Mosquitoes are stealthy pests, but red itchy bumps are not the only thing they leave behind. Mosquitoes can transmit viruses like the one that causes eastern equine encephalitis (EEE or Triple E). Although outbreaks of EEE are not widespread across the United States, the disease can be devastating.
An Uncommon but Serious Disease
Eastern equine encephalitis is an uncommon but serious and potentially fatal infection. In areas affected by the virus, equines and emus are susceptible, as are llamas, bats, reptiles, pigs, rodents and amphibians. The virus is also dangerous for humans. There are only a few cases reported in humans each year, but in humans with the encephalitic form (causing swelling of the brain), the mortality rate is 30 percent (CDC, 2019). Many of those who survive an EEE infection can develop mild to severe brain damage. Annual testing and case summary reports are published by USDA APHIS. The CDC tracks human incidence.
A Bridge to Infection
EEE does not spread by person-to-person contact, people to animals or animals to people. It is spread by the sting of certain species of mosquitoes. The Culiseta melanura mosquito, which feeds almost exclusively on birds, passes the virus among birds living in freshwater swamps. Different species of mosquitoes that feed on both birds and other animals act as the bridge transmitting the EEE virus from infected birds to uninfected animals.
After infection, the virus can replicate and travel through the bloodstream. In rare cases, the virus will spread to the brain, resulting in inflammation. Infection from the EEE virus can appear asymptomatic in equine species, but most of the time clinical signs are present (USDA APHIS, January 2008).
Signs of EEE
Clinical signs in both humans and equine species can include:
- Behavior changes
- Muscle twitches
- Inability to swallow
- Head pressing in equines
There are no vaccines or other treatments to cure EEE in humans. A vaccine is available for horses, but it needs to be repeated annually.
Biosecurity to Control Insect Transmitted Diseases
Biosecurity measures should be implemented to reduce the chance of an infection in equine species. The main biosecurity measures include vaccinating horses and reducing the mosquito population (CDC, 2019). Methods to reduce a mosquito population include:
- Using insect repellent.
- Using screens on windows and doors with no holes.
- Drilling holes in the bottom of water holding containers.
- Cleaning gutters regularly.
- Turning over wheelbarrows when not in use.
- Changing water in troughs and buckets at least once a week.
Mosquitoes can carry many other diseases beside EEE, including West Nile virus and Zika virus. During 2018, mosquito-borne diseases affected thousands of individuals (Elflein, 2019). Biosecurity measures that reduce mosquito populations help to decrease mosquito-borne diseases as well. These measures are also effective against other disease carrying insects such as flies and midges (Malone 2019).
Centers for Disease Control and Prevention (CDC). November 2019. Eastern Equine Encephalitis. Retrieved from https://www.cdc.gov/easternequineencephalitis/index.html
United States Department of Agriculture, Animal and Plant Health Inspection Service (USDA, APHIS). January 2008. Eastern Equine Encephalomyelitis. [PDF File]. Retrieved from https://www.aphis.usda.gov/publications/animal_health/content/printable_version/fs_eastern_equine_enceph.pdf
Elflein J. August 2019. Mosquito-borne diseases in the U.S. – Statistics and Facts. Retrieved from https://www.statista.com/topics/4264/mosquito-borne-diseases-in-the-us/
Malone M. 2019. Disease Carrying Insects List. Retrieved from https://animals.mom.me/disease-carrying-insects-list-4628.html
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