Control of malaria and West Nile virus vectors

Numerous medically important parasites and pathogens, such as viruses, bacteria, protists and nematode worms, that cause serious diseases in humans are transmitted by mosquitoes. Malaria, a disease caused by parasitic protists belonging to the genus Plasmodium, and West Nile virus (WNV), an arbo-virus classified within the Japanese Encephalitis serological complex, rank among the most important of them.

Human malaria continues to be one of the most important global health problems, with an estimated number of 216 million annual cases and 655,000 deaths in 2010 [1]. Greece was declared malaria-free in 1974; however, 40 cases of Plasmodium vivax infection with an endemic configuration were reported in 2011, mostly in Eurotas, Lakonia [2]. Human malaria represents a typical example of a single-cycle disease that includes one single host (Homo sapiens), one parasite (Plasmodium species) and one insect vector (Anopheles mosquitoes). Only 30–40 species of Anopheles mosquitoes (out of 400 identified) are considered to be important vectors of the four Plasmodium species (P. falciparum, P. vivax, P. ovale and P. malariae) causing human malaria [5, 6]. At least two blood meals, one to gain the pathogen and a second to transmit the disease, are required for a female mosquito to act as a vector. The transmission cycle of Plasmodium species involves sexual replication in mosquito vectors and asexual replication in humans. Malaria transmission is mainly regulated by vector distribution, abundance, life expectancy, host feeding preference, feeding rate and competence [6].

Since 1994 WNV, the most widely distributed arbo-virus in the world, has started to become frequent in Mediterranean regions [3]. WNV cases in humans were documented in Greece for the first time in 2010 in central Macedonia, northern Greece [4]. The persistence of WNV in nature is achieved through an enzootic transmission cycle involving mainly ornithophilic (‘amplification vector’) mosquitoes (primarily Culex spp.) and birds. Mosquitoes with mixed feeding preferences (both humans and birds) carry the virus from infected birds to secondary, incidental and usually dead-end hosts, such as humans, horses and other non-avian vertebrates. Although WNV has been isolated in >60 species of mosquitoes, only members of the genus Culex are considered to be major amplification vectors [3].

The most efficient way to keep populations of mosquito vectors of malaria and WNV at low levels is through an integrated mosquito management (IMM) program that combines all available control methods in the most effective, economical and safe way [5].

A consistent surveillance program (including both larvae and adults) determining mosquito species composition, relative abundance and population dynamics in relation to climatic conditions, and mapping of seasonal breeding sites, is a prerequisite for an effective control program and evaluation of its success.

Larval mosquito sampling in aquatic habitats usually serves as a tool for estimating the relative abundance of a vector species, and assessing its population density before and after the application of larvicidal insecticides.

Adult mosquito surveillance verifies the presence of a mosquito vector species, assesses its relative abundance, sets action thresholds for control activities, and assesses the success of control actions [7]. A wide array of sampling techniques, including netting, collection with aspirators, direct human bait catches, suction traps and traps utilizing attractants (e.g. CO2 traps), are used for adult mosquito sampling.

Altering or eliminating mosquito larval habitats represents a permanent, effective and economical method of mosquito control [7]. This strategy, which may significantly reduce breeding sites, includes single sanitation measures, such as proper disposal of used tires, cleaning up of illegal dump sites and rain gutters, as well as more complicated water management projects carried out at a regional scale, such as impoundment and open marsh water management.

When reduction of larval habitats fails to maintain populations of mosquito vector species below a threshold level, implementation of larvicidal activities is necessary. Larviciding aims to keep the transmission risk low by suppressing the target mosquito population before it reaches adulthood and has the ability to disperse [7]. Because mosquito larvae are often concentrated in limited habitats, the application of larvicides is restricted to smaller areas compared with adulticides. Mapping breeding habitats represents a valuable tool for effective larviciding, because habitats of even limited size may result in large mosquito broods.

Adulticiding is recommended when accurate surveillance data show that the risk of disease transmission is high and that therefore a significant proportion of the adult mosquitos needs to be eliminated. Indoor residual spray (IRS) and insecticide-treated nets (ITNs) represent two of the most important tools for controlling malaria vectors. IRS involves the application of stable insecticide formulations to the interior sprayable surfaces (walls, roofs and ceilings) of houses, and remains the most effective way of interrupting malaria transmission, especially in epidemiological settings (recently also in malaria-endemic areas) with seasonal transmission where mosquito vectors mainly rest indoors [8]. A prerequisite for the success of IRS is the co-operation of the inhabitants so that complete coverage of their houses is achieved. ITNs (which only recently became widely used) impregnated with pyrethroids decrease the physical contact between humans and mosquito vectors and kill some of the endophagic nocturnal-biting Anopheles mosquitoes [8].

Adulticiding operations against mosquito vectors of WNV should be determined according to the (1) general ecology and topography of the region, (2) relative abundance, distribution, flight range and age structure of the target mosquito population, (3) time period since the mosquito species was found to be infected with WNV, (4) flight range of bird WNV-amplifying hosts, (5) human demographic profile, and (6) persistence of WNV [7];  (7) accurate knowledge of the daily pattern of activity of the mosquito species is also necessary to achieve maximum control benefits. Finally, a strategy to manage resistance of mosquito vectors to insecticides is necessary in IMM programs.

The use of predators, parasites, pathogens, competitors and toxins from micro-organisms to suppress mosquito populations while avoiding toxicological effects in non-target organisms could play an important role as part of an IMM program. Numerous organisms (aquatic insects, entomopathogenic nematodes, entomopathogenic fungi and mosquitocidal bacteria such as Bacillus thurigensis) have been used as biological control agents; however, the larvivorous fish Gambusia affinis represents the best-known aquatic predator [5].

Evaluation of control efforts is a necessary component of an effective operational program against adult mosquitoes, while similar approaches (when feasible) may be followed for the evaluation of larvicide operations [7].

Control of both malaria and WNV vectors is a multidimensional, continuous process involving long-term planning, high levels of co-ordination, adoption of novel technologies, public awareness and the availability of funds.

References:

  1. World Health Organization (WHO). World Malaria Report 2011. Geneva: WHO; 2011.
  2. European Center for Disease Control and Prevention (ECDC) and WHO. Mission Report. Joint ECDC/WHO mission related to local malaria transmission in Greece in 2011. Summary September/October 2011. Stockholm: ECDC/WHO; 2012.
  3. Blitvich BJ. Transmission dynamics and changing epidemiology of West Nile virus. Animal Health Res Rev 2008;9:71-86.
  4. Papa A, Danis K, Baka A, et al. Ongoing outbreak of West Nile virus infections in humans in Greece, July–August 2010. Euro Surveill 2010;15:pii:19644.
  5. Becker N, Petric D, Zgomba M, et al. Mosquitoes and Their Control, 2nd edn. Berlin and Heidelberg: Springer-Verlag; 2010.
  6. Gullan PJ, Cranston PS. The Insects: An Outline of Entomology, 4th edn. Hoboken, NJ: Wiley-Blackwell; 2010.
  7. CDC. Epidemic/Epizootic West Nile Virus in the United States: Guidelines for Surveillance, Prevention, and Control, 3rd revision. CDC; 2003. Available at: http://www.cdc.gov/ncidod/dvbid/westnile/resources/wnv-guidelines-aug-2003.pdf.
  8. Karunamoorthi K. Vector control: a cornerstone in the malaria elimination campaign. Clin Microbiol Infect 2011;17:1608–1616.

Alexandros D. Diamantidis and Nikos T. Papadopoulos, Laboratory of Entomology and Agricultural Zoology, University of Thessaly, Greece

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