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La Crosse Encephalitis

Last updated Aug. 13, 2018

Approved by: Maulik P. Purohit MD, MPH

La Crosse (LAC) Encephalitis is a mosquito-borne virus that was first described in La Crosse, Wisconsin in 1963. Since then, it has been reported in several Midwestern and Mid-Atlantic states.


What are the other Names for this Condition? (Also known as/Synonyms)

  • Californian Encephalitis
  • LAC Encephalitis

What is La Crosse Encephalitis? (Definition/Background Information)

  • La Crosse (LAC) Encephalitis is a mosquito-borne virus that was first described in La Crosse, Wisconsin in 1963. Since then, it has been reported in several Midwestern and Mid-Atlantic states
  • The LAC virus (LACV) is one of many mosquito-transmitted viruses that can cause an inflammation of the brain (encephalitis). About 80-100 cases of this condition are reported each year in the United States. Most cases occur in children younger than age 16
  • While most people who become infected have no symptoms, those who do become ill may have fever, headache, vomiting and lethargy (tiredness). Severe cases develop encephalitis accompanied by seizures. Coma and paralysis occur in some cases
  • There is no specific treatment for LAC Encephalitis. Supportive therapy is provided to those who develop severe cases of the disease

(Source: La Crosse Encephalitis; Genetic and Rare Diseases Information Center (GARD) of National Center for Advancing Translational Sciences (NCATS), USA.)

Who gets La Crosse Encephalitis? (Age and Sex Distribution)

  • La Crosse Encephalitis is a rare mosquito-borne infection, with 80-100 cases reported annually in the United States
  • The presentation of symptoms may begin between 5-15 days after an infected mosquito bites an individual
  • LACV infection can occur in individuals of all ages, although most reported cases are in children under 16 years of age
  • Both males and females may be affected
  • The infection is primarily reported from upper Midwestern and Mid-Atlantic, and Southeastern regions of the United States of America

What are the Risk Factors for La Crosse Encephalitis? (Predisposing Factors)

The risk factors for La Crosse Encephalitis may include:

  • Living in or visiting upper Midwestern and Mid-Atlantic, and Southeastern regions of the United States of America or woodland habits in the summer months (when infected mosquito population is likely to be higher)
  • Children under the age of 16 years
  • Weak immunity, which may occur in 
  • Very young infants 
  • Elderly individuals
  • Those who have an immune disorder (such as HIV infection/AIDS)
  • Those who have undergone organ transplantation and are under immune-suppressing medication

It is important to note that having a risk factor does not mean that one will get the condition. A risk factor increases one’s chances of getting a condition compared to an individual without the risk factors. Some risk factors are more important than others.

Also, not having a risk factor does not mean that an individual will not get the condition. It is always important to discuss the effect of risk factors with your healthcare provider.

What are the Causes of La Crosse Encephalitis? (Etiology)

  • La Crosse Encephalitis is caused by the bite(s) of a mosquito infected with the La Crosse Encephalitisy virus (LACV)
  • In most cases, Aedes triseriatus or the ‘treehole mosquito’ is the vector that transmits the virus to humans. The treehole mosquito is an aggressive mosquito that bites primarily during the day
  • Once inside the human bloodstream, the virus multiplies and causes the characteristic signs and symptoms
  • La Crosse Encephalitis is not contagious, and does not spread from one human to another

What are the Signs and Symptoms of La Crosse Encephalitis?

Typically, the signs and symptoms of La Crosse Encephalitis develop from 5-15 days after an infected mosquito bite, and may vary among affected individuals in type and severity. Whereas some of those infected may be asymptomatic, others may develop a range of signs and symptoms, such as the following: 

  • Fever
  • Headache
  • Vomiting 
  • Tiredness

How is La Crosse Encephalitis Diagnosed?

La Crosse Encephalitis is diagnosed on the basis of the following information:

  • Complete physical examination
  • Thorough medical history evaluation
  • Assessment of signs and symptoms
  • Laboratory tests: Immunological testing of blood and cerebrospinal fluid to check for antibodies against LACV
  • Imaging studies
  • Biopsy studies, if necessary

Many clinical conditions may have similar signs and symptoms. Your healthcare provider may perform additional tests to rule out other clinical conditions to arrive at a definitive diagnosis.

What are the possible Complications of La Crosse Encephalitis?

The complications of La Crosse Encephalitis may include:

  • Encephalitis
  • Seizures
  • Coma 
  • Paralysis

Complications may occur with or without treatment, and in some cases, due to treatment also.

How is La Crosse Encephalitis Treated?

  • There is no specific treatment for La Crosse Encephalitis, since it is a viral infection
  • If serious signs and symptoms are present, hospitalization and supportive therapeutic methods may be necessary, including: 
  • Respiratory support to ease breathing
  • IV fluids to replenish fluids
  • Prevention of secondary infections

How can La Crosse Encephalitis be Prevented?

La Crosse Encephalitis is a preventable infection, which may be achieved by:

  • Use of mosquito repellent creams or permethrin on clothes - sprays containing permethrin can be used on clothes; sprays containing DEET can be used on the skin
  • Sleeping under mosquito nets, or using screens for doors and windows to keep mosquitos out
  • Wearing full-length pants, long-sleeved shirts, etc.
  • Maintenance of good sanitary conditions, to prevent mosquitoes from breeding. When possible, eliminate all potential mosquito habitats that contain standing water, where mosquitoes breed and lay their eggs, like water contained in temporary pools, old tires, discarded vessels, birdbaths, pots, rainwater puddles, etc.
  • Since the Aedes triseriatus mosquitos commonly use tree holes for breeding, plugging the tree holes in the vicinity of one’s home with soil may help minimize the mosquito population
  • Educating oneself when traveling from a non-endemic region to an endemic region, and be aware of the risk factors and basic preventive methods

What is the Prognosis of La Crosse Encephalitis? (Outcomes/Resolutions)

  • The prognosis of La Crosse Encephalitis is dependent upon the severity of the signs and symptoms and associated complications, if any
  • Individuals with mild conditions have better prognosis than those with severe symptoms and complications
  • In severe cases, the recovery may take weeks or months. Permanent brain damage may occur in some affected individuals, leading to recurrent seizures years after infection
  • The permanent brain damage in school-age children may manifest as low IQ and poor school perfomance as well
  • Rarely, the infection may be fatal

Additional and Relevant Useful Information for La Crosse Encephalitis:

Although humans get infected with the virus through an infected mosquito bite, they do not develop high concentrations of the virus particles in their bodies, and therefore, are unable to infect mosquitoes who feed on their blood. Therefore, humans are known as “dead-end” hosts for LACV.

What are some Useful Resources for Additional Information?


References and Information Sources used for the Article:


Helpful Peer-Reviewed Medical Articles:


Reviewed and Approved by a member of the DoveMed Editorial Board
First uploaded: Aug. 13, 2018
Last updated: Aug. 13, 2018