‘Tis the season…

As summer progresses, mosquito-borne infections become more common. While West Nile virus typically peaks later in the summer or early fall, Eastern equine encephalitis (EEE), a much more deadly viral neurological disease, tends to start cropping up earlier in the season.

Recently, EEE was diagnosed in two horses in Horry and Marion Counties in South Carolina, where the virus is mainly transmitted by a type of swamp mosquito, commonly known as the blacktailed mosquito. The outcome isn’t stated in the report, but most likely the horses died, since mortality rates for EEE are extremely high.

Finding EEE in a region indicates that the virus is circulating in the bird and mosquito populations. Knowing this is important for a couple of reasons:

  • It means that horses might be exposed to the virus by mosquito bites. Therefore, it’s a good idea to reduce mosquito exposure by a variety of methods. Vaccination also needs to be considered, but given the fact that vaccines don’t protect horses immediately, waiting until the first cases of the year are diagnosed may be too late. Ideally vaccination should be performed a month or so before the time when exposure is likely.
  • It also means that humans may be exposed to the virus in the same way – by mosquito bites. EEE in humans is rare but devastating. If EEE is in an area, people need to take proper mosquito bite prevention measures. There’s currently no vaccine against this virus for people. People cannot get EEE directly from horses or birds.

Horse owners need to be aware of infectious disease risks for their area (and anywhere they may take their horses), and they need to talk to their veterinarian about the risks and risk mitigation. Not every horse in North America requires EEE vaccination, but in some regions it’s very important and should be a core vaccine.