Mosquitoes are the most important insect pests that affect the health and wellbeing of humans and domestic animals worldwide. They can cause a variety of health problems due to their ability to transfer (vector) viruses and other disease-causing pathogens, even in the arid Southwest U.S.
Female mosquitoes usually require a blood meal from a vertebrate animal for egg production. Different species of mosquitoes have particular host animal preferences for blood-feeding. Some specialize on birds, some on humans, and still other mosquitoes feed on a wide range of hosts. During feeding the mosquito injects her saliva into the host’s skin, which can generate an itchy reaction. If she has acquired a disease pathogen from an earlier blood meal, and sufficient time has elapsed for the pathogen to develop inside the mosquito (incubation period), the pathogen may be transmitted to a new host. Incubation periods are different for each pathogen and can range from days to weeks.
Mosquito-vectored diseases of humans include: arboviral encephalitis (brain inflammation), dengue fever, chikungunya, Zika, yellow fever, malaria and filariasis. Mosquitoes are not known to transmit blood-borne pathogens such as hepatitis or HIV. Different mosquitoes vector specific diseases, and many mosquito species are not vectors of any human disease.
West Nile virus (WNV) is the most common mosquito- borne disease affecting humans in Arizona, as of this writing. WNV is vectored by Culex mosquitoes (described below). The virus is primarily a disease of birds, but can also be transmitted to humans and horses. St Louis encephalitis (SLE) and Western equine encephalitis (WEE) viruses are enzootic (native to and present in animal populations) in Arizona, and can occasionally result in local human infections through the bite of infected mosquitoes. Western equine encephalitis in Arizona has been very rare since 2000, but human cases of SLE have occurred in Arizona in recent years.
West Nile has occurred in Arizona since 2003, and is an endemic (meaning usually found among people or in a region) disease; the other diseases are less commonly reported in humans in Arizona. Currently (2019), dengue, chikungunya and Zika viruses are considered emerging diseases in the U.S. A disease is classified as emerging when the number of cases has increased over the past 20 years and could potentially increase in the future. Emerging diseases account for more than 12% of all human disease causing pathogens. Dengue, chikungunya and Zika viruses are not currently transmitted by mosquitoes in Arizona, but the mosquito vector, Aedes aegypti, is present in many cities and towns in Arizona. The viruses are circulating in Mexico, so there is a risk of outbreak in Arizona.
The most susceptible to the effects of these mosquito- borne pathogens are children and the elderly. However, in some instances life-threatening illness and/or permanent debilitation can occur in infected human hosts of any age.