Scientific review on mosquitoes and mosquito‐borne diseases

Isolation, characterization and functional analysis of a bacteriophage targeting Culex pipiens pallens resistance-associated Aeromonas hydrophila

BACKGROUND

Culex pipiens pallens is a well-known mosquito vector for several diseases. Deltamethrin, a commonly used pyrethroid insecticide, has been frequently applied to manage adult Cx. pipiens pallens. However, mosquitoes can develop resistance to these insecticides as a result of insecticide misuse and, therefore, it is crucial to identify novel methods to control insecticide resistance. The relationship between commensal bacteria and vector resistance has been recently recognized. Bacteriophages (= phages) are effective tools by which to control insect commensal bacteria, but there have as yet been no studies using phages on adult mosquitoes. In this study, we isolated an Aeromonas phage vB AhM-LH that specifically targets resistance-associated symbiotic bacteria in mosquitoes. We investigated the impact of Aeromonas phage vB AhM-LH in an abundance of Aeromonas hydrophila in the gut of Cx. pipiens pallens and its effect on the status of deltamethrin resistance.

METHODS

Phages were isolated on double-layer agar plates and their biological properties analyzed. Phage morphology was observed by transmission electron microscopy (TEM) after negative staining. The phage was then introduced into the mosquito intestines via oral feeding. The inhibitory effect of Aeromonas phage vB AhM-LH on Aeromonas hydrophila in mosquito intestines was assessed through quantitative real-time PCR analysis. Deltamethrin resistance of mosquitoes was assessed using WHO bottle bioassays.

RESULTS

An Aeromonas phage vB AhM-LH was isolated from sewage and identified as belonging to the Myoviridae family in the order Caudovirales using TEM. Based on biological characteristics analysis and in vitro antibacterial experiments, Aeromonas phage vB AhM-LH was observed to exhibit excellent stability and effective bactericidal activity. Sequencing revealed that the Aeromonas phage vB AhM-LH genome comprises 43,663 bp (51.6% CG content) with 81 predicted open reading frames. No integrase-related gene was detected in the vB AH-LH genome, which marked it as a potential biological antibacterial. Finally, we found that Aeromonas phage vB AhM-LH could significantly reduce deltamethrin resistance in Cx. pipiens pallens, in both the laboratory and field settings, by decreasing the abundance of Aeromonas hydrophila in their midgut.

CONCLUSIONS

Our findings demonstrate that Aeromonas phage vB AhM-LH could effectively modulate commensal bacteria Aeromonas hydrophila in adult mosquitoes, thus representing a promising strategy to mitigate mosquito vector resistance.

Toxicity and possible mechanisms of action of honokiol from Magnolia denudata seeds against four mosquito species

This study was performed to determine the toxicity and possible mechanism of the larvicidal action of honokiol, extracted from Magnolia denudata seeds, and its 10 related compounds against third-instar larvae of insecticide-susceptible Culex pipiens pallens, Aedes aegypti, and Aedes albopictus and Anopheles sinensis resistant to deltamethrin and temephos. Honokiol (LC₅₀, 6.13–7.37 mg/L) was highly effective against larvae of all of the four mosquito species, although the toxicity of the compound was lower than that of the synthetic larvicide temephos. Structure–activity relationship analyses indicated that electron donor and/or bulky groups at the ortho or para positions of the phenol were required for toxicity. Honokiol moderately inhibited acetylcholinesterase and caused a considerable increase in cyclic AMP levels, indicating that it might act on both acetylcholinesterase and octopaminergic receptors. Microscopy analysis clearly indicated that honokiol was mainly targeted to the midgut epithelium and anal gills, resulting in variably dramatic degenerative responses of the midgut through sequential epithelial disorganization. Honokiol did not affect the AeCS1 mRNA expression level in Ae. aegypti larvae, but did enhance expression of the genes encoding vacuolar-type H⁺-ATPase and aquaporin 4, indicating that it may disturb the Na⁺, Cl⁻ and K⁺ co-transport systems. These results demonstrate that honokiol merits further study as a potential larvicide, with a specific target site, and as a lead molecule for the control of mosquito populations.

The challenges posed by equine arboviruses

Equine populations worldwide are at increasing risk of infection by viruses transmitted by biting arthropods, including mosquitoes, biting midges (Culicoides), sandflies and ticks. These include the flaviviruses (Japanese encephalitis, West Nile and Murray Valley encephalitis), alphaviruses (eastern, western and Venezuelan encephalitis) and the orbiviruses (African horse sickness and equine encephalosis). This review provides an overview of the challenges faced in the surveillance, prevention and control of the major equine arboviruses, particularly in the context of these viruses emerging in new regions of the world.

Potential vectors of equine arboviruses in the UK

There is growing concern about the increasing risk of disease outbreaks caused by arthropod-borne viruses (arboviruses) in both human beings and animals. There are several mosquito-borne viral diseases that cause varying levels of morbidity and mortality in horses and that can have substantial welfare and economic ramifications. While none has been recorded in the UK, vector species for some of these viruses are present, suggesting that UK equines may be at risk. The authors undertook, therefore, the first study of mosquito species on equine premises in the UK. Mosquito magnet traps and red-box traps were used to sample adults, and larvae were collected from water sources such as tyres, buckets, ditches and pools. Several species that are known to be capable of transmitting important equine infectious arboviruses were trapped. The most abundant, with a maximum catch of 173 in 72 hours, was Ochlerotatus detritus, a competent vector of some flaviviruses; the highest densities were found near saltmarsh habitats. The most widespread species, recorded at >75 per cent of sites, was Culiseta annulata. This study demonstrates that potential mosquito vectors of arboviruses, including those known to be capable of infecting horses, are present and may be abundant on equine premises in the UK.