Mosquito-borne Viral Disease in the U.S.

Most people are aware of the spread of the mosquito-borne viral disease, Zika, in many countries in South and Central America.  Worldwide, the most important disease transmitted by mosquitos is malaria.  Instead of being caused by a virus, however, malaria is caused by a parasite that infects the Anopheles mosquito. The World Health Organization (WHO) reports that 214 million cases of malaria occurred in 2015 with 438,000 deaths attributed to the disease.  The disease is passed on from a malaria-infected person to the next person via the mosquito bite.
 
The most common mosquito-borne viral disease globally is Dengue fever.  In 2015, nearly 2.4 million cases of this viral illness were reported by the WHO.  Both of these infections occur more commonly in tropical regions of the world. In the U.S., we are fortunate that local exposure to malaria has essentially been eliminated, and with the exception of a few cases affecting U.S. citizens living in topical settings such as Hawaii and Puerto Rico, nearly all dengue cases reported in the U.S. are acquired elsewhere by travelers or immigrants.

As of July 13^th of this year, the U.S. Centers for Disease Control (CDC) reports that there have been no locally acquired cases of Zika virus disease in the U.S. There have been, however, 1,305 travel-associated cases of this disease reported by the CDC in the U.S. since January 1, 2015. Infection with the Zika virus is usually quite mild.  A serious concern, however, is the link between a Zika virus infection in a pregnant woman and the development of microcephaly in newborns, a congenital defect of cranium and brain size resulting in profound neurological defects.

The most prevalent mosquito-borne diseases that develop from mosquito bites occurring in the U.S. include West Nile virus disease, Eastern Equine Encephalitis, Japanese encephalitis, and St. Louis encephalitis.

West Nile Virus Disease (WNVD) is primarily spread by the bite of a mosquito that has fed on an infected bird.  Over 300 species of birds have been found to be infected with the West Nile virus including common songbirds, crows, blackbirds, blue jays, doves, and pigeons.  Once a bird becomes infected, a mosquito can then transfer the virus from the bird's blood stream to humans, setting the stage for the infection.  In a very small number of cases, WNV also has been spread through blood transfusions, organ transplants, breastfeeding and even during pregnancy from mother to baby.  After reaching a peak at 9862 cases in 2003, the CDC reports that the number of cases appears to be decreasing.   Most people infected with West Nile virus will not have any symptoms. About 1 in 5 people who are infected will develop a fever and other symptoms. Less than 1% of those infected develop a serious, sometimes fatal, neurologic illness.

Eastern equine encephalitis (EEE) Most persons infected with the eastern equine encephalitis virus have no apparent illness with an average of 8 people per year developing a severe form involving inflammation of the brain (encephalitis).  Most cases have occurred in the Atlantic and Gulf Coast states.  Symptoms of EEE begin with the sudden onset of headache, high fever, chills, and vomiting. The illness may then progress into disorientation, seizures, or coma. EEE is one of the most severe mosquito-transmitted diseases in the United States with an approximately 33% mortality rate.

La Crosse encephalitis (LACV) causes inflammation of the brain (encephaltitis) in approximately 80 to 100 people in the U.S. each year. Most LACV infections, however, are much less severe.  Most cases of this disease have been reported from upper Midwestern, mid-Atlantic and southeastern states. Like the other illnesses being discussed, the risk for developing LACV is highest for people who live, work or recreate in woodland habitats, because of greater exposure to potentially infected mosquitoes.

St. Louis encephalitis virus infection primarily affects individuals living in eastern and central U.S. As with the other viral infections discussed, most people infected with this virus have no apparent illness. Severe neurological disease (often involving encephalitis, an inflammation of the brain) occurs more commonly in older adults. In rare cases, long-term disability or death can result. There is no specific treatment for SLEV infection; care is based on symptoms.

From the descriptions of these mosquito-borne viral illnesses affecting U.S. residents, several similarities become apparent: 

• Most people have no symptoms or are only mildly symptomatic.
• When symptoms do occur they are often similar in nature with headache, fever, chills, nausea, vomiting predominating.  
• A very small percentage of those infected will develop a more severe form of the disease, usually resulting in a brain infection (encephalitis) which is sometimes fatal.
• There is no specific treatment available for these infections.  Severe illnesses are treated by supportive therapy which may include hospitalization, respiratory support, IV fluids, and prevention of other infections.
• With the exception of a vaccine against the Japanese encephalitis virus (not typically given in the U.S.), immunizations are not available for these viral diseases.  
• Prevention measures for these illnesses center around avoiding mosquito bites
  • When outdoors, use an insect repellent that contains an EPA-approved active ingredient such as DEET, Picaridin, IR3535, and Oil of Lemon Eucalyptus.
  • Since mosquitoes are most active at dusk and dawn, along with using a repellent, consider wearing long sleeves and pants during these times.
  • Be sure that your window and door screens are intact.
  • Remove sources of standing water around the home that serve as mosquito breeding sites.

Sources for article:
West Nile Virus from the Centers for Disease Control and Prevention
Eastern Equine Encephalitis from the Centers for Disease Control and Prevention
Saint Louis Encephalitis from the Centers for Disease Control and Prevention
La Crosse Encephalitis from the Centers for Disease Control and Prevention