In April 2000, a Ventura County, Calif., yearling was found down, unresponsive, with abnormal eye movements and flailing leg motions. He was taken to a veterinary clinic but euthanized 24 hours later when he remained comatose.
The horse had been castrated 90 days earlier with no complications and hadn’t been ill otherwise. He was immunized seven days earlier with a combination WEE/EEE/flu and tetanus vaccine.
Tests for western equine encephalitis (WEE), Venezuelan equine encephalitis (VEE), rabies, herpes virus and West Nile virus were negative. Postmortem studies confirmed eastern equine encephalitis virus. Blood confirmed high antibody titers against EEE. No other animals on the premises were ill.
This was the first case of EEE infection/virus activity west of the Rocky Mountains, and it set off an intensive investigation by the CDC (Centers for Disease Control). A check of all the animals that competed at the California and Utah shows the yearling attended revealed no EEE.
California’s excellent virus surveillance systems showed no evidence of EEE virus activity. Intensive mosquito monitoring and trapping around the farm were negative. Comparison of antibody levels in the dead gelding and horses from the farm that had been vaccinated at the same time showed only the gelding was positive for IgM by ELISA testing (indicating active infection). Even the possibility of “criminal mischief”/bioterrorism was investigated.
The remaining possible source was the vaccination. Three vials of vaccine remaining on the farm, as well as others from the same lot that the manufacturer had in mandatory storage, were tested for live virus. Nothing. However, this didn’t exclude the possibility that other vials from that batch contained live virus, a situation that has occurred with human vaccines containing viruses that had been inactivated with the same formalin inactivation procedures.
The next step was nuclear sequencing of the RNA from the vaccine, from the virus obtained from the yearling and several other virus strains of EEE that had been collected from natural infections. On partial sequencing, there was a 100% match between the virus from the horse and the virus in the vaccine, compared to a 99% match with the other strains. On more extensive sequencing, there was a 99.5% match between the horse and the vaccine, 98.3 to 99.4% match with the other strains.
Viruses of this type are notorious for producing small mutations in their genetic makeup when grown in the lab in tissue cultures or when they are multiplying inside the host. Because the genetic matching between horse and vaccine fell just short of 100%, researchers couldn’t unequivocally state the infection came from the vaccine, but that remains the most likely explanation.
The fact this was a young animal likely being vaccinated/exposed to EEE for the first time would make him more susceptible than an adult being revaccinated.
In addition, even if the vaccine was related to the EEE case, it doesn’t mean errors were made in the production of the vaccine. It also doesn’t mean you shouldn’t vaccinate. Incidents like this are rare. However, if a horse with little likelihood of exposure shows signs of disease shortly after vaccination, a link with the vaccine should be investigated.