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2021-04-28
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Inference of the worldwide invasion routes of the pinewood nematode Bursaphelenchus xylophilus using approximate Bayesian computation analysis

Recommended by based on reviews by Aude Gilabert and 1 anonymous reviewer

Extracting the maximum historical information on pine wood nematode worldwide invasion from genetic data

Redistribution of domesticated and non domesticated species by humans profoundly affected earth biogeography and in return human activities. This process accelerated exponentially since human expansion out of Africa, leading to the modern global, highly connected and homogenized, agriculture and trade system (Mack et al. 2000, Jaksic and Castro 2021), that threatens biological diversity and genetic resources. To accompany quarantine and control effort, the reconstruction of invasion routes provides valuable information that help identifying critical nodes and edges in the global networks (Estoup and Guillemaud 2010, Cristescu 2015). Historical records and genetic markers are the two major sources of information of this corpus of knowledge on Anthropocene historical phylogeography. With the advances of molecular genetics tools, the genealogy of these introductions events could be revisited and empowered. Due to their idiosyncrasy and intimate association with the contingency of human trades and activities, understanding the invasion and domestication routes require particular statistical tools (Fraimout et al. 2017).

Because it encompasses all these theoretical, ecological and economical implications, I am pleased to recommend the readers of PCI Zoology this article by Mallez et al. (2021) on pine wood nematode invasion route inference from genetic markers using Approximate Bayesian Computation (ABC) methods.

Economically and ecologically, this pest, is responsible for killing millions of pines worldwide each year. The results show these damages and the global genetic patterns are due to few events of successful introductions. The authors consider that this low probability of introductions success reinforces the idea that quarantine measures are efficient. This is illustrated in Europe where the pine-worm has been quarantined successfully in the Iberian Peninsula since 1999. Another relevant conclusion is that hybridization between invasive populations have not been observed and implied in the invasion process. Finally the present study reinforced the role of Asiatic bridgehead populations in invasion process including in Europe.

Methodologically, for the first time, ABC was applied to this species. A total of 310 individual sequences were added to the Mallez et al. (2015) microsatellite dataset. Fraimoult et al. (2017) showed the interest to apply random forest to improve scenario selection in ABC framework. This method, implemented in the DiYABC software (Collin et al. 2020) for invasion route scenario selection allows to handle more complex scenario alternatives and was used in this study. In this article by Mallez et al. (2021), you will also find a clear illustration of the step-by-step approach to select scenario using ABC techniques (Lombaert et al. 2014). The rationale is to reduce number of scenario to be tested by assuming that most recent invasions cannot be the source of the most ancient invasions and to use posterior results on most ancient routes as prior hypothesis to distinguish following invasions. The other simplification is to perform classical population genetic analysis to characterize genetic units and representative populations prior to invasion routes scenarios selection by ABC.

Yet, even when using the most advanced Bayesian inference methods, it is recognized by the authors that the method can be pushed to its statistical power limits. The method is appropriate when population show strong inter-population genetic structure. But the high number of differentiated populations in native area can be problematic since it is generally associated to incomplete sampling scheme. The hypothesis of ghost populations source allowed to bypass this difficulty, but the authors consider simulation studies are needed to assess the joint effect of genetic diversity and number of genetic markers on the inference results in such situation. Also the need to use a stepwise approach to reduce the number of scenario to test has to be considered with caution. Scenarios that are not selected but have non negligible posterior, cannot be ruled out in the constitution of next step scenarios hypotheses.

Due to its interest to understand this major facet of Anthropocene, reconstruction of invasion routes should be more considered as a guide to damper biological homogenization process.

References

Collin, F.-D., Durif, G., Raynal, L., Lombaert, E., Gautier, M., Vitalis, R., Marin, J.-M. and Estoup, A. (2020) Extending Approximate Bayesian Computation with Supervised Machine Learning to infer demographic history from genetic polymorphisms using DIYABC Random Forest. Authorea. doi: https://doi.org/10.22541/au.159480722.26357192

Cristescu, M.E. (2015) Genetic reconstructions of invasion history. Molecular Ecology, 24, 2212–2225. doi: https://doi.org/10.1111/mec.13117

Estoup, A. and Guillemaud, T., (2010) Reconstructing routes of invasion using genetic data: Why, how and so what? Molecular Ecology, 9, 4113-4130. doi: https://doi.org/10.1111/j.1365-294X.2010.04773.x

Fraimout, A., Debat, V., Fellous, S., Hufbauer, R.A., Foucaud, J., Pudlo, P., Marin, J.M., Price, D.K., Cattel, J., Chen, X., Deprá, M., Duyck, P.F., Guedot, C., Kenis, M., Kimura, M.T., Loeb, G., Loiseau, A., Martinez-Sañudo, I., Pascual, M., Richmond, M.P., Shearer, P., Singh, N., Tamura, K., Xuéreb, A., Zhang, J., Estoup, A. and Nielsen, R. (2017) Deciphering the routes of invasion of Drosophila suzukii by Means of ABC Random Forest. Molecular Biology and Evolution, 34, 980-996. doi: https://doi.org/10.1093/molbev/msx050

Jaksic, F.M. and Castro, S.A. (2021). Biological Invasions in the Anthropocene, in: Jaksic, F.M., Castro, S.A. (Eds.), Biological Invasions in the South American Anthropocene: Global Causes and Local Impacts. Springer International Publishing, Cham, pp. 19-47. doi: https://doi.org/10.1007/978-3-030-56379-0_2

Lombaert, E., Guillemaud, T., Lundgren, J., Koch, R., Facon, B., Grez, A., Loomans, A., Malausa, T., Nedved, O., Rhule, E., Staverlokk, A., Steenberg, T. and Estoup, A. (2014) Complementarity of statistical treatments to reconstruct worldwide routes of invasion: The case of the Asian ladybird Harmonia axyridis. Molecular Ecology, 23, 5979-5997. doi: https://doi.org/10.1111/mec.12989

Mack, R.N., Simberloff, D., Lonsdale, M.W., Evans, H., Clout, M., Bazzaz, F.A. (2000) Biotic Invasions : Causes , Epidemiology , Global Consequences , and Control. Ecological Applications, 10, 689-710. doi: https://doi.org/10.1890/1051-0761(2000)010[0689:BICEGC]2.0.CO;2

Mallez, S., Castagnone, C., Lombaert, E., Castagnone-Sereno, P. and Guillemaud, T. (2021) Inference of the worldwide invasion routes of the pinewood nematode Bursaphelenchus xylophilus using approximate Bayesian computation analysis. bioRxiv, 452326, ver. 6 peer-reviewed and recommended by Peer community in Zoology. doi: https://doi.org/10.1101/452326

2021-01-05
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Do substrate roughness and gap distance impact gap-bridging strategies in arboreal chameleons?

Recommended by based on reviews by Simon Baeckens and 2 anonymous reviewers

Gap-bridging strategies in arboreal chameleons

Until now, very little is known about the tail use and functional performance in tail prehensile animals. Luger et al. (2020) are the first to provide explorative observations on trait related modulation of tail use, despite the lack of a sufficiently standardized data set to allow statistical testing. They described whether gap distance, perch diameter, and perch roughness influence tail use and overall locomotor behavior of the species Chamaeleo calyptratus.
Peterson (1984) described already the pattern how and when the tail is moved when bridging the distance from one perch to another. The study by Luger et al. (2020) further explores how this bridging distance, as well as other perch parameters modulate this behavior and the importance of tail use in it. Zippel et al. (1999) study the underlying musculoskeletal anatomy of the tail in chameleons, showing that chameleons have a strikingly different tail anatomy than other prehensile squamates. The difference is (partially) to be seen in the capacity of tail autotomy, that has been lost in chameleons.
Luger et al. (2020) describe the role the tail has in bridging a gap, and show that challenging and acrobatic movements to bridge large gaps, or when grasping on not so rough surfaces, relies heavily on a strong tail. Full body suspension with the tail can explain why tail autotomy has been lost, thus explaining the diverging tail musculature. They speculate on the role of this behavior for sexual selection for males. Sexual selection for males with a higher gripping performance could explain why male chameleons perform better for their size. In addition, boldness could have played a role. The authors state that exploring personality and its links to morphology, performance, and behaviors like grap-bridging would be a worthwhile avenue for future research on sexual selection in reptiles.

References

Luger, A.M., Vermeylen, V., Herrel, A. and Adriaens, D. (2020) Do substrate roughness and gap distance impact gap-bridging strategies in arboreal chameleons? bioRxiv, 2020.08.21.260596, ver. 3 peer-reviewed and recommended by PCI Zoology. doi: https://doi.org/10.1101/2020.08.21.260596
Peterson, J. A. (1984). The locomotion of Chamaeleo (Reptilia: Sauria) with particular reference to the forelimb. Journal of Zoology, 202(1), 1-42. doi: https://doi.org/10.1111/j.1469-7998.1984.tb04286.x
Zippel, K. C., Glor, R. E., and Bertram, J. E. (1999). On caudal prehensility and phylogenetic constraint in lizards: the influence of ancestral anatomy on function in Corucia and Furcifer. Journal of Morphology, 239(2), 143-155. doi: https://doi.org/10.1002/(SICI)1097-4687(199902)239:2%3C143::AID-JMOR3%3E3.0.CO;2-O

2020-07-22
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The open bar is closed: restructuration of a native parasitoid community following successful control of an invasive pest.

Recommended by based on reviews by Kévin Tougeron and Miguel González Ximénez de Embún

Raise and fall of an invasive pest and consequences for native parasitoid communities

Host-parasitoid interactions have been the focus of extensive ecological research for decades. One the of the major reasons is the importance host-parasitoid interactions play for the biological control of crop pests. Parasitoids are the main natural regulators for a large number of economically important pest insects, and in many cases they could be the only viable crop protection strategy. Parasitoids are also integral part of complex food webs whose structure and diversity display large spatio-temporal variations [1-3]. With the increasing globalization of human activities, the generalized spread and establishment of invasive species is a major cause of disruption in local community and food web spatio-temporal dynamics. In particular, the deliberate introduction of non-native parasitoids as part of biological control programs, aiming the suppression of established, and also highly invasive crop pests, is a common practice with potentially significant, yet poorly understood effects on local food web dynamics (e.g. [4]).
In their study, Muru et al. [5] took advantage of an existing biological control program focusing on the Asian chestnut gall wasp Dryocosmus kuriphilus, an invasive (and highly damaging) pest of chestnut trees. The species is currently a successful invader in many geographic regions, including southern France, where local parasitoid communities failed to provide an adequate control since its widespread establishment in 2010 [6]. In response, the non-native parasitoid species Torymus sinensis, which is highly-specific to the Asian chestnut gall wasp, was massively released in commercial chestnut orchards across several regions in France and the island of Corsica. The pest population outbreak was successfully contained, and thanks to the vast amount of host-parasitoid interaction data collected as part of the program, the authors were able to explore the effects of the large fluctuations in Asian chestnut gall wasp natural abundances on native parasitoid communities, immediately before, and up to five years following the introduction of its natural enemy T. sinensis.
Using co-occurrence and clustering analyses, Muru et al. [5] demonstrate that the invasion and the consecutive (efficient) control of the Asian chestnut gall wasp by the parasitoid T. sinensis have a significant impact on the structure of local parasitoid food webs. In particular, following decline in the Asian chestnut gall wasp’s populations, native parasitoids markedly switched to alternative hosts, most likely due to their respectively higher relative abundances. This pattern seemed to be driven by the degree of generalism in native parasitoid species. Indeed, when its abundances were still relatively high, the Asian chestnut gall wasp was primarily attacked by species capable of exploiting a broad range of hosts, while at low population densities only specialist parasitoids such as Mesolobus sericeus were able to persist and compete with the non-native T. sinensis.
The current study is important for two major reasons. First, it underscores the value of long-term species interaction data in order to understand the dynamic nature of food webs, namely their structural flexibility in response to changes in the environment or, as in this case, large fluctuation in abundances of a major pest species. In this context, biological control programs could be a great source of data for exploring long-term, large-scale dynamics of species interactions, and their use in ecological studies deserves to be further emphasized. Second, the study adds to the increasing empirical evidence that mobile generalist foragers can display adaptive, frequency-dependent switching behaviour ([1], [7]), which has been suggested to act as a key stabilizing mechanism in food webs by buffering fluctuating population dynamics at larger spatial scales ([8- 10]).
However, the timing of such buffering seems important, especially in systems such as commercial chestnut orchards. Despite their capacity to adaptively switch their foraging behaviour, the response of the native parasitoid communities to the new, unfamiliar resource was not fast enough in order to contain the primary outbreak under an appropriate damage threshold, thus requiring the introduction of the more specialized parasitoid T. sinensis. Nevertheless, based on current ecological theory, results presented by Muru et al. [5] suggest that the response of native parasitoid community to fluctuating host dynamics – i.e. shifts in parasitoid foraging behaviour based on their traits – could be predictable. This is encouraging considering the growing impact of biological invasions and insect pest outbreaks, but also the need to implement efficient, yet sustainable strategies for crop protection. Future studies would show at what extent observations by Muru et al. [5] are generalizable over longer time periods or other model systems. Noticeably, better understanding about population dynamics and interactions with the broader community of hosts available across habitats should allow to fine-tune predictions about parasitoids’ response to fluctuating resources.

References

[1] Eveleigh ES, McCann KS, McCarthy PC, Pollock SJ, Lucarotti CJ, Morin B, McDougall GA, Strongman DB, Huber JT, Umbanhowar J, Faria LDB (2007). Fluctuations in density of an outbreak species drive diversity cascades in food webs. Proc. Natl. Acad. Sci. USA 104, 16976-16981. doi: 10.1073/pnas.0704301104
[2] Tylianakis JM, Tscharntke T, Lewis OT (2007). Habitat modification alters the structure of tropical host–parasitoid food webs. Nature 445, 202-205. doi: 10.1038/nature05429
[3] Murakami M, Hirao T, Kasei A (2008). Effects of habitat configuration on host–parasitoid food web structure. Ecol. Res. 23, 1039-1049. doi: 10.1007/s11284-008-0478-0
[4] Geslin B, Gauzens B, Baude M, Dajoz I, Fontaine C, Henry M, Ropars L, Rollin O, Thébault E, Vereecken NJ (2016). Massively introduced managed species and their consequences for plant–pollinator interactions. Adv. Ecol. Res. 57, 147-199. doi: 10.1016/bs.aecr.2016.10.007
[5] Muru D, Borowiec N, Thaon M, Ris N, Viciriuc M I, Warot S, Vercken E (2020) The open bar is closed: restructuration of a native parasitoid community following successful control of an invasive pest. bioRxiv, 2019.12.20.884908, ver. 6 peer-reviewed and recommended by PCI Zoology. doi: 10.1101/2019.12.20.884908
[6] Borowiec N, Thaon M, Brancaccio L, Warot S, Vercken E, Fauvergue X, Ris N, Malausa J-C (2014). Classical biological control against the chestnut gall wasp 'Dryocosmus kuriphilus' (Hymenoptera, Cynipidae) in France. Plant Prot. Q. 29, 7-10.
[7] Bartley TJ, McCann KS, Bieg C, Cazelles K, Granados M, Guzzo MM, MacDougall AS, Tunney TD, McMeans BC (2019). Food web rewiring in a changing world. Nat. Ecol. Evol. 3, 345–354. doi: 10.1038/s41559-018-0772-3
[8] Kondoh M (2003). Foraging adaptation and the relationship between food-web complexity and stability. Science. 299, 1388-1391. doi: 10.1126/science.1079154
[9] McCann KS, Rooney N (2009). The more food webs change, the more they stay the same. Philos. Trans. R. Soc. Lond. B Biol. Sci. 364, 1789-801. doi: 10.1098/rstb.2008.0273
[10] Valdovinos FS, Ramos-Jiliberto R, garay-Narváez L, Urbani P, Dunne JA (2010). Consequences of adaptive behaviour for the structure and dynamics of food webs. Ecol. Lett. 13, 1546-1559. doi: 10.1111/j.1461-0248.2010.01535.x

2020-07-03
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The 'Noble false widow' spider Steatoda nobilis is an emerging public health and ecological threat

Recommended by based on reviews by Michel Dugon and 2 anonymous reviewers

How the noble false widow spider Steatoda nobilis can turn out to be a rising public health and ecological concern

"The noble false widow spider Steatoda nobilis is an emerging public health and ecological threat" by Clive Hambler (2020) is an appealing article discussing important aspects of the ecology and distribution of a medically significant spider, and the health concerns it raises.
By contrast to previous studies (Dunbar et al., 2018; Warell et al., 1991; Bauer et al., 2019; BBC 2013, 2018), this article, with its extensive media and scientific literature review, shows that S. nobilis (Thorell, 1875) is now an important health concern in Britain. Indeed, the author shows that the population of this spider has significantly increased, at least since 1990, in both southern Britain and Ireland where it has remained greatly under-recorded. In these areas, S. nobilis is now often the dominant spider on and in buildings, in places in which there is a high a risk of bites, some of which are likely to be severe, in humans, with these bites largely under-recorded. According to Clive Hambler "There is thus a possibility of bites being left without adequate rapid treatment and monitoring - with a low but non-trivial risk of necrosis or sepsis".
The author points that one of the reasons for the lack of awareness of the risk is that arachnologists typically have a conflict of interest between the conservation of the species they study and raising concerns about spiders. This may lead them to understate the risk. Clive Hambler therefore calls for a closer, appropriately weighted attention to the frequency and risk of bites, based on all the information available, rather than being "dismissive of the possibilities of bites and impacts simply because many media reports contain major errors or alarmism". He also argues that the British Arachnological Society’s guidance on "false widow spiders" "needs substantive revision, both in terms of the likelihood of bites and the severity of effects."
Indeed, the author demonstrates that many inaccuracies have been published (see Table 3 of his manuscript) and, for each, he provides a correction and/or an alternative opinion. At the end of this MS (see Table 4), he provides testable speculations and hypotheses. As he rightly points out, testing is very important to fuel the debate, because "It will be very difficult to get a balanced and proportionate debate and response for such a confused and emotive issue, especially with the many misleading popular reports." He also suggests that research will require interdisciplinary collaboration between experts in many domains, including pathologists, immunologists, clinicians, ecologists, arachnologists, psychologists, physiologists, climatologists and epidemiologists.
This preprint is clearly descriptive and speculative, but well-written, interesting and certainly useful in terms of a review of the biology, ecology, potential dangerousness and distribution of S. nobilis, particularly for future studies. There is no doubt that arachnologists, the medical community and the media will be interested in this article, which is intended to sound the alarm. Naturalists in general will also be interested in this manuscript because it is an original and successful attempt to increase knowledge about a particular taxon based on diverse information sources.
The structure of the MS is a bit odd, with a certain toing-and-froing between the ecology/biology/distribution of the spider and the risks, dangerousness and venom of bites, but this is not problematic, as shown by the reviews of the manuscript - three reviews (available below) were written, two by specialists in this noble false widow (Michel Dugon and another researcher who wished to remain anonymous).
Despite the controversy surrounding certain of the statements made in this article, I therefore strongly recommend it and look forward to seeing the identified research priorities addressed.

References

[1] Hambler, C. (2020). The “Noble false widow” spider Steatoda nobilis is an emerging public health and ecological threat. OSF Preprints, axbd4, ver. 4 peer-reviewed and recommended by PCI Zoology. doi: 10.31219/osf.io/axbd4
[2] Dunbar J.P., Afoullouss S., Sulpice R., Dugon M.M. (2018) Envenomation by the noble false widow spider Steatoda nobilis (Thorell, 1875) - five new cases of steatodism from Ireland and Great Britain. Clin Toxicol (Phila). 56(6):433-435. doi: 10.1080/15563650.2017.1393084
[3] Warrell D.A., Shaheen J., Hillyard P.D., Jones D. (1991) Neurotoxic envenoming by an immigrant spider (Steatoda nobilis) in southern England. Toxicon. 29(10):1263-5. doi: 10.1016/0041-0101(91)90198-Z
[4] Bauer, T., Feldmeier, S., Krehenwinkel, H., Wieczorrek, C., Reiser, N. and Dreitling, R. (2019) Steatoda nobilis, a false widow on the rise: a synthesis of past and current distribution trends. NeoBiota 42: 19–43. doi: 10.3897/neobiota.42.31582
[5] BBC (2013). False widow spider bites footballer Steve Harris. http://www.bbc.co.uk/news/uk-england-devon-24470023 Accessed 1 November 2018.
[6] BBC (2018). False widow spider infestation schools to remain shut. https://www.bbc.co.uk/news/uk-england-london-45761046 Accessed 19 December 2018.

2020-07-01
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Sub-lethal insecticide exposure affects host biting efficiency of Kdr-resistant Anopheles gambiae

Recommended by based on reviews by Etienne Bilgo, Thomas Guillemaud, Niels Verhulst and 1 anonymous reviewer

kdr homozygous resistant An. gambiae displayed enhanced feeding success when exposed to permethrin Insect-Treated Nets

Malaria is a vector-borne parasitic disease found in 91 countries with an estimated of 228 million cases occurred worldwide during 2018. The 93% (213 million) of those cases were reported in the African Region (WHO 2019). Six species of Plasmodium parasites can produce the disease but only P. falciparum and P. vivax are the predominant species globally. More than 40 species of Anopheles mosquitoes are important malaria vectors (Asley et al. 2018). Intrinsic (genetic background, parasite susceptibility) and extrinsic (feeding host preference, host diversity and availability, mosquito abundance) factors affect the capacity of mosquitoes to vector the disease (Macdonald 1952). Malaria is prevented by chemoprophylaxis, vaccination, bite-avoidance and vector-control measures. The mainstays of vector control are long-lasting insecticide (pyrethroid) treated nets and indoor residual spraying with insecticides (Asley et al. 2018). The widespread use of pyrethroid insecticides forced the emergence of insecticide resistance in malaria vectors reducing the insecticidal effect. Mosquitoes can modify their behaviour avoiding insecticide contact and so potentially reducing vector control tools efficacy. In this sense, Diop et al. (2020) investigated whether pre-exposure to an Insecticide-Treated Net (ITN) modulates the mosquito ability to take a blood meal in Anopheles gambiae. By means of video recording experiments the authors analyzed how the feeding/bitting behaviour was affected by kdr mutation genotypes (homozygous susceptible – SS-, heterozygotes -RS- and homozygous resistant -RR-) when exposed to two different insecticides (permethrin and deltamethrin). According to the results, the blood-feeding success did not differ between the three genotypes in the absence of insecticide exposure. However, authors observed differences in the feeding duration and blood meal size. In example, RR mosquitoes spent less time taking their blood meal than RS and SS. On the other hand, RS mosquitoes took higher blood volumes than RR females. These differences can affect the mosquito fitness by decreasing/increasing the likelihood to be killed by the host defensive behavior or increase the oogenesis so enhancing fecundity. Regarding the effect of exposition to insecticides authors detected a strong relationship between kdr genotype and Knock Down (KD) phenotype when mosquitoes were exposed to Permethrin. Previously, the authors have evidenced that RR mosquitoes prefer a host protected by a permethrin-treated net rather than an untreated net and that heterozygotes RS mosquitoes have a remarkable ability to find a hole into a bet net (Diop et al. 2015, Porciani et al. 2017). With data here obtained, they demonstrated that kdr homozygous resistant An. gambiae displayed enhanced feeding success when exposed to permethrin ITN. The changes observed in the feeding/biting mosquito behaviour can affect their fitness shaping the evolution of the insecticide resistance in mosquitoes’ natural populations. Moreover, this may also alter parasite transmission dynamics by modifying vector/host interactions and so vector capacity.

References

World Health Organization (2019). World malaria report 2019. Geneva: World Health Organization; 2019. ISBN 978-92-4-156572-1
Ashley EA, Pyae Phyo A, Woodrow CJ (2018). Malaria. Lancet. 391(10130):1608‐1621. doi: 10.1016/S0140-6736(18)30324-6
Macdonald G (1952). The analysis of equilibrium in malaria. Trop Dis Bull 49: 813-828.
Diop MM, Chandre F, Rossignol M, Porciani A, Château M, Moiroux N and Pennetier, C. (2020). Sub-lethal insecticide exposure affects host biting efficiency of Kdr-resistant Anopheles gambiae. bioRxiv 653980, ver. 4 peer-reviewed and recommended by PCI Zoology. doi: 10.1101/653980
Diop MM, Moiroux N, Chandre F, Martin-Herrou H, Milesi P, Boussari O, et al. (2015) Behavioral cost and overdominance in Anopheles gambiae. PLoS ONE. 10(4):e0121755. doi: 10.1371/journal.pone.0121755
Porciani A, Diop M, Moiroux N, Kadoke-Lambi T, Cohuet A, Chandre F, et al. (2017) Influence of pyrethroïd-treated bed net on host seeking behavior of Anopheles gambiae s.s. carrying the kdr allele. PLOS ONE. 12(7):e0164518. doi: 10.1371/journal.pone.0164518

2020-01-10
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Culex saltanensis and Culex interfor (Diptera: Culicidae) are susceptible and competent to transmit St. Louis encephalitis virus (Flavivirus: Flaviviridae) in central Argentina

Recommended by based on reviews by 2 anonymous reviewers

Multiple vector species may be responsible for transmission of Saint Louis Encephalatis Virus in Argentina

Medical and veterinary entomology is a discipline that deals with the role of insects on human and animal health. A primary objective is the identification of vectors that transmit pathogens. This is the aim of Beranek and co-authors in their study [1]. They focus on mosquito vector species responsible for transmission of St. Louis encephalitis virus (SLEV), an arbovirus that circulates in avian species but can incidentally occur in dead end mammal hosts such as humans, inducing symptoms and sometimes fatalities. Culex pipiens quinquefasciatus is known as the most common vector, but other species are suspected to also participate in transmission. Among them Culex saltanensis and Culex interfor have been found to be infected by the virus in the context of outbreaks. The fact that field collected mosquitoes carry virus particles is not evidence for their vector competence: indeed to be a competent vector, the mosquito must not only carry the virus, but also the virus must be able to replicate within the vector, overcome multiple barriers (until the salivary glands) and be present at sufficient titre within the saliva. This paper describes the experiments implemented to evaluate the vector competence of Cx. saltanensis and Cx. interfor from ingestion of SLEV to release within the saliva. Females emerged from field-collected eggs of Cx. pipiens quinquefasciatus, Cx. saltanensis and Cx. interfor were allowed to feed on SLEV infected chicks and viral development was measured by using (i) the infection rate (presence/absence of virus in the mosquito abdomen), (ii) the dissemination rate (presence/absence of virus in mosquito legs), and (iii) the transmission rate (presence/absence of virus in mosquito saliva). The sample size for each species is limited because of difficulties for collecting, feeding and maintaining large numbers of individuals from field populations, however the results are sufficient to show that this strain of SLEV is able to disseminate and be expelled in the saliva of mosquitoes of the three species at similar viral loads. This work therefore provides evidence that Cx saltanensis and Cx interfor are competent species for SLEV to complete its life-cycle. Vector competence does not directly correlate with the ability to transmit in real life as the actual vectorial capacity also depends on the contact between the infectious vertebrate hosts, the mosquito life expectancy and the extrinsic incubation period of the viruses. The present study does not deal with these characteristics, which remain to be investigated to complete the picture of the role of Cx saltanensis and Cx interfor in SLEV transmission. However, this study provides proof of principle that that SLEV can complete it’s life-cycle in Cx saltanensis and Cx interfor. Combined with previous knowledge on their feeding preference, this highlights their potential role as bridge vectors between birds and mammals. These results have important implications for epidemiological forecasting and disease management. Public health strategies should consider the diversity of vectors in surveillance and control of SLEV.

References
[1] Beranek, M. D., Quaglia, A. I., Peralta, G. C., Flores, F. S., Stein, M., Diaz, L. A., Almirón, W. R. and Montigiani, M. S. (2020). Culex saltanensis and Culex interfor (Diptera: Culicidae) are susceptible and competent to transmit St. Louis encephalitis virus (Flavivirus: Flaviviridae) in central Argentina. bioRxiv 722579, ver. 6 peer-reviewed and recommended by PCI Entomology. doi: 10.1101/722579