Genetic control and transgenic mosquitoes

Public Health Significance of Urban Pests Mosquitoes 369 368 • cytoplasmic incompatibility natural incompatibility of some allopatric populations of the same species; and • chromosomal translocation sterility inherited by crossing normal individuals with heterozygotes having a translocation. Except for some trials that used irradiated males, the theoretical prediction of the long- term impact of these methods on suppressing nuisance mosquitoes was not conclusive in field applications. Among the various possible reasons for this failure are: • an incomplete mixing of the populations released with the natural populations or beha- vioural differences in reproduction between these two populations; • the density threshold that allows regulation of the population after the introduction of the genetic modification was not reached; • the absence of compensation, by an increase in the number of released genetically modi- fied individuals when the natural population grows or the lack of competitiveness of the released sterile males; and • immigration of fertile females towards the trial area from nearby areas. At present, research is concentrating on the production of transgenic mosquitoes. These are genetically modified to: • prevent pathogen development in and transmission by the insects Blair, Adelman Olson, 2000; Christophides, 2005; • foster insecticide susceptibility or prevent insecticide resistance Carlson et al., 1995; Collins James, 1996; Hemingway, 1999; or • reduce mosquito populations Benedict Robinson, 2003; Pates Curtis 2005. Apart from ethical concerns about the release of uncontrollable transgenic organisms, it is doubtful whether the modified mosquitoes are sufficiently fit to replace the natural populations Spielman, 1994; Catteruccia, Godfray Crisanti, 2003.

11.9. Economic burden of mosquitoes

The economic burden of controlling mosquitoes and mosquito-borne diseases is very dif- ficult to assess, and few data from North America and Europe are available. Doubtless, expenditures in the United States have increased considerably since the advent of WNV, as control and preventive measures have been intensified in many states.

11.8.5.1. Space repellents

The smoke from burning basil-type herbs, seeds of the neem tree Azadirachta indica, or tree wood and resin of aromatic trees has been used with various levels of success to repel insects. Burning mosquito coils that contain pyrethrum significantly reduce mosquito biting rates outdoors, such as on terraces they should not be used indoors; other types of pyrethrum dispensers include electric vaporizing mats, electric liquid vaporizers and aerosol spray cans.

11.8.5.2. Personal repellents

Personal repellents are useful when visiting areas where severe biting is likely to be encountered. Among the most efficient repellents applied in most parts of the EU and United States are DEET N,N-diethyl-meta-toluamide, lemon eucalyptus oil p-men- thane-3,8-diol, ethyl butylacetylaminopropionate 3-[N-acetyl-N-butyl]aminopropionic acid ethyl ester and icaridin 1-piperidinecarboxylic acid 2-2-hydroxyethyl-1-methyl- propylester; of course, for safe use, some precautions have to be followed. Under special and extreme circumstances, a hooded jacket and fur clothing treated with permethrin are efficient repellents, as was demonstrated during studies undertaken in the Florida Everglades Schreck, Haile Kline, 1984.

11.8.5.3. Bednets

Bednets are simple to install, lightweight and durable. To be effective, they should cover sleepers completely yet be large enough so that sleepers do not come in contact with the fabric. Holes in bednets should be mended immediately, and nets should be let down before darkness and tucked beneath the sleeping mat or mattress. WHO has initiated programmes to reduce the transmission of tropical diseases, using insecticide-impregnated bednets Rozendaal, 1997; WHO, 2001. Insecticide-treated bed- nets have been successful in reducing the number of malaria infections in villages where the level of transmission of malaria is low or moderate. In some villages where the level of transmission is high, the use of insecticide-impregnated bednets has reduced the para- sitic load of infected people and, as a consequence, reduced morbidity by 50 and mor- tality by 20.

11.8.6. Genetic control and transgenic mosquitoes

Diverse approaches have been considered for genetic control of nuisance or vector mos- quito populations. Genetic control consists of the release of genetically modified indivi- duals in the field, to reduce or modify the composition of natural populations of target insects. It is based on a number of approaches Asman, McDonald Prout, 1981; Grover, 1985; Alphey et al., 2002: • the introduction of sterility in males by irradiation or chemicals the so-called sterile insect technique; • sterility induced by hybridization sterility inherited by crossing sibling species; Public Health Significance of Urban Pests Mosquitoes 371 370 of WNV transmission seasonal screening of individual samples and restricting screening to blood donations designated for immunocompromised recipients was most cost effec- tive Korves, Goldie Murray, 2006. Efforts are in progress to develop a vaccine against WNV Chang et al., 2004; Hall Khromykh, 2004. However, a cost–effectiveness analysis calculated for the United States indicated that universal vaccination would be unlikely to result in societal monetary savings, compared with the cost per case of illness under the present disease incidence rates Zohrabian, Hayes Petersen, 2006. According to this analysis, at a cost of US 8.7 billion in a hypothetical population of 100 million people, vaccination would prevent 256000 cases of illness including neuroinvasive diseases, lifetime disabilities and deaths from WNV during a 10-year period, given an average cost per case of illness of about US 34000. Under these assumptions, a universal vaccination programme would be cost effective only when the incidence of disease increased substantially or the costs of vacci- nation were below US 12 per person in the analysis, baseline vaccination costs were assumed to be US 100. The economic impact of mosquitoes, in particular of mosquito-borne diseases, must also be expanded to the animal industry. Horses are highly susceptible to infection with WNV, and many infections end in their death Murgue et al., 2001. Thus, for example, WNV cost the equine industries in Colorado and Nebraska more than US 1.25 million in 2002, with an additional US 2.75 million estimated for preventive measures USDA, 2003. Older data, from 1965, tell that American losses from mosquito attacks on livestock rea- ched US 25 million that year, including a US 10 million decline in milk production Rodhain Perez, 1985.

11.10. Benchmarks