California EEE Case Investigation Complete

Final results have been released from a study that investigated the only case of Eastern equine encephalitis (EEE) ever documented in California. The infected Quarter Horse yearling died in the spring of 2000 (See Article Quick Find  #1102 for an archived article). The disease is mostly seen on the East Coast, and is rarely diagnosed west of the Mississippi and Texas. While the source of the infection was not definitively identified, it is possible that an incompletely inactivated vaccine virus infected the horse.

The study was published in a recent issue of the Centers for Disease Control’s online peer-reviewed journal, Emerging Infectious Diseases. The article can be accessed at Research team members were from the Humphrey, Giacopuzzi & Associates Equine Hospital in Somis, Calif.; The University of California-Davis, the California Department of Health Services in Sacramento, the National Veterinary Service laboratory in Ames, Iowa; and the U.S. Army Medical Research Institute of Infectious Diseases in Fort Detrick, Md.

The study states that California’s Arboviral Encephalitis Surveillance Program is among the most comprehensive in the United States. The farm where the EEE case was found participated in the program and had a sentinel chicken flock located adjacent to the farm (sentinel chicken flocks are screened periodically for mosquito-borne diseases to detect any threat of epidemics). Locally infected mosquitoes were apparently not the source of exposure, and there was no evidence of spread from the infected horse to the local mosquito populations based on the screening of area sentinel flocks and mosquito pools.

After excluding the possibilities of natural infection, bioterrorism, and importation, the researchers had to consider incomplete formalin (a clear aqueous solution of formaldehyde containing a small amount of methanol) inactivation of the EEE virus (EEEV) in the vaccine that had been given to the horse prior to disease onset.

“Attempts to isolate live EEEV from residual and stored vaccine were unsuccessful,” said the researchers. “However, this does not eliminate the possibility that the horse received live virus with its immunization.” Researchers say that if inactivated viruses were in the vaccine, they were probably present in undetectable levels during vaccine development and testing. Also, live viruses were probably distributed sporadically throughout a vaccine lot, allowing for only one recognized case. Since the horse was so young, it was likely exposed to EEE for the first time, which is noteworthy considering the small amount of the virus involved in the vaccine.

“If (the vaccine) or portions of the lot contained live virus, many exposed horses may have not been susceptible because of previous immunization or presence of maternal antibodies,” the report states. “In addition, cases may have been unrecognized or unreported. If (the vaccine) was the source of infection for this case or other cases, it was probably a rare event.

“A definitive source for infection may never be revealed in this case,” the researchers say in the paper. “However, the case illustrates the need to maintain awareness that EEE can occur outside its normal geographic boundaries; it also underscores the importance of prompt diagnosis, reporting, and surveillance for arboviral (transmitted chiefly by arthropods such as mosquitoes) encephalomyelitides.” 

About the Author

Stephanie L. Church, Editor-in-Chief

Stephanie L. Church, Editor-in-Chief, received a B.A. in Journalism and Equestrian Studies from Averett College in Danville, Virginia. A Pony Club and 4-H graduate, her background is in eventing, and she is schooling her recently retired Thoroughbred racehorse, Happy, toward a career in that discipline. She also enjoys traveling, photography, cycling, and cooking in her free time.

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