What Diseases Do Mosquitoes Carry?

Jorge P Parada, MD, MPH

People rarely enjoy the annoying buzz of mosquitoes at night. Worse yet is when one of those flying bloodsuckers bites you, leaving an itchy red welt behind. Of course, the inconvenience of the bite pales in comparison to the health risks these pests pose.

In fact, according to the U.S. Centers for Disease Control and Prevention, mosquitoes are the “world's deadliest animal, responsible for more than 700,000 deaths worldwide each year." In the United States, it’s rare to encounter the more deadly diseases carried by mosquitoes like malaria, dengue, Yellow Fever, or other common tropical infections like zika, or chikungunya. Still, mosquitoes pose a significant health threat to those living in the U.S. 

Mosquito-borne Diseases

Nearly all mosquito-borne diseases of importance in the United States are due to arbovirus infections. So, let us start by reviewing arboviral encephalitides. These are mosquito-transmitted viral diseases causing brain inflammation/encephalitis which affects the central nervous system. In the U.S., the best known and most common of which is West Nile virus (WNV) infection – the most common deadly disease transmitted by mosquitos in the country.

Fortunately, in most cases, WNV is a mild and self-limited infection. Symptoms may be so light as to go unnoticed, or present as a “summer flu,” with mild body and headaches and low-grade fever. In rare and extreme cases, WNV is a potentially life-threatening infection, with symptoms including higher fever, head and body aches, confusion and worsening weakness/progressive ascending paralysis. Such symptoms should prompt you to seek medical attention.

Other arbovirus infections , caused by mosquito-borne diseases, also range from unapparent to mild, nonspecific illnesses (fever, headache, musculoskeletal pain, and malaise), to occasionally severe illness of the central nervous system, also called neuroinvasive infection. This form of infection presents with headache, mental status changes, and confusion, and may progress to coma.

Unfortunately, neuroinvasive disease/encephalitis may result in permanent neurologic damage and possibly death. Treatment is symptomatic and supportive, and severe cases require medical attention.

The other four major types of encephalitis in the U.S. include St. Louis encephalitis (SLE), California encephalitis (CE primarily includes the LaCrosse virus [LAC]), eastern equine encephalitis (EEE), and western equine encephalitis (WEE). SLE is found throughout the United States, WEE occurs west of the Mississippi River, and EEE occurs east of the Mississippi River but mostly along the Atlantic and Gulf coasts as well as the north-central states. Finally, CE occurs in California and the eastern United States. Human cases of arbovirus infection have a seasonal occurrence from mid- to late summer.

Preventing Mosquito Bites

Mosquitoes bite most frequently around dawn and dusk. If you must or want to be outside during those times, it’s best to be inside a screened-in porch or dressed in clothing that leaves less skin exposed as a target for mosquitoes. Your best protection will be insect repellant containing at least 20% DEET, picaridin or oil of lemon-eucalyptus. This will lower the likelihood of mosquito bites and as a result, the transmission of diseases carried by mosquitoes.

If you plan to stay indoors, you will need to ensure that the mosquitoes stay outside. Remember to tour your house and check the status of the window screens and the seal of your doors. It's recommended to screen all windows and doors and patch up even the smallest tears to reduce your risk of exposure to mosquito-borne diseases.

A mosquito bite typically results in a pink, itchy bump. As tempting as it may be, avoid scratching it as this only agitates the venom and increases the itching and swelling. In addition, over-scratching might cause breaks in the skin that can serve as a port of entry for bacterial infections. Although less common, some people can be more sensitive to mosquito bites and have more severe reactions, such as welts or hives. All bites should be washed with soap and cold water. Benadryl and over the counter 1% hydrocortisone cream may be indicated for intense itching and the larger reactions. If there are signs and symptoms of infection, you may need to see your doctor for antibiotics.