The arrival of monsoon season also provides mosquito breeding opportunities when rainwater gathers in drainage areas, clogged storm drains, unattended pools, and anything else that can hold even a small amount of water. Mosquito-borne viruses are found every year in Arizona, with the West Nile virus being the most common. Health officials throughout Arizona have been monitoring mosquito-borne virus activity for decades in an effort to detect, monitor, and control mosquito-borne diseases.
Mosquito surveillance involves trapping mosquitoes, counting them, identifying the species involved, and testing the appropriate species for viruses. These surveillance methods are used to better identify areas where mosquito control efforts are needed. Detection and control of mosquito breeding sites depend upon integrated efforts among state, county, and tribal agencies as well as private citizens.
Every month from March through October, over 300 samples of mosquitoes are collected by health officials throughout Arizona and tested at the Arizona State Public Health Laboratory, with many more local vector control departments conducting their own testing. Dead birds and horses are tested for mosquito-borne viruses at the University of Arizona’s Veterinary Diagnostic Laboratory. Reporting of mosquito-borne diseases in humans and horses by physicians and veterinarians is critical for detecting and responding to mosquito-borne viruses statewide. This surveillance data allows public health officials to identify areas of higher risk so appropriate measures can be taken to prevent future occurrences.
Starting in 2013, chikungunya virus expanded into the western hemisphere. Meanwhile, dengue and Zika viruses have had increased incidences and geographic expansions. In light of these developments, there is an ongoing effort to conduct surveillance and control for their mosquito vectors as well, namely, Aedes aegypti. Although there have been no locally acquired cases of these viruses identified in Arizona, the presence of the vector species increases the risk for locally acquired cases to occur.
Zika is a flavivirus transmitted by mosquitoes and first identified in Uganda in 1947. In 2015, Zika virus circulation in the Americas began in Brazil. On February 1, 2016, the World Health Organization declared a Public Health Emergency of International Concern because of clusters of microcephaly and other neurological disorders in some areas affected by Zika. To date, Zika has not been spread by mosquitoes in the continental United States; however, lab tests have confirmed Zika virus in travelers returning to the United States and in some non-travelers who got Zika through sex with a traveler.
Zika virus spreads to people primarily through the bite of an infected Aedes species mosquito (A. aegypti and A. albopictus). Because the mosquitoes that spread Zika are found throughout the world, it is likely that outbreaks will spread to new countries.
Most people infected with Zika have no symptoms, but for those who do, the most common symptoms are fever, rash, joint pain, and conjunctivitis (red eyes), lasting from several days to a week.
Because of the lack of virus symptoms, travelers returning to the United States from an area with Zika should take steps to prevent mosquito bites for three weeks after returning so they do not spread Zika to mosquitoes.
Why Zika Is Risky for Some People
Zika infection during pregnancy can cause fetuses to have a birth defect of the brain called microcephaly. Other problems have been detected among fetuses and infants infected with Zika virus before birth, such as defects of the eye, hearing deficits, and impaired growth. There have also been increased reports of Guillain-Barré syndrome, an uncommon sickness of the nervous system, in areas affected by Zika.
Zika Virus Facts
West Nile Virus
- No vaccine exists to prevent Zika.
- Mosquitoes that spread Zika virus bite mostly during the daytime.
- Mosquitoes that spread Zika virus also spread dengue and chikungunya viruses.
West Nile virus (WNV) is a mosquito-borne virus found throughout the United States and is the most common mosquito-borne disease in Arizona. It is not spread by person-to-person contact or directly from animals to people.
West Nile virus was first detected in the United States in 1999 and in Arizona in 2003. It is not known how it was introduced into the United States, but it may have entered in an infected traveler, bird, or mosquito.
The virus can cause illness in people, horses, certain types of birds, and other animals. Mosquitoes become infected with the virus when they feed on birds that are infected. After a few days, infected mosquitoes can then transmit the virus to other birds, humans, and horses. Only certain species of mosquitoes carry the WNV; the most common are the Culex mosquitoes, which are common throughout Arizona.
WNV is most common in crows, ravens, and jays but has been reported in more than 280 bird species. Horses are also susceptible to WNV infection, but a vaccine is available through veterinarians. WNV infections affecting the nervous system have also been reported in squirrels, goats, sheep, and llamas, but the susceptibility of these animals to WNV is not well known. WNV illness in dogs and cats is rare.
People of all ages can get sick from WNV, but 4 in 5 people have no symptoms. Those affected have flu-like symptoms, such as fever, headache, body aches, swollen glands, and muscle weakness. Symptoms can last for days or weeks. In rare cases, West Nile virus can cause more serious conditions such as encephalitis (an inflammation of the brain) and/or meningitis (an inflammation of the linings of the brain and spinal cord).
There is no specific treatment for West Nile virus infection. As with other viral illnesses, supportive care and symptomatic therapy are given.
West Niles Virus Facts
St. Louis Encephalitis
- No vaccine exists to prevent WNV.
- It is the most common mosquito-borne illness in Arizona.
- Culex mosquitoes tend to bite from dusk to dawn.
Human disease cases of St. Louis encephalitis are very rare in Arizona and in the United States in general, but due to mosquito surveillance, public health professionals know that it’s still circulating in some mosquito populations. The Culex mosquito is the main carrier of St. Louis encephalitis.
The virus can cause illness in people, horses, certain types of birds, and other animals. Mosquitoes become infected when they feed on birds that are infected with the virus. After a few days, infected mosquitoes can then transmit St. Louis encephalitis to other birds, humans, and horses.
Signs and Symptoms
In the one percent of infected people that do become ill, symptoms start to show 5–15 days following the bite of an infected mosquito. Disease typically starts with fever, malaise, headache, dizziness, and nausea. Illness typically gets worse over a period of a week and then spontaneously resolves. Occasionally, the illness progresses to a more serious stage. This typically includes signs of central nervous system infection such as dizziness, stiff neck, confusion, disorientation, and altered mental status.
Dengue fever is primarily transmitted by Aedes aegypti and Aedes albopictus mosquitoes, which can also transmit chikungunya, Zika, and yellow fever viruses. Currently, there have been no locally acquired cases of dengue identified in Arizona; however, cases are brought into Arizona every year by travelers who pick up the virus while abroad.
Arizona is at risk for local transmission of dengue virus because of the presence of the Ae. aegypti vector mosquito, which is widespread throughout Arizona. This would most likely occur if an individual was infected while traveling outside of Arizona but then returned to Arizona and was bitten by an Ae. aegypti mosquito. If this mosquito then took a blood meal from a new host, it could transmit the virus to this next host. This person would be considered a locally acquired case.
Signs and Symptoms
An individual bitten by an infected mosquito usually develops signs and symptoms of disease 4–7 days following exposure. This typically starts with a sudden high fever accompanied by facial flushing, rash, body aches, myalgia, arthralgia, and headache. Sore throat, anorexia, nausea, and vomiting are also common. These symptoms will typically last 2–7 days.
Mild hemorrhagic manifestations are not uncommon and include petechial and mucosal membrane bleeding. Gastrointestinal bleeding or vaginal bleeding in women of childbearing age is also possible, especially in more severe disease.
In December 2013, the first cases of chikungunya in the western hemisphere were reported in the Caribbean. There have been locally acquired cases of chikungunya fever in Florida as well as a chikungunya positive mosquito found in the Houston area.
The introduction of chikungunya virus to the United States increases the risk of importation and local transmission to Arizona because of the presence of the mosquito Ae. aegypti. The possibility also exists for infected mosquitoes to travel across state and national borders.
Signs and Symptoms
Acute chikungunya fever usually lasts 3–10 days and is characterized by a sudden onset of high fever (usually greater than 102º F) and severe joint pain. Fever can last from several days to up to a week and is sometimes intermittent. Joint pain is usually symmetric and most commonly seen in the hands and feet but can manifest in other joints as well.
Other signs and symptoms can include headache, diffuse back pain, myalgia, nausea, vomiting, polyarthritis, tenosynovitis, rash, and conjunctivitis. In about 50 percent of patients, a rash occurs 2–5 days after fever onset. It is typically maculopapular and involves the trunk and extremities.
Together, as a community, we have a responsibility to each do our part to monitor and eliminate mosquito breeding opportunities. Check your yard and neighborhood for standing or stagnate water. Notify your HOA or city street maintenance department to report drainage issues that allow water to accumulate.
Maricopa County responds to mosquito concerns in residential areas and will schedule fogging appropriately. They can be reached at 602-506-0700. If you notice mosquito problems in local parks, contact the appropriate city park division.
Sources upon request.
Photo credit: Douglas Allen/iStock, Rattikankeawpun/iStock, elenaleonova/iStock
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