Date Published: July 5, 2019
Publisher: Public Library of Science
Author(s): Krijn Paaijmans, Marco Brustollin, Carles Aranda, Roger Eritja, Sandra Talavera, Nonito Pagès, Silvie Huijben, Vincent CORBEL.
A range of mosquito species that belong to the Culicidae family are responsible for the worldwide transmission of infectious arboviral diseases such as dengue fever, Zika, West Nile fever and Chikungunya fever. Spain is at risk of arbovirus outbreaks, as various arboviral diseases are frequently introduced and it has established competent vector populations. Autochthonous human cases of West Nile virus have been reported infrequently since 2004, and since October 2018 three autochthonous human case of dengue fever have been confirmed. In response to an outbreak of any arboviral disease, space spraying or fogging will be implemented to control adult mosquito populations. To ensure adulticiding is cost-effective, the insecticide susceptibility status of vectors throughout Catalonia, an autonomous region in north-eastern Spain, was assessed through standardized WHO tube and CDC bottle bioassays. All Culex pipiens populations tested were resistant to at least one of the pyrethroids tested, whereas Aedes albopictus populations were susceptible to all pyrethroids tested. More detailed studies on the Cx. pipiens populations from the Barcelona area (the capital and largest city of Catalonia) revealed resistance to all four classes of public health insecticides available (pyrethroids, carbamates, organophosphates and organochlorides). All Ae. albopictus populations were susceptible to those classes, except for one of the tests performed with pirimiphos-methyl (an organophosphate). Pyrethroids are currently the first line chemical class to be used in space spray operations in response to an outbreak of an arboviral disease. While pyrethroids can be effective in reducing Ae. albopictus populations, this class may not be effective to control Cx. pipiens populations.
A range of mosquito species that belong to the Culicidae family are responsible for the worldwide transmission of infectious arboviral diseases such as dengue fever, Zika, West Nile fever and Chikungunya fever. On the European continent, autochthonous disease cases (i.e. infections acquired by mosquito transmission in a currently non-endemic area) have been observed in several Mediterranean countries. Autochthonous transmission of dengue virus (DENV) has occurred in France in 2010 , Croatia in 2010  and on the Portuguese archipelago Madeira in 2012 , with more than 2000 local cases reported in the latter. Autochthonous West Nile virus (WNV) cases have been seen in France [4, 5] and Italy in 2011  and the same countries experienced outbreaks of Chikungunya virus (CHIKV), Italy in 2007  and 2017  and France in 2010  and 2017 .
This paper combines different insecticide resistance datasets that have been collected in different geographical regions by different research teams for different mosquito species over the past 5 years. The methodology and results sections are presented per mosquito species for clarity.
Here we provide evidence that several Cx. pipiens populations from Catalonia and Barcelona, which are potential vectors of arboviruses like WNV, USUV and RVFV [26, 27, 42] are resistant to some of the pyrethroid insecticides evaluated. We observed spatial variation in resistance phenotype, with high levels of resistance in Bellvis and to a lesser extent in Gava. One explanation for this observation could bethe extensive use of pyrethroids in the agricultural activity in the past and at present. In contrast, no large and repetitive adult and larval control have been applied in the area with these products nor in the past nor at present. Since the 1980’s, mosquito control in the area has been conducted with larvicides, formerly Temephos and since 1992 with Bti. No pyrethroids have been used as larvicides. Few adulticides have been conducted with Malathion, except for occasional adulticiding with pyrethroids. The only possible continuous application of adulticides using this family of insecticides originates from domestic insecticide devices that are largely used by public in general. Moreover, the Cx. pipiens populations from the Barcelona area are resistant to insecticides belonging to all four classes of public health insecticides available: Resistance is clearly demonstrated for DDT (organochloride), pirimiphos-methyl (organophosphate), propoxur (carbamate) and various pyrethroids. Again, one explanation is the widely use of DDT in agriculture and public health in the past in all Spain until 1977 when it was banned and the possible illegal use has been certainly almost inexistent. On the other hand, insecticide resistant genes may have migrated from other parts of Europe and/or the world, as extensive and long-distance migration of genes has been observed in Cx. pipiens .
At the moment insecticides remain one of the corner stones of arboviral disease outbreak control and prevention. The results presented here indicate that Ae. albopictus populations across Catalonia can be controlled by space spraying with pyrethroids and maybe -at least in the Barcelona area- with organophosphates, the two chemical classes approved for space spraying by WHO . For residual spray applications (of e.g. mosquito resting areas inside homes or cattle stables/sheds) the carbamate class can be used (again only confirmed for Barcelona). Having said that, the number of chemical classes approved for adulticiding by the European Union is limited, with pyrethroids being exclusively recommended in local and national guidelines . Unfortunately, the situation for Cx. pipiens appears worse (but note the discussion on appropriate diagnostic doses/times) as resistance to all four classes of public health insecticides has been observed. As we rely heavily on a very limited choice of chemicals (pyrethroids are currently the first line chemical class for adulticiding in response to outbreaks of any arboviral disease), proper resistance management strategies should be in place  for any vector species that is targeted. This necessitates continuous entomological surveillance to monitor vector population dynamics and their insecticide susceptibility status .