Morning Rounds Series

Mosquito-Borne Illnesses


  Chapter 151.

Zoonotic encephalitis is most often an arboviral infection transmitted hematologically by an arthropod vector and animal host. Often the vector is a mosquito or tick and the animal host is a small animal or bird. The one exception is rabies (see Chap. 147), which follows peripheral nerve tracts after inoculation from an infected animal's bite. Additionally, encephalitis may be seen in the nonviral zoonotic infections of Bartonella henselae, Brucella canis, borreliosis, Coxiella burnetii, Ehrlichia spp., listeriosis, leptospirosis, Lyme disease, RMSF, psittacosis, and toxoplasmosis.9,29,30

The clinical presentation is a prodromal illness with malaise, myalgia, fever, and occasionally parotitis. This prodromal phase advances to a sudden decline in mental status associated with headache and fever. However, there are no specific signs and symptoms that determine the exact etiology and causative organism of encephalitis. The CSF is often abnormal, showing a slightly elevated opening pressure, normal to slightly elevated protein concentration, normal glucose levels, and predominance of lymphocytes. The electroencephalogram is abnormal with diffuse bilateral slowing interrupted by occasional spike activity. CSF viral cultures are frequently sterile, and the infectious agent is rarely isolated from the CSF.29–32 ELISA of serum can be used to detect most arboviral infections causing encephalitis. Computed axial tomography is often negative with no evidence of focal findings or abnormalities. Magnetic resonance imaging often does reveal evidence of encephalitis by the imaging of cerebral swelling. If the CT scan were negative, following up with an MRI scan would be the prudent course of investigation. Treatment is supportive and directed toward decreasing elevated intracranial pressure.

West Nile Virus Encephalitis

The West Nile virus was first isolated in the West Nile District of Uganda in 1937, and since then was commonly found in vertebrates in Africa, Eastern Europe, West Asia, and the Middle East.33 It is a member of the Japanese encephalitis serocomplex of the Flaviviridae family of viruses.34 In 1999 it was detected in the western hemisphere.

The mode of transmission involves mosquitoes, primarily of the Culex species, that feed on infected birds, which serve as the natural reservoir for the virus. Over 110 species of birds have been identified as being infected by the West Nile virus in the United States. Most of these birds are crows, ravens, and jays.34

Infected mosquitoes transmit the virus to humans and other animals when taking a blood meal.

There is an incubation period of 3 to 15 days after an infected bite. About 20 percent of those bitten develop fever and a flu-like illness, "West Nile fever." The classic presentation for West Nile fever is that of a mild dengue-like illness of sudden onset with fever, lymphadenopathy, headache, abdominal pain, vomiting, rash, conjunctivitis, eye pain, and anorexia. Duration is typically 3 to 6 days.34

Meningoencephalitis occurs in approximately 1 in 150 patients that become infected and is more common than meningitis. Risk factors for the development of meningoencephalitis are immunocompromised host (HIV, TB, or malaria), or advanced age. Complaints of weakness out of proportion to physical examination findings are common and myoclonus is nearly a universal finding. Interestingly, complete flaccid paralysis may occur and can be confused with Guillain-Barré syndrome. Other neurologic findings include parkinsonian-like signs, ataxia, extrapyramidal signs, cranial nerve abnormalities, myelitis, optic neuritis, and seizures. Treatment for West Nile virus is supportive.34

Laboratory findings for West Nile virus infection are limited to a total leukocyte count that is either normal or slightly elevated. Hyponatremia has been occasionally associated with patients with encephalitis. Cerebrospinal fluid demonstrates mostly lymphocytes, protein elevation, and a normal glucose. Computed tomography of the brain shows no evidence of acute disease, though with progressive meningoencephalitis cerebral edema may be noted.

Diagnosis is made by identifying West Nile virus (WNV)-specific IgM antibody in serum or CSF specimens. IgM-specific antibody is an acute-phase identifier that can persist in serum for up to 12 months postinfection. IgG WNV-specific antibody is found in the convalescent phase in both serum and CSF. As of this writing, PCR testing is still in an experimental stage.34

Methods to reduce the exposure risk for West Nile virus include use of mosquito repellents, clothing with long sleeves and long pants, and avoiding the outdoors during dawn and dusk. The insect repellent recommended is DEET (N,N-diethyl-m-toluamide and related compounds). Personal insect repellents should not be used on infants under 6 months of age. Use of mesh covers on strollers is recommended for this age group, along with protective clothing.34

Zoonotic Meningitis

Zoonotic meningitis can be caused by brucellosis, listeriosis, plague, salmonellosis, tularemia, leptospirosis, Lyme disease, ehrlichiosis, Q fever, RMSF, and psittacosis. CSF is almost always abnormal, showing a slightly elevated opening pressure, normal to slightly elevated protein concentration, normal glucose levels, and predominance of lymphocytes. Treatment is directed toward the specific organism cultured from the CSF. However, empiric antibiotic coverage should be administered immediately in any presumptive case of meningitis in an effort to reduce mortality and morbidity.31 See Chap. 235 for specific empiric recommendations.


Arboviral Encephalitides

Arboviral encephalitides are mosquito-transmitted viral diseases capable of causing encephalitis. Varieties found in the United States include eastern equine encephalitis (EEE), western equine encephalitis (WEE), St. Louis encephalitis (SLE) and La Crosse (LAC) encephalitis, and West Nile Virus encephalitis (WNV). Eastern equine encephalitis is particularly deadly, with a 30-35% fatality rate, according to the U.S. Centers for Disease Control and Prevention. However, cases are an average of four a year in the U.S., according to the CDC.

Most viral encephalitis infections either don't produce any symptoms at all or only lead to general flu-like symptoms such as fever, headache, muscle ache and malaise. Symptoms of severe encephalitis include high fever, stiff neck or back, sensitivity to light, vomiting and confusion.

Worldwide Arboviral Encephalitides

West Nile Virus

West Nile virus (WNV) is a potentially serious illness. Experts believe WNV is now established as a seasonal epidemic in North America that flares up in the summer and continues into the fall. The most serious manifestation of WNV infection is fatal encephalitis.

According to the CDC, only about one in 150 people infected with WNV will develop severe illness. Most people infected with the virus will not develop any symptoms at all. The risk of severe illness and death is highest for people over 50 years old.

The first appearance of WNV in North America was in 1999, according to the CDC, with encephalitis reported in humans and horses. From 1999 through 2001, there were 149 cases of West Nile virus human illness in the United States reported to CDC and confirmed, including 18 deaths.


Public Service Announcement:


Introduction (3.85 MB Real Audio .rm file)
How the Virus works (3.086 MB Real Audio .rm file)
Human Disease (6.072 MB Real Audio .rm file)
Prevention (9.714 MB Real Audio .rm file)
Animal Infection (10.440 MB Real Audio .rm file)
Conclusion (2.514 MB Real Audio .rm file)


West Nile Virus Factsheet


Malaria is a major international public health problem, causing 300–500 million infections worldwide and approximately one million deaths annually, according to the CDC. Confined primarily to large areas of Central and South America, Hispaniola, Africa, Asia, Eastern Europe, and the South Pacific, malaria is a disease spread by the bite of an infected female Anopheles mosquito.


This disease is characterized by a broad range of symptoms, from fever and body ache to serious infection leading to seizures, coma, and death. Though contracting malaria can have serious consequences, illness and death are largely preventable. Vaccinations against malaria currently do not exist, but taking an appropriate drug regimen and protection against mosquito bites can help prevent malaria. Regardless of precautions, however, travelers are still at some risk of contracting the disease.

Dengue Fever

Dengue fever is primarily a disease of the tropics, and the viruses that cause it are maintained in a cycle that involves humans and Aedes aegypti, a domestic, day-biting mosquito that prefers to feed on humans. Infection with dengue viruses produces a spectrum of illness ranging from vague sickness to fatal blood disease. No dengue vaccine is available.


Map: Distribution of Aedes aegypti in the Americas in 1970, at the end of the mosquito eradication program, and in 2002

Map: American countries with laboratory-confirmed hemorrhagic fever, prior to 1981 and from 1981 to 2003

Map: World distribution of dengue viruses and their mosquito vector, Aedes aegypti, in 2005

Dengue Surveillance Report


Yellow Fever

Only occurring in South America and Africa, yellow fever is a viral disease transmitted between humans by a mosquito. Yellow fever is a very rare cause of illness in travelers, but most countries have regulations and requirements for yellow fever vaccination that must be met prior to entering the country. Travelers should also follow general precautions to avoid mosquito bites.

U.S. Centers for Disease Control and Prevention, U.S. Department of Agriculture