Aedes (Stegomyia) albopictus (Skuse), the Asian tiger mosquito, was first introduced to the United States in 1985. The first species was recorded in Harris County, TX. This introduction has prompted a variety of research in order to determine potential risks and associated control measures. Since this original finding, populations have spread throughout the United States with established numbers in approximately 25 states. Aedes albopictus mosquitoes have been proven to successfully survive and proliferate in a wide range of habitats. This adaptability and tolerance has created a great deal of concern to the public health sector as they have been proven competent vectors of a number of arboviruses and established populations ultimately magnify current risks in associated areas (Ali and Nayer 1997). From a global perspective, Aedes albopictus has been recognized as the most invasive mosquito, spreading to approximately 36 countries over the last three decades (Andreadis 2009). The introduction of this species into the United States as well as many other countries has been through tire trade practices and expanded innovations in the transportation industry. Other advancements in worldwide trade have also contributed to not only Aedes albopictus introductions, but a multitude of invasive species. These unwanted introductions prompt further research and legislation in order to control the spread of disease and minimize overall environmental impacts.
The introduction of Aedes albopictus to the United States
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Aedes albopictus populations were reported initially in 1985 at a tire reprocessing plant in Houston, TX. The newly introduced strain had established itself by the fall of 1986. The rapid and successful growth of these populations initiated discussion among health organizations and local governments in order to determine the direction needed in order to control the spread of disease. Due to the serious implications associated with their arrival, expanded surveillance efforts put forth uncovered established populations in an additional 12 states within the U.S. (Reiter 1998). Some hypothesis regarding mode of entry include containerization and the Lighter Aboard Ship (LASH) system that are new to the transportation industry as well as standard tire and automobile transport coming from Asian countries (Reiter 1998). Obvious concerns of additional invasive pest introductions indicate a need for tighter laws on incoming trade commodities.
Industry reports have shown that there are approximately 2 billion scrap tires being stored and an additional 240 million tires being generated each year in the United States (Reiter 1998). Studies at U.S. ports indicate that approximately 25% of arriving tires have water in them, making them attractive oviposition sites as well as a preferential place for several other species that do not require the presence of water in order to contain viable eggs. Aedes albopictus is just one of many species that prefers to lay their eggs in tires and containers. Such knowledge means that not only could these tires have existing invasive pests in them, but will undoubtedly present higher risk of future infestations.
The primary concerns are a result of studies proving Ae. albopictus to be a competent vector of several alphaviruses and flavaviruses. The spread of this species across the country in such a successful way shows that they are highly adaptable and have the ability to overwinter and survive at temperatures other than their native isothermal requirements (Novak 1995). These populations are now established in the United States therefore requiring measures to understand and control their spread as well as protect the transmission of disease throughout the country.
General biology and habitat requirements of Aedes albopictus
Mosquitoes undergo complete metamorphosis, meaning they pass through several life stages in order to reach maturity. The duration of time required to complete the entire life cycle varies from species to species. Aedes albopictus, as well as several other mosquito species, have the unique ability to lay their eggs on a dry substrate and remain viable for up to two years. Egg hatching is triggered by a number of external cues, such as light availability and duration, oxygen content in the water, periods of desiccation, humidity and temperature. The dormant state of the egg makes control of such populations particularly difficult in that the majority of control measures cannot affect unhatched eggs. Laboratory experiments on the initial stage of development of Ae. albopictus have proven the temperate strains to be among the more cold tolerant species, exhibiting the ability to overwinter as diapausing eggs (Delatte et al. 2009).
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Mosquitoes in general are poikilothermic, making them susceptible to outside temperature variations. Several temperature related studies have been conducted by Delatte et al. uncovering very useful requirement parameters to support Ae. albopictus proliferation. One particular study reviewed life expectancy as it relates to temperature, as well as development duration and success. Adult survival was shown to be inversely correlated with temperature with adult females outliving males at all temperatures. Delatte et al. discovered optimal temperatures for larval development at 29.74° C. and established populations at high altitudes to be proof of their ecological plasticity. They found that females, under favorable conditions, laid their eggs over several days. This might suggest that Ae. albopictus can lay their eggs in more than one oviposition site from a single gonotrophic cycle (Delatte 2009).
Aedes albopictus populations have varied blood-feeding behaviors with preferences to mammalian hosts; however a fair amount of avian blood has been detected in sampling efforts (Richards et al. 2006). These studies showed humans to be the primary sources of blood followed by several domestic pets in urban settings. This information suggests that Ae, albopictus can sufficiently serve as a bridge vector of several arboviruses and flaviviruses as well as adapt readily to a range of habitats (Richards et al. 2006). This supports the finding of populations in urban structures such as tires, flower pots and vases, buckets, gutters and bird baths as well as natural containers such as tree holes and tank bromeliads (Ali and Nayer 1997). Although albopictus populations tend to be weak fliers, they will diligently seek out adequate resources; with this adaptability risk of spreading sylvatic restricted pathogens such as Yellow Fever to Urban modified environments results (Maciel-de-Freitas et al. 2006).
Determining the reasons for such reproductive success are key to determining how best to control existing populations as well as understanding the potential expansion possibilities. It has been shown that when host availability is high, an increase in offspring results. This means that oviposition will occur in many sites, thus enhancing survivorship. This behavior is known as "skip oviposition" and could be a key contributor to Ae. albopictus success in some environments (Maciel-de-Freitas et al. 2006). Additional research has shown Ae. albopictus to competitively displace several local mosquito populations, such as Aedes aegypti in many areas. Such interspecific competition has been shown to increase development times on occasion but did not prove oviposition site selection of albopictus mosquitoes to change (Black et al. 1989).
Public Health concerns and associated risks
The rapid establishment of Ae. albopictus in areas across the United States has created concerns in the public health sector due to its potential vectoring capabilities. The host seeking behaviors exhibited by albopictus populations indicate their ability to spread disease readily from not only sylvatic to urban environments, but also from avian hosts to mammalian ones. Aggressive biting tendencies increase risk of transmission of flavaviruses such as Japanese Encephalitis, West Nile Virus and Yellow Fever. Laboratory studies had shown that Ae. albopictus is capable of vertical and horizontal transmission of alphaviruses such as Ross River Encephalitis, Western Equine Encephalitis, Chickungunya, Eastern Equine Encephalitis, Mayaro, Venezualan Equine Encephalitis, Sindbis as well as dog-heartworm, Dirofilaria immitis (Ali and Nayer 1997). With this lengthy list, it has been an aggressive goal of professionals to keep populations under control.
The Asian tiger mosquito is able to pass pathogens via a transovarial route. This mode of transmission makes Ae. albopictus a competent maintenance host for many viruses effecting man. The viruses transmitted by this mosquito have not responded to prophylaxis or antibiotics, therefore making detection and prevention critical (Ali and Nayer 1997).
Current and Future Control Practices
With Ae. albopictus populations arriving to the United States at alarming rates via the tire industry and commercial trade practices, we have been forced to refocus our attentions on control measures as opposed to simple prevention measures. Although tires have proven to be the primary habitat for larval development, it is important to consider various strategies for control.
An obvious first step is a movement to control the number of tires available to mosquitoes and another is to find reasonable ways to dispose of or eliminate these sources. Any comprehensive control plan should begin with source reduction measures. It is the most effective step, but not always the easiest.
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Following source reduction measures is a strong surveillance program. Trapping, identification, testing and location of breeding sites are critical in determining the level of risk present as well as the degree of control that needs to be taken. These steps often are limited by funding and man power so the ability to prioritize treatments is imperative.
There are a number of strategies available for control and treatment. Such strategies include the use of biological control agents such as predatory mosquitoes from the genus Toxorhynchites, copepods and various bacterial larval formulations. Chemical options include products such as organophosphates, pyrethroids and insect growth regulators (IGRs) and various adulticides such as resmethrin and malathion (Ali and Nayer 1997). The final step critical to any control program is pursuing ongoing education, training, public relations and legislation/enforcement policies.
Legislation and enforcement
The arrival of this mosquito gave many the opportunity to take a closer look at the solid waste issues seen across the country as well as the growing tire problem. This has presented a challenge to public health as well as the recycling industry. It is obvious that some sort of action is necessary in order to slow the spread of invasive species.
For this reason, the U.S. government has enforced legal requirements for disinsection of all used tires from Asia. They must be certified as dry, clean and free of insects. The enforcement responsibilities fall on the Division of Quarantine (Reiter 1998). Disinsection must be done by the exporter via fumigation, steam cleaning, or cleaning with a pressurized chemical mixture. This process needs to be documented and supplied to the importer during clearance by U.S. Customs. Non compliance to this process can result in holding of commodities and treatment at the shipper's expense, however the law does not currently allow for monetary penalties to be assessed (Reiter 1998).
This is clearly a lofty responsibility and one that has faults. The labor-intensive job of packing and unpacking results in many shipments passing customs without complete inspection. Even small percentages of loads that escape scrutiny can result in serious invasive pest introductions.
As mentioned, prevention and source reduction components of control often come with large price tags and a require cooperation of efforts and resources. It is critical the government and local entities work together in order to reduce risk. One such example of efforts to control tire waste and promotion of recycling and environmentally effective practices was put forth by the Governor of Illinois. The establishment of the Waste Tire Act was a multi-level approach to solving this very complicated, but critical problem.
Case Study: Illinois Waste Tire Act
Individuals were appointed to a task force specifically in place to address the drafting of legislation in order to reduce the overall number of tires, determine the opportunities for funding and coordinate management efforts. The goals of the Illinois Waste Tire Act were: 1) to promote collection and disposal of waste tires for recycling, 2) oversee removal of used and waste tire dumps, 3) to encourage the use of waste tires in environmentally safe ways, such as in energy recovery, 4) promote research and further efforts involved in disease reduction(Novak 1995).
The combined efforts of several regulatory agencies such as the EPA, the Pollution Control Board, the Department of Public Health and the Department of Agriculture as well as cooperation with the Department of Energy and Natural Resources were required. The Department of Energy held a research mandate, such efforts included studies on optimal chemical application methods, development of a disease detection programs, and genetic manipulation experiments utilizing diapause and overwintering traits among Ae. albopictus. A second phase was established to develop models to assist in determining mosquito abundance and transmission risk and to develop pest management strategies (Novak 1995).
Components of the act included a ban on open dumping and burning of waste tires. There were regulations set forth regarding storage, transportation and altering tires as well as requirements for businesses to obtain permits. A challenge of finding appropriate funding revenue as well as a division of resources was required. This was accomplished through a $0.50 increase in Automobile title fees and new tire taxes (Novak 1995). Caps on revenue and allocation of funding was required and employed.
Enforcement of this legislation was put under the control of the Illinois pollution control board. And additional training and education fell under the oversight of the Department of Agriculture.:
They determined the following markets for used tires in the United States; tire derived fuel, rubberized asphalt, playground cover, retreading and recapping, road bed subsurfaces and railroad subsurfaces as well as uses in erosion control.
This intricate and thorough plan has slowed the spread of Ae. albopictus in Illinois. The comprehensive nature and cooperative efforts reflect the importance of control through an integrated approach.
Eradication of Ae. albopictus from the United States is certainly not an achievable goal, nor is the prevention of all other invasive species, however the pressure that exists as a result of these introductions have forced professionals in various related industries to think differently about the way we once conducted business. Changing current practices can clearly slow the spread of disease and ultimately reduce risk. Prevention of such introductions as well as control of their dispersal will not only protect our country, but minimize the possibility of genetic variation and resulting new diseases.
Tighter legislation and support of enforcement practices is an ongoing goal, allowing local governments the autonomy to review individual needs and risks based on location and resources. Cooperation and support from national programs and resources is required to establish standardized procedures and ensure consistency throughout the country.
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