A female wood duck with four ducklings together on a branch above a pond.

Getting Stung: Mosquitoes, EEE and Biosecurity

Samantha ShieldsAnimal Health, Livestock diseases Leave a Comment

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:

  • Fever
  • Depression
  • Behavior changes
  • Muscle twitches
  • Paralysis
  • Inability to swallow
  • Convulsions
  • 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). Retrieved from https://www.aphis.usda.gov/livestock-poultry-disease/equine/encephalitides

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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About the Author

Samantha Shields

Samantha Shields was an undergraduate student at the University of Vermont, studying for a Bachelor of Science in Biological Science, and a minor in Animal Science. She worked with ADBCAP Director Julie M. Smith, DVM, PhD, as an online outreach assistant intern. Samantha assisted with content development for the Healthy Farms Healthy Agriculture website by evaluating, summarizing, and presenting information about protecting animal health. At the University of Vermont, she was an active member of many programs including RALLYTHON, University of Vermont Program Board, Campus Recreation, and the Women’s Club Hockey. Samantha is interested in pursuing the field of epidemiology after graduation. She has a particular interest in the research of different factors that result in diseases, as well as public health emergency planning and response.

About the Editor

Joanna Cummings

Joanna Cummings received a Bachelor of Science in Horticulture from The Pennsylvania State University (PSU), with a specialization in vegetable crop and greenhouse production. At PSU, she was a research technician on no-tillage vegetable crop experiments, and a greenhouse assistant in the All-American Selections Research Gardens. Her career in the agriculture industry includes field research, work with dairy and vegetable farms, and as a greenhouse manager, estate gardener, landscaping and market garden entrepreneur. She transitioned to the science communication field after receiving a master of science degree in environmental communications from Antioch University New England. At Antioch, Joanna was a field botany laboratory teaching assistant, manager of the herbarium, and editor of the department's student newsletter, Notes and Niches. She currently works with Research Professor Julie M. Smith as a communications professional in the University of Vermont Animal and Veterinary Sciences Department. She is the webmaster for the Healthy Farms Healthy Agriculture website.

About the Editor

Dr. Julie Smith

Dr. Julie Smith is a research professor at the University of Vermont. Julie received her B.S. in biological sciences, D.V.M., and Ph.D. in animal nutrition at Cornell University. Since 2002, she has applied her veterinary background to programs in the areas of herd health, calf and heifer management, and agricultural emergency management. She has taught courses in animal welfare, calf biology and management, and ABCs of biosecurity to undergraduates, mostly in the Department of Animal and Veterinary Sciences. As a veterinarian and spouse of a dairy farmer, Julie is well aware of the animal health and well-being concerns of dairy animals. Julie has conducted trainings for Extension educators, livestock producers, and community members on the risks posed by a range of animal diseases, whether they already exist in the United States, exist outside of the United States, or pose a risk to both animal and human health. In all cases, she emphasizes the importance of awareness and prevention. In addition, she has led a number of projects on biosecurity and emergency disease preparedness.


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