The topic of mosquito-borne disease is complex, and presentation of information pertaining to it at times requires the use of technical terms. These terms are depicted in bold type and are explained in the glossary.
Mosquitoes are vectors of many different disease agents around the world. Depending on the species, mosquitoes are vectors of the protozoa that cause malaria, the nematode worms that cause filariasis, and a large number of arboviruses, which is the shortened term for arthropod-borne viruses. Arboviruses cause a number of diseases including two of great impact in tropical and subtropical regions of the world, namely, yellow fever and dengue fever.
This section will be limited in coverage to mosquito-borne viruses transmitted in Indiana. West Nile is the most recent arbovirus to be established, but other mosquito-borne viruses occur in the state. Arboviruses cause flu-like illness in the majority of patients, but they can cause severe disease such as encephalitis and meningitis, and may even cause death. In addition, survivors of an arbovirus infection can suffer life-long neurological damage known as "sequelae." The most serious sequelae include paralysis, learning and speaking difficulties, and mental retardation.
Nearly everyarbovirus is a zoonosis, including all that occur in Indiana. This means that arboviruses exist year after year in a non-human animal reservoir, usually birds or wild mammals, in which they have little or no ill effect. Arboviruses are transmitted naturally among reservoir animals by mosquitoes, but an infected mosquito can transmit an arbovirus to a human during the taking of a blood meal subsequent to feeding on an infected reservoir animal. Although humans can be infected and may suffer disease, we are considered incidental hosts of arboviruses in Indiana because we play no role in their maintenance and transmission.
It is important for the reader to know that there are currently no human vaccines or anti-viral drugs available for arbovirus infections in Indiana. You are encouraged to learn more about mosquito biology (see E-242-W "The Biology and Medical Importance of Mosquitoes in Indiana " (PDF 744KB)) and mosquito-borne diseases (see E-240-W "West Nile Virus in Indiana" (PDF 920KB)). This will enable you to make more informed decisions about health risks and how to protect yourself and your family.
Numerous factors determine the ability of a mosquito to transmit arboviruses that cause disease. First, a mosquito must be a competent vector. "Vector competence" refers to the ability of a mosquito to acquire an arbovirus from a reservoir host and later transmit the arbovirus to a susceptable host during the act of taking another blood meal. This is dependent on the mosquito being a suitable host in which the arbovirus survives, undergoes essential development, multiples, and eventually reaches and infects the salivary glands. Transmission of an arbovirus occurs by injection of salivary secretions during the act of feeding. It may surprise some people to learn that relatively few mosquito species are suitable hosts of arboviruses and therefore most species are not competent vectors.
Second, a mosquito species must possess "vector capacity." Vector capacity is determined by a number of factors such as vector competence, and mosquito population density, host preferences, and biting rates. Physiological factors also are important, including mosquito immunity to infection by specific arboviruses. Mosquitoes may be competent, but are not effective vectors if they have low vector capacity. Lack of competence and low capacity combined are the reason that relatively few species of mosquitoes are effective vectors of arboviruses in Indiana and the U.S.
Accurate identification and detailed knowledge of the biology of mosquito species and their potential involvement in arbovirus transmission is extremely important. For example, with regard to West Nile virus, competent vectors include species of mosquitoes in the genus Culex, but not common floodwater mosquitoes in the genus Aedes and the genus Ochlerotatus. Females of floodwater mosquitoes often exist in extremely high numbers and feed readily on humans, but they are not competent vectors of West Nile virus. Among competent Culex species, C. restuans has high vector capacity to birds, but low vector capacity to humans because female C. restuans feed almost exclusively on birds. In contrast, if abundant enough, C. salinarius has high vector capacity to humans because females are competent vectors and feed readily on both birds and mammals. Clearly, efforts to reduce transmission of West Nile virus among birds and potentially to humans should focus on the control of Culex mosquitoes, with the elimination of larval habitats or controlling larvae in these habitats being the most effective approach (see E-26-W "Mosquitoes in and Around the Home" (PDF 379KB), E-52-W "Mosquito Management by Trained Personnel" (PDF 283KB), and E-242-W "The Biology and Medical Importance of Mosquitoes in Indiana" (PDF 744KB)). In contrast, attempts to control floodwater mosquitoes will have little or no effect on the transmission of West Nile virus in Indiana.
Numerous species of mosquitoes are vectors of arboviruses in Indiana (see E-242-W"The Biology and Medical Importance of Mosquitoes in Indiana" (PDF 744KB)), including those in the genus Culex that are vectors of WNV and SLE virus. With regard to Culex control, people can greatly assist in source reduction of larvae by filling in wheel ruts and by emptying or removing a variety of containers that hold water in their yards and on their business properties. Source reduction is a highly effective means of controlling Culex mosquitoes and is very important because adult females typically seek a blood meal near the site in which larvae completed development.
The reader is advised that the application of insecticides to kill mosquito larvae in water is best left to professionals. The same is true with regard to applying insecticides for control of adult mosquitoes. Readers interested in mosquito control are directed to Purdue Extension publications E-26-W "Mosquitoes In and Around the Home" (PDF 379KB) and E-52-W "Mosquito Control By Trained Personnel" (PDF 283KB) and the websites listed at the bottom of this page.
There is a great deal of interest in mechanical devices that are purported to control mosquitoes. Devices featuring only ultraviolet light, known as "bug zappers," have been shown in scientifically controlled studies out-of-doors to be ineffective both in controlling mosquitoes and in reducing the biting rate of females. A new generation of devices that release chemicals such as carbon dioxide and other mosquito attractants have been shown to be more effective in killing adult mosquitoes in tests conducted under laboratory conditions. However, in the absence of scientific controlled studies out-of-doors, there are no data to show that these devices reduce the biting rate of vector mosquitoes. Readers are advised to use a recommended repellent when facing possible exposure to mosquito bites.
Mosquito repellents appear to act by masking odor cues given off by a person or by direct repellency to female mosquitoes seeking a blood meal. Repellent products containing DEET, picaridin, and oil of lemon eucalyptus are recommended by the Centers for Disease Control and Prevention (CDC). The CDC also recommends treating clothing with permethrin, which acts both as a repellent and an insecticide. Various other compounds such as citronella, garlic, and vitamin B are purported to be repellents, but research does not support this. Readers interested in detailed information regarding the effectiveness and proper use of mosquito repellents and permethrin are directed to the CDC website.
The disease links at the top of this page provide specific information pertaining to mosquito-borne diseases in Indiana. The key points are presented as "bullets" for quick reference. Additional information on mosquito biology and mosquito-borne diseases can be found in the references and at the websites presented at the bottom of this page. More accurate diagnosis and better reporting of arbovirus infections to CDC began in 1964, but, with the exception of WNV infections, data on number of cases since the mid to late 1990's are not generally available.
Many websites provide information on mosquito biology and control. The following websites are recommended as accurate and current sources of information.
A large number of books and other written publications are devoted to mosquitoes. Here we mention one that covers the biology, distribution, importance, and identifications of species of mosquitoes in Indiana.