephalitis (EEE) and Western equine encephalitis (WEE) are mosquito-borne viral diseases that primarily affect horses east and west of the Mississippi River, as their names imply.  The Centers for Disease Control and Prevention (CDC) last reported cases of EEE and WEE in horses, other animals, and humans in 1997.  Since then, only summaries of human encephalitis cases are reported by CDC.  These numbers likely underestimate the human cases since viral encephalitis is not a reportable disease in all states.
In 2001, the National Veterinary Services Laboratories (NVSL), Ames, Iowa tested 899 diagnostic serum samples for antibodies to EEE and WEE, and performed virus isolation attempts on 224 brain tissue or whole blood samples.  

The majority of samples were of equine origin, with less than 10% of avian origin.  EEE virus isolates obtained by cell culture or mouse inoculation were confirmed by complement fixation assay using Eastern, Western and Venezuelan equine encephalitis virus reference antigens and sera.  All horse brain submissions tested negative for rabies virus antigen.

Of the samples submitted, 95 horses and two birds had laboratory evidence of EEE infection. All EEE virus isolates were obtained from brain tissue samples.  These EEE-positive samples were negative for West Nile virus RNA by nested RT-PCR testing.

Serologic identifications of EEE cases were based on several testing procedures along with reported clinical signs and vaccination history.

Although it is rare for NVSL to receive paired serum for encephalitis testing, the combination of testing procedures, vaccination history, geographic location and other factors lead diagnosticians to highly probable positive or negative findings.

No submitted samples were suggestive of WEE or Venezuelan equine encephalitis virus.

The NVSL testing results do not fully encompass the national picture for the incidence of EEE or WEE, since not all neurologic horses are tested, some animals are tested at a state or local laboratory, and WEE is not a reportable disease to the state veterinarian’s office in many states.

To obtain further information on the 2001 occurrence of EEE and WEE, state veterinarians were surveyed.  More than 22 have responded to date, and additional information is included on Figure 3 (see web site link at end of article).  None of the states reported WEE cases in 2001.

According to the Arbovirus Diseases Branch of the CDC, Fort Collins, CO, eight human cases of EEE were confirmed in 2001.  These cases were from Florida (three), Georgia (two), Louisiana, Michigan, and Texas.  No cases of human WEE were reported.

Because of the zoonotic nature of EEE and WEE (their ability to cause disease in humans and horses), and the unusual occurrence of EEE cases in Wisconsin, Iowa, and Minnesota during 2001, all horses in the continental United States should be vaccinated for these diseases.  Horses in the southeastern United States and in endemic areas should be vaccinated more frequently according to veterinary recommendations.

Horse owners are encouraged to have veterinarians examine all neurologic horses and submit samples for diagnostic tests.  Mosquito control measures, which have been strongly recommended for control of West Nile virus, also serve to reduce the numbers of EEE and WEE vectors and need to be implemented.

Kathryn M. Moser, Microbiologist;
Dr. Eileen N. Ostlund, 515/663-7551, Eileen.N.Ostlund@usda.gov
National Veterinary Services Laboratory, Ames, Iowa;
Dr. Roberta M. Dwyer, 859/257-4295, rmdwyer@uky.edu
Department of Veterinary Science, University of Kentucky, Lexington, Kentucky

This article was reprinted from the April 2002 issue of Equine Disease Quarterly, funded by Underwriters at Lloyd's, London, Brokers, and their Kentucky Agents. The entire article with graphics can be viewed at http://www.uky.edu/Agriculture/VetScience/q_apr02/q_apr02.htm, by clicking on the EEE/WEE in 2001 link in the left column.


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