Introduction

La Crosse virus (LACV) was originally discovered in the city of La Crosse in Wisconsin, USA, hence the name. It was discovered in the brain of a 4-year-old child who died in 1960.⁴ The mosquito most commonly transmitting the virus is known as Aedes triseriatus, also known as the eastern treehole mosquito as they live in the holes of trees. Usually, these trees are in deciduous woodland areas (i.e. trees that lose their leaves in autumn). Other species of mosquitoes known as Ae. albopictus and Ae. japonicus have also been identified to be carrying the virus.³ These mosquitoes are most active during the summer and early autumn months. The LACV is part of a group of viruses known as the ‘California serogroup’ which comprises 18 different mosquito-borne viruses which are genetically similar to one another.⁷

The majority of those infected with LACV have mild to no symptoms. Mild symptoms are mostly of fever, fatigue or malaise (feeling generally unwell), these usually come on within 5-15 days of infection. These can resemble flu-like symptoms and most people will make a full recovery. Those under 16 who are infected with LACV are more susceptible to severe illness. This can include having encephalitis (inflammation of the brain), meningitis (inflammation of the outer linings of the brain) or a combination of both - meningoencephalitis. Symptoms a patient would experience in these cases include headaches (that won’t go away), seizures, disorientation, sickness or acute onset paralysis (meaning paralysis that comes on suddenly).¹

LACV infections are reportable diseases in the USA meaning a local health authority or the CDC should be informed of a case. La Crosse encephalitis is very rare, with 1,281 cases reported between 2003 and 2019 (note the keyword being ‘reported’ - there will be other cases which are not reported. Those with mild symptoms are likely to have been managed at home or without seeking a diagnosis. The reported cases are more likely to be those who are in hospital. Studies estimate that LACV infections are largely unreported). According to a 2023 study, the annual average number of LACV cases is between 31 and 84.⁸

Cases reported were between the ages of 1 month to 95 years old, with the median age being 8 years old, showing that the age of severe illness is strongly skewed towards paediatric patients. Of the 1,281 cases, there were 15 deaths- just over 1% fatality rate. In those that survived, some children had chronic (long-term) symptoms of seizures and reduced mental function- including speech problems, memory problems and lower IQ scores. Chronic symptoms occurred in 6-15% of diagnosed cases.⁵

Though the virus was discovered in Wisconsin, it has been found in other states in the Appalachian region. Since the early 2000s, most cases have been from eastern Tennessee, Ohio, West Virginia, and western North Carolina.⁴ Cases have not been reported outside of the USA (and no cases in Hawaii or Alaska).⁵ However, climate change is a threat and can change the distribution of the cases. In Canada, although there have not been any reported cases of LACV infections, the Aedes triseriatus mosquito has been found in Ontario, New Brunswick and Quebec, all border states of the USA which have had cases.⁷

Transmission of the virus

La Crosse virus usually originates in woodland rodents such as eastern grey squirrels, and chipmunks. Groundhogs, cottontail rabbits and red foxes can also be hosts to the virus. Mosquitoes feed on the blood of these animals and so become infected with the virus themselves.⁸ These animals all live in and around trees, especially holes in trees where water can collect and remain for longer periods of time. The rodents can sometimes be called ‘reservoir hosts’ or ‘amplifying hosts’ of the virus as it can stay in them in large numbers. Mosquitoes are known as ‘vectors’- this is the term used for an organism that transmits the virus from one to another, such as from rodents to humans or rodents to rodents. When mosquitoes feed on the infected animals, the virus enters their blood from their gut. The virus then replicates in other tissues including muscle, fat and saliva. As the mosquito takes in blood when feeding, it leaves behind salivary secretions into the host’s skin and blood. This is how the virus can spread from mosquitoes to other organisms.^2,3

Female mosquitoes can pass on the virus to their offspring, known as ‘vertical transmission’, from one generation to the next. Infected female mosquitoes have a mating advantage over non-infected females too, and this further spreads the virus. Eggs of unborn mosquitoes can survive during the winter months until they are ready to hatch in summer, and so the virus can survive with the eggs too. 

Humans cannot pass on the virus to one another (i.e. being in physical contact with someone who is infected with LACV will not pass the virus on), as the virus does not replicate in enough numbers within humans to be transmitted. Humans are therefore known as ‘end hosts’ or ‘dead end hosts’ as the transmission stops with them.⁸

When the mosquito bites a human, the virus is transmitted via the bite, usually into a muscle. Viruses cannot replicate on their own; the virus integrates with a cell of the human and uses the human cell to replicate itself. Once it has replicated within a muscle, it can enter the cells in the bloodstream. After entering the blood, it can invade the nerves and the brain (called neuroinvasion), where it can cause inflammation of the brain. Diagnosis of a LACV infection is by a lab based blood test which detects antibodies, which are part of the body’s immune response to the virus. Diagnosis can be challenging as the virus does not replicate in large numbers in humans, and because symptoms can resemble those of other viral illnesses.¹

There is no treatment specific to LACV or La Crosse encephalitis - usually treatment is supportive, i.e focusing on symptom relief. This usually involves pain relief and anti-sickness medication. Those in hospital will receive more stringent monitoring as they would be much more unwell.¹

The rates and pattern of transmission relies on a number of factors, which makes it difficult to monitor and predict from year to year. These factors include temperature and humidity. Unlike most mosquitoes that prefer dusk and dawn for activity, Aedes triseriatus mosquitoes prefer daytime. Distribution of host organisms and availability of habitat for the mosquitos- especially nesting sites where they lay eggs also affects the disease transmission.⁷

Prevention of La Crosse virus

There is no vaccine against LACV but there are preventative measures that can be taken. As mentioned previously, LACV is usually only found in certain regions of the USA in summer and early autumn time, and so this is where the advice will be most relevant, and especially for children. Precautions include avoiding wooded areas- especially abandoned trees and open water bodies where water can be still for long periods of time.² 

If in these areas, covering up (wearing long sleeved shirts and long trousers) to minimise exposed skin, and using mosquito repellent on areas of exposed skin is also important. Examples of repellents include products containing DEET and lemon eucalyptus oil. Permethrin is a repellent that can be used on clothes. Using mosquito nets in bed, having air conditioning, and keeping windows closed at night can also help to reduce exposure risk.²

Other precautions include filling tree holes, removing abandoned trees and where possible, removing areas of stagnant water. Even containers like outdoor bins which can hold water can be sites of mosquito breeding, so these should be emptied and cleaned regularly. Tyre swings or other play equipment should also have drain holes to prevent water from gathering.

Healthcare providers in the affected areas should be aware of the symptoms of the virus and can test it. Other public health measures involve awareness campaigns to advise parents and children to take precautionary advice for mosquito-borne diseases.⁶

Summary

The La Crosse virus (LACV) is a virus that is transmitted to humans by mosquitoes in the Appalachian states of the USA (Ohio, West Virginia, Tennessee, North Carolina)- although there have been very rare cases outside of these states also. In most cases, LACV causes mild flu-like symptoms or no symptoms at all. La Crosse encephalitis (encephalitis is an inflammation of the brain) is a rare outcome of a LACV infection. Those under 16 are most susceptible to severe symptoms of LACV infections. Precautions like using mosquito repellents and avoiding wooded areas (especially areas of stagnant water in trees) where these mosquitoes live in summer months will reduce the risk of exposure.