Transmission of Eastern Equine Encephalitis Virus

Key points

  • Eastern equine encephalitis is caused by a virus in the genus Alphavirus.
  • The virus is maintained in the environment by Culiseta melanura mosquitoes and avian hosts.
  • Humans and other animals are typically infected by bridge vectors and are considered dead-end hosts.
  • Rare transplant-associated transmission of eastern equine encephalitis virus has been documented.
Electron microscope image of eastern equine encephalitis virus

Virus

Eastern equine encephalitis virus is a member of the genus Alphavirus, family Togaviridae.

Eastern equine encephalitis virus has a single-stranded, positive-sense RNA genome. The virus particles are spherical and have a diameter of 60–65 nm.

Electron microscopic image of eastern equine encephalitis virus particles in mosquito salivary gland tissue.
Transmission electron microscopic image of numerous eastern equine encephalitis virus particles in mosquito salivary gland tissue.

There are four lineages of the eastern equine encephalitis virus antigenic complex.

  • Group I consists of only eastern equine encephalitis virus and is endemic in North America and the Caribbean and causes most human disease cases.
  • The other three groups (IIA, IIB, and III) are caused by related Madariaga virus (formerly known as South American eastern equine encephalitis virus) and cause primarily equine illness in Central and South America.

Transmission

Eastern equine encephalitis virus is maintained in a cycle between Culiseta melanura mosquitoes and avian hosts in freshwater hardwood swamps. Cs. melanura is not considered to be an important vector of eastern equine encephalitis virus to humans because it feeds almost exclusively on birds. Transmission to people requires another mosquito species to create a "bridge" between infected birds and uninfected mammals, such as people or horses. Most of the bridge species are within the Aedes, Coquillettidia, and Culex genera. Eastern equine encephalitis virus has been documented to be transmitted through solid organ transplantation, with one organ donor transmitting the infection to three organ transplant recipients.

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Image of eastern equine encephalitis virus transmission cycle. A Culiseta melanura mosquito and group of birds are shown on opposite sides of a circle with arrows connecting them to represent how eastern equine encephalitis virus cycles between mosquitoes and birds. Another arrow points from the birds to another mosquito and then to the figures of a horse and person. People and horses can become infected when a bridge vector, a mosquito species that feeds on many kinds of mammals and birds, then bites people or horses. People and horses are considered dead-end hosts because, unlike birds, they can't pass the virus on to other biting mosquitoes.
Eastern equine encephalitis virus transmission cycle.

Horses are susceptible to eastern equine encephalitis virus infection and many cases are fatal. Eastern equine encephalitis virus infections in horses, however, do not increase risk for human infections because horses (like humans) are considered to be “dead-end” hosts for the virus (i.e., the concentration of virus in their bloodstreams is usually insufficient to infect mosquitoes). There is a vaccine to prevent eastern equine encephalitis virus infection in horses.

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Content Source:
National Center for Emerging and Zoonotic Infectious Diseases (NCEZID)