Haplotype H1 of Culex pipiens Implicated as Natural Vector of Dirofilaria immitis in an Endemic Area of Western Spain

Various studies in Europe have reported several species of mosquitoes infected by D. immitis larvae, including Cx. pipiens in Spain, Italy, and Turkey; Cx. theileri in Madeira, Portugal, and the Canary Islands, Spain; Ae. vexans in Turkey; and Ae. albopictus, Ae. caspius, An. maculipennis, and Cq. richiardii in Italy.

“Various studies in Europe had reported several species of mosquitoes infected by D. immitis larvae such as Cx. pipiens in Spain (Morchón et al., 2007), Italy (Cancrini et al., 2006), and Turkey (Yildirim et al., 2011); Cx. theileri in Madeira, Portugal (Santa-Ana et al., 2006), and on the Canary Islands, Spain (Morchón et al., 2011a); Ae. vexans in Turkey (Biskin et al., 2010; Yildirim et al., 201”

Aedes albopictus and Cx. pipiens are recognised vectors of heartworm.

“Among the mosquito species that best adapt to these kinds of changes are Aedes albopictus and Cx. pipiens, which are recognised vectors of heartworm”

Larvae of D. immitis have been found in Cx. pipiens in the Iberian Peninsula and in Cx. theileri in the Canary Islands.

“Furthermore, larvae of D. immitis in Cx. pipiens in the Iberian Peninsula [14,15] and in Cx. theileri in the Canary Islands [16] have been found.”

The incrimination of vectors of Dirofilaria in Spain has so far led to the detection of Dirofilaria immitis DNA in the abdomen of two Culex pipiens (haplotype H1) in Salamanca and in Culex theileri in the Canary Islands, and no mature infections have been found.

“The incrimination of vectors of Dirofilaria in Spain has led so far to the detection of D. immitis DNA in the abdomen of two Culex pipiens (haplotype H1) in Salamanca (western Spain) and in Cx. theileri in the Canary Islands, and no mature infections have been found, as no DNA was detected in the head-thorax of the analysed specimens ( Morchón et al., 2007 and Morchón et al., 2011).”

The detection of Dirofilaria immitis in a single Culex pipiens specimen indicates a low prevalence in this species (0.16%), in line with previous reports from mainland Spain, although with a higher infection rate (0.3%).

“The detection of D. immitis in a single Cx. pipiens specimen is indicative of the nematode low prevalence in this species (0.16%), in line with previous reports from mainland Spain, although with a higher infection rate (0.3%) ( Morchón et al., 2007).”

Analysis of nuclear rDNA ITS-2 sequences allows validation of the classification and haplotype characterization of mosquitoes.

“According to Bargues et al. (2006) and Morchón et al. (2007), the analysis of nuclear rDNA ITS-2 sequences allows validation of the classification and haplotype characterization of mosquitoes.”

As a vector-borne disease, the distribution of D. immitis is influenced by climate, with regions with more favorable climatic conditions allowing proliferation of vector mosquitoes and higher prevalences.

“As it is a vector-borne disease, its distribution is influenced by the climate, so that those regions with more favorable climatic conditions allow the proliferation of vector mosquitoes and, therefore, presence of higher prevalences (9, 25, 26).”

Among the culicid mosquito species, Culex pipiens, Cx. quinquefasciatus and Ae. albopictus are known to be infected with Wolbachia and considered as vectors for Dirofilaria.

“Among the culicid mosquito species, Culex pipiens, Cx. quinquefasciatus and Ae. albopictus are known to be infected with Wolbachia [32, 67] and considered as vectors for Dirofilaria [13–15, 28, 45, 50, 69].”

Several previous studies have declared genetic and morphological differences responsible for the susceptibility or resistance of various mosquito species to Dirofilaria parasites.

“Several previous studies have declared genetic and morphological differences responsible for the susceptibility or resistance of various mosquito species to Dirofilaria parasites [56, 57]”

Cx. theileri, Cx. pipiens s.l., and An. maculipennis s.l. have already been implicated as vectors of Dirofilaria in Spain, Italy, Turkey, and Iran.

“Culex theileri, Cx. pipiens s.l. and An. maculipennis s.l. have already been implicated as vectors in countries such as Spain [15,20], Italy [16,17], Turkey [18] and Iran [21].”

Cx. pipiens infection rates in Spain, Italy, and Turkey have been reported as 0.3%, 0.048-0.54%, and 0.12%, respectively.

“Infection of Cx. pipiens in Spain has been detected with a rate of 0.3% [15], in Italy it ranges from 0.048% [17], much lower than this present study, to 0.26% and 0.54% [16,44], similar to our values, with an intermediate prevalence rate in Turkey of 0.12% [18].”

A study in a western region of Spain where D. immitis is endemic found D. immitis larvae only in the H1 haplotype of Culex pipiens but not in haplotypes H2 and H3.

“a study performed in a western region of Spain where D. immitis is endemic found D. immitis larvae only in the H1 haplotype of Culex pipiens but not in haplotypes H2 and H3 (286).”

C. pipiens is considered a potential primary vector for dirofilariasis wherever it resides because it is highly abundant in various regions of endemicity in Europe and Asia, remains active throughout summer, and has been found infected by Dirofilaria spp.

“For example, C. pipiens, which has been found to be infected by Dirofilaria spp., is highly abundant in various regions of endemicity in Europe and Asia, and its populations remain active throughout the summer; consequently, C. pipiens is considered a potential primary vector for dirofilariasis wherever it resides (10,30,286), although other species in the same areas can contribute to transmission”