Are mosquito-borne viruses becoming more common and more severe? A clinical virologist explains the risk

In recent weeks, several deaths from Eastern Equine Encephalomyelitis have been reported in the Northeast. Dr. Anthony Fauci was hospitalized in late August after contracting West Nile virus and is now recovering. And this week, it was reported that a New Hampshire resident was hospitalized after testing positive for EEE, West Nile virus, and St. Louis encephalitis virus. This has many wondering, “Are mosquito-borne viruses increasing and becoming more severe?”

Which viruses are transmitted by mosquitoes in the USA and what are the symptoms?

Although many consider mosquitoes to be nothing more than a nuisance, they are actually the deadliest animals in the world. Mosquitoes can transmit a number of fatal infections, including malaria, dengue fever, yellow fever, chikungunya, Zika, West Nile fever, and EEE. It is estimated that at least 1 million people worldwide die each year from diseases caused by a mosquito bite.

Fortunately, not all types of mosquito-borne diseases are endemic in the United States, but that could change over time. Climate change, human activities (e.g., development of natural habitats), and changes in the geographic distribution of animals have led to an increase in mosquito- and tick-borne infections over the past decade. Currently, West Nile virus is the most common mosquito-borne infection in the United States, although cases of EEE, dengue fever, St. Louis encephalitis virus, Jamestown Canyon virus, and malaria have also been attributed to local transmission. Typically, states in the Northeast and upper Midwest experience higher rates of mosquito-borne infections, but malaria and dengue fever have been detected in Texas and Florida, and West Nile virus can occur nationwide.

The vast majority of people infected with a mosquito-borne virus do not develop symptoms. However, when symptoms do occur, they are often flu-like and nonspecific and may include fever, headache, chills, rash, and joint pain. In dengue fever, the limb and joint pain can be so severe that the illness is commonly referred to as breakbone fever. In rare cases of West Nile virus, EEE, and St. Louis encephalitis, the disease can progress to neurological disorders such as encephalitis and meningitis, or be fatal.

Are there tests for these viruses?

While testing for mosquito-borne viruses does exist, it is mainly limited to large reference laboratories and public health facilities. Unlike testing for COVID-19 and influenza, molecular testing (e.g., PCR) is not a good way to diagnose mosquito-borne viruses. This is because viruses such as West Nile virus and EEE are present in the bloodstream for a relatively short time, and PCR tests can be negative until a doctor is seen.

For this reason, serological (i.e. antibody-based) tests are the most common means of diagnosing infections caused by mosquito-borne viruses. In the first one to two weeks after an infection, a type of antibody called IgM is produced, which is an early immune response to a virus. IgM antibodies typically stay in a person's bloodstream for three to six months. Serological tests that detect IgM can indicate a recent infection. However, these tests are often not specific enough, meaning false positive and cross-reactive results can occur.

For example, a person infected with West Nile virus may also test positive for St. Louis encephalitis or dengue on IgM tests. This is because West Nile virus, St. Louis encephalitis and dengue fever all belong to the same family of viruses (i.e., flaviviruses), and therefore IgM antibodies produced in response to West Nile virus can cross-react with tests designed to diagnose other members of the flavivirus family.

Other serological tests are designed to detect IgG antibodies, which take longer to produce in the body but provide longer-term immunity. Tests that look for IgG antibodies are more specific (i.e., they do not show the level of cross-reactivity seen in IgM tests), but it may take two to three weeks after infection for IgG antibodies to reach detectable levels. This limits the usefulness of IgG-based serological tests in diagnosing acute mosquito-borne viral infection.

What steps can you take to prevent mosquito-borne infections?

Unfortunately, vaccines to prevent mosquito-borne infections are not widely available. Currently, vaccines are available for chikungunya virus, yellow fever, Japanese encephalitis virus, and dengue fever. To prevent other mosquito-borne viral infections (e.g., West Nile virus, EEE, St. Louis encephalitis), the best measure is to reduce mosquito breeding sites and the likelihood of a bite.

Mosquitoes lay their eggs in standing water, so pour the water into old tires, flower pots and birdbaths. Avoid the times of day when mosquitoes are most active, including before sunrise and after dark. And if you must be outside when mosquitoes are present, wear a long-sleeved shirt and pants and apply insect repellent. These measures can help protect yourself and others from mosquito-borne diseases, which will continue to pose a risk until cooler temperatures arrive.